VPP 322: Aquatic Animal Diseases, Health Care and Management (1+1)

Chemical parameters - Hardness

HARDNESS

Principle

• The concentration of calcium and magnesium expressed as equivalent of CaCO₃ is considered as a measure of total hardness. The calcium and magnesium ions of the sample are titrated with ethylene diamine tetra cetic disodium salt (EDTA) to form the stable Ca EDTA and Mg EDTA. A small quantity of eriochrome black-T is added to a water sample and buffered at pH 10 which would form a soluble wine-red complex with some of the calcium and magnesium ions. During titration, the EDTA will first complex all of the free Ca²⁺ and Mg²⁺ and the solution turns blue. The calcium and magnesium would then dissociate from their complexes with eriochrome black-T to form more stable complexes with EDTA. The following equations summarize the titrations.

Ca²⁺ + Mg² + Ca and Mg eriochrome black T + EDTA (wine red)

Ca EDTA + Mg + eriochrome black T (blue)

Reagents

Buffer solution

• 67.5 of NH₄Cl is dissolved in 570ml of NH₄OH and is diluted to 1 litre in a volumetric flask with distilled water.

Eriochrome black-T indicator

• 4.5 g of hydroxylamine hydrochloride and 0.50 g of eriochrome black-T are dissolved in 100 ml of 70% ethanol. This indicator should be prepared freshly for use.

Standard calcium solution (0.01M)

• 1 g of calcium carbonate (CaCO₃) is taken in a 1 litre beaker and to it 50% HCl is added slowly until the CaCO₃ dissolves. It is then diluted to 200ml with distilled water. The solution is boiled for 5 - 10 min to expel carbon dioxide followed by cooling and adjusting the pH to 7 using NH₄OH solution. This is then made up to 1 litre in a volumetric flask with distilled water.

Standard EDTA titrant

• 4 g of EDTA and 100 mg of MgCl₂ 6H₂O are dissolved in distilled water in a volumetric flask before making up the solution to 1 litre. 10 ml of the standard calcium solution is taken in a 250 ml beaker and to that 90 ml of distilled water is added. The calcium of the standard solution is titrated with the above EDTA solution following the procedure given below. The molarity of the EDTA is computed from equation NV=N’V’. EDTA solution prepared according to the procedure above should be slightly stronger than 0.01 M, but it may be diluted to the exact molarity if desired.

Procedure

• 100ml of water sample is taken in a flask and to it 2 ml of the buffer solution is added. After stirring, 8 drops of eriochrome black-T indicator are added and the solution is titrated against the EDTA solution. A colour change form wine red to pure blue is the end point. The total hardness is calculated as follows:

Hardness as CaCO3 mg/L = ( Volume of EDTA required / Sample taken (ml) ) * 100