Chemical amounts (A2)
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Why is KOH used in complexometric titrations with EDTA to determine the amount of calcium in a vitamin tablet?
Why is it important not to leave the solution standing after KOH is added in the tablet solution, thanks.
Igloo writes:
When analysing for calcium ions using a complexometric titration with EDTA, magnesium ions interfere, since they too form a complex with EDTA. As you will know, calcium and magnesium ions are often found together, and, being in the same group of the Periodic Table, share similar chemical properties.
However, if the titration is carried out at high pH, e.g. in the presence of an alkali such as potassium hydroxide, the magnesium ions precipitate out as the hydroxide, since Mg(OH)2 is not very soluble in water. Calcium hydroxide, however, remains in solution, provided that the concentration of calcium ions is not too high - as will be the case under the conditions of your experiment - and so the calcium ions alone remain available for complexing with the EDTA.
I do not know why the solution should not be allowed to stand too long once the alkali has been added.
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updated: 29 September 2006
