The Ferrates. By Claude A. O. Rosell. August 2, 1895.

According to an article in the J. Prakt. Chem., 32, 448, G.E. Stahl discovered that saltpeter ignited with iron and added to water produced a purple or amethyst color as the KOH resulting from the decomposition of the KNO3 held part of the iron in solution. I have verified the accuracy of this statement.

In the course of investigating metallic oxides, Fremy rediscovered the solution of ferric oxide in fused potassium hydroxide. He investigated the matter more thoroughly than did Stahl or Ekeberg (who accidentally and independently rediscovered potassium ferrate). Fremy's work was published in a number of different journals, beginning in January, 1841, when he stated that the compound of ferric oxide and potassium hydroxide which is soluble in water corresponds in composition to the manganates.  He called the new compound ferric acid.

Besides the dry fusion of ferric oxide and potassium hydroxide, Fremy also produced potassium ferrate by electrolyzing KOH solution with a cast iron anode and by bubbling chlorine into ferric hydroxide suspended in a potassium hydroxide solution. He also produced it by heating iron with potassium peroxide.

Bloxam found that calcium ferrate may be produced in solution by gently heating a solution of bleaching powder (calcium hypochlorite) with a small amount of ferric chloride, but this procedure is very delicate and often fails.

Sodium ferrate is more difficult to produce than potassium ferrate. Bloxam prepared a solution of sodium ferrate by adding bromine to ferric hydroxide suspended in a sodium hydroxide solution. I have prepared sodium ferrate by Bloxam's method and by fusing sodium peroxide with ferric oxide in an iron crucible. The fused mass, once cooled, must be treated with ice in order that the heat produced from its reaction with water does not decompose the sodium ferrate formed.

If barium chromate is digested with a solution of sodium ferrate, barium ferrate is formed and the solution changes color from red to yellow due to the formation of sodium chromate. Barium ferrate has very low solubility in water. It is the most stable and predictable of all the ferrates. While still wet it is decomposed by any acid, including carbonic acid, but once it has been thoroughly dried it is not quite so unstable. It is still readily attacked by acids, especially hydrochloric acid, which easily decomposes it completely with the evolution of chlorine.

The only use thus far suggested for ferrates  is the production of oxygen, described in British Patent No. 85, Jan. 10, 1886, in which it is stated that ferrates may be decomposed by steam and reformed by a current of air at high temperatures.

Ferrates can be reduced by nitrites, tartrates, glycerol, oxalates, alcohol, ether, ammonia, urea, and many other soluble organic compounds, one notable exception being the acetates. They are also decomposed by some insoluble organic materials such as paper and insoluble carbohydrates, but most water-insoluble organic compounds (such as paraffins and benzene) decompose them only very slowly.

The preceding information was taken from an article in J. Am. Chem. Soc.; 1895; 17(10); 760-769 and condensed/edited by Polverone.

Preparation and Purification of Potassium Ferrate. VI. By G. W. Thompson, L. T. Ockerman and J. M. Schreyer. Received August 9, 1950.

Numerous investigators have reported wet methods for preparing impure potassium ferrate. A procedure producing pure potassium ferrate has been reported by Schreyer. It involved bubbling chlorine gas through hydrous ferric oxide suspended in 8 molar potassium hydroxide solution maintained at a temperature of 50-55 degrees. The procedure was low-yield and laborious.

Hrostowski and Scott reported the preparation of 96.9% potassium ferrate by a similar method, using sodium hypochlorite as the oxidizing agent and precipitating potassium ferrate from the sodium ferrate solution obtained by adding solid potassium hydroxide until the solution was saturated. This method had yields of 10%-15% of the theoretical.

Previous methods of potassium ferrate purification removed chloride by taking advantage of the solubility of potassium chloride in a concentrated potassium hydroxide solution that had precipitated the potassium ferrate. Brönsted's data (Brönsted, J. Am. Chem. Soc., 49, 1448-1454 (1920)) show a pronounced increase in the solubility of both potassium chloride and potassium nitrate as the potassium hydroxide molarity is decreased. It seems that the bulk of the potassium ferrate would be precipitated by 11 molar potassium hydroxide solution while the bulk of the potassium chloride and potassium nitrate impurities would remain in solution.

Combined Preparation and Purification of Potassium Ferrate:
This method is advantageous in that it minimizes mechanical losses and time consumed. The following procedure is recommended.

30 g of NaOH is dissolved in 75 ml of water. Chlorine is bubbled through the cooled solution with vigorous stirring while maintaining the temperature under 20 degrees. Chlorination is complete when the solution has gained 20 g in weight. 70 g of solid NaOH is added slowly with stirring. The temperature may rise as high as 30 degrees to aid solution of the NaOH. The mixture is then cooled to 20 degrees and precipitated sodium chloride removed with a fritted glass filter.

To the filtered solution is slowly added 25 g of ferric nitrate at 25-30 degrees. The temperature is maintained at 30 degrees while saturating with NaOH. The mixture is then filtered with suction through a coarse fritted glass filter.

The sodium ferrate filtrate is placed in a 250 ml beaker and immersed in a 20 degree water bath. 100 ml of saturated KOH solution is added with stirring. Stirring is continued for 5 minutes, finally filtering through a fritted glass filter of medium porosity, the filtrate being discarded.

The precipitate is leached on the filter with 4-5 10 ml portions of 3 molar KOH solution. The residue remaining on the filter has a light gray cast and is discarded.

The filtrate is transferred to a 250 ml beaker and 50 ml of chilled, saturated KOH solution is added. Any solid potassium ferrate still on the filter disk is washed out with a few ml of saturated KOH solution. The final solution is approximately 11 molar in KOH. The solution is stirred for 5 minutes and then filtered through medium porosity fritted glass.

The precipitate remaining on the filter is washed with 13 ml of benzene. 3-5 20 ml portions of 95% ethanol are drawn through the filter and the precipitate is transferred to to a beaker containing 1000 ml of 95% ethanol and stirred for 20 minutes. This washing is repeated 3 times. The precipitate is then removed by filtration and dried by drawing 30 ml of ethyl ether through the filter. A calcium chloride drying tube is used to protect the potassium ferrate from atmospheric moisture during the drying process. Suction is continued for 20 minutes and the precipitate is given a final drying in a vacuum dessicator. Dry potassium ferrate is stable and should be kept in a dessicator.

Numerous samples were prepared by this method, giving yields of 44.1%-76.4% of theory and purities of 92.34%-96.3% potassium ferrate, the highest purities being obtained when centrifugation was substituted for filtration.

The preceding information was taken from an article in J. Am. Chem. Soc.; 1951; 73 (3); 1379-1381 and condensed/edited by Polverone.

Other journal articles (with too little interesting information to be summarized here) detailed the preparation of relatively pure barium, cesium, and rubidium ferrates, using soluble salts of the proper metal, reacting them with sodium or potassium ferrate, and precipitating them with the corresponding hydroxide. Barium ferrate seemed an especially easy target, since it has low solubility and can be easily prepared precipitated with good purity by double exchange with a carbonate-free solution of sodium or potassium ferrate.