DIVERS: PREPARATION OF PURE ALKALI NITRITES. 85 XII1.--PrepcLration of Pure Alkali Nitrites. By EDWARD .DIVERS. WHEN pure sodium or potassium nitrite is wanted, it is customary to prepare silver nitrite from crude alkali nitrite, and convert this again into alkali nitrite by means of sodium or potassium chloride. The crude nitrite must be nearly free from sulphate, and either before or after adding the silver nitrate to it, nitric acid must beadded until all the hydroxide and carbonate are neutralised. The silver nitrite is got in the most convenient form for washing by precipitating it from con- centrated solutions. Silver chloride is very sensibly soluble in a concentrated solution of alkali nitrite, and when the solution is no longer clouded by the addition of more alkali chloride, it already contains this salt in excess.Therefore, somewhat large dilution is necessary, and this entails, of course, much evaporation afterwards. The silver nitrite process is evidently not a very satisfactory one, and when sodium nitrite is wanted pure, it is better to recrystallise, three times over, the commercial 96 per cent. sodium nitrite, well draining each time on the suction funnel. A concentrated solution of the crude salt should be left to clear from lead turbidity for two days, or be filtered cold through a fine filter; the lead carbonate is more soluble in the hot nitrite solution than in the cold, After separating the lead, the solution should be fully neutralised with nitric acid before evaporation for crystallising. Potassium nitrite is too soluble and deliquescent to be conveniently purified in a similar way.A most satisfactory and simple process for preparing either sodium or potassium nitrite, when the pure hydroxide or carbonate is at command, is to saturate this with red nitrous fumes under appropriate conditions. That nitrites can be thus obtained is known to every chemist, was known to Gay-Lussac in 1816, and was described by Fritzsche in 1840, but it has hitherto been stated and believed that much nitrate is then unavoidably formed along with the nitrite, That is a mistake, and therefore this note is published. If obvious precautions of the simplest kind are taken, so little nitrate, if any, is formed as to be hardly detectable with certainty in presence of so much nitrite. Consequently, if the quantity of pure alkali taken is known, a solution of given strength in nitrite is perhaps better prepared in this way than in any other.86 DIVERS: PREPARATION OF PURE ALKALI NITRITES.Avoiding, so far as practicable, the use of cork and caoutchouc, nitrous gases, from nitric acid and starch or arsenious oxide, are passed into the concentrated solution of the hydroxide or carbonate until the alkali is quite neutralised. Sodium carbonate alone is somewhat inconvenient, because of its sparing solubility, but this may be circumvented by adding it, finely divided and in sufficient quantity, t o its own saturated solution, just before passing the gases and by often shaking the vessel during their absorption. To prevent free access of air, the nitrite is prepared in a flask with its mouth kept loosely closed while the gases are passing; it is not necessary to cool the flask.The strength of -the nitric acid and the temperature of the generator of the nitrous gases must be so regulated that just a little nitric oxide is in excesfi of the nitric peroxide, and therefore is passing unabsorbed, as a guarantee that the latter does not act on the solution in absence of its equivalent of the former and thus produce some nitrate. To free the gases from volatilised nitric acid, they may be passed through a bottle or tube, either empty or packed loosely with cotton. The finished solution must be almost neutral, and if acid must be boiled until neutral, before exposing it to the air. A concen- trated solution of alkali nitrite dissolves a little nitrous acid without decomposing it, as water alone would.To get the salt in the solid state or to crystallise out the sodium nitrite where it is necessary to be sure of absence of all nitrate in it, the solution may be freely evaporated, even at a boiling heat, without decomposing or oxidising it. The alkali nitrites have been very imperfectly described, and need examination. I n the meantime, some points in their description are here given. Sodium nitrite and potassium nitrites are distinctly though faintly yollow, and give markedly yellow solutions in a little water. They are very slightly alkaline to litmus. At 1 5 O , 5 parts of sodium nitrite requires 6 parts of water to dissolve it; potassium nitrite is soluble in about one-third of its weight of water.Sodium nitrite melts at 27 1' (mercury-thread immersed). Sodium nitrite is moderately deliquescent, remaining dry in winter-cold air ; potassium nitrite is exceedingly deliquescent, and is obtained in very small, thick, prismatic crystals, whilst sodium nitrite crystallises in very thin, flattened prisms, often very large. Sodium nitrite is well known to be anhydrous; not so potassium nitrite, crystals of which are reputed to contain $H20. I have, however, examined small, but distinct and separate, crystals taken from the upper part of some kilos. of the commercial salt, which had become well drained by long standing. They were removed in very dry weather and weighed, and then found to lose hardly 1 per cent. on fusion. The anhydrous character of the potassium salt was further ascertained by testing a cake of minute crystals, prepared by myself, which had been pressed,REDUCTION OF AN ALKALI NITRITE BY AN ALKALI METAL. 87 under cover, between porous tiles, in cold, dry air ; the loss of weight on heating, much above looo, was a little over 1 per cent., and the percentage of potassium was 45.30, instead of 45.88, required for the anhydrous salt. Somewhat remarkably, the point as to hydration of potassium nitrite was examined independently is the same year, 1863, by Lang and by Hampe, with identical results, indicating the composition expressed by 2KNO,+H,O, but both these chemists made the determination on a magma of indistinct crystals, which had been dried in a vacuum desiccator.