Chloride in Water (exp:8) - assignment - Q-A

Determination of Chloride in Water
(1) Q: Discuss the environmental significance of “chloride” in water.
Ans: Chlorides in reasonable concentrations are not harmful to human. At concentrations above 250 mg/L they give a salty taste to water that is objectionable to many people. For this reason, chlorides are generally limited to 250 mg/L in supplies intended for public use. In many areas of the world where water supplies are scarce, source containing as much as 2000 mg/L are used for domestic purposes without the development of adverse effects, once the human system becomes adapted to the water. According to Bangladesh Environment Conservation rules (1997), drinking water standard for chloride is 150-600 mg/l; but for coastal regions of Bangladesh, the limit has been relaxed to 1000 mg/l.
Before the development of bacteriological testing procedures, chemical testes for chloride and for nitrogen, in its various forms, served as the basis of detecting contamination of groundwater by wastewater. Chlorides are used to some extent as tracers in sanitary engineering practice; however, they have been replaced to a great extent by organic dyes.
(2) Q: What precautions should be taken in the determination of chloride concentration by Mohr method? Explain why.
Ans: Several precautions should be taken in the determination of chloride concentration by Mohr method:
A uniform sample size should be used, preferably100 ml (or 50ml), so that ionic concentrations needed to indicate the end point will be constant.
The pH should be in the range of 7 to 8 because Ag+ is precipitated as AgOH at high pH levels and the CrO42- is converted to Cr2O72- at low pH levels.
A definite amount of indicator should be used to provide a certain concentration of CrO42- otherwise AgCr2O4may form too soon enough.
(3) Q: In determination of chloride, why an indicator blank or error is subtracted from the amount of silver nitrate used in titration? Explain.
Ans: In titration, chloride is precipitated as a white silver chloride as equ.
Ag+ + Cl- = AgCl (white precipitate)
The end point of titration cannot be detected visually unless an indicator capable of demonstrating the presence of excess Ag is present. The indicator normally used in potassium chromate, which supplies chromate ions. As the concentration of Cl- ions becomes exhausted, the silver ion concentration increases and a reddish brown precipitate of silver chromate is formed
2Ag+ + CrO42- = Ag2CrO4 (reddish brown precipitate)
This taken as evidence that all chloride has been precipitated. Since an excess Ag+ is needed to produce a visible amount of Ag2CrO4, the indicator error (blank or error) is subtracted from all titration.

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