CLARK'S METHOD FOR HARDNESS ESTIMATION

CLARK'S METHOD FOR HARDNESS ESTIMATION
II - 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.
Procedure:
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.
Calculations:
Step - 1: Standardisation of soap solution:
50mL of standard hard water  (V1-V4) mL of soap solution
50mL of standard hard water contains 50mg of CaCo3
(Standard hard water contains 1g/L or
1mg/mL CaCo3)
Thus, (V1-V4) mL of soap solution = 50mg of CaCO3 eq.
1 mL of soap solution = )(
50
 VV 41
mg of CaCO3 eq.
Step - 2: Determination of total hardness of water:
50mL of hard water  (V2-V4)mLof soap solution
 (V2-V4) x )(
50
VV 41
mg of CaCO3 eq.
1000mL or 1L of hard water
Determination of hardness of water:
1)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.
EDTA METHOD: • COMPLEXOMETRIC TITRATI...

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