THE DETERMINATION OF FERROUS IRON IN SILICATES BY TITRATION WITH DICHROMATE
Journal of the American Chemical Society
The permanganate titration has been used for the determination of ferrous iron in silicates almost, if not quite, exclusively. This study introduces dichromate for this purpose in a method which is proven to be accurate. A dichromate titration of the ferrous iron obtained by extraction of a silicate with hydrofluoric acid has several points in its favor. A dichromate solution is more stable than the permanganate solution, which is ordinarily used for this purpose. Moderate quantities of
... antities of hydrochloric acid do not interfere with the dichromate titration, thus allowing the use of this acid, if it is desired, in the extraction process. Dichromate is not as easily reduced by organic matter as is permanganate. However, ferrous iron cannot be titrated accurately with dichromate in a hydrofluoric acid solution. Further, ferrous iron in fluoride solution oxidizes so rapidly that a slow titration, such as accompanies the use of an outside indicator on a spot plate, may introduce a considerable error. Boric acid' has been found to remove hydrofluoric acid from solution, interaction forming metafluoboric acid. The use of boric acid also permits an accurate titration of ferrous iron to be made with dichromate (Table I) . In Experiments 3 and 4 the dichromate was added until, upon placing a drop of the solution in contact with a drop of 0.05% potassium ferricyanide solution, the development of color on the spot plate was very slow. The time required for the development of the ferrous iron test is designated in the table. More dichromate was then added and another test applied. The more the dichromate added the longer became the time required for the development of the color test. These two experiments show that an enormous error may be introduced by the presence of fluorides. In Experiments 5 and 6 boric acid was present and hydrochloric acid absent. In 7 and 8 boric acid was added previous to titration in the presence of hydrochloric acid. Excellent results were obtained in the four experiments. Constant quantities of iron in the presence of constant amounts of hydrochloric acid in constant volume of solution were titrated with variable amounts of hydrofluoric acid both with and without the addition of boric acid in excess preceding titration (Experiments 9-20 inclusive). Below approximately 0 . 1 5 N the effect of hydrofluoric acid is found to be small when titrating moderate amounts of iron. Above 0.15 N the error becomes progressively larger as the concentration of the Barnebey, THIS JOURNAL, 37, 1481 (1915).