The rate at which the iron solution passes through the zinc should not exceed that prescribed, but the rate may be increased somewhat when the wash-water is added.  It is well to allow the iron solution to run nearly, but not entirely, out of the funnel before the wash-water is added.  If it is necessary to interrupt the process, the complete emptying of the funnel can always be avoided by closing the stopcock.

It is also possible to reduce the iron by treatment with zinc in a flask from which air is excluded.  The zinc must be present in excess of the quantity necessary to reduce the iron and is finally completely dissolved.  This method is, however, less convenient and more tedious than the use of the reductor.]

[Note 3:  The dilute sulphuric acid for washing must be warmed ready for use before the reduction of the iron begins, and it is of the first importance that the volume of acid and of wash-water should be measured, and the volume used should always be the same in the standardizations and all subsequent analyses.]

[Note 4:  The end-point is more permanent in cold than hot solutions, possibly because of a slight action of the permanganate upon the manganous sulphate formed during titration.  If the solution turns brown, it is an evidence of insufficient acid, and more should be immediately added.  The results are likely to be less accurate in this case, however, as a consequence of secondary reactions between the ferrous iron and the manganese dioxide thrown down.  It is wiser to discard such results and repeat the process.]

[Note 5:  The potassium permanganate may, of course, be diluted and brought to an exactly 0.1 N solution from the data here obtained.  The percentage of iron in the iron wire must be taken into account in all calculations.]

!Method B!

!Oxalate Standards!

Procedure.—­Weigh out two portions of pure sodium oxalate of 0.25-0.3 gram each into beakers of about 600 cc. capacity.  Add about 400 cc. of boiling water and 20 cc. of manganous sulphate solution (Note 1).  When the solution of the oxalate is complete, heat the liquid, if necessary, until near its boiling point (70-90 deg.C.) and run in the standard permanganate solution drop by drop from a burette, stirring constantly until an end-point is reached (Note 2).  Make a blank test with 20 cc. of manganous sulphate solution and a volume of distilled water equal to that of the titrated solution to determine the volume of the permanganate solution required to produce a very slight pink.  Deduct this volume from the amount of permanganate solution used in the titration.

From the data obtained, calculate the relation of the permanganate solution to the normal.  The reaction involved is: 

5Na_{2}C_{2}O_{4} + 2KMnO_{4} + 8H_{2}so_{4} —­> 5Na_{2}so_{4} + K_{2}so_{4} + 2MnSO_{4} + 10Co_{2} + 8H_{2}O

[Note 1:  The manganous sulphate titrating solution is made by dissolving 20 grams of MnSO_{4} in 200 cubic centimeters of water and adding 40 cc. of concentrated sulphuric acid (sp. gr. 1.84) and 40 cc. or phosphoric acid (85%).]