[Note 1:  The reactions given on page 61 are those which take place in the presence of an excess of acid.  In neutral solutions the reduction of the permanganate is less complete, and, under these conditions, two gram-molecular weights of KMnO_{4} will furnish only 48 grams of oxygen.  A normal solution for use under these conditions should, therefore, contain 316.0/6 grams, or 52.66 grams.]

[Note 2:  Potassium permanganate solutions are not usually stable for long periods, and change more rapidly when first prepared than after standing some days.  This change is probably caused by interaction with the organic matter contained in all distilled water, except that redistilled from an alkaline permanganate solution.  The solutions should be protected from light and heat as far as possible, since both induce decomposition with a deposition of manganese dioxide, and it has been shown that decomposition proceeds with considerable rapidity, with the evolution of oxygen, after the dioxide has begun to form.  As commercial samples of the permanganate are likely to be contaminated by the dioxide, it is advisable to boil and filter solutions through asbestos before standardization, as prescribed above.  Such solutions are relatively stable.]

COMPARISON OF PERMANGANATE AND FERROUS SOLUTIONS

Procedure.—­Fill a glass-stoppered burette with the permanganate solution, observing the usual precautions, and fill a second burette with the ferrous sulphate solution prepared for use with the potassium bichromate.  The permanganate solution cannot be used in burettes with rubber tips, as a reduction takes place upon contact with the rubber.  The solution has so deep a color that the lower line of the meniscus cannot be detected; readings must therefore be made from the upper edge.  Run out into a beaker about 40 cc. of the ferrous solution, dilute to about 100 cc., add 10 cc. of dilute sulphuric acid, and run in the permanganate solution to a slight permanent pink.  Repeat, until the ratio of the two solutions is satisfactorily established.

STANDARDIZATION OF A POTASSIUM PERMANGANATE SOLUTION

!Selection of a Standard!

Commercial potassium permanganate is rarely sufficiently pure to admit of its direct weighing as a standard.  On this account, and because of the uncertainties as to the permanence of its solutions, it is advisable to standardize them against substances of known value.  Those in most common use are iron wire, ferrous ammonium sulphate, sodium oxalate, oxalic acid, and some other derivatives of oxalic acid.  With the exception of sodium oxalate, these all contain water of crystallization which may be lost on standing.  They should, therefore, be freshly prepared, and with great care.  At present, sodium oxalate is considered to be one of the most satisfactory standards.

!Method A!