total hardness of water

7/28/2019 Total Hardness of Water

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EXPERIMENT: DETERMINATION OF THE TOTAL HARDNESS (PERMANENT AND

TEMPORARY) OF WATER.

THEORY:

The hardness of water is generally due to dissolved calcium and magnesium salts and may be

determined by complexometric titration. EDTA is a good complex forming agent. When

EDTA reacts with metal there is the formation of complex.

Indicator standard EDTA

Metal ions metal ions indicator complex metal ion EDTA

complex + indicator.

This process is known as complexometric titration. Buffer solution is used to maintain the pH

value about 10. The efficiency of complex formation with EDTA varies with change in pH of

the solution. Basic medium is most favorable for complex formation.

HOOC-CH2 CH2-COOH

N-CH2-CH2-N + M2+ M-EDTA

HOOC-CH2 CH2-COOH

The EDTA solution may be standardized by using standard Zn ion solution, using metal ion

indicator, Erichrome black -T. It forms the complex with the Zn ion which is red in color.

When it is titrated with EDTA, it removes the metal ion indicator forming metal EDTA

complex which gives blue solution.

M-In (wine red) + EDTA M-EDTA + In (blue)

The total hardness of water can be determined by titrating the buffer solution with EDTA

whose strength is known. The end point corresponds to cation Ca2+ and Mg2+ forming EDTA

complex at pH 10.0.

The total hardness is expressed in parts of CaCO3per million of water. If the solution does

not contain Mg2+ ion, little known amount of Mg2+ should be added which forms the wine

red colored complex,Ca2+ cannot form a stable complex with the indicator. Mg2+ indicator

later on adding EDTA gives more stable Mg2+

EDTA complex leaving free indicator which isblue in color. To remove the interfere due to traces of other metals Co, Ni, Cu, Zn, Hg, Mn


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little hydroxyl ammonium chloride is added which reduces some of metals to their lower

oxidation states so cannot form complex.

PROCEDURE:

1. Preparation of about 0.01 M EDTA solution

0.9 gm of pure EDTA is weighed out roughly and dissolved in 250 ml volumetric flask with

deionosed water up to mark.

2. Preparation of standard (0.01 M ) Zinc ion solution

Weigh out accurately 0.163gm of Zinc and dissolve it in 5 ml conc. HCl and 5 ml water in a

500ml beaker and allow it to react. Neutralize it by adding NaOH solution drop by drop until

it gives turbidity. Stir it properly. When turbidity appears add a drop of dil. HCl to neutralize

excess NaOH. Make the solution upto 250 ml.

3. Standardization of EDTA solution with Zinc ion solution

Pipette out 25ml of Zinc ion solution in a 250ml conical flask. Add 2ml buffer solution and

30 to 40 mg Erichrome black -T. Titrate with EDTA solution until color changes from wine

red to deep blue color. Repeat the process to get concurrent readings.

4. Determination of total hardness of water ( permanent and temporary)

Take 50ml sample of water to be tested in a 250 ml conical flask. Add 1ml buffer solution

(ammonium hydroxide /ammonium chloride, pH 10) and 10 to 12 drops of Erichrome black

-T indicator (30-40 mg). Titrate with standard EDTA solution (0.01M ) until color changes

from red to pure blue, there should be no magnesium present in the sample of water it is

necessary to add 0.1ml magnesium-EDTA solution (0.1M ) before adding the indicator.


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OBSERVATION:

1. Titration of standard Zinc ion solution Vs EDTA

No. of obs. Vol. of Zn++ solution

(ml)

Initial burette

reading (ml)

Final burette

reading (ml)

Difference of burette

reading (ml)

Concurrent

reading (ml)

2. Titration of EDTA vs. water

No. of obs. Vol. of water

sample (ml)

Initial burette

reading (ml)

Final burette

reading (ml)

Difference of

burette reading (ml)

Concurrent

reading (ml)

CALCULATION:

Strength of EDTA =

50ml of water contains...................gm of CaCO3

1000ml of water contains..................gm of CaCO3.

total hardness of water

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