To determine the Hardness of given sample of water

To determine the Hardness of given sample of water

DISCUSSION:

Hardness is water Is due to the natural accumulation of salts from contact with soil and geological formations or it may enter from direct pollution by industrial effluents. Calcium and magnesium are the principle cations causing hardness. Iron, aluminum, manganese, strontium, and zinc also cause hardness but to a relatively little extent or to negligible amount. The term total hardness indicates the concentration of calcium and magnesium ions only. However if presents in significant amounts the other metallic ions should also be included. The total hardness is expressed in terms of calcium carbonate .

PRINCIPLE:

Calcium and magnesium ions react with EDTA to form soluble complexes and the completion of reaction is indicated by the colour change of a suitable indicator such Erichrome Black-T.

REAGENTS:

1. Calcium standard Solution:
   

Weigh accurately 1gram of pure calcium carbonate (previously dried ate 1500C) and place

In 250ml. conical flask using 50ml. distilled water. Place a funnel in the neck of the flask.

Add 20.5ml NHCL warm until the solution is complete. Cool and transfer to a 1000ml.

Volumetric flask and make up to the mark with distilled water

1. Standard EDTA titrant 0.02N, (0.01M):
   

Weigh accurate 3.723 g. AR grade Disodium ethylene Diamine tera acetate dehydrate, also called (ethylene dinitrilo)-tetra acetic acid disodium salt EDTA NazH2CioHi2OsN2.2H20, and dissolve in distilled water and wake up to 1000m. check the titer by standardizing against standard calcium solution as in the ” Procedure for clear samples

1 ml of exactly 0.02N EDTA== I mg CaCO3

1. Ammonia–ammonium Chloride Buffer:
   

Dissolve 16.9g ammonium Chloride NH4CI n 143 mi conc. Ammonia solution, add1250% magnesium salt of EDTA and 780mg MgsO4.7FH20 or 644mg MgCl2.6Ha0 in 50ml distilled water, Add this solution to 143ml.conc. Ammonia solution in which 16 .9gm
NH4CI has already been dissolved. To attain highest accuracy, It has to be adjusted o exact equivalence by the appropriate addition of a Small amount of EDTA Or Sulphate (or Chloride).

4. Sodium Sulphide inhibitor:

Dissolve 5g NazS.9H20 or 3.7g NazS.SH2O in 100ml. distilled water. This inhibitor Deteriorates on oxidation and air must be excluded. Keep the bottle tightly fitted with a rubber stopper. When appreciable concentration of heavy metals is present, they give a Sulphide precipitate which obscures the end point.

1. Erichrome Black-T Indicator:
   

Alcoholic Solution of this dye are not so stable and one of the following methods may be adopted:

(i) Mix 0.5 to Z of the dye in 100g of triethanolamine

(ii) Mix together 0.5g of the dye and 100g sodium chloride to prepare a dry powder mixture.

PROCEDURE:

Clear samples:

1. Place a suitable volume of sample (filtered if necessary) in a conical flask and dilute to 50ml.
2. Add 1 ml of buffer solution per 50ml. volume. The pH of the titre should be 10.0+0.1.
   

Add 1 ml Sodium Sulphide-inhibitor if necessary. Add 1 drop of indicator solution or an appropriate amount of dry powder.

1. Indicator used as Erichrome Black-T
   
2. Titrate with standard EDTA solution slowly, until a reddish tings appears and add the last few drops within 3-5 seconds. At the end point the solution will be blue. The whole titration procedures should be completed within 5 minutes after the addition of buffer.
   

Samples Containing Suspended Matter and Colloidal Organic:

Matter:

Evaporate an aliquot of the sample to dryness on a steam bath. Heat it in a muffle furnace to S50°C until the organic matter is oxidized. Dissolve the residue in 20ml IN. HCI, neutralize to (pH=7) with IM NaOH and dilute to 50ml. with distilled water. Cool to room temperature and proceed from step No.2 of the procedure describes for clear samples.

Low Hardness Sample:

Ton exchange effluents or softened waters and some natural waters will have only low hardness (less than 5mg/l). For those samples take a large volume for titration ranging from 100 to 1000ml. Add proportionately higher amounts of buffer, inhibitor, and indicator. Titrate with EDTA slowly, using a micro, burette. Carry out a blank using the same volume of deionized or distilled water.

Calculation:

OBSERVATIONS: