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determination of aluminum by back titration

Determination of aluminum  (Al+3) by back titration using EDTA titration


               Complexometric back titration generally performs when the metallic ions form a
stable complex with EDTA in a slow reaction or when a metal ion blocks an indicator. The
blocked indicator cannot release metallic ions, thus no color change will be observable at
the endpoint of the complexometric direct titration. Both conditions exist in the case of
aluminum ion, thus the ion is best determined by complexometric back titration along
with heating to enhance the complexation of Al-EDTA.

In this analysis, an excess known amount of EDTA is added to the Aluminum
sample solution followed by a pH 10 buffer solution where the Al-EDTA complex can
form. To prevent aluminum ion blocking the indicator and to facilitate the Al-EDTA
complexation, heating the mixture prior to adding the indicator Eriochrome black T wherein
its free form is a blue color. The amount of unchelated EDTA can be then determined using
complexometric back titration with standardized zinc or Magnesium solution. At the endpoint,
a change to wine-red color is observable. This change is due to the formation of Zn- or Mg-
Eriochrome black T complex
determination of aluminum by edta titration


1- Aluminum ions solution (analyte)
2- Eriochrome black T indicator 
3- Standardized 0.01 M zinc sulfate solution (titrant).
4- 0.01 M EDTA solution


1- Pipette 25.0 mL of Aluminum ions solution into a 250.0 mL and add an
excess known amount of 0.01 M EDTA solution (V EDTA) and add sufficiently
buffer solution pH 10 just to make the solution ammoniacal.
2- Boil the mixture for a few minutes to ensure complete complexation of
Aluminum. Then, cool to room temperature and adjust the pH to 10.
3- Add 50mg of Eriochrome black T indicator and titrate rapidly with standard
0.01 M zinc sulfate solution until the color changes from blue to wine red.
Record the used volume (as V Zn).
4- Repeat the titration twice.


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