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I am doing my individual investigation and am testing the best method to soften water. I am using sodium carbonate as a softener but when I titrate using edta it doesn’t seem to have an effect (the same volume of titre as with hard water). Does sodium form a complex with edta? If so, is there any way to overcome this?
Igloo writes
EDTA does not form complexes with sodium ions, but it does so with many others, including Ca2+ and Mg2+, the ions commonly responsible for the hardness in water.
Sodium carbonate is a well known water-softening agent, and if after adding an excess of this reagent you carry out an efficient filtration, there should be no Ca2+ or Mg2+ ions left in the filtrate to react with the EDTA, and the titre should be close to zero. If you are getting comparable results with the original water, I can only assume that the water is already “soft”.
Alternatively you may well be using a solution of EDTA which is too concentrated for the conditions. Remember that it is conventional in titrations to use solutions of comparable concentrations, so that sensible titres are obtained. Have you thought of this? Even a very “hard” water contains about 0.006 mol dm-3 of calcium and magnesium ions, and naturally “soft” water is reckoned to be about one-tenth of this, so have you made up your EDTA solution to a concentration of about 0.001-0.002 mol dm-3?
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Risk assessment
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updated: 01 December 2006
