Determination of hardness of given sample of water

DETERMINATION OF HARDNESS

OBJECTIVE: To determine the hardness of a given Sample of water

INTRODUCTION:

Hard water are generally considered to be those waters that require considerable amount of soap to produce a foam or lather and that also produce scale in hot water pipes, heaters, boilers and other unit in which the temperature of water is increased materially. Hardness are caused by multivalent metallic cat ions. Such ions are capable of reacting with soap to form precipitates and with certain anion present in the water to form scale. The principle hardness causing cat ions are the divalent calcium, magnesium, however strontium, ferrous ion, manganous ions which are normally present in trace quantities in natural water, also cause hardness. The degree of hardness of drinking water has been classified in terms of the equivalent CaCO₃ concentration as follows:

Soft	0-60 mg/l
Medium	60-120 mg/l
Hard	120-180 mg/l
Very hard	>180 mg/l

PRINCIPLE OF EDTA TITRIMETRIC METHOD:

Ethylenediamine tetra acetic acid (EDTA) and its sodium salts form a chelated soluble complex when added to a solution of certain metal cat ions. If small amount of dye such as Erichrome Black T indicator added to an aqueous solution containing calcium and magnesium ions at a pH 10.0 ± 0.1, the solution will becomes wine red. When EDTA is added as a titrant, the Ca⁺⁺ and Mg⁺⁺ divalent ions get chelated and sharp change of colour from wine red to blue which indicates complete chelation. This is the end point of titration.

REAGENTS:

1.       Buffer solution: Dissolve 16.9 gm ammonium chloride, NH₄Cl, in 143 ml conc.ammonium hydroxide, NH₄OH, add 1.25 g of magnesium salt of EDTA and dilute to 250 ml with distilled water.

2.       Standard EDTA solution: Dissolve 3.723 g of disodium salt of EDTA in 1 liter distilled water ( this solution may be standardized against standard calcium solution)

3.       Erichrome Black T indicator: Mix 0.125 g dye with 100 g hydroxylamine hydrochloride. Dissolve this mixture in 50 ml of 95% ethyl or isopropyl alcohol.

PROCEDURE:

1.       Take 50 ml of sample (or aliquot) in an Erlenmeyer flask.
2.       Add 1 or 2 ml buffer solution and three drop of Erichrome Black T indicator.
3.       Titrate with standard EDTA solution till wine red colour changes to blue. Record the volume of titrant used.

RESULT:

SAMPLE NO.	BURETTE READING		ML OF TITRANT USED
	INITIAL	FINAL
1

CALCULATION:-

Hardness (EDTA) as mg/l as CaCO₃ = [(A x B x 1000)/ml of sample ]       ML of sample = 25ml (tap water)      A= ml of titrant =         ml      B=MgCaCO₃ equivalent to 1ml of EDTA        Hardness (EDTA)= [(A x 1 x 1000)/25] mg/l as CaCO₃