Finally, dilute the solution to 200-300 cc., heat the solution to a temperature just below boiling, add 15 cc. of a manganese sulphate solution and while hot, titrate for the excess of the oxalic acid with standard permanganate solution (Notes 4 and 5).

From the corrected volume of the solution required, calculate the amount of oxalic acid undecomposed by the pyrolusite; subtract this from the total quantity of acid used, and calculate the weight of manganese dioxide which would react with the balance of the acid, and from this the percentage in the sample.

[Note 1:  The success of the analysis is largely dependent upon the fineness of the powdered mineral.  If properly ground, solution should be complete in fifteen minutes or less.]

[Note 2:  A moderate excess of oxalic acid above that required to react with the pyrolusite is necessary to promote solution; otherwise the residual quantity of oxalic acid would be so small that the last particles of the mineral would scarcely dissolve.  It is also desirable that a sufficient excess of the acid should be present to react with a considerable volume of the permanganate solution during the titration, thus increasing the accuracy of the process.  On the other hand, the excess of oxalic acid should not be so large as to react with more of the permanganate solution than is contained in a 50 cc. burette.  If the pyrolusite under examination is known to be of high grade, say 80 per cent pure, or above the calculation of the oxalic acid needed may be based upon an assumption that the mineral is all MnO_{2}.  If the quality of the mineral is unknown, it is better to weigh out three portions instead of two and to add to one of these the amount of oxalic prescribed, assuming complete purity of the mineral.  Then run in the permanganate solution from a pipette or burette to determine roughly the amount required.  If the volume exceeds the contents of a burette, the amount of oxalic acid added to the other two portions is reduced accordingly.]

[Note 3:  Care should be taken that the sides of the beaker are not overheated, as oxalic acid would be decomposed by heat alone if crystallization should occur on the sides of the vessel.  Strong sulphuric acid also decomposes the oxalic acid.  The dilute acid should, therefore, be prepared before it is poured into the beaker.]

[Note 4:  Ferrous ammonium sulphate, ferrous sulphate, or iron wire may be substituted for the oxalic acid.  The reaction is then the following: 

2 FeSO_{4} + MnO_{2} + 2H_{2}so_{4} —­> Fe_{2}(so_{4})_{3} + 2H_{2}O

The excess of ferrous iron may also be determined by titration with potassium bichromate, if desired.  Care is required to prevent the oxidation of the iron by the air, if ferrous salts are employed.]

[Note 5:  The oxidizing power of pyrolusite may be determined by other volumetric processes, one of which is outlined in the following reactions: