By E. RAYMOND.
The method in question is recommended as easy, expeditious, and
accurate. It consists in precipitating all the manganese in the state of
peroxide, dissolving it in a ferrous solution so as to bring back the
manganese to the manganous slate, and determining volumetrically, by
means of potassium permanganate, the quantity of ferrous salt which
has been converted into ferric. The method of rapidly precipitating
manganese peroxide is peculiar. If we act upon cast-iron or steel with
nitric acid and potassium chlorate in certain proportions, and boil
the mixture, the manganese is completely precipitated in the state of
peroxide insoluble in nitric acid, but retaining a small quantity of
ferric oxide. Suppose that we have a sample of steel or manganiferous
cast-iron containing less than 7 per cent of manganese. Three grammes
are treated in a small flask with 40 c. c. of nitric acid, of sp. gr.
1.20, added little by little. The liquid is stirred, and ultimately
heated to complete solution. It is withdrawn from the fire, and 15
grammes potassium chlorate are added, and then 20 c. c. of nitric acid
at sp. gr. 1.40. It is boiled for about fifteen minutes, until the
escape of chlorine ceases; all the manganese is found thrown down
as peroxide; hot water is added, the mixture is filtered, and the
precipitate washed with boiling water. To dissolve the manganese
peroxide thus obtained we measure exactly 50 c. c. of an acid solution
of ferrous sulphate, made up with 40 grammes ferrous sulphate to 750 c.
c. water and 230 c. c. sulphuric acid (full strength). The 50 c. c. are
poured into the flask in which the sample has been dissolved, and
to which a little peroxide adheres, and it is then poured upon the
precipitate and the filter in a Berlin-ware capsule. The manganese
peroxide dissolves very readily, transforming its equivalent of ferrous
sulphate into ferric sulphate. The liquid is then diluted to 100 or 150
c. c. for the next operation. We then take a solution of permanganate
formed by the same proportions as are used in determining iron by the
process of Margueritte (5.65 grammes of the crystalline salt per liter
of water), and determine its standard exactly. By means of this liquid
we determine volumetrically the quantity of ferrous sulphate remaining
in the solution of manganese. We take then 50 c. c. of the original
solution of ferrous sulphate diluted as above, and determine the total
ferrous salt.
The difference between the two determinations corresponds to the ferrous
salt which has been peroxidized by the manganese peroxide. The quantity
of iron thus peroxidized multiplied by 0.491 gives the quantity of
manganese contained in the portion operated upon. In the case of a
steel or cast iron containing but little manganese it is convenient to
dissolve the peroxide in 25 c. c. only of the ferrous solution. Small
Gay-Lussac burettes may then be used in the titration of only 0.010
meter internal diameter, and graduated into one-twentieth c. c., which
allows of great exactitude in the determination. For a spiegeleisen
not more than 1 gramme of the sample should be taken, and for a
ferro-manganese 0.3 gramme.
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