pH Measurement

Introduction

pH is a measure of  the acidity or basicity of a solution. It is defined as the co logarithm of the activity of dissolved hydrogen ions (H+). Hydrogen ion activity coefficients cannot be measured experimentally, so they are based on theoretical calculations. The pH scale is not an absolute scale; it is relative to a set of  standard solutions whose pH is established by international agreement. The lower case letter "p" in pH stands for the negative common (base ten) logarithm, while the upper case letter "H" stands for the element hydrogen. Thus, pH is a logarithmic measurement of  the number of  moles of  hydrogen ions (H+) per litre of  solution. Incidentally, the "p" prefix is also used with other types of  chemical measurements where a logarithmic scale is desired, pCO2 (Carbon Dioxide) and pO2 (Oxygen) being two such examples. ‐

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The logarithmic pH scale works like this: a solution with 10 12 moles of  H+ ions per liter has a pH of  12; a solution with 10 3 moles of  H+ ions per liter has a pH of  3. While very uncommon, there is such a thing as an acid with a pH measurement below 0 and a caustic with a pH above 14. Such solutions, understandably, are quite concentrated and extremely reactive. [1] ‐

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History The concept of pH was first introduced by Danish chemist Søren Peder Lauritz Sørensen at the Carlsberg Laboratory in 1909. It is unknown what the exact definition of  p is. Some references suggest the p stands for “Power”, others refer to the German word “Potenz” (meaning power in German), and still others refer to “potential”. Jens Norby published a paper in 2000 arguing that p is a constant and stands for “negative logarithm”; which has also been used in other works. H stands for Hydrogen. Sørensen suggested the notation "PH" for convenience, standing for "power of  hydrogen", using the cologarithm of  the concentration of  hydrogen ions in solution, p[H] Although this definition has been superseded p[H] can be measured if  an electrode is calibrated with solution of  known hydrogen ion concentration. [1]

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Measurement methods There are two methods for measuring pH; a probe and meter, and litmus paper. The most accurate and reliable method is the probe and meter. This method is no less convenient than the other method, but requires a more expensive piece of  equipment.

1. Litmus Paper: The term litmus comes from an Old Norse word meaning “to dye or color”. This is fitting since the lichens used to make litmus have also been used to dye cloth for hundreds of years. Very little information is available about the beginnings of  litmus. There is some data that suggest that litmus paper was developed by J.L. Gay Lussac, a French chemist during the early 1800s. ‐

Figure(1).Litmus paper.

Litmus paper is the most recognized member of  chemical indicators. Litmus changes color when exposed to an acidic or basic solution. The simple pH scale ranges from 0 14 with 0 being the most acidic, 7 being neutral, and 14 being the most basic or alkaline. Litmus paper is commonly used in educational science classes. Because it has such wide recognition, it has become a cultural reference in our society as well. It is common to use the term litmus test when referring to a test in which a single factor determines the outcome. While litmus paper is effective at indicating whether a substance is acidic or basic, it cannot report an exact numerical pH value. ‐

The Future

Litmus paper will most certainly continue to be used extensively in education due to its reasonable cost and ease of  use. However, some varieties of  lichens are becoming extinct. As a result, it is possible that manufacturers of  litmus paper may switch to synthetic materials in the future. This is already being done by manufacturers of  other types of pH papers. Additionally, because litmus cannot give quantitative results, it cannot replace other pH papers and pH meters. In fact, the trend is to make pH indicators that are even more accurate and less subjective. One such trend is to utilize fiber optic probes in pH meters in order to make them even more sensitive. [3]

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2. Potentiometry method: All potentiometry does is to measure (meter) the voltage (potential) caused by the hydronium ion: H3O+. This new method gave accuracy, reliability and fast results. Tools for measuring pH:

1. A pH meter (to accurately measure and transform the voltage caused by hydronium ion into a pH value); 2. A pH electrode (to sense all the hydronium ions and to produce a potential). 3. A reference electrode (to give a constant potential no matter what the concentration of  hydronium ion is). The pH meter

Basically, a pH meter measures the potential between the pH electrode (which is sensitive to the hydronium ions) and the reference electrode (which doesn't care what's in the solution). The pH electrode

The pH electrode's potential changes with the H3O+ ion concentration in the solution. The clever bit is that the pH electrode only senses the hydronium ions. This means that any voltage produced is from hydronium ions only. This way we can relate the potential directly to the hydronium concentration. Figure(2). pH electrode

The reference electrode

The reference electrode supplies a “constant” value against which we measure the potential of  the pH electrode. That's the thing about potentials, they have to be in Pairs to produce a voltage. [2] The function of  the diaphragm on a pH electrode:

Figure(3).reference electrode

The diaphragm on a pH electrode allows a flow of  electrolyte from the reference solution into the solution being measured. This completes the electrical circuit between the reference and sensing electrode and gives the potential difference which is measured by the pH meter. The type of  diaphragm used will influence the outflow of  electrolyte into the sample, which will in turn affect the response speed of  the electrode. The common diaphragm types are listed in table(B.1) in appendix(B) below along with their inherent properties. For most common applications a platinum diaphragm is the one of  choice.

