Determination of Alumina in Manganese Ore the Thiosulphate -Phosphate Method


Outline of the Method:  The Aluminium is precipitated in presence of Acetic Acid by boiling the solution with diammonium Phosphate and Sodium thiosulphate, excess thiosulphate reduces iron into the ferrous state and precipitates alumina as phosphate. The precipitate is ignited and weighed as Aluminium phosphate.

Aluminium cation reaction:

In the presence of Diammonium Phosphate & Thiosulphate Aluminium Ion produce a white precipitate, Reaction as follows:

2 Al3++ 3 S2O32-+ 3 H2O → 3SO2+ 3S + 2 Al (OH)3

      Al (OH)3 + (NH4)2 HPO4→AlPO4



  1. 1.1 gm homogeneous sample was taken in 250 ml beaker, digested with 30 to 40 ml HCl and baked over hot plate till evaporated to dryness. After removal of silica from the system, residue was fused and mixed in the mother aliquot and volume made up to 250ml volumetric flask.
  1. 2.50ml sample aliquot i.e., (0.20gm) was taken in 600ml beaker and volume was made up to 200ml by adding distilled water. 
  1. 3. 30 ml of Diammonium phosphate solution was added with 2 drops of methyl orange indicator into the above solution, neutralized with Ammonium hydroxide till to precipitation started.
  1. 4.Now concentrated hhydrochloric acid was added drop by drop with constant stirring until the pprecipitate just disappeared. About 1 ml of concentrated hhydrochloric acid was added in excess.
  1. 5.The solution was heated for 20 min. and about 50 ml sodium thiosulphate solution and a small amount of macerated ashless filter paper was added and heated until it started boiling. 
  1. 6.After 15 to 20 min of boiling about 50 ml of acetate-acetic acid buffer was added and boiled to coagulate the precipitate. 

7. The precipitate of aluminium phosphate was transferred to a weighed silica or platinum crucible and heated gently until its contents became dry. Then the precipitate was ignited at a lower temperature ( 450°C to 500°C ) at first and then at 800°C until constant weight is obtained.


 Alumina, percent = A x 41.78/B


A = weight in g of aluminium phosphate obtained

B = weight in g of the sample represented by the aliquot taken

Results :

The experiment was carried out in triplicate by using 2 Certified reference Materials & one sample .  In all the cases, same quantity of sample and same amount of chemicals were used. The results were also compared with the declared value in certificates of the Certified Reference Materials. Following are findings and are found close to the certified values.  

Sl No. CRM -1 B.C.S No.176/1 CRM-2 CSIR-NML CRM No166.3A Unknown Sample
1 3.98% 5.12% 4.40%
2 4.14% 5.04% 4.32%
3 4.08% 5.18% 4.46%
Declared Value (%) 4.10 5.06 4.36


Analysis of Alumina in Manganese Ore by wet Chemical method is a fast, accurate and less time-consuming method.  The reagents used are also less expensive. Moreover, the repeatability of the method is quite good with low standard deviation. Consequently this method is very much applicable to determine Alumina in Manganese Ore .

Contributed by: Sukhenjit Datta  & Sourav Bharati