chem(urgent) help~~

2007-12-01 1:23 am

the experiment about(Hardness of water by EDPA titration)
Q1.Can the first indicator be used for the second titration?
(the indicator: Erichrome Black T and hydroxynaphthol
The first titration is about determination of total hardness(Mg2+ and Ca2+)
Erichrome Black T is used for the first titration
The second titration is about determination of concentration of Ca2+
hydroxynaphthol is used for the second titration)

回答 (2)

Carson

2007-12-01 11:08 pm

✔ 最佳答案

If the first indicator be used for the second titration, you will not get a satisfactory result.
Eriochrome Black T indicator does not give a satisfactory color change at high pH. Hydroxy naphthol indicator is similar to Eriochrome Black T, and functions in the same manner, but at high pH.Procedure for Calcium Only. If the pH of the sample solution is raised to about 13, magnesium will precipitate as the very insoluble hydroxide which will not react with EDTA, and calcium alone can be titrated using calcon indicator. Since the total hardness titration gives the sum of both Ca and Mg in the sample, and titration at pH 13 gives Ca only, Mg can be obtained by difference.
I hope this can help with your understanding. =)

參考: My knowledge

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用戶42902

2007-12-02 1:54 am

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