TITANOUS CHLORIDE AND S I T R I C ACID BY M. COBLENS A N D J. K. B E R X S T E I S
Milligan' showed that, when the temperature is kept down, stannous chloride reduces nitric acid to hydroxylamine with over 9 0 7 ~of the theoretical yield. Ferrous sulphate, a weaker reducing agent, and titanous chloride, a stronger reducing agent, give nitric oxide only, showing that s0ii.e disturbing reaction takes place. At the suggestion of Professor Bancroft, we have repeated and amplified a small portion of Milligan's work. Stannous chloride was prepared and standardized at first by heating 3 j cc concentrated hydrochloric acid with twenty grams of tin, and letting the mixture stand over night, the excess tin then being filtered off. The sclut'on of tannous chloride was diluted and divided into two parts. To one portion an excess of iodine (15 grams) dissolved in potassium iodide was added. At first a red precipitate of stannous iodide forms but this soon disappears. The excess of iodine was then titrated with a sodium thiosulphate solution whose titer was known in terms of iodine. It is not necessary to add starch as an indicator as the solution becomes colorless a t the end-point, except for a white precipitate, presumably stannic oxide. To the other portion nitric acid was added and, after the reaction was complete, the hydroxylamine was reduced to ammonia with titanous chloride. The excess titanous chloride was destroyed with copper sulphate, twenty grams of caustic soda were added and the ammonia was distilled into z j cc of approximately normal sulphuric acid by the Kjeldahl process, using a Chamot tube and a spiral condenser. To be certain that no ammonia remained in the distilling flask, some sodium sulphide was added towards the end and the distillation continued for a while after that. The excess sulphuric acid was determined by back titration with a standard caustic soda solution. RiIaking up small lots of stannous chloride was not very satisfactory, SO we prepared a liter at a time, keeping it in an atmosphere of nitrogen. Under these conditions it remained standardized indefinitely, After we had become familiar with the manipulations we had no difficulty in getting 90-99% yields of hydroxylamine. M7e next determined qualitatively the effect of adding a ferrous sulphate solution to the stannous chloride solution, varying the amount added from one drop up to an equal volume. Even the addition of one drop of the ferrous sulphate solution caused the evolution of nitric oxide, showing that the ferrous salt accelerates the decomposition of one of the intermediate reduction products into nitric oxide. Though titanous chloride will reduce hydroxylamine quantitatively to ammonia, Milligan found, as has been stated, that it reduced nitric acid only to nitric oxide. The disturbing factor is not the oxidation product of titanous c
J. Phys. Chem. 28, 744 (1924)
TITASOUS CHLORIDE A T D NITRIC ACID
751
chloride. Milligan showed that if an excess of nitric acid is added to a titanous chloride solution and if the resulting solution is then allowed t o react with a stannous chloride solution, the excess nitric acid is reduced to hydroxylamine by the stannous chloride, The disturbing factor is therefore the titanous chloride. On the assumption that nitrous acid was the intermediate reduction product which gave rise to the nitric oxide, a sodium nitrite solution was added drop by drop t o an acid solution of titanous chloride. Nitric oxide was given off at once and no ammonia was formed, I n order to prove that the evolution of nitric oxide was due to the titanous chloride and not to a reaction between the sodium nitrite and the acid, a sodium nitrite solution was added drop by drop to an acid solution of stannous chloride. Reduction to hydroxylamine took place, as was proved by subsequent addition of titanous chloride and analysis for the resulting ammonia. Kitric acid was next allowed to react with an excess of titanous chloride in a closed vessel, shaking continually so as to bring about intimate contact between the excess titanous chloride and the evolved nitric oxide. Reduction to ammonia occurred, showing that the reason we do not get ammonia with nitric acid and titanous chloride under ordinary conditions is that the gas escapes from the solution before it can be reduced. It seemed desirable to find out whether the catalytic deconiposition by titanous chloride was confined to the nitrous acid stage and whether titanous chloride would reduce hyponitrous acid to ammonia. Sllver hyponitrite was prepared by the sodium amalgam method, which consists in treating sodium amalgam with potassium nitrate a t low temperature and precipitation of the hyponitrite by means of silver nitrate. Our first attempt was a failure, the precipitate being black. We concluded that this was due to the oxidation of the hyponitrite by air. Working in a vacuum seemed to be rather a nuisance and we found that satisfactory results could be obtained by covering the solution with a layer of coal oil which was inert to sodium. This of course doe not keep out the air entirely; but it decreases enormously the rate at which air is taken up by the solution. The following method of preparation was found satisfactory. About three grams of sodium were dissolved in 100-110grams of mercury. The pasty amalgam was added a little at a time to IOO cc potassium nitrate solution, the molecular ratio of potassium to sodium being about one to four. The beaker was kept in an ice-bath. When the evolution of gas ceased, the solution was neutralized with acetic acid and silver nitrate was added in slight excess. After the precipitate had settled, the supernatant liquid was decanted, the precipitate dissolved in dilute nitric acid, and reprecipitated with sodium carbonate. The coal oil was then drawn off from the top and the mercury from the bottom. The solution was filtered and the precipitate was washed thoroughly with cold water. The precipitate was transferred to a watch-glass and dried in a vacuum desiccator over sulphuric acid. From the time the silver is introduced, the work is done in the dark. The precipitate thus obtained is a beau-
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BY &I. COBLEKS A N D J . I