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Glass bulb at the bottom of  electrodes

The glass membrane on pH electrodes comes in a variety of  different shapes based on the fitness for purpose of  the electrode. The most common membrane shapes in use today are outlined in table (B.2) in appendix (B). In practice there is a trade off  between the speed of  response of  the electrode and its ruggedness, thus the application the electrode is being selected for will govern the choice of  membrane design. The classical sphere shaped membrane offers low electrical resistance due to its large surface area but is relatively fragile. The cone shaped membrane is seen as a universal design as it is robust, of  medium resistance and is easy to clean. ‐

The difference between the various types of  membrane glass for pH electrodes

As pH is measured in a wide variety of  solutions at varying temperatures it is not possible to formulate a single pH membrane glass which can give accurate and reliable pH measurements in all conditions. For this reason different types of  membrane glass have been developed to give good performance in a variety of  measurement conditions. The most common membrane glass types found in Schott electrodes are as outlined in table (B.3) in appendix (B). [5] The classical set up ‐

The classical set up for measuring pH consisted of  a pH meter; a pH electrode and a reference electrode. [2] ‐

The modern set up ‐

Figure(4).Classical setup

Although you can perfectly measure pH using the "classical" set up, it was soon realized that the two electrodes could be built into the same probe (although there still are two totally separate electrodes). This is nowadays called the combined pH electrode, which is much more practical. ‐

The formula for pH calculation

Figure(5).Modern setup

From the above we know that in order to measure pH we measure potential differences. Nernst studied the relation between the mV (millivolts) reading produced by a pH electrode against a standard hydrogen electrode and the pH of  the measuring solutions. Now, a standard hydrogen electrode is a rather complicated piece of  equipment. Suffice to say that it gives very well defined and reproducible

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mV readings over a wide range of  pH values and strictly responds only to hydronium ions. Nernst equation:

U =U 0 + (2.303RT/ziF) log(H3O+) Where: ‐

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U: is the potential measured between indicator and reference electrode. U0: is the standard potential of the electrode assembly, depends on its construction (well made electrodes give 0 mV with pH 7 – neutral solution). R: is the gas constant (8.31441 J K 1 mol 1). T: is the absolute temperature in K (273.15 + t in °C). Zi: is the charge of  the hydronium ion (1+) (each ion carries a single positive charge with it). F: is Faraday constant (96 484.56 C * mol 1). ‐

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The pH meter basically uses this equation to calculate the pH value. However, all the individual parts within the equation have to be known (not everything is constant). The potential is measured by the electrodes and the temperature (T) needs to be measured as well and if  there isn't any temperature sensor connected to the meter, the temperature (T) value has to be entered. [2]

pH measurement in theory and in practice In theory a pH electrode is so constructed that its potential in buffer pH = 7 equals 0 mV. (A buffer is a solution which keeps a given concentration of  hydronium ions and therefore a given pH). This potential is called the zero potential of  a pH electrode. This is one of the two main characteristics of a pH electrode. The second characteristic is the electrode slope. The slope of  an electrode gives the change of  potential measured between two buffers with a pH difference of  1 in comparison to what is expected by theory (from Nernst). Nernst calculated this value as 59.16 mV/pH at 25 degrees Celsius. But this is just theory and as we all know the practice always looks a little different. In practice the zero point as well as the slope of  the electrode differ from what they should be. This difference has to be known so that the meter can compensate for any changes in electrode response. This way you get “true” results. Determining the deviation of  the zero point and the slope from their theoretical values and saving these values into the meter (it works automatically) is called calibration. [5]

How to calibrate your pH meter 1. Take the pH electrode and connect it to the meter. Remove the storage vessel and rinse the sensing area thoroughly with distilled water. Dab the electrode with a soft paper tissue to remove the rest of  water (don't rub the electrode surface).

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2. Dip the electrode into buffer pH = 7, so that the diaphragm of  the electrode is well immersed. If  the pH electrode doesn't have an integrated temperature sensor, connect a separate temperature sensor to the meter and dip it also into the buffer solution. If  you don't have a temperature sensor which you can connect to the meter, use a conventional sensor and enter the temperature into the meter when required. 3. Start the calibration on the meter (check instructions for use of  the meter). You will notice that the temperature is measured first and then a mV reading is taken, which should be around 0 mV. The difference to 0 mV will be registered by the meter and taken into account during later measurements. 4. Rinse the sensors thoroughly with distilled water and dab the electrode with a soft paper tissue to remove the rest of  water (don't rub the electrode surface). Dip in buffer pH = 4 and repeat the procedure. 5. Having two mV readings, this and the previous one, the meter can establish a linear function U = f  (pH) and also calculate the slope of  the electrode. The real value differs again from the theory of 59.16 mV/pH. The meter saves the difference and takes it into account during later measurement. (see Appendix(A)). [2]

Temperature compensation Temperature plays a key role in pH measurement. There are two reasons for this: 1. The pH value of  the solution changes with the temperature. 2. The slope (theoretical slope) of  the electrode changes with the temperature. pH measurement should always be performed together with temperature measurement because only pH values measured at the same temperature can be compared. Of  course there is one exception which confirms the rule above. This is when you calibrate. In this case both problems mentioned above are eliminated. [2]

The response time of  an electrode The response time of  an electrode is the length of  time necessary to get a stable reading when the electrode is moved from one solution to another of  different pH or temperature. Response time is dependent on the electrode type, the measuring sample, temperature, the magnitude and direction of concentration change and the presence of  interfering substances. A slow response time can be indicative of  the incorrect selection of  electrode for the intended application. [5]

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Paper vs. Meter comparison Litmus paper •

Advantages:

1. Easy for young children to use. 2. Does not need calibration. •

Disadvantages:

1. Resolution is not as good as meters (reads in 0.5 pH unit increments). 2. It is not temperature compensated. pH meter •

Advantages:

1. Measures to 0.1 pH units. 2. Can be temperature compensated. Note: avoid using meters with one point calibration. •

1. 2. 3. 4.

