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
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
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
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
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.
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
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