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Notes 1st Year 2020 : Chemistry

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Aditya Vyawahare
Vidya Valley School, Pune
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Estimation of total, permanent and temporary hardness of water (EDTA method) Expt. No.: Date: Aim: To estimate the amount of total, permanent and temporary hardness in the collected sample of water. A standard solution of . EDTA is provided. Principle: Hardness in water is due to the presence of dissolved salts of calcium and magnesium. It is unfit for drinking, bathing, washing and it also forms scales in boilers. Hence it is necessary to estimate the amount of hardness producing substances present in the water sample. Once it is estimated, the amount of chemicals required for the treatment of water can be calculated. The estimation of hardness is based on complexometric titration. Hardness of water is determined by titrating with a standard solution of ethylene diamine tetra acetic acid (EDTA) which is a complexing agent. Since EDTA is insoluble in water, the disodium salt of EDTA is taken for this experiment. EDTA can form four or six coordination bonds with a metal ion. 1. Total hardness Total hardness is due to the presence of bicarbonates, chlorides and sulphates of calcium and magnesium ions. The total hardness of water is estimated by titrating the water sample against EDTA using Eriochrome Black-T (EBT) indicator. Initially EBT forms a weak EBTCa2+/Mg2+ wine red coloured complex with Ca2+/Mg2+ ions present in the hard water. On addition of EDTA solution, Ca2+/Mg2+ ions preferably forms a stable EDTACa2+/Mg2+ complex with EDTA leaving the free EBT indicator in solution which is steel blue in colour in the presence of ammonia buffer (mixture of ammonium chloride and ammonium hydroxide, pH 10). Page 1 of 6

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