Disadvantages: The meter must be calibrated with buffer solutions before each use. More expensive than litmus paper. Performance deteriorates over time. Better meters have at least a two point calibration and have an automatic temperature compensation (ATC). Buffer solutions may be ordered in liquid or powdered form. The liquid is more expensive and has a shorter shelf  life, but may be more convenient than mixing the powdered buffers. Most meters require the small, flat ‘watch type’ batteries. Although the batteries last a long time, if  the meter is turned off  when not in use, it is a good idea to have an extra set of  batteries on hand. [6] ‐

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Some application areas: 1. pH measurement in food industry pH clearly plays an important role in food processing. Among the reasons for measuring pH in food processing include: • To produce products with consistent well defined properties. • To efficiently produce products at optimal cost. • To avoid causing health problems to consumers. • To meet regulatory requirements. The pH range of  foods varies considerably with some typical values of  food and related solutions. Due to the logarithmic nature of  the measurement, even small changes in pH are significant. The difference between pH 6 and pH 5 represents a ten fold increase in acid concentration; a change of  just 0.3 represents a doubling of  acid concentration. Variations of  pH can impact flavor, consistency, and shelf  life. [4] ‐

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Examples: •

pH Measurement in Corn Wet Milling

BACKGROUND

Corn wet milling is the most common method used to process corn. In this process, corn kernels are separated into their component fractions and then further treated to yield cornstarch, corn sweeteners, corn oil, and animal feed. ‐

pH measurements are made throughout the milling process to optimize the product yield. The bulk of the water used for rinsing and washing usually needs pH adjustment to prevent altering the nominally acidic pH present during the various steps. Rinse waters are generally recycled upstream, but must be eventually treated (and neutralized) before disposal. Specific pH applications include the following: 1. The pH in the steeping tank is used to control the addition of  the sulfur dioxide (or other acid) that begins to release the starch from the corn. Too much acid may release the starch prematurely and causes corrosion of  stainless steel process lines. Not adding enough will not prepare the corn kernels adequately. 2. Starch modification is used to lower the viscosity of  the starch product. This process must be con ducted under controlled pH to meet product specifications. 3. The enzymes used in starch conversion are expensive and function best at well defined pH levels (typically in the 3 5 range depending on the enzyme used). pH levels higher than normal will not utilize the enzymes efficiently, while operation at lower pH may allow the reaction to proceed too far and cause plugging of  ‐

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downstream filters. High pH values cause the product to set into a viscous gel that is difficult to process. [7]

•

pH of  Cheese

pH, together with temperature, rank as the most important indicators of  food quality and safety. The pH of  cheese is measured to ensure that quality standards have been properly met. pH is also monitored at different stages of  cheese processing and transformation to guarantee safety and improve production. Along with temperature and water activity, pH is an important determinant in the shelf  life of  foods. The pH value of  cheese varies not only among types of  cheese, but also between batches of the same variety. There is also variation throughout the maturation process, decreasing early on and increasing again as the cheese matures. Typically, the pH of  cheese ranges from 5.1 to 5.7 with a few exceptions. Increasingly pressure is being put on cheese makers to supply end product which has been pH tested. [8]

2. Ph measurements in agriculture: Example: •

Soil pH Measurement with a Portable Meter

Background:

Soil pH is a very useful measurement, indicative of  soil chemical and biological properties important to plant growth. The availability of  plant nutrients, activities and nature of  microbial populations, solubility toxic substances and activities of  certain pesticides are all influenced by soil pH. Considerations Involved in Measuring Soil pH: 1. All meters should be calibrated once each day before use with standard pH buffer solutions at two pH points. Standardize the pH meter until the instrument reading agrees with both buffer pH’s. 2. It is best to make pH measurements at approximately room temperature because pH reading sere temperature sensitive. If  possible, adjust the instrument to the temperature of  the solution being measured. 3. the most common procedure is to place the soil into a paper cup and add one tablespoon of  distilled water. Stir the sample for 15 seconds and let stand for 30 minutes.

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4. The electrode tip must be placed in the soil slurry and not in the overlying solution. Stir the slurry carefully and read the pH immediately. 5. To avoid abrasion of  the sensing surface, prevent the electrode tip from touching the bottom of the sample cup. Some instruments have electrode holders to prevent abrasion. Rinse the electrodes after each soil sample. [9]

Future research and development in pH sensors The selection of  the optimum sensor(s) and analyzer for a control system is as important as its proper configuration. pH measurement is not simple and furthering the problem is the fact that many, if  not most, applications requiring pH measurement are challenging environments involving high temperatures and/or process elements that can foul, coat, or poison a pH sensor. In the past, this has resulted in the need for frequent cleaning and replacement of  the pH sensor, sometimes once a day in very harsh circumstances, making the measurement costly in both equipment and time. While there is no perfect pH sensor yet today, the technology is advancing to the point that the pH sensor is a relatively trouble free part of  the process, so long as ‐

one selects carefully and performs appropriate maintenance too. The two major areas of  advance have been extending the life of  the pH sensors and extending their range of  applications. The most obvious area of  advance in pH technology is in the area of  glass durability. New glass formulations and low stress handling techniques provide exceptional ‐

resistance to thermal and caustic degradation. This translates to less breakage from thermal stress or shock and improved speed of  response at near theoretical levels and minimal hysteresis for fast accurate calibrations even after months of  service. In addition to the more durable glass, new sensor designs also mount the glass bulbs in protected tips that shield the glass from direct impact while in service or during calibration. For applications where glass tipped sensors cannot serve, ion selective ‐

field effect transistor (ISFET) technology may be the best choice. The response time of these sensors can be as much as 10 times faster than traditional glass electrodes. This response combined with stability make ISFET sensors appropriate for use in cold processes like brine or water for cooling.

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Aging effects caused by temperature fluctuations are also less with ISFET sensors providing longer intervals between maintenance and calibration. However, the ISFET sensor does not typically survive in harsh alkaline or strongly acidic solutions as well as glass does. It is non linear at the ends of  the pH spectrum; hence, the ISFET sensor ‐

has a range of  two to 12 pH versus zero to 14 pH for the glass electrode sensor. If  the application requires fast response in a relatively benign environment, or does not allow the use of  glass, then an ISFET sensor is the answer. Despite the best technology developments, sensors still need cleaning in dirty applications. Spray cleaners attach to the sensor and provide a periodic burst of  cleaner onto the sensor electrode, removing the coating that can degrade performance and cause dead time. If  cleaning is critical and the application is sufficiently benign to not require frequent sensor replacement then a mechanical or pneumatically activated retraction system may be used. These systems easily and automatically remove the sensor from the process for cleaning and then return it smoothly, reducing impact on employee time and reducing downtime. The systems are costly, so a cost/benefit analysis is appropriate; a long life sensor that is reliable and easy to install may prove to be a ‐

better choice. Many of  these improvements significantly forward the operation of  pH sensors in a wide range of  applications. Over the last decade, more and more steps to build in sensor diagnostics that alert the operator before a critical failure occurs have come about. With advance notice of sensor of sensor degradation, orderly maintenance or shutdowns are now possible. Most analyzers today are modular in design, allowing the same analyzer to be used with a simple change of a circuit board for multiple applications (pH, ORP, conductivity, chlorine, turbidity, and others), which can significantly speed training and operation. In addition, many analyzers allow multiple inputs, permitting a single analyzer to control more than one sensor. Finally, pH analyzers can be equipped with advanced digital communications (HART, Profibus, Foundation fieldbus, and others) to allow integration with central databases eliminating “islands of  automation,” speeding record keeping and trouble

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shooting, and complying with requirements in many industries. [10]

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References: [1] http://en.wikipedia.org/wiki/PH [2] http://www.brinkmann.com/utilities/download_pdf.asp?DR=downloads %2Fpdf%2Ffaqs%2Felectrodes&FN=pH-background.pdf.. %2Fpdf%2Ffaqs%2Felectrodes&FN=pH-background.pdf [3] http://www.madehow.com/Volume-6/Litmus-Paper.html [4] http://www.foodmanufacturing.com/scripts/Products The Importance of  pH ‐

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Measurement.asp [5] http://www.reagecon.com/TechPapers/phfaqv4.pdf [6] http://www.globe.gov/tctg/hydro_prot_ph.pdf?sectionId=152 [7] http://www.emersonproce http://www.emersonprocess.com/raihome/docume ss.com/raihome/documents/Liq_AppData_ nts/Liq_AppData_2000 2000

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84.pdf  [8] http://www.qclscientific.co.uk/ph measurement 150 c.asp ‐

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[9] www.rce.rutgers.edu [10]http://www.isa.org/InTechTemplate.cfm?Section=Article_Index1&template=/Content Management/ContentDisplay.cfm&ContentID=67497

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Appendices APPENDIX (A): Calibration of  pH meter

Zero point adjustment

Here is a typical calibration report:

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APPENDIX (B): diaphragm types and membrane shapes •

Table(B.1): common diaphragm types

•

Table(B.2): The most common membrane shapes in use today

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•

Table(B.3): membrane glass types

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APPENDIX (C): Data sheets

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pH Measurement  Electrode Basics  pH electrode technology hasn’t changed much in the past 50 to 60 years. With all the technological advancements of the last 30 to 40 years, pH electrode manufacturing remains an art. The special glass body of the electrode is blown to its configuration by glass blowers. Not a terribly advanced nor “high tech” process but a very critical and important step in the electrode manufacturing. In fact, the thickness of the glass determines its resistance and affects its output. MDS37, $750, see page B-29.

pH electrodes are constructed from a special composition glass which senses the hydrogen ion concentration. This glass is typically composed of alkali metal ions. The alkali metal ions of the glass and the hydrogen ions in solution undergo an ion exchange reaction generating a potential difference. In a combination pH electrode, the most widely used variety, there are actually two electrodes in one body. One portion is called the measuring electrode, the other the reference electrode. The potential that is generated at the junction site of the measuring portion is due to the free hydrogen ions present in solution. The potential of the reference portion is produced by the internal element in contact with the reference fill solution. This potential is always constant. In summary the measuring electrode delivers a varying voltage and the reference electrode delivers a constant voltage to the meter. The voltage signal produced by the pH electrode is a very small, high impedance signal. The input impedance requires that it only be interfaced with equipment with high impedance circuits. The input impedance required is greater than 1013 ohms. This is the reason why pH electrodes do not interface directly will all equipment. pH electrodes are available in a variety of styles for both laboratory and industrial applications. No matter their status, they are all composed of glass and are

B-3 

therefore subject to breakage. Electrodes are designed to measure mostly aqueous media. They are not designed to be used in solvents, such as CCI 4, which does not have any free hydrogen ions. The pH electrode due to the nature of its construction needs to be kept moist at all times. In order to operate properly the glass needs to be hydrated. Hydration is required for the ion exchange process to occur. If an electrode should become dry, it is best to place it in some tap water for a half hour to condition the glass. pH electrodes are like batteries; they run down with time and use. As an electrode ages, its glass changes resistance. This resistance change alters the electrode potential. For this reason, electrodes need to be calibrated on a regular basis. Calibration in a pH buffer solution corrects for this change. Calibration of any pH equipment should always begin with buffer 7.0 as this is the “zero point.” The pH scale has an equivalent mV scale. The mV scale ranges from +420 to -420 mV. At a pH of 7.0 the mV value is 0. Each pH change corresponds to a change of ±60 mV. As pH values become more acidic, the mV values become greater. For example a pH of 4.0 corresponds to a value of 180 mV.

PHB-212, $420, see page B-23.

