Determination of Calcium-Magnesium Hardness of Water

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Aim

To determine the Calcium-Magnesium hardness of given water sample.

Apparatus

Burette, pipette, conical flask.

Reagents

  1. EDTA (0.01 M)
  2. 1 N NaOH solution
  3. Muroxide

Significance

Small amounts of Ca combat corrosion of metal pipes by forming a protective coating where as appreciable Ca content, On the other hand, forms scales in boilers, pipes, utensils etc.

Principle

When EDTA is added to water containing both Ca2+ and Mg2+, it combines with Ca2+ in preference to Mg2+. When the pH is sufficiently high (12 to 13), Mg2+ is largely precipated as hydroxide and Ca2+ can be determined directly using EDTA.

Procedure

  1. Pipette out 25m1 of water sample into a conical flask.
  2. Add 2m1 of 1 N NaOH, followed by 2 to 3 drops of Muroxide indicator and shake well. The solution turns pink in colour.
  3. Titrate this solution against Std EDTA solution taken in a burette and discontinue the titration when the colour changes from pink to purple.
  4. Note down the volume of EDTA run down from the burette.

Observation

Trial No Initial Reading Final reading Difference
1      
2      
3      
4      

Calculation

Calcium Hardness = [A × B × 1000] / ml of sample =                    mg/l as CaCO3 Where,

A = Volume of EDTA consumed.

B = 1 (mg of CaCO3 equivalent to 1ml of EDTA titrant)

Magnesium Hardness = Total hardness – Calcium hardness =            mg/l as CaCO3 Concentration of Ca or mg/l of Ca = {A × B × 400.8} / ml of sample.

Concentration of Mg or mg/l of Mg = 0.243 × Calcium hardness.

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