3. Determination of total, temporary and permanent hardness of water by EDTA Method

3. DETERMINATION OF TOTAL, TEMPORARY AND PERMANENT HARDNESS OF WATER BY EDTA METHOD

(EDTA Method - I)

Expt. No.

Date:

AIM

To determine the total hardness, permanent hardness, temporary hardness in the given sample of hard water by EDTA method. A standard hardwater and a EDTA solution are provided.

PRINCIPLE

Disodium salt of Ethylene Diamine Tetra Acetic acid (EDTA) is a well known complexing agent. Its structure is shown in the figure I.

Disodium salt of EDTA is used to determine the various hardness of the given hard water containing Ca²⁺ and Mg²⁺  ions. When EDTA is added to hard water, it reacts with calcium and magnesium ions present in hard water to form stable EDTA metal complexes. From the volume of EDTA consumed the hardness can be calculated. Eriochrome Black - T is used as an indicator. The indicator forms a weak complex with the metal ions present in the hard water and gives wine red colour.

When EDTA is added into the hard water, the metal ions form a stable metal complex with EDTA by leaving the indicator. When all the metal ions are taken by EDTA from the indicator metal ion complex, the wine red colour changes into steel blue, which denotes the end point. The metal EDTA complex is stable at pH 8-10. This pH range can be maintained by adding ammoniacal buffer (NH₄Cl + NH₄OH).

Temporary hardness can be removed by boiling and filtering the given hard water sample. During boiling soluble bicarbonates are converted into insoluble carbonates.

 

Step II: DETERMINATION OF TOTAL HARDNESS OF HARDWATER SAMPLE

TITRATION - II

Hardwater Sample Vs. Std. EDTA

Calculation of the total hardness of hard water

Volume of hard water = 20ml

Volume of EDTÀ consumed, V₂ = ................ ml (titre value)

20 ml of the given hardwater sample consumes V₂ ml of EDTA

ie., 20 ml of the given hardwater sample contains 20 / V₁ × V₂ mg of CaCO₃

1000 ml of the given hardwater sample contains

:. Total hardness of the given sample of hardwater = 642.85 ppm.

The filtrate is collected in the conical flask, EBT indicator and buffer solution are added. It is then titrated against the EDTA.

Temporary hardness = Total hardness - Permanent hardness

 

MATERIALS REQUIRED

1. EDTA solution 2. Standard hand water 3. EBT indicator 4. Buffer solution 5. Burette, pipette, conical flask, 250 ml beaker, 100 ml std. flask 6. Sample hard water.

 

PROCEDURE

TITRATION - 1

Standardisation of EDTA

Step I

The burette is washed well with the distilled water and rinsed with a little amount of the given EDTA solution. It is then filled with the same EDTA solution upto the zero level without air bubbles. Initial reading of the burette is noted. 20ml of standard hard water solution is pipetted out into a clean conical flask. 5ml of ammonia buffer solution and 2 drops of Eriochrome Black - T indicator are added. The solution turns wine red in colour and it is then titrated against EDTA taken in the burette. The change of wine red colour to steel blue colour is the end point. The final reading in the burette is noted. The difference in the burette reading gives the volume of the EDTA solution. The titration is repeated to get concordant values.

Let the volume of EDTA be V₁ ml.

 

Step III: DETERMINATION OF PERMANENT HARDNESS

TIRTATION - III

Boiled hardwater sample Vs std EDTA

Calculation of the permanent hardness of the hard water

Volume of the boiled hard water = 20 ml

Volume of EDTA solution consumed, V₃ = 6 ml (titre value)

20 ml of boiled hard water sample consum V₃ ml of EDTA

ie., 20 ml of boiled hard water sample contains 20 / V₁ × V₃ × 1000 / 20 mg of  CaCO₃

V₃  / V₁ × 1000 mg of  CaCO₃

…….. × 1000 mg of  CaCO₃

…………….

= ……….. mg of  CaCO₃ equivalent

Permanent hardness of hard water sample = ………..  ppm.

 

Step IV: Calculation of the temporary hardness of the hard water

Temporary hardness of the given sample of water } = Total hardness – permanent hardness

= 642.85  - 214 . 28

=.428 . 57 ppm.

 

Step II

TITRATION – II

Determination of Total Hardness of Hardwater Sample

20 ml of the given hardwater sample is pipetted out into a clean conical flask. 5 ml of ammonia buffer solution and 2 drops of Eriochrome Black - T indicator are added. The solution turns wine red in colour. This solution is titrated against EDTA solution taken in the burette. The change of wine red colour into steel blue colour is the end point. The titration is repeated to get concordant values.

Let the volume of EDTA be V₂ ml

 

Step III

TITRATION - III

Determination of Permanent Hardness

100 ml of the given sample of hardwater is taken in a clean 250 ml beaker and boiled for 10-15 minutes. It is then cooled and filtered. The filtrate is collected in a 100 ml standard flask. 20 ml of this made up solution is pipetted out into a clean conical flask. 5ml of ammonia buffer solution and 2 drops of Eriochrome Black - T indicator are added. The solution turns wine red in colour. This solution is titrated against the EDTA taken in the burette. The change in colour from wine red to steel blue is the end point. The titration is repeated to get concordant values.

Let the volume of EDTA be V₃ ml Step ml

 

Step IV

Determination of Temporary Hardness

Temporary hardness of the water sample is calculated by subtracting permanent hardness from total hardness.

Temporary hardness = Total hardness – Permanent hardness.

 

RESULT

1. Amount of total hardness of the given sample of water = 642 . 85 ppm.

2. Amount of permanent hardness of the given sample of water = 214 . 28 ppm.

3. Amount of temporary hardness of the given sample of water = 428.57 ppm.

Structure of Disodium salt of EDTA (Fig-I)

Step 1: STANDARDISATION OF EDTA

TITRATION - I

Standard Hard water Vs. EDTA

Calculation

Volume of standard hard water V₁ = 20 ml

Strength of standard hard water N₁ = ..... N

Volume of EDTA  V₂ = ............ ml

Strength of EDTA N₂ = .............. ?

According to the law of volumetric analysis