260 QUANTITATIVE AGRICULTURAL ANAL

The manganese content of a large number of different legumes
(aerial portion) vas determined by Jones and Bullis,1 who found

alsike clover to have the greatest amount, averaging 0.068 per
cent, while alfalfa had the least, with 0.023 per cent.

Work on the effect of manganese has been done also by Kelley,2
who concluded that manganese is a plant food, when present in
small amounts, but that in larger quantities it becomes toxic.

In some Hawaiian soils the per cent of manganese is so high
as to interfere with the growth of the pineapple, causing a depres-
sion in iron assimilation.3

The bismuthate method for the determination of manganese
is one of the best. It is based upon the use of sodium bisrnuthate
to oxidize bivalent manganese to heptavalent manganese in the
form of permanganic acid. When a solution of manganous
nitrate is treated with sodium bisrnuthate the reaction proceeds
thus:

2Mn(N03)2 + 5NaBi03 + 14HN03 -* 2NaMn04 + 3NaN03 +

5Bi(NO3)3 + 7H20.

Sodium permanganate so produced is reduced by means of a
standard reducing agent, the excess of which is then titrated
with standard permanganate solution.

Persulphate Method for Manganese. — Manganese may be
oxidized by ammonium persulphate, in the presence of silver
nitrate, from a bivalent to a heptavalent condition, producing
permanganic acid:

> Ag2S2Os + 2NH4N03, (1)
Ag2S208 + 2H2O -> 2H2SO4 H- Ag2O2, (2)
5Ag202 4- 2Mn(NOs)2 + 6HNO3 -+2HMn04 + lOAgN03 +
2H2O. (3)
The manganese is determined in an extract from the soil fusion
by comparing the intensity of color produced in this manner with
that of a manganese solution of known concentration, similarly
1 J. Ind. Eng. Chem., 13, 6 (1921). See also McHABGUB, J. Am. Chem.
Soc.t 44, 1592 (1922).
2 Hawaii Exp. Sta. Bull 26 (1912).
3 JOHNSON, Ibid., 9, 1 (1917).