ENVIRONMENTAL ENGINEERING LABORATORY REPORT
Azaz Ahmed, Roll No- CIB_09_015, B. Tech 5th Semester, Department of Civil Engineering, School of Engineering, Tezpur University, Assam, India, Pin-784028 Email:
[email protected]
EXPERIMENT NO. 1 Instrument: Turbidity meter Introduction: Turbidity meter operates on the nephelometric principle of turbidity measurement. It is used to measure how much light is either absorbed or scattered by suspended matter in water.
Fig.: Digital Turbidity Meter (Image source: http://www.ei-instrument.com/digital-turbidity-meter-341-331.htm) Turbidity: Turbidity is an optical characteristic or property of a liquid, which in general terms describes the clarity, or haziness of the liquid. According to EPA“The cloudy appearance of water caused by the presence of suspended and colloidal matter. In the waterworks field, a turbidity measurement is used to indicate the clarity of water. Technically, turbidity is an optical property of the water based on the amount of light reflected by suspended particles. Turbidity cannot be directly equated to suspended solids because white particles reflect more light than dark-colored particles and many small particles will reflect more light than an equivalent large particle.” Principle of instrument: When particles are suspended in water and a light is shined through the sample, not all of the light will pass straight through the sample. Instead, the light will reflect off of the suspended particles and some of the light will exit at a right angle to the direction of entry into the sample. The turbidity meter uses a laser pointer as a light source, and two photodiodes as detectors for the intensity of the transmitted and refracted light. The basic setup is shown in figure 2.
Fig: Schematic concept of turbidity meter.
By measuring the voltages off of both of the photo diodes, we can derive a function which calculates turbidity from the ratio of the voltage across the 90 degree sensor to the voltage across the 180 degree sensor. Light scattering/transmission method: Light radiated to the sample cell and the scattered light resulting from the suspended substances in the sample plus the light which passes through the sample are measured. The difference between the two values is in proportion to the concentration of suspended substances in the sample. Using this relation, the turbidity is obtained.
Fig.: Principle of Turbidity Meter
EXPERIMENT NO. 2 Instrument: pH Meter Introduction: A pH meter utilizes voltage to measure the acidity or alkalinity of chemical substances. Most pH meters operate in a straightforward manner, with a digital screen that displays the pH value. Some have additional functions such as conductivity and temperature measurement. A pH meter generally has three parts: the cell electrode, the reference electrode and a resistance thermometer. The cell electrode measures the pH of the substance, while the reference electrode serves as the standard or reference range of pH. The resistance thermometer compensates for temperature changes to ensure a precise measurement. A pH reading below 7 indicates acidity. Solutions that measure above 7 are alkaline, and solutions that measure 7 such as pure water are neutral.
Fig.3: A pH Meter
Principle of Instrument: The principle behind pH meters is that electrical voltage is generated between two solutions with different pH concentrations when they are separated by a thin glass wall. The two solutions are linked via the electrodes; the cell electrode is dipped in the solution of unknown pH, while the reference electrode is dipped in a standard solution with a known pH. pH meter calibration: pH meter is to be calibrated before each use. Each pH electrode used for measurements is slightly different and its characteristic changes with aging. Without proper pH meter calibration results will be usually off by at least several tenths of the unit. pH meter calibration procedure calls for use of two or three pH calibration buffers of exactly known pH (these can be bought as either solutions or in solid form) in which pH electrode is dipped and pH meter indications are corrected. Depending on the pH meter type it may either recognize buffer automatically and perform calibration procedure almost on its own (just asking for buffer change when needed) or it is to be calibrated using knobs and changing buffers once each calibration step is completed. In both cases underlying principle is the same - gain and offset parameters are set assuming linear dependence between pH and electrode voltage.
EXPERIMENT NO. 3 Instrument: Spectrophotometer Introduction: Spectrometer is used for measuring the transmittance or reflectance of a substance.
Fig: Spectrophotometer
Principle of instrument: A Spectrophotometer is commonly employed to study the absorption quantity of a light beam which a sample object absorbs. It is simply an optics structured electronic unit which is composed of a pair of individual devices, particularly a Spectrometer plus a Photometer. A spectrometer is needed as being a tool to generate the light beam from a preferred wave length additionally; the Photometer is needed as a sensor to study the power of the light beam. These resources are set up such in which the Spectrometer can dispatch the light beam to the test object at the same time the Photometer will be able to determine both concentration of transmitted and original light. The photometer delivers its output as a voltage signal to a display device, usually to a galvanometer. Display for the original light intensity and transmitted light intensity is different. Later by comparing these two displays an electromagnetic spectrum is plotted to evaluate the different properties of the sample object. The step by step principle of Spectrophotometer is as follows:
Firstly, the Spectrometer sends a light beam through a blank. A blank is nothing more than a solution identical to the sample object solution except that it does not absorb light. This is done in order to calculate the original light intensity. Here light intensity means the number of photons per second. The photometer measures the number of photons per second and gives an output as a voltage signal which is displayed in a galvanometer. Secondly, the Spectrometer sends another light beam of the same intensity through the sample object solution. The sample solution absorbs some of the photons carried by the light beam which results in a major change in the light intensity. Similarly a Photometer measures the transmitted light intensity and delivers output as a voltage signal. Finally, the displayed data is used to calculate the absorbance and transmittance capacity of the sample object. Here transmittance means the fraction of light that passes through the sample solution and received by the Photometer. The rest of the photons that cannot pass are absorbed by the sample solution and this phenomenon is termed as absorbance. Mathematical formula for transmittance (T) and absorbance (A) are: T = I/I 0 and A = -log10T. Whereby, I = Transmitted light intensity and I0 = Original light intensity. In practice, Spectrophotometer instruments measure light power (P) as opposed to the light intensity. Here power means the energy per second that is only the product of the quantity of photons (light intensity) and energy per photon. T = P/P0 Where, P = Transmitted light power and P 0 = Original light power.
EXPERIMENT NO. 4 Instrument: Flame photometer. Introduction: Flame photometry is a technique whereby the concentration of a metal in solution may be determined by spraying the solution into the flame and comparing the emission intensity of the sample with a standard solution of metal. The instrument employed for this test is the flame photometer.
Fig: Flame photometer device.
Principle of instrument: Flame photometer uses a flame that evaporates the solvent and also sublimates and atomizes the metal and then excites a valence electron to an upper energy state. Light is emitted at characteristic wavelengths for each metal as the electron returns to the ground state that makes qualitative determination possible. Flame photometers use optical filters to monitor for the selected emission wavelength produced by the analyte species. Comparison of emission intensities of unknowns to either that of standard solutions (plotting calibration curve), or to those of an internal standard (standard addition method), allows quantitative analysis of the analyte metal in the sample solution.
The intensity of the light emitted could be described by the Scheibe-Lomakin equation: I=k×cn Where, c = the concentration of the element and k = constant of proportionality n ~1 (at the linear part of the calibration curve), Therefore the intensity of emitted light is directly proportional to the concentration of the sample. Because of the very narrow and characteristic emission lines from the gas-phase atoms in the flame plasma, the method is relatively free of interferences from other elements. Therefore the flame photometry (as with other atomic spectroscopy methods) is very sensitive; measuring concentration of ppm magnitude (part per million) usually does not cause any problem. The optimal concentration range of the solutions for the measured metal ion is 10-3-10-4 mol dm-3 Typical precision for analysis of dilute aqueous solutions are about ±1-5% relative.There is no need for source of light, since it is the measured constituent of the sample that is emitting the light. The energy that is needed for the excitation is provided by the temperature of the flame (2000-3000 ºC), produced by the burning of acetylene or natural gas (or propane-butane gas) in the presence of air or oxygen. By the heat of the flame and the effect of the reducing gas (fuel), molecules and ions of the sample species are decomposed and reduced to give atoms.
Fig: Inside of a flame photometer.
The most sensitive parts of the instrument are the aspirator and the burner. The gases play an important role in the aspiration and while making the aerosol. The air sucks up the sample (according to Bernoulli’s principle) and passes it into the aspirator, where the bigger drops condense and could be eliminated. The monochromator selects the suitable (characteristic) wavelength of the emitted light. The usual optical filters could be used. The emitted light reaches the detector. This is a photomultiplier producing an electric signal proportional to the intensity of emitted light