As pH values become more basic, the mV values become more negative; pH=9 corresponds to -120 mV. Dual calibration using buffers 4.0 or 10.0 provide greater system accuracy. pH electrodes have junctions which allow the internal fill solution of the measuring electrode to leak out into the solution being measured. This junction can become clogged by particulates in the solution and can also facilitate poisoning by metal ions present in the solution. If a clogged junction is suspected, it is best to soak the electrode in some warm tap water to dissolve the material and clear the junction.

Our pH Field and Lab Instruments product line  continues to expand, visit  omega.com  for new details! 

pH electrodes should always be stored in a moistened condition. When not in use, it is best to store the electrode in either buffer 4.0 or buffer 7.0. Never store an electrode in distilled or deionized water as this will cause migration of the fill solution from the electrode.

pH electrodes have a finite lifespan due to its inherent properties. How long a pH electrode will last will depend on how it is cared for and the solutions it is used to measure. Typically, a gel-filled combination pH electrode will last 6 months to 1 year depending on the care and application. Even if an electrode is not used it still ages. On the shelf, the electrode should last approximately a year if kept in a moistened condition. Electrode demise can usually be characterized by a sluggish response, erratic readings or a reading which will not change. When this occurs, an electrode can no longer be calibrated. pH electrodes are fragile and have a limited lifespan. How long an electrode will last is determined by how well the probe is maintained and the pH application. The harsher the system, the shorter the lifespan. For this reason it is always a good idea to have a back-up electrode on hand to avoid any system down time. Calibration is also an important part of electrode maintenance. This assures not only that the electrode is behaving properly, but that the system is operating correctly. correctly. In summary these are the “electrode facts of life.”

For Sales & Service U.S.A. and Canada

Each electrode supplied with either BNC (shown) or US standard connector.

PHE-1304 economy, $32.

PHE-1417 economical with double PTFE junction, $90.

PHE-1411 general purpose for samples requiring double junction, $70.

PHE-2385 rugged puncture tip for meats, cheeses and leathers, $85.

ORE-1411 double junction ORP for interfering ions, $101.

PHE-1311 general purpose, $57.

PHE-1335, extra long test tubes (PHE-1335-D detachable detachable style shown), $89 + 5 = $94. All electrodes shown smaller than actual size.

ORE-1311 general purpose ORP, $87.

See C-13 of the GREENBOOK ®  for all electrodes shown here.

Glass Electrode Error in pH Units  10°C (50°F) 15°C (59°F) 20°C (68°F) 25°C (77°F)

Temperature

30°C (86°F) 35°C (95°F) 40°C (104°F)

Compound Sulfuric Acid Limes Wines Oranges Beer Blood, Human Egg whites Sodi So diu um Bi Bic car arb bon onat ate e Ammonia Sodium Hydroxide

Automatic Temperature Compensation becomes more critical as the temperature changes from 25°C (77°F), or the pH from 7.0

59 mV per decade at 25°C (77°F) 52 mV per decade at 0°C (32°F) 74 mV per decade at 100°C (212°F)

+

pH mV @ 25°C (77°F)

H (Hydrogen Ions) Acid 1 2 3 4

0 + 414

Order Online

+ 355

+ 296

+ 237

omega.com  omega.com

+ 177

5

6

Neutral 7

+ 118

+ 59

00

® 

Over 100,000 Products Available! 

pH Value

8

9

– 59

– 118

0 .3 2.8-3.8 3 .0 3.0-4.0 4.0-5.0 7.3-7.5 7.6-8.0 8.4 8. 4 11.6 14.0

OH – (Hydroxyl Ions) Alkaline 10 11 12 13 – 117

– 237

– 296

– 355

To download information and to order pH Field and  Lab Instrument products online, visit  omega.com 

14 – 414

B-4 

Temperature Compensation  pH Measurement Error  The millivolt output of all pH electrodes varies with temperature in a manner predicted by theory. The magnitude of this variation is a function of temperature and of the pH value of the system being measured.

At a pH of 7 and a temperature of 25°C (77°F), temperature error is approximately zero. So, at any temperature when the pH is about 7, there is no temperature error. And, at any pH when the temperature is at 25°C (77°F), there is no error. When the

temperature is other than 25°C (77°F) and the pH other than 7, the temperature error is: 0.03 pH error/pH unit/10°C (18°F). The following table illustrates this combined effect:

pH TEMPERATURE ERROR TABLE pH Value

Temperature, °C

2

3

4

5

6

7

8

9

10

11

12

5

–0.30

–0.24

–0.18

–0.12

–0.06

0

+0.06

+0.12 +0

+0.18

+0.24 +0.30

15

–0.15

–0.12

–0.09

–0.06

–0.03

0

+0.03

+0.06 +0

+0.09

+0.12 +0.15

25

0

0

0

0

0

0

0

0

0

0

0

35

+0.15

+0.12

+ 0.09 +0

+0.06

+0.03

0

–0.03

–0.06 –0

–0.09

–0.12

–0.15 –0

45

+0.30

+0.24

+ 0.18 +0

+0.12

+0.06

0

–0.06

–0.12 –0

–0.18

–0.24

–0.30 –0

55

+0.45

+0.36

+ 0.27 +0

+0.18

+0.09

0

–0.09

–0.18 –0

–0.27

–0.36

–0.45 –0

65

+0.60

+0.48

+ 0.36 +0

+0.24

+0.12

0

–0.12

–0.24 –0

–0.36

–0.48

–0.60 –0

75

+0.75

+0.60

+ 0.45 +0

+0.30

+0.15

0

–0.15

–0.30 –0

–0.45

–0.60

–0.75 –0

85

+0.90

+0.72

+ 0.54 +0

+0.36

+0.18

0

–0.18

–0.36 –0

–0.54

–0.72

–0.90 –0

NOTE:  Values in light blue are less than 0.1 error and may not require temperature compensation. Values in gray are temperature and pH in which there 

is no error i n pH from temperature.

