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ESTIMATION OF CHLORIDE AND SALT IN GIVEN WATER SAMPLES

AIM

To estimate the amount of chloride and salt content in the given water samples A and B.

REAGENTS

0.005N silver nitrate solution and 10% potassium chromate.

PRINCIPLE

When potassium chromate is added as an indicator to the water sample, it dissolves in water and the chromate ions give yellow colour to the sample. Sodium chloride is present in the dissolved state in the given sample of water. When this is titrated against silver nitrate, the silver ions react first with the chloride ions present in the sample and form silver chloride precipitate and sodium nitrate.

When all the chloride ions in the sample are precipitated, the excess silver nitrate present reacts with potassium chromate and forms a pale red precipitate of silver chromate.

The appearance of the pale red colour indicates that all chloride ions have been precipitated and indicates the end point of titration. From the titre values, the amount of chloride and salt present in the sample is calculated.

PROCEDURE

Take a 50 ml burette and wash it with tap water and distilled water and then rinse it with 0.005 N silver nitrate solution. Fill the burette with the 0.005 N silver nitrate solution and note down the initial reading. Pipette out 10 ml of the given water sample with a clean 10 ml pipette into a clean and dry conical flask. Add two to three drops of potassium chromate as the indicator. The solution in the conical flask turns to yellow colour. Titrate this solution against the 0.005 N silver nitrate solution taken in the burette. The appearance of a pale red colour is the end point of titration. Note down the final burette reading. Repeat the titration until consecutive concordant values are obtained. From the titre values, calculate the amount of chloride and salt present in the given water sample using the given formulae.

CALCULATIONS

Sample A

Sample B

RESULT

Amount of chloride present in sample A = _____ g/L

Amount of salt present in sample A = _____ g/L

Amount of chloride present in sample B = _____ g/L

Amount of salt present in sample B = _____ g/L.

DISCUSSION

All types of natural water contain various amounts of salts such as potassium, sodium, magnesium, calcium, sulphates, phosphates, nitrates, and chlorides. Among these salts, sodium chloride occurs in larger quantities. All these salts are responsible for the salinity of water. Salinity has been defined as the total amount of dissolved salts in 1L of water and is denoted by ‘S’. Salinity is higher in sea water than in fresh water. The average salt content of sea water is known to be approximately 35%, whereas that of fresh water normally may not exceed 0.5%. Salt content of various water resources has significance in understanding better the regulatory mechanism of various aquatic animals.

Sea derives salts from the fresh water. During rain, water percolates into the soil where salts get dissolved in the water. The excess rain water run down into the rivers and from there it reaches the sea. The river water is non-salty, whereas the sea water is salty. This is mainly because of the various animals living in the sea water. These animals require calcium carbonate (CaCO₃) to form their protective covering. As a result, the percentage of calcium carbonate in the sea water becomes reduced, whereas that of sodium chloride increases. Moreover, the rate of evaporation is also greater in sea water, which results in the accumulation of salts in it.

VIVA QUESTIONS

1. What is the colour of chromate ions?
2. What is the chemical reaction between silver nitrate and sodium chloride?
3. Sea water is more salty. Explain the reason.
4. Give the equivalent weight of chloride.
5. What is the end point here?