Page 89 - Chemistry - XI

Investigatory Projects
8 Investigatory Projects

Any kind of investigation which is formulated, designed and carried out in a laboratory constitutes
investigatory science project. It may be a simple one like collection of diff erent colour salts to as diffi cult
as production of innovative products. Investigatory project exposes the students to science laboratory and
to think science in everything around them.

PROJECT - 1
Introduction

Sulphide ions are present in water when anaerobic bacteria decomposes organic matter or reduce sulphates.
In stagnent water, these are found.

Objective
To test the bacterial contamination of water through sulphide ion concentration.

Materials Required
Cadmium acetate 50 g, zinc acetate 50 g, distilled water 500 mL, iodine solution (0.025 M), conc. HCl,
Na S O (sodium thiosulphate) 0.05 M, and starch solution as an indicator
2 2 3
Principle/Theory

Sulphide ions are readily oxidised, therefore, care should be taken at the time of sampling to exclude air
by fl ushing it with nitrogen or carbon dioxide. This is a diffi cult process. The best way is to fi x the sample
immediately after solution.

Procedure
1. Take 25 g of cadmium acetate and 25 g of zinc acetate and dissolve it in 1.0 L of water.

2. Neutralise the solution with little excess of alkali.
3. Take 10 mL of cadmium-zinc acetate solution and add 80 mL of sample of given water to obtain a total
volume of about 100 mL.
4. Take 100 mL of fi xed sample solution in a titration fl ask.
5. Add 10 mL of 0.025 M iodine solution.

6. Add 15 mL of 50% HCl (1 :1) solution in water immediately.
7. Add starch solution as an indicator.
8. Titrate the excess of iodine against 0.05 M Na S O .
2 2 3
9. Calculate the amount of S ions in the original samples from the amount of iodine used in reaction
–2
with H S.
2
10. Repeat the same procedure with other samples of water.

End Point
Blue to colourless
87
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