VOLUMETRIC ESTIMATION OF MANGANESE BY POTASSIUM PERMANGANATE. (Volhard’s Method)
Aparatus. Reagents.—The usual apparatus. For the standard solution K2Mn2O8 is required. Zinc oxide free from Mn is used for neutralisation. For analysis the student may take a ferro-manganese ore (containing Fe and Mn).
Method, Reactions.—If a dilute, neutral or faintly acid solution of a manganese salt be heated to boiling, and a solution of K2Mn2O8 added, a brownish red precipitate of hydrated MnO2 is formed—
K2Mn2O8 + 3MnSO4 + 2H2O = K2SO4 + 5MnO2 + 2H2SO4
and as soon as the permanganate is in excess the characteristic rose colour appears. The reaction is exact in a neutral solution containing no iron or chromium, therefore excess of acid is to be avoided, and any iron and chromium present must be removed before titration. This condition is obtained by adding an emulsion of zinc oxide (pure) in slight excess, the acid being neutralised and ferric hydroxide precipitated.
Preparation of Standard Solution.—The N/10 K2Mn2O8 previously used for iron will serve again if any remain. If not prepare a N/10 permanganate solution as before.
Checking the Standard.—If the solution previously used is available it should again be checked by titration against metallic iron (wire). If the solution be freshly prepared it will also be checked in this way—
1 c.c. N/10 K2Mn2O8 should equal .00165 gm. Mn. The value in terms of Mn may be obtained by multiplying the value in terms of Fe by .2946.
The Analysis.—Weigh out .5 gm, of the sampled and finely powdered ore. Transfer to a casserole or porcelain dish. Add 2 c.cs. 10E. HCl, 4 c.cs. 16E. HNO3, and 6 c.cs. 10E. H2SO4. Heat on the iron plate till dense white fumes are evolved, and continue at this heat about three minutes. Transfer the contents of the porcelain to a 500 c.c. flask. If any particles of the ore appear undissolved (of the original colour) repeat the acid treatment and combine the solutions. Wash out the casserole or dish with hot water, and to the contents of the flask add an emulsion of zinc oxide till the acid is neutralised and the iron is precipitated as Fe2(HO)6. Violent agitation hastens the precipitation of the iron. Test the progress of the precipitation by removing a drop of the solution and adding to it in a small test tube a little of the zinc oxide emulsion. Shake, and notice if a precipitate forms. Pour back the test into the main solution.
When the precipitation is complete any appreciable excess of zinc oxide must be avoided.
Dilute with distilled water to the mark. Shake and let settle. When settled remove with a pipette 100 c.cs. of the clear liquid to a 300 c.c. flask. Heat to boiling over a flame, and titrate with N/10 K2Mn2O8 shaking after each addition until a permanent pink is obtained, visible at the edges of the clear liquid by holding a sheet of white paper behind the beaker. If this titration has taken some time the solutions should again be heated to boiling. If the pink tint disappears a little more K2Mn2O8 is required.
If the percentage of manganese is low it will be advisable to take 200 c.cs. or more for titration. When titrating hot liquids the student must be careful that the steam does not appreciably heat the burette. As a rule, if the titration be performed quickly there is little danger of thus introducing error, but if prolonged the student had better make a connection leading sideways from the burette jet, so that the burette is not vertically above the rising steam.
The results of this analysis are calculated in percentages as usual, and should agree to .1%.