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Sodium nitroprusside, Na2[Fe(CN)5NO]
Sodium nitroprusside, Na2[Fe(CN)5NO].2H2O, which is usually obtained by decomposing the potassium salt with sodium carbonate. Potassium carbonate is exceedingly soluble in water, so that sodium nitroprusside is readily obtained in pure form by crystallisation.
It may be conveniently prepared by heating potassium ferrocyanide with 50 per cent, nitric acid solution on a water-bath until a drop of the solution gives no colour with ferrous sulphate. The whole is cooled, the liquid poured from the precipitate, neutralised with sodium carbonate, and taken to dryness. Extraction with water, filtration from the insoluble iron compounds, and crystallisation from the clear aqueous solution yields the salt in ruby-coloured, rhombic prisms. The reactions involved may be represented as follows: -
2K4Fe(CN)6 + 3HNO3 = 2K3Fe(CN)6 + 2KNO3 + H2O + HNO2.
2K3Fe(CN)6 + 3HNO2 = 2K2[Fe(CN)5NO] + 2KCN + H2O + HNO3.
K2[Fe(CN)5N0] + Na2CO3 = Na2[Fe(CN)5NO] + K2CO3.
Another method consists in dissolving 34.5 grams of sodium nitrate in 150 grams of water at 70° C., and mixing with 216 grams of sodium hydrogen sulphite and a solution of 82 grams of potassium ferricyanide in 250 grams of water, also at 70° C. The mixture is boiled until evolution of gas ceases and the cooled liquid mixed with 54 grams of sodium hydrogen sulphite. The deep red Jiquid deposits crystals of sodium nitroprusside.
When potassium ferricyanide is warmed with a solution of bleaching powder to 70° C. a considerable evolution of gas takes place, and a reddish deposit of ferric oxide and calcium carbonate is formed. The filtered solution is concentrated and the potassium nitroprusside extracted with alcohol, and converted into the insoluble copper salt by addition of cupric chloride. This latter is decomposed with sodium hydroxide, yielding the sodium salt, which may be further purified by dissolving in a little water, addition of alcohol, and subsequent evaporation after filtering off any insoluble material. The constitution to be assigned to sodium nitroprusside in particular, and hence to nitro-prussides in general, has been a subject of debate. Browning's formula is: -
whilst Friend suggests
in harmony with his shell theory of complex salts. From a combined study of the electric conductivity in dilute aqueous solution and the depression of the freezing-point in water, it has been concluded that the salt under these conditions yields the following ions: -
Na•, Na•, and Fe(CN)5NO''.
The suggestion is therefore made that the salt is more correctly represented by the single formula, Na2[Fe(CN)5NO], than by the double formula given above. In order to harmonise the double cyclic formula with this, it is merely necessary to assume that the double negative radicle [Fe(CN)5NO]2'''' itself dissociates into two Fe(CN)5NO'' ions in solution.
This assumption has its parallel in the case of triphenylmethyl, C(C6H5)3, which in solution has the simple formula, but is regarded as associated in the solid condition to hexaphenyl ethane, (C6H5)3C - C(C6H5)3.
Further, a study of the dehydration of the solid sodium salt supports the suggestion that the salt has the double formula [Na2Fe(CN)5NO]2.4H2O, for its complete dehydration is a matter of some difficulty, the last molecule of water clinging tenaciously to the salt. If the single formula, Na2Fe(CN)5NO.2H2O, be assumed, it is necessary to postulate the existence of half a molecule of water.
It may be conveniently prepared by heating potassium ferrocyanide with 50 per cent, nitric acid solution on a water-bath until a drop of the solution gives no colour with ferrous sulphate. The whole is cooled, the liquid poured from the precipitate, neutralised with sodium carbonate, and taken to dryness. Extraction with water, filtration from the insoluble iron compounds, and crystallisation from the clear aqueous solution yields the salt in ruby-coloured, rhombic prisms. The reactions involved may be represented as follows: -
2K4Fe(CN)6 + 3HNO3 = 2K3Fe(CN)6 + 2KNO3 + H2O + HNO2.
