kimia analitik . determination of aluminium

8/14/2019 kimia analitik . Determination of Aluminium

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Determination ofAluminium

as Aluminium Oxyde


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Procedure

1. Weigh out accurately about 1,8 gramaluminium sample salt

2. Rinse and dissolve in 200 mL of water then

add 5 gram of ammonium chloride, a fewdrops of methyl red indicator and heat justto boiling.

3.

Add pure dilute ammonia solution (1:1) dropwise from a burette untill the color of thesolution change to a distinct yellow


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4. Boil the solution for 1 or 2 minute and filter at

once through a quantitative filter paper

(Number 41)

5. Wash the precipitate thoroughly with hot

2 percent ammonium nitrate or chloride

solution made neutral with ammonia solutionto methyl red indicator.

6. Place the paper with the precipitate in a

constant weighed crucible.


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7. Dry, char and ignite for 10-15 minutes with a

Fisher or Meker high temperature burner.

8. Allow the crucible covered with a well fitting

lid to cool in a desiccator and weigh as soon

as cold. Ignite to constant weight.

9. Calculate the percentage of aluminium.


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Discussion

The aluminium is precipitated as the hydrated

oxide by means of ammonia solution in the

presence of ammonium chloride. The

gelatinous precipitate is washed, converted

into the oxide by ignition and weighed as

Al2O3


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Aluminium hydroxide is amphoteric in

character :

Al(OH)3(s) + 3H+

(aq) Al3+(aq) + 3H2O(l)

Al(OH)3(s) + 3OH-(aq)

AlO2-(aq)

+ 2H2O(l)

Precipitation commences at approximately

pH 4 and is complete between pH 6,5

7,5.

These pH range can be adjusted with the aid

of methyl red as indicator.

The pH employed for precipitation must becontrolled.


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Condition of Precipitation

This pH controlling is achieved by theaddition of ammonium chloride which exertsa buffering effect and also assists the

coagulation of initially colloidal precipitate. The presence of ammonium salt reduces to a

minimum co precipitation of the divalentmetal such as calcium and magnesium.


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A readily filterable precipitate is obtained by

precipitation in hot solution.

The precipitate cannot be washed with hotwater, for aluminium hydroxide is readily

peptised and will run through the filter.

A 2 % neutral ammonium nitrate or chloridesolution is satisfactory, the presence of

ammonium chloride in the precipitate causes

no appreciable volatilisation aluminium during

ignition.


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Igniting the Precipitation

The aluminium oxide obtained by igniting

aluminium hydroxide is hygroscopic unless

the temperature has been raised at least

12000C. For this reason the precipitate

should be ignited in a silica crucible

(porcelain is slightly hygroscopic when

heated to a high temperature) over a Fisheror Meker burner. The best procedure is in an

electric furnace.


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kimia analitik . determination of aluminium

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