Demonstration of Some Properties of Prussian Blue MORITZ KOHN,' Hawno, Cuba
THE SIMPLE reactions described in the following lines can be carried out as lecture demonstrations or in the laboratory. In order to prepare Prussian Blue for the following experiments, it is advisable to use a nearly neutral solution of FeCls, and to precipitate i t with the necessary quantity of a solution of RaFe(CN)s. I. Reduction of Prussian Blue to white ferrous ferrocyanide: Fer[Fe(CNhla
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Fe.Fe(CNh
(a) This reduction can be carried out in the quickest and most complete way by treating Pmssian Blue with a solution of sodium hyposulfite, Na2%04,and adding a few drops of dilute hydrochloric acid. When larger quantities of Prussian Blue are reduced, the mixture should be heated gently until its color becomes white. (b) It is also possible, and even useful, to demonstrate that i t is the hyposulfurous acid, HzSeOa, which is the powerful reducing agent. A solution of acid sodium sulfite, NaHSOa, mixed with finely divided zinc and a few drops of dilute hydrochloric acid, immediately reduces the Pmssian Blue to the white ferrous compound. (c) For the same purpose, a solution of neutral sodium sulfite, NazSOa,can be used as well, adding dilute hydrochloric acid and zinc dust. After the reduction has been completed with a sufficient quantity of the reducing agent, it is possible to preserve the white precipitate, even in an uncovered test tube, the mixture remaining white for some time without turning blue. But Pmssian Blue is formed immediately again by addition of bromine water or hydrogen peroxide.
' Formerly of the University of Vienna.
II. Renction of neutral polassium oxalate on Prussian Blue. As Graham found out some eighty years ago, the well-known blue colloidal solution of Prussian Blue can be prepared by adding a small quantity o6,nentral potassium oxalate, &Ca04, and shaking the mixture in a glass-stoppered bottle. This colloidal solution can be kept without changing for a long time, if well protected from light. Then if a part of this colloidal solution is put into a second glass-stoppered bottle, larger quantities of neutral potassium oxalate are added, and the whole is shaken again until the color disappears, a clear solution results as the following reaction takes place :
If the reaction is to be carried out with larger quantities of Prussian Blue, the bottle should be opened after the addition of KaCtO4, and gently heated in a water bath to accelerate the reaction. From these experiments, it can he concluded that the "peptizatiou" of insoluble Pmssian Blue to the colloidal solution is an intermediate state before the chemical reaction resulting in potassium ferrocyanide and potassium femcoxalate takes place, by adding larger quantities of KzCe04: Insoluble Prussian slue
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Peptization with small quantities of KsC,O, to the eolloidal solution
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Disappearance of the blue color because of the reactions (1) and (2)
