CZAR 2CPh Reviewer
pH Measurement and Buffer Preparation Colorimetry the science and technology used to quantify and describe physically the human color perception used to determine the concentration of colored compounds in solution liquid indicators Electrometry art or process of taking electrical measurements
pH meter pH low concentration of hydrogen ions (directs biomolecules’ function and conformation) Buffer Solution WA & CB, WB & CA Resists change in pH (neutralize excess hydrogen/ hydroxyl ion) Henderson-Hasselbach equation Relates pH to pK and the ratio of the concentrations of the acid and its conjugate base Calculate acid and CB (salt) based on desired pH Reagents Conc. H3PO4 (85% w/w, sg: 1.7, MM: 98) Conc.HCl (37.3% w/w, sg: 1.18, MM: 36) Glacial Acetic Acid (99.7% w/w, sg: 1.05, MM: 60) NaOH pellets (MM: 40) NaH2PO4.H2O (MM:138) NaH2PO4.7H2O (MM: 268) Acid-Base Indicators:
o Bromophenol blue o Bromcresol green o Bromcresol purple o Bogen indicator o Phenol red o Methyl reed o Methyl orange o Phenolphthalein o Thymol blue Preparation of Reagents 500 mL of 6M (HCl, NaOH) Buffers 250 mL of 0.1 M buffer solutions: Buffer Solutions Desired pH Phosphate; pK = 2.12 2 Phosphate; pK = 2.12 3 Acetate; pK = 4.70 5 Phosphate; pK = 7.21 7 Phosphate; pK = 7.21 7.5 Phosphate; pK = 7.21 8 Phosphate; pK = 12.32 11 Weak Acid and Conjugate Base? Electrometric Determination of pH pH meter: 4, 7, 10 pH of 20 mL o Buffer Solutions o Distilled Water o Sample [H+]? Adjust pH of prepared buffer solutions by adding 1 M (HCl, NaOH) Colorimetric Determination of pH Color Standards 6 Vials/ Test Tubes +5 mL of a buffer and 2 gtts of an acid-base indicator then shake (for all) 6 vials/ TT, repeat step 2
CZAR 2CPh Reviewer
Acid-Base Indicators Thymol blue Bogen indicator Bromphenol blue Methyl red Bromcresol green Methyl orange Bromcresol purple Phenolphthalein Phenol red
pH Determination o Use Distilled water, sample o 5 ml sample + 2 gtts acid-base indicator Buffer Capacity Determination pH meter: 4, 7, 10 100 ml 0.10 M (HCl, NaOH) + 0 .10 M (HCl, NaOH) in 1 mL portion to 20 mL of the buffer Pre-Lab sheet pH negative of the logarithm of the H3O+ ion concentration Importance o in marine systems, it will offer a natural, antiseptic effect, helping fish resist illness and also keep coral from calcifying at an accelerated speed. pOH the negative of the logarithm of the OH- ion concentration. Ka is the dissociation constant for an acid in water a measure of how much H3O+ is produced The higher the Ka, the stronger the acid.
Kb the dissociation constant for a base in water a measure of how much OH- is produced The higher the Kb, the stronger the base. pKa tells us how acidic (or not) a given hydrogen atom in a molecule is the ability of substances to ionize the negative base-10 logarithm of the acid dissociation constant of a solution. pKb the negative base-10 logarithm of the base dissociation constant of a solution. Kw the dissociation constant for water reacting with itself: pKw the negative log of the water ion product , Kw Molarity most commonly used measure of concentration expressed as the number of moles of solute per liter of solution. Normality a measure of concentration that is equal to the gram equivalent weight per liter of solution Buffer Importance o help to reduce major changes in pH levels of body fluids o aqueous solutions that require a constant pH in order to assure the stability and clinical effectiveness of a medicines
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Maintain some drug or medicine in ionized form as ionized forms are more soluble in aqueous solutions. Maintain some drug or medicine in un-ionized form as un-ionized forms are more soluble in lipids. Maintain the stability of drugs in different aqueous solutions as many drugs are vulnerable to hydrolysis of aqueous solutions. Maintain the pH of most of the drugs or medicine near to neutral otherwise that specific drug or medicine may cause irritation in body tissues. used in foods to maintain the acidity of the food in order to preserve the flavor and appearance of food maintain the physical, chemical and microbiological stability of foods giving color to different fabrics Printing industries as pH of paper and inks must be controlled to assure proper penetration and drying of the ink. Electroplating industries as some alloys can be plated if very strict pH control is maintained. Leather industries as narrow range of pH control of tanning and dyeing baths determine the texture and color of the finished product. Glue and gelatin manufacturing industries as
properties of gelatin and glue vary rapidly with a very slight change in pH during manufacture. Indicator pH Color Thymol blue 1.2– A: R (Acid) 2.8 B: Y Bromphenol blue Bromcresol 3.8 A: Y green 5.4 B: B Bromcresol purple Phenol red 6.4– A: Y 8.0 B: R Thymol blue 8.0– A: Y (alkaline) 9.6 B: B Bogen indicator Methyl redmethylene blue Methyl orange Phenolphthalein
3.1– 4.4 8.3– 10.0
A: R B: Y A: C B: F
Henderson-Hasselbalch Equation
CZAR 2CPh Reviewer Buffer Capacity Buffer capacity, β, is a quantitative measure of the resistance of a buffer solution to pH change on addition of hydroxide ions. It can be defined as follows.
Where dn is an infinitesimal amount of added base and d(p[H+]) is the resulting infinitesimal change in the cologarithm of the hydrogen ion concentration.
Post-Lab Why is there a discrepancy between the calculated pH and the experimental pH of the prepared buffer solution? When measuring the pH of the solution, a pH meter is used to do so. However, the Glass electrodes found in common pH meters respond not to the concentration of hydrogen ions, but to their activity, which depends on several factors, primarily on the ionic strength of the media. When you titillate the solution (experimental pH) you use a compound to determine the point of neutralization. If the solution has a buffer the exact moment of change will be delayed, and you will use a bigger amount of the solution, creating a difference between the calculated (theorical) pH and the experimental one. Importance of Calibrating the pH meter Accurate pH measurements cannot be accomplished with a pH meter unless the meter has been calibrated against standardized buffer. Without a proper calibration the meter has no way to determine the pH value of the solution you are testing.
Why is the electrometric method of pH determination more accurate than the colorimetric method? Electrometric methods are more precise than colorimetric methods, simply because it is easier to measure a change in voltage, current, and/or electrical resistance than it is to see some color vs. a reference. Electrical conductivity is better because electrical conductivity can be measured with higher accuracy. What is the effect of dilution on the pH of a buffer and the buffer capacity? Dilution will not alter the pH of a buffer solution. It will, however, decrease the buffering capacity, since the concentration has been reduced. Given: 80 mL of 0.2 M KH2PO4 120 mL of 0.2 M K2HPO4 Calculate the pH:
