(3) K₂S₂O₇ = K₂SO₄ + SO₃.
At a sufficiently high temperature the two preceding reactions may take place in one, thus:
2KHSO₄ = K₂SO₄ + H₂O + SO₃.
At the temperature at which these reactions take place, the water that is set free does not recombine with the sulfur trioxid nor with the sulfuric acid that is present in excess, but is expelled from the mixture; hence the mixture becomes more concentrated during the digestion. The sulfur trioxid set free acts upon the organic matter in the powerful manner peculiar to it, and the potassium sulfate formed in the last reaction above unites with another molecule of sulfuric acid, and the same round of reactions is repeated continuously so long as there is an excess of free sulfuric acid present in the mixture. As the liquid becomes more concentrated with the continuation of the digestion, the boiling-point increases so that the effect is the same as heating under pressure. The danger of too great concentration and risk of consequent loss of nitrogen is avoided by using increased proportions of sulfuric acid.
As compared with the kjeldahl the gunning method presents the following advantages:
(1) The gunning method requires fewer reagents. As no form of mercury is used no potassium sulfid is needed, and there is no risk of loss from the presence of mercurammonium compounds.
(2) The solution to which caustic soda is added is clear, so that in neutralizing, it is an easy matter to avoid great excess of alkali, and so, in most cases, to avoid foaming and bumping in distillation.
(3) In the blank determinations less nitrogