Kimia Analitik . Determination Of Aluminium

Determination of Aluminium as Aluminium Oxyde

Procedure 1. 2.

3.

Weigh out accurately about 1,8 gram aluminium sample salt Rinse and dissolve in 200 mL of water then add 5 gram of ammonium chloride, a few drops of methyl red indicator and heat just to boiling. Add pure dilute ammonia solution (1:1) drop wise from a burette untill the color of the solution change to a distinct yellow

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 solution to methyl red indicator. 6. Place the paper with the precipitate in a constant weighed crucible.

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.

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

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 be controlled.

Condition of Precipitation  This

pH controlling is achieved by the addition of ammonium chloride which exerts a buffering effect and also assists the coagulation of initially colloidal precipitate.  The presence of ammonium salt reduces to a minimum co precipitation of the divalent metal such as calcium and magnesium.

A

readily filterable precipitate is obtained by precipitation in hot solution.  The precipitate cannot be washed with hot water, for aluminium hydroxide is readily peptised and will run through the filter.  A 2 % neutral ammonium nitrate or chloride solution is satisfactory, the presence of ammonium chloride in the precipitate causes no appreciable volatilisation aluminium during ignition.

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 Fisher or Meker burner. The best procedure is in an electric furnace.