Add the appropriate error factor to correct uncompensated uncompensate d readings. Correct uncompensated uncompensate d readings as follows (the factors from the table assume that the electrodes were calibrated in buffer at 25°C/77°F):

Effects of Temperature on pH Temperatures Above 25°C: temperature compensation lowers high pH and raises low pH, resulting in value closer to neutral. Temperatures Below 25°C: temperature compensation raises high pH (more basic) and lowers low pH (more acidic), resulting in values further away from neutral. Whether or not temperature compensation need be used is a matter of the needed

B-5 

pH accuracy. For example, if the accuracy requirement is ±0.1 pH, at a pH of 6 and a temperature of 45°C (113°F), the error is 0.06, well within the accuracy requirements.. On the other hand, with requirements the same ±0.1 pH accuracy requirement, operating at pH 10 and 55°C (131°F) would give an error of 0.27 pH and compensation should be used. When compensation is required, it can be done in one of two ways. If the temperature fluctuates, then an automatic compensator should be used. If the temperature is constant within several degrees C, then a manual compensator can be used. If no compensator is needed, a fixed resistor can be installed across the temperature compensator terminals.

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Any of the above devices–automatic compensation, manual compensation or fixed resistor–operate resistor–operate as a function of the pH meter’s electronic circuit. As such, information and parts should be obtained from the meter manufacturer. If automatic compensators are used, they should always be at the same location as the pH electrode. When electrodes are calibrated in buffer, the temperature compensator also should be in the buffer. In a similar way, a manual temperature compensator should be adjusted to reflect the temperature to which the pH electrode is exposed during both calibration and operation. Reproduced Reproduce d with permission of Sensorex 

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pH Measurement Tips  The measurement of pH is very important in many laboratories and industries. Below are a few guidelines to aid in making accurate and precise pH readings.

PHB-600R, $570, with pH electrode, temperature probe, 2 buffer solutions and electrode holder (all included), shown smaller than actual size. See page B-25 for more information.

Meter Calibration Frequency For accurate results a pH meter should be calibrated at least once per 8-hour shift. Buffers Standard buffers should always be used for meter calibrations. Buffers can be purchased already prepared or in capsule form. Standard buffers usually are available in three pH valuespH 4.00, pH 7.00, and pH 10.00. Buffers should be stored away from heat and in tightly sealed containers. Always use freshly poured buffers for meter calibration. All buffers should be used at room temperature, 25°C (77°F).

Types of Calibration

5. Rinse the electrode with distilled or deionized water. (This would be the procedure for a one-point calibration. Continue through step 8 for a two-point calibration.)

One-Point Calibration Meter calibration using only one buffer. The value of buffer used should be that closest to value anticipated for sample. Two-Point Calibration Two-Point Meter calibration using two buffers, one of which should always be 7.00. The second buffer used would depend on the application.

6. Place electrode into the second buffer, either pH 4.00 or pH 10.00.

Method Here is a general method for most pH meters. Some pH meters require slightly different techniques. Please read the instructions for their particular procedures. 1. The temperature setting on the meter must correspond to the temperature of the buffers used, or an automatic temperature compensator must be employed. 2. Turn pH meter to “pH” or “ATC” if automatic temperature compensation is used. 3. Place clean electrode into fresh, room temperature pH 7.00 buffer. 4. Adjust the pH reading to exactly 7.00 using the ZERO OFFSET, STANDARDIZED or SET knob.

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7. Adjust the pH reading to display the correct value using the SLOPE, CALIBRATE, or GAIN controls (coarse ajust). 8. Adjust the pH reading to read the correct value using the SLOPE knob (fine ajust). The pH meter is now calibrated and ready to use.

Electrode Care Over 80% of pH measurement difficulties are due to electrode problems. Proper storage, use, and maintenance increases accuracy.

Use and Maintenance Electrodes should be used in a vertical position. Electrodes should be rinsed between samples with distilled or deionized water. NEVER wipe an electrode to remove excess water. Just blot the end of the electrode with a lint-free paper. Wiping the electrode can cause spurious readings due to static charges. The level of filling solution in refillable electrodes should be kept at least 2/3 full. The filling hole should be open during use. pH electrodes are fragile. A proper electrode holder should be used to provide support and aid in raising and lowering the probe into solutions.

Storage Electrodes should be stored in an acidic solution with a low salt content. Commercial soaking solutions are available or you can make your own by mixing a 1M KCI solution adjusted to pH 4.0.

® 

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pH sensor AH-300-K-1-1

pH sensor AH-300-K-1-1

Higher life expectancy by refilling Low flow rate by small junction Low alcaline error

applications Process Water

Cooling And Boiler Feed Water

description The Kuntze AH-300-K-1-1 is a combination electrode with a 1mm ceramic junction for pH measurement. The sensor is liquid filled, thus blocking resistant and particulary suitable for lightly polluted media.

technical data characteristic measuring parameter measuring range ambient conditions max. pressure min. condutctivity temperature

pH-value 0 .. 1 4 p H

6 ba r adjustable by internal pressure > 50 µS -5 .. +100°C

Version 02/2009

pH sensor AH-300-K-1-1

mechanical construction junction shaft material electrode material reference system internal buffer mechanical connection electr ectriical conne nnection

Keramik glass AH glass ball Ag/AgCl/3M KCl pH 7 S7 plug 2-po -pole screw connection

Article Numbers Na m e

Description

AH-300-K-1-1

pH sensor: KCl liquid filled, 1 mm ceramic junction, S7 plug

Article Number 24132040K

accessories accessories pH-buffer solutions

The slope of pH sensors change over the time (depending on the measuring media), we recomment a regulary calibration with our buffer solutions.  We offer in a 1000 ml package pH 2, pH 3,56, pH 4, pH 7 and pH 9,22 and in 50 ml package pH 4 and pH 7.

