Total Hardness Solution

Test for Hardness of water: 

Reagents: 0.01 M EDTA Buffer solution 

Eriochrome Black T indicator 

Standard EDTA solution 

Disodium dihydrogen ethylenediamine tera acetate (A.R.) is available commercially and can be used as a primary standard. However, many samples contain moisture which is difficult to remove through drying. In such cases the solution is standardized against standard zinc or magnesium solution. Its molecular weight is 372.25 and after drying at 80oC it can be weighed out a accurately. 

1.Eriochrome Black T: This substance is sodium (I-hydroxy-2- naphthylazo)-6-nitro-2-naphthol-4-sulphonate). It is also known as Solochrome black or WDFA. etc. In strongly acid solution Eriochrome Black T (EBT) polymerises to a red brown product, and the metal complexes are usually red in colour. 
 

The colour at different pH given by Eriochrome black T are: 

Red - below pH 5.5
Blue - between pH range 7-11
Yellowish orange - above pH 11.5

Between pH range 7-11, when metallic salts are added, the colour of the indicator changes from blue to red. At the end point in EDTA titration the indicator will be set free and colour change will be from red to blue. 

Preparation of Eriochrome Black T as indicator 

(i) the solution of indicator is prepared by dissolving 1 g of Eriochrome Black T in 15 ml of trienthanol amine and adding 5 ml of absoluta alcohol. It can also be prepared by dissolving 0.2 g of Eriochrome Black T in 20 ml of absolute alcohol. Use about 3 to 4 drops of the indicator solution. 

(ii) 1 g of the indicator is grinded with 100 g of NaCl to a very fine powder. This mixture is stored in a well stoppered bottle. Use about 0.2 g of indicator in each titration. 

Procedure: 

. 20 drops of sample water was take in a dry test tube with a calibrated pipette. 

. Adding 2 drops of butter solution to the tube.
. Adding a pinch of Eriochrome Black T indicator to the tube. 

. Stir the solution. The colour of the solution will became wine red. 

. Adding EDTA with another pipette till the colour changes to blue. . Counting the drops of EDTA. 

Calculation: 

Volume of EDTA = no. of drops of EDTA x drop value of pipette = V1 Molarity of EDTA = M1
Volume of water = no. of drops of water x drop value of water = V2 Hardness (mg/L) = V1 x M1 x 105/V2 

Result: The hardness of sample water is ___________ 

To determine magnesium against EDTA using Eriochrome Black T as an indicator. 

Material required 

(i) 0.1 M EDTA solution: Dissolve 37.225 g of EDTA into 1000 ml of volumetric flask, in redistilled water and make upto the mark with the same water. 

(ii) Eriochrome Black T (EBT) indicator solution. 

(iii) Buffer solution (pH 10): Add 57 ml of concentrated ammonia solution (sp. gr. 0.88) to 7 g of NH4Cl (A.R.) and dilute to 100 ml with distilled water. 

(iv) Magnesium ion solution (0.1M): Weigh accurately 0.61 g of magnesium turning and dissolve in HCl. Neutralise the resulting solution with M-sodium hydroxide. Transfer in to a 250 ml volumetric flask and dilute the solution with distilled water up to the mark. MgSO4.7H2O can be employed for preparing the solution. Weigh accurately 7.10 g magnesium sulphate heptahydrate and dissolve in water and dilute to 250 ml. 

MgSO4 + Na2C10H14O8N2  MgC10H14O8N2 + Na2SO4 

Procedure 

Take 250 ml of the magnesium solution in a conical flask, dilute to about 100 ml with distilled water. Add 2 ml of the buffer solution (pH10). Add 4-5 drops of indicator solution and titrate against 0.1M EDTA solution with constant solution stirring until the colour changes from red to blue at the end point. 

1 ml of 0.1M EDTA = 2.432 mg of Mg. 

Precautions: 

1. EDTA should be dried be remove moisture in the preparation of its solution. 

2. Redistilled water should be employed for preparing the EDTA solution. 

3. Polyvalent cations, if present, interfere and hence should be removed as hydroxides. 

4. The colour change near the end point is very slow. Care should be taken to avoid over titration. 

To determine calcium against EDTA using Murexide as an indicator. Materials required
(i) 0.01 M EDTA solution:
(ii) Murexide indicator solution: 

(iii) NaOH solution: Prepare 1M aqueous solution. 

(iv) Calcium ion solution (0.01 M): Weigh accurately 0.50 g A.R. CaCO3 and transfer the salt in a 500 ml measuring flask. Dissolve the salt in minimum quantity of 6M HCl adding drop by until effervescences stop and the solution become clear. Make up the mark. 

Procedure 

Talk 25 ml of the calcium solution, dilute to 100 ml with water and add 2 ml of 1 (M) NaOH solution to have a solution of pH about 12. Add few drops of murexide indicator and titrate against standard EDTA solution. The colour chang at the end point is from red to purple. 

Calculations 

1 ml of 0.01 M EDTA = 0.4008 mg Ca 

To determine Ca2+ and mg+ ions in a given solution and estimate total hardness of water 

The mixture having ca2+ & mg2+ ions is titrated in one aliquot of pf the sample solution and ca2+ ions are titrated in a further aliquot in strong alkaline medium in which Mg2+ ions get precipitated. Mg2+ ions are then evaluated from the difference. 

Materials required 

0.01M Ca2+ and Mg2+ ions solution, 0.01M EDTA, 2M NaOH, buffer pH. 10, Murexide and Erio T indicators. 

Procedure 

(i) Pipette out 25 ml of the solution (Ca2+ and Mg2+ ions) into a conical flask. Add 5 ml of buffer (pH = 10) and dilute to about 50 ml of redistilled water. Now add 3 – 4 drops of Erio T indicator. Warm the solution upto 600C and titrate with EDTA until the colour of the solution changes from wine red to clear blue. Titrate slowly near the end point. Suppose A ml be the volume (in ml) of EDTA used. 

(ii) Pipette out 25 ml of the solution (Ca2+ and mg2+ ions) into a conical flask. To this add 5 ml of 2M NaOH solution and 2 – 3 drops of Murexide indicator. After diluting the solution to about 25 ml with distilled water, titrate with EDTA until the colour changes from red to purple. Suppose B be the volume (in ml) of EDTA used. 
 

Calculations 

B x 0.4008 = mg of Ca 
(A – B) x0.2532 = mg of Mg. 

In order to obtain the total amount of these ions present in solution, the number are to be multiplied by 100/Q, where Q is the volume (in ml) of the aliquot, i.e. 25 ml.
Estimation of the hardness of water: It is expressed in parts per million, ppm. of Ca. 

B/x x 0.4008 M = ppm Ca where B=ml of EDTA 

x = ml of tap water
M = Molarity of EDTA