An Introductory Course of Quantitative Chemical Analysis eBook

This eBook from the Gutenberg Project consists of approximately 220 pages of information about An Introductory Course of Quantitative Chemical Analysis.

!Zimmermann-Reinhardt Procedure!

!Method (B)!

Procedure.—­Grind the mineral to a fine powder.  Weigh out two portions of about 0.5 gram each into small porcelain crucibles.  Proceed with the solution of the ore, treat the residue, if necessary, and reduce the iron by the addition of stannous chloride, followed by mercuric chloride, as described for the bichromate process on page 56.  Dilute the solution to about 400 cc. with cold water, add 10 cc. of the manganous sulphate titrating solution (Note 1, page 68) and titrate with the standard potassium permanganate solution to a faint pink (Note 1).

From the standardization data already obtained calculate the percentage of iron (Fe) in the limonite.

[Note 1:  It has already been noted that hydrochloric acid reacts slowly in cold solutions with potassium permanganate.  It is, however, possible to obtain a satisfactory, although somewhat fugitive end-point in the presence of manganous sulphate and phosphoric acid.  The explanation of the part played by these reagents is somewhat obscure as yet.  It is possible that an intermediate manganic compound is formed which reacts rapidly with the ferrous compounds—­thus in effect catalyzing the oxidizing process.

While an excess of hydrochloric acid is necessary for the successful reduction of the iron by stannous chloride, too large an amount should be avoided in order to lessen the chance of reduction of the permanganate by the acid during titration.]



Pyrolusite, when pure, consists of manganese dioxide.  Its value as an oxidizing agent, and for the production of chlorine, depends upon the percentage of MnO_{2} in the sample.  This percentage is determined by an indirect method, in which the manganese dioxide is reduced and dissolved by an excess of ferrous sulphate or oxalic acid in the presence of sulphuric acid, and the unused excess determined by titration with standard permanganate solution.

Procedure.—­Grind the mineral in an agate mortar until no grit whatever can be detected under the pestle (Note 1).  Transfer it to a stoppered weighing-tube, and weigh out two portions of about 0.5 gram into beakers (400-500 cc.) Read Note 2, and then calculate in each case the weight of oxalic acid (H_{2}C_{2}O_{4}.2H_{2}O) required to react with the weights of pyrolusite taken.  The reaction involved is

MnO_{2} + H_{2}C_{2}O_{4}(2H_{2}O) + H_{2}so_{4} —­> MnSO_{4} + 2Co_{2} + 4H_{2}O.

Weigh out about 0.2 gram in excess of this quantity of !pure! oxalic acid into the corresponding beakers, weighing the acid accurately and recording the weight in the notebook.  Pour into each beaker 25 cc. of water and 50 cc. of dilute sulphuric acid (1:5), cover and warm the beaker and its contents gently until the evolution of carbon dioxide ceases (Note 3).  If a residue remains which is sufficiently colored to obscure the end-reaction of the permanganate, it must be removed by filtration.

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An Introductory Course of Quantitative Chemical Analysis from Project Gutenberg. Public domain.
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