Version 02/2009

ABB pH Sensor Model TB557 Holds Up in Harsh Environments Where Other Sensors Fail Polychemie was having diffi ficul culty ty produc pro ducing ing a sensitive hemispherical  dif precursor monomer product glass electrode is due to high pH sensor rugged enough to failure rate occurring within withstand an abrasive its batch reactors. The and strong caustic sensors Polychemie was environment. It is rated  using were exposed to for a true 0 – 14 pH  high temperatures as well range at temperatures as corrosive and abrasive to 284 degrees F.  The conditions, making them body of the sensor is inoperable. ABB solved ® Kynar polyvinylidene the problem by providing fluor uoride ide thermo the rmopla plastic stic Polychemie with its pH strengthened by a Sensor Model TB557, titanium sheath; which proved to be virtually immune to the challenging both materials hold  reactor environment. up well to the harsh

The ABB A BB sensor’s pH-

process conditions.

measurement controls caustic additions o maintain close-tolerance pH levels. he pH sensor must respond quickly and withstand high temperatures as well as corrosive and abrasive conditions, which can lead to numerous sensor failures. Over the course of the process, the reactor’s pH sensor is subjected to an environment ranging from 2 to 10 pH, and emperatures near 180 degrees F. Steam coils within the vessel’s jacket maintain he high temperatures.

easurement of pH within a process batch reactor is critical to the production of coagulant coagul ant and solution solu tion fl occula occulants. nts. Polychemie, located in Pearlington, ississippi, manufactures these chemicals or its parent company, SNF Inc., which is one of the world’s largest suppliers of  water-soluble polymers widely used for  water and wastewater treatment and a wide range of specialty applications.

Control Functions

Early in the chemical process, the pH measurement controls the initial raw material additions at low pH levels. Later, the pH

he pH measurement also comes into play  during the th e final process proc ess phase. phas e. Here, the th e

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During the initial phase, two components ow into the agitated reactor, mixing with a third material already present. A pH value controls con trols the t he fl ow rate of o f one of the th e wo components to maintain the required stoichiometric ratio of the raw materials in he reactor. The setpoint for this control loop keeps the reactor solution highly  acidic – about 2 pH.

s e ir o t S s s e c c u S

process strips away water and residual organic material that is not part of the monomer product. The plant recycles the excess organic materials. During this phase, the reactor operates under  vacuum at elevated temperature, and with a tight tolerance on alkalinity  (9 – 10 pH).

rangement allows insertion or removal of  he sensor without disturbing the process.

ABB supplies the sensor complete with a ball valve assembly that bolts to a standard standar d flanged port por t on the reactor. re actor. he sensor has a patented reference comprised of a matrix of immobilized KCI salt fronte fronted d by a poro porous us Tefl Teflon® The pH value is used to control the rate of  unction. This “solid state” reference is virtually immune to poisoning, plugging caustic added to the reactor to maintain and pumping problems that plague the high alkalinity required for stripping. conventional liquid, slurry and gel-based Salts formed during this phase account references. Without this reference for the abrasive nature of the pH sensor’s environment. environmen t. At the beginning of this echnology the sensor would be quickly  phase, the sensor must respond virtually  destroyed during the vacuum and extreme high pH process phases. across its full range within 30 seconds –  from high-acidic to highly alkaline. Signals from the pH sensor’s transmitter run to a distributed control system (DCS). The DCS employs PID control algorithms, developing signals for the control valves that set the rate of raw  material and caustic additions. The endpoint of a monomer batch occurs when the analysis of the stripped extraction samples reaches a predetermined value.

The Search for the Right Sensor Initially, Polychemie tried several types Initially, of pH sensors in its reactors, but none could last long in the harsh environment. Shortly after installation, sensor  response times slowed to a point that rendered the pH reading useless. Even flowing reference re ference sensors se nsors with w ith both conventional glass and unique glass/steel measuring electrodes quickly degraded. Eventually, the plant found success with the TB557 pH sensor from ABB Instrumentation. The TB 557 Model consists of a hot tap retractable sensor  that is conveniently inserted into and out of the reactor through a ball valve. This ar-

PH Sensor Model TB 557 Characteristics

he ABB sensor’s pH-sensitive hemispherical glass electrode is rugged enough to withstand an abrasive and strong caustic environment. It is rated or a true 0 – 14 pH range at temperaures to 284 degrees F. The body of  ® he sensor is Kyn Kynar  ar  polyvinylidene uoride thermoplastic strengthened by a itanium sheath; both materials hold up well to the harsh process conditions. An integral cable and temperature element complete this rugged pH sensor. he Polychemie plant has been using ABB TB557 pH sensors for three years. Polychemie calibrates the pH sensors and transmitters for its reactors on a weekly schedule. To To do so, so , technicians simply retract the sensor, close the ball valve, and then remove the sensor for  eld calibration. Technicians check both bo th he sensor and transmitter as well as he sensor response time. Because pH measurement is so critical to successful batch production of this monomer  process, Polychemie replaces sensors every six months as a matter of policy po licy..

Copyright © 2005 ABB 3BUS340006R0001 Tefl on® is a registered trademark of DuPont. Kynar® is a registered trademark of Arkema.

pH Sensor

 

DT016A

The pH sensor can be connected to the Nova5000, MultiLogPRO or TriLink data loggers. The pH sensor is capable of measuring the entire range of 0 - 14 pH and is used for various experiments in Biology, Chemistry and Environmental Science. This sensor can replace the traditional pH meter and in addition, it automatically collects the pH data and the pH changes during chemical reactions and displays these changes in a graph. The pH sensor (DT016A) consists of the Fourier Systems adaptor (DT017) and a pH electrode (DT018) and is equipped with an automatic temperature compensation system.

