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General Test Procedure (GTP) for Total Hardness

PURPOSE
  • To lay down the procedure for determining the total Hardness in water samples.
  • Ethylenediaminetetraacetic acid and its sodium salts (abbreviated EDTA) form a chelated soluble complex when added to a solution of certain metal cations. if a small amount of dye such as Eriochrome Black T or Calmagite is added to an aqueous solution containing calcium and magnesium ions at pH of 10.0 + 0.1, the solution becomes wine red. if EDTA is added to an aqueous solution containing calcium and magnesium ions These ions will be complex and the solution turns from wine red to blue, marking the end point of titration.

APPARATUS
Burette
Conical flask
Pipettes

REAGENTS
Indicators
Buffers
Ethylene Diamine Tetra Acetic Acid

PREPARATION OF BUFFERS
  • Dissolve 16.9g of ammonium chloride (NH4Cl) in 143 ml conc. ammonium hydroxide (NH4OH) Add 1.25g magnesium salt of EDTA and dilute to 250ml with distilled water
  • Store solution in a borosilicate glass container for no longer than 1 month. Stopper tightly to prevent the loss of ammonia or absorption of carbon dioxide from the atmosphere. Discard the buffer when the addition of 1 or 2 ml to the sample fails to produce the desired pH of 10.0 to 11.0 at the titration endpoint.

PREPARATION OF INDICATORS

Eriochrome Black T
Dissolve 0.5 g dye in 100 ml. of triethanolamine (also called ethylene glycol monoethyl or add 1.0 or 2.0 (about 0.5 mg) crystals of dry powder into the water sample.


PREPARATIONS OF 0.01M EDTA SOLUTIONS
  • Weigh 3.723g of analytical reagent grade EDTA dissolve in distilled water, and dilute to 1000ml.

PREPARATION OF STANDARD CALCIUM SOLUTION
Weigh accurately 1.0g anhydrous CaCo3 powder into a 500ml Erlenmeyer flask. Place a funnel in the flask neck and add, a little at a time, 1+1 HCl until all the CaCo3 has dissolved. add 200ml distilled water and boil for a few minutes to expel CO2 cool. add a few drops of methyl red indicator .and adjust to the intermediate orange color by adding 3N NH4OH or 1+1HCl, as required. transfer quantitative and dilute to 100 ml with distilled water,1ml=1.00mg CaCo3.

PROCEDURE
  • Take 100 ml of water sample in a clean 250 ml conical flask.
  • Add 2.0 ml of buffer solution to obtain a pH of 10 .0 to 10 .1
  • Add 2 drops or 50 mg of dry Eriochrome Black T indicator.
  • Add 0.01 EDTA slowly and continue stirring.
  •  The solution will become blue at the endpoint.

CALCULATION

Where 
A = ml titrant for Sample
B = mg CaCO3 equivalent to 1.00 ml of EDTA titrant.

REFERENCE
American Public Health Association Page No. 2-37 to 2-39

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