2K3Fe(CN)6 + 3HNO2 = 2K2[Fe(CN)5NO] + 2KCN + H2O + HNO3.
K2[Fe(CN)5N0] + Na2CO3 = Na2[Fe(CN)5NO] + K2CO3.
Another method consists in dissolving 34.5 grams of sodium nitrate in 150 grams of water at 70° C., and mixing with 216 grams of sodium hydrogen sulphite and a solution of 82 grams of potassium ferricyanide in 250 grams of water, also at 70° C. The mixture is boiled until evolution of gas ceases and the cooled liquid mixed with 54 grams of sodium hydrogen sulphite. The deep red Jiquid deposits crystals of sodium nitroprusside.
When potassium ferricyanide is warmed with a solution of bleaching powder to 70° C. a considerable evolution of gas takes place, and a reddish deposit of ferric oxide and calcium carbonate is formed. The filtered solution is concentrated and the potassium nitroprusside extracted with alcohol, and converted into the insoluble copper salt by addition of cupric chloride. This latter is decomposed with sodium hydroxide, yielding the sodium salt, which may be further purified by dissolving in a little water, addition of alcohol, and subsequent evaporation after filtering off any insoluble material. The constitution to be assigned to sodium nitroprusside in particular, and hence to nitro-prussides in general, has been a subject of debate. Browning's formula is: -
whilst Friend suggests
in harmony with his shell theory of complex salts. From a combined study of the electric conductivity in dilute aqueous solution and the depression of the freezing-point in water, it has been concluded that the salt under these conditions yields the following ions: -
Na•, Na•, and Fe(CN)5NO''.
The suggestion is therefore made that the salt is more correctly represented by the single formula, Na2[Fe(CN)5NO], than by the double formula given above. In order to harmonise the double cyclic formula with this, it is merely necessary to assume that the double negative radicle [Fe(CN)5NO]2'''' itself dissociates into two Fe(CN)5NO'' ions in solution.
This assumption has its parallel in the case of triphenylmethyl, C(C6H5)3, which in solution has the simple formula, but is regarded as associated in the solid condition to hexaphenyl ethane, (C6H5)3C - C(C6H5)3.
Further, a study of the dehydration of the solid sodium salt supports the suggestion that the salt has the double formula [Na2Fe(CN)5NO]2.4H2O, for its complete dehydration is a matter of some difficulty, the last molecule of water clinging tenaciously to the salt. If the single formula, Na2Fe(CN)5NO.2H2O, be assumed, it is necessary to postulate the existence of half a molecule of water.
Properties of Sodium nitroprusside
An aqueous solution of sodium nitroprusside deposits Prussian blue on exposure to light. In the presence of alkali sulphides - as, for example, ammonium sulphide - it yields a beautiful purple colour, which is very characteristic, and so sensitive that the presence of 0.0000018 gram of hydrogen sulphide in 0.004 c.c. can easily be detected. Ammonium hydroxide does not hinder the colour formation, but caustic alkalies destroy it. It gradually fades on standing, in consequence of oxidation of the sulphide to sulphite. The composition of the purple substance is uncertain, but Hofmann suggests the formula Na3[Fe(CN)5(O:N.SNa)], since, by the action of thio-urea, CS(NH2)2, upon sodium nitroprusside, he obtained the complex derivative Na3[Fe(CN)5(O:N.SCNH.NH2)], as a carmine-red powder, closely similar to the substance under discussion.
Hydrochloric acid decomposes sodium nitroprusside, yielding hydrogen cyanide and ferric chloride. The ferric chloride then reacts with excess of the nitroprusside to form ferric nitroprusside.
Concentrated sulphuric acid decomposes the salt, but the reactions involved are very complex. In part they appear to proceed as follows: -
2Na2Fe(CN)5NO + 7H2SO4 + 5H2O = 2H(NO)SO4 + NaHSO4 + NH4HSO4 + FeSO4 + 2(NH4)2SO4 + 5CO + Na3Fe(CN)5.