Typical Experiments •

Acid – Base titration

•

Investigation of water quality

•

Acidification of milk and others foods

•

pH measurements in tissue extracts

•

Alcoholic fermentation in yeast

How it Works The pH electrode contains two half-cells. One contains a reference element of known H+-concentration. The other, at the bottom of the electrode, is an H+- sensitive glass membrane. The adaptor measures and amplifies the potential difference between the

1

two half-cells. The pH level (pH= – log (H+)) is calculated from the potential difference.

Sensor Specification Range:

0 - 14 pH 2% over entire range, after temperature compensation

±

Accuracy: Resolution (12-bit):

0.004 pH

Temperature Compensation:

Yes

Operating Temperature: Temperature:

0 °C – 50 °C

Response Time for 95% of Reading:

10 seconds

Default Sample Rate:

10 samples per second

Feature:

Equipped with an offset calibration screw

Sensor Storage:

Store the pH electrode in its storage solution when not in use

Contents •

Adaptor & electrode set

•

Adaptor only

•

Electrode only with a bottle containing pH 4.01 buffer

 

   

 

 

 

   

DT016A  

DT017 DT018

Adaptor & electrode set DT-016A       Adaptor only DT-017        Electrode only DT-018 

2

Equipment Setup 1. Connect the electrode to the adaptor. 2. Connect the adaptor to the data logger's input.

Required Equipment •

Nova5000, MultiLogPRO or TriLink data loggers

•

Wash bottle with distilled or deionized water

•

Several clean beakers

•

Lab wipes

Technical Notes •

If other electrochemical type sensors (Oxygen and Conductivity) are placed in the same solution at the same time and connected to the same data logger, they can interfere with each other’s signals. Keep the sensors as far apart as possible - the distance required will depend on the conductivity of the solution. If there is still a problem, try connecting the sensors to different data loggers or take readings using one sensor at a time.

•

In order to use temperature compensation a Temperature sensor must be connected to the data logger along with the pH sensor (the Temperature sensor must be plugged into Input 1).

Preparation of the Electrode for Initial Use 1. Remove the protective bottle or cover cover from the electrode and thoroughly rinse the electrode with distilled water. Wipe carefully with a clean lab wipe. 2. During shipment, air bubbles may have migrated into the electrode sensing bulb. Hold the electrode up to the light and inspect the sensing bulb for air bubbles. If air is seen, carefully shake the electrode downward (like a thermometer) to dispel the air bubble from the sensing bulb at the tip of the electrode.

Calibration The pH sensor is shipped fully calibrated. For experiments that require very accurate calibration, however, the pH sensor is equipped with an offset calibration screw. The screw is located at the back of the sensor case. Place the electrode in a reference solution (buffer of pH 7) and start recording. Insert a flat screwdriver to the calibration hole and slowly turn the calibration screw until the reference value is reached.

3

Using the pH Sensor with Fourier Data Loggers and MultiLab Software 1. Rinse the electrode with distilled water and blot with a lab wipe. 2. Launch the MultiLab software (from either your PC or Nova5000). 3. Connect the Temperature Temperature sensor (DT029) to the data logger’s first sensor input I/O-1. 4. Connect the pH sensor to I/O-2. 5. The Temperature and pH sensors are are automatically recognized by the MultiLab software. 6. Click Setup on the main toolbar and program the data logger’s sample rate and number of samples. 7. Place the electrode in a beaker containing the sample and a stir bar. 8. Click Run on the main toolbar to start the measurement.

Maintenance •

At the end of the measurement, remove the electrode from the sample; rinse the electrode with distilled water over the waste  beaker. Blot the electrode dry with a lab wipe. The electrode is now ready to read the pH of other samples.

•

When not in use, store the pH electrode in the supplied bottle containing the storage solution.

•

The recommended storage solution is comprised of 50% ph 4 buffer and 50% 4M KCl salt. If this storage solution isn't available then a fresh ph 4 buffer can be used as well.

Cleaning the Electrode Do not use strong solvents (e.g. acetone, carbon tetrachloride, etc.) to clean the pH electrode. Be sure to recalibrate the electrode after cleaning. 1. If the electrode has become coated with oil or grease, carefully wash the electrode under warm tap water using dish-washing detergent. Rinse thoroughly with fresh tap water followed by a rinse with distilled water. Soak the electrode in pH electrode storage solution for 30 minutes after this cleaning procedure. Recalibrate the electrode before use.

4

2. If the electrode has been exposed to protein or similar materials, soak in acidic pepsin, ASI part number CS 0003, for 5 minutes. Rinse thoroughly with distilled water. Soak in storage solution for 30 minutes prior to recalibration. 3. If the previous cleaning procedures fail to restore response, soak the electrode in 0.1 N HCI for 30 minutes. Rinse thoroughly with distilled water. Recalibrate before use. 4. If electrode response is still not restored, replace the electrode.

An Example of using the pH Sensor Acidification of milk  In this experiment we follow pH changes in milk, kept in a thermos, during 30 hours of incubation.

Figure 1: Acidification of milk

Troubleshooting If the pH displays values which are out of the sensor range, verify that the sensor cables are properly connected.

5

Technical Support Please contact Fourier technical support as follows: Web: http://www.fourier-sys.com/support_support.html Email: [email protected] Consult the FAQs before contacting technical support: http://www.fourier-sys.com/support_faq.html

Copyright and Warranty All standard Fourier Systems sensors carry a one-year warranty, which states that for a period of twelve months after the date of delivery to you, it will be substantially free from significant defects in materials and workmanship. This Warranty does not cover breakage of the product caused by misuse or abuse. This Warranty does not cover Fourier Systems consumables such as electrodes, batteries, EKG stickers, cuvettes and storage solutions or buffers.

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