Potassium ferro-dinitroso thiosulphate, KFe(NO)₂S₂O₃

Potassium ferro-dinitroso thiosulphate, KFe(NO)₂S₂O₃.H₂O, results when nitric oxide is passed into a concentrated mixed solution of ferrous sulphate and potassium thiosulphate at room temperature. The solution becomes deep brown in colour, and reddish brown crystals of the potassium salt are obtained on concentration. The salt is but slightly soluble in cold water, and is insoluble in water-free alcohol and in ether. Concentrated sulphuric acid dissolves it without decomposition, yielding a greenish yellow solution.

Alkalies partly decompose the salt in the warm, whilst ammonia effects its complete decomposition.

The sodium salt, NaFe(NO)₂S₂O₃.2H₂O, is obtained in a similar manner to the preceding salt. It yields either laminated or needle- shaped crystals, which are glistening black in appearance. They are appreciably more soluble in water than those of the potassium salt, and yield a deep brown solution. The salt is fairly stable below 0° C., but above that temperature continuously evolves nitric oxide. When its aqueous solution is boiled, sulphur dioxide is expelled and ferric hydroxide precipitated. Upon concentrating the clear solution obtained by filtering, crystals of sodium ferro-heptanitroso sulphide, NaFe₄(NO)₇S₃.H₂O, separate out.

The ammonium salt, (NH₄)Fe(NO)₂S₂O₃.H₂O, may be prepared in an analogous manner. It may be recrystallised from warm water, when it yields flat prisms, black in colour.

The rubidium and caesium salts have also been prepared, namely RbFe(NO)₂S₂O₃.H₂O and CsFe(NO)₂S₂O₃. Thallium, apparently, does not yield an analogous salt under like conditions.