Estimation of hardness of water
Estimation of hardness of water:
Method of estimation of hardness of water:
Hardness of water can be estimated by the following methods.
A.Temporary hardness is determined by finding the alkalinity of water before boiling and that left after boiling ( temporary hardness is removed by boiling)
Procedure: Pipette out 50ml of water sample in a conical flask, 2-3 drops of methyl orange indicator is added and titrated against N/50 HCl. The volume of acid used is ‘V1ml. 100ml of water sample is taken in a Pyrex beaker and evaporated to dryness. About 50ml of distilled water is added, warned, stirred to dissolve the soluble portions left. The solution is filtered, washed the residue with distilled water. The solution is made up to 100mL. 50mL of this solution is taken in a conical flask and titrated against N/50HCl using methyl orange as indicator. The volume of acid used is ‘V2’mL.
B.Estimation of permanent hardness is based on the fact that it can be removed by boiling with a known excess of standard Na2CO3. The chloride and sulphates of calcium and magnesium form insoluble carbonates. The residual sodium carbonate is determined by titrating against a standard acid. The decrease in the amount of soda added is equivalent to permanent hardness.
Procedure: 50ml of the given water sample is taken in the beaker and 50ml of N/50 Na2CO3 solution is added. Boiled the solution for 15minutes. Cooled the solution, filtered the precipitate and the filtrate is collected in a conical flask. Titrated the unused Na2CO3 present in the flask against N/50 HCL. Using methyl orange as an indicator. The titre value is ‘V’.
2.Soap Titration Method: (or) Clarks Method:
Principle: When soap solution is added to hard water, the hardness causing ions (Ca2 , Mg2 and other heavy metal ions) are first to react to form a precipitate of insoluble calcium and magnesium soap is obtained.
The process takes place continuously until there is hardness in the sample of water. When all the hardness causing ions have been precipitated, further addition of soap solution gives lather.
Step – 1: Standardisation of sample solution: The burette is first rinsed and filled with soap solution. In a narrow mouth 250mL glass-stoppered bottle, 50mL of standard water is taken (The hardness of standard hard water is 1g/L). Soap solution is added 0.2mL at a time to this standard hard water solution. After each addition, the contents of the stoppered bottle are shaken vigorously until lather formation starts.
Then the addition of soap solution is made at the rate of 0.1mL at a time till a lather is formed that persists for 2 minutes. Let the volume of the soap solution used is ‘V1’mL.
Step- 2: Determination of total hardness of water: The above procedure is repeated by taking 50mL of given water sample (Sample to be analysed.). The volume of the soap solution used in this is ‘V2’mL.
Step -3: Determination of permanent hardness of water: 250mL of the water sample is taken in 500mL beaker. It is then boiled till the volume reduces to 50mL. (Boiling causes all the bicarbonates to decompose respectively into insoluble CaCo3 and Mg(OH)2). The precipitate is filtered and washed with distilled water. Simultaneously, the filtrate and washings are quantitatively collected in a 250mL conical flask.
The volume is then made 250mL by adding distilled water. 50mL of this sample is taken and procedure of Step – 1 is repeated titre value of the soap solution be ‘V3’mL.
Step -4: Determination of lather factor: 50mL of distilled water of zero hardness is titrated with soap solution. The titration is continued till lather is obtained which lasts for 2 minutes. Let the titre value be V4mL. This volume must be subtracted from all the litre values.
Determination of hardness of water:
Soap solution method (or) soap titration method clark’s method:
Soluble soaps consist of sodium (or) potassium salts of higher fatty acids. When soap solution is added to hard water, the hardness causing ions first react to form precipitate of calcium and magnesium soaps which are insoluble in water.
Thus, after all the hardness causing metal ions present in the hard water sample are precipitated, the further addition of soap gives lather.
The total hardness of water sample can be determined by titrating an aliquot of the sample against a standard soap solution in alcohol. The appearance of a stable lather persisting even after shaking for about 2 min marks end point.
The water sample is boiled for 30min to remove temporary hardness and then it is titrated with standard soap solution, the titre value corresponds to permanent hardness of sample.
The difference between the two measurements corresponds to the temporary hardness.