Technology and Engineering CHEMICAL ANALYSIS

Determination of alluminium SMK Negeri 13 Bandung

Objectives…. After this session, the students are expected to be able : - to explain the steps of determination of alluminium - to explain the objectives of every step - to explain the function of reagents - to arrange the planning of determination of alluminium according to the job sheet - to conduct the determination of alluminium - to do the calculation in the determination of alluminium - to arrange the report of the determination of alluminium Teknologi dan Rekayasa

Procedure Weigh out accurately about 1,8 gram alluminium 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 until the color of the solution change to a distinct yellow Teknologi dan Rekayasa

Procedure…. 4. Boil the solution for 1 or 2 minute and filter at once through a quantitative filter paper (what number?) 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. Teknologi dan Rekayasa

Procedure… 7. Dry, char and ignite for 10-15 minutes with a Fisher or Meker high temperature burner. (12000C) 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 alluminium. Teknologi dan Rekayasa

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Discussion….. The alluminium 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 Teknologi dan Rekayasa

Discussion….. alluminium hydroxide is amphoteric in character : Al(OH)3 + 3H+  Al3+ + 3H2O Al(OH)3 + 3OH-  AlO2- + 2H2O Teknologi dan Rekayasa

Precipitation Begin at pH 4 Complete between pH 6,5 – 7,5. The pH employed for precipitation must be controlled The pH range adjusted with the aid of methyl red as indicator. Teknologi dan Rekayasa

The Ammonium chloride pH controlling as a buffering agent assists the coagulation of initially colloidal precipitate reduces to a minimum co precipitation of the divalent metal such as calcium and magnesium. Teknologi dan Rekayasa

Readily filterable precipitate Hot precipitation Readily filterable precipitate Slow/gradual Big size Co agullated Teknologi dan Rekayasa

Washing of precipitate Use no pure water because alluminium hydroxide will readily peptized and will run through the filter. Use 2 percent neutral NH4Cl or NH4NO3 solution . Teknologi dan Rekayasa

Washing solution…. NH4Cl or NH4NO3 electrolyte, prevent the peptisation, stabilize the co agulated precipitate Neutral prevent the dissolving reaction to be Al+3 or AlO2- Hot Co agullation (Help to stabilized the coagulated precipitate) Teknologi dan Rekayasa

Ignition At least 12000C 2 Al(OH)3 Al2O3 Ignite in a silica crucible (porcelain is slightly hygroscopic when heated to a high temperature) Use Fisher or Meker burner. (The best procedure is in an electric furnace.) Teknologi dan Rekayasa

Ignition Alluminium oxide is formed at 9000C, but hygroscopic A stable and pure alluminium oxide is formed at 12000C Teknologi dan Rekayasa