The pentacyanide derivative unites with the carbon monoxide to yield sodium carbonyl ferrocyanide, Na3[Fe(CN)5.CO], which again interacts with the ferrous sulphate to yield the ferrous derivative, Fe3[Fe(CN)5CO]2, characterised by its violet colour.
Neutral potassium permanganate is without action on the salt. Sodium amalgam reduces it to sodium ferrocyanide and ammonia. Thus: -
12Na2[Fe(CN)5.NO] + 16Na + 54H = 10Na4Fe(CN)6 + 12NH3 + Fe2O3 + 9H2O.
Reduction in neutral solution with a zinc-copper couple causes the evolution of nitrogen. In acid solution (dilute sulphuric) sodium amalgam causes the formation of Prussian blue.
Sodium hydroxide solution converts the salt into the nitrito derivative, Na4[Fe(CN)5.NO2].10H2O, whilst ammonium hydroxide yields, at 0° C., the ammoniate Na3[Fe(CN)5.NH3], and at the ordinary temperature Na2(NH4)[Fe(CN)5.NH3].
When heated to 440° C. in a vacuum, sodium nitroprusside decomposes, evolving nitric oxide and cyanogen.
Sodium nitroprusside is a convenient reagent to use in preparing nitroprussides of the heavy metals, such as copper and nickel, which yield insoluble nitroprussides, and may therefore be estimated volumetrically in this manner by titration, using sodium sulphide as indicator. In this connection it is well to note that solutions of sodium nitroprusside may be preserved for months unaltered if stored in the dark in a bottle of non-alkaline glass. A trace of sulphuric acid greatly enhances the stability of the solution against the effect of light and of alkali from glass. Light rapidly induces decomposition.
The di-ammoniate, Na2Fe(CN)5.NO.2NH3, has been prepared.
Hydrochloric acid decomposes sodium nitroprusside, yielding hydrogen cyanide and ferric chloride. The ferric chloride then reacts with excess of the nitroprusside to form ferric nitroprusside.
Concentrated sulphuric acid decomposes the salt, but the reactions involved are very complex. In part they appear to proceed as follows: -
2Na2Fe(CN)5NO + 7H2SO4 + 5H2O = 2H(NO)SO4 + NaHSO4 + NH4HSO4 + FeSO4 + 2(NH4)2SO4 + 5CO + Na3Fe(CN)5.
The pentacyanide derivative unites with the carbon monoxide to yield sodium carbonyl ferrocyanide, Na3[Fe(CN)5.CO], which again interacts with the ferrous sulphate to yield the ferrous derivative, Fe3[Fe(CN)5CO]2, characterised by its violet colour.
Neutral potassium permanganate is without action on the salt. Sodium amalgam reduces it to sodium ferrocyanide and ammonia. Thus: -
12Na2[Fe(CN)5.NO] + 16Na + 54H = 10Na4Fe(CN)6 + 12NH3 + Fe2O3 + 9H2O.
Reduction in neutral solution with a zinc-copper couple causes the evolution of nitrogen. In acid solution (dilute sulphuric) sodium amalgam causes the formation of Prussian blue.
Sodium hydroxide solution converts the salt into the nitrito derivative, Na4[Fe(CN)5.NO2].10H2O, whilst ammonium hydroxide yields, at 0° C., the ammoniate Na3[Fe(CN)5.NH3], and at the ordinary temperature Na2(NH4)[Fe(CN)5.NH3].
When heated to 440° C. in a vacuum, sodium nitroprusside decomposes, evolving nitric oxide and cyanogen.
Sodium nitroprusside is a convenient reagent to use in preparing nitroprussides of the heavy metals, such as copper and nickel, which yield insoluble nitroprussides, and may therefore be estimated volumetrically in this manner by titration, using sodium sulphide as indicator. In this connection it is well to note that solutions of sodium nitroprusside may be preserved for months unaltered if stored in the dark in a bottle of non-alkaline glass. A trace of sulphuric acid greatly enhances the stability of the solution against the effect of light and of alkali from glass. Light rapidly induces decomposition.
The di-ammoniate, Na2Fe(CN)5.NO.2NH3, has been prepared.
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