A Low-Cost Laboratory Experiment Using a Nephelo-Turbidity ~ e t e r Ishwar Das, S. S. DaS, and Anal Pushkama Gorakhpur University, Gorakhpur-273009, India An enormous amount of research that involves turbidity measurements is currently being conducted in the fields of chemical kinetics, analytical chemistry, and water pollution control. The existing commercial nephelo-turbidity meters are fairly costly and not very useful for teaching purposes. A low-cost nephelometer has been designed in the laboratory, keeping in view the following learning objectives: 1. to demonstrate the principle of the nephela-turbidity meter at
undergraduate and postgraduate levels. 2. to have acquaintance with the principle of the photocell. 3. to study the kinetics of decomposition of sodium thiosulfate by
mineral acids, and 4. to analyze the data by least-squaresmethod. Principle of the Nephelo-turbidity Meter When light is passed through the suspension, part of the incident radiant energy is dissipated by absorption, reflection, and refraction while the remainder is transmitted. The term scattered light is used when the suspension is viewed a t right angles to the incident light. The measurement of the intensity of the scattered light as a function of the concentration of the dispersed phase is the principle of nephelometric analysis.' Description of the Apparatus
The apparatusconsists of a light source L. An electric bulb (25 W) has been used (Fig. 1).Light passing through a slit having an aperture of 6.0 cm falls on the Corning glass vessel A, which contains suspension a t a fixed temperature. The vessel A covered with a blackened glass plate is kept a t a distance of 15.0 cm from the light source. The photocell B is closely placed (distance not more than 0.5 cm) to the glass vessel. The entire assembly is closed in a wooden box fitted with a thermometer. A thermostat is not used because the maximum variation in temperature does not exceed +0.2 "C during the course of the experiment. The intensity of the scattered light in terms of photovoltage has been measured with the help of a photocell and a digital multimeter DM (HIL, India). Klnetlc Studles: DecomposRion of Sodlum Thlosullate by Mlneral Acids The apparatus has been successfully employed for the kinetic studies of decomposition of sodium thiosulfate by mineral acids, which produces colloidal sulfur according to the reaction, As the reaction proceeds, concentration of colloidal sulfur increases. The kinetics of this reaction may he studied by recording different times required for solutions of different concentrations to attain the same degree of turbidity. I t was observed that the time is approximately inversely propor-
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Bassen. J.; Denney, R. C.; Jeffrey, G. H.; Mendham, J. Vogel's Textbook of Quantitative Inorganic Analysis. 4th ed. (ELBS): Longman: 1979.
Figure 1. Experimeml setup: (L) Light source. (A) Reaction vessel. (6)Photocell, and (DM) Digital multimeter.
tional to the original concentration of sodium thiosulfate in the presence of excess HC1. The reaction is shown to obey first-order kinetics. Theory Let x be the concentration of sulfur necessary to make the solution appear distinctly turbid. Since x is same in each set of experiments, i t is a constant. If a is the original concentration of sodium thiosulfate and the reaction obeys the firstorder kinetics, then we can write
where k is the rate constant. On rearranging and integrating, the above equation can he written as
[-log(= - x)loY = [kt],' + A where A is the integration constant or
On expansion, the above equation can be written in the following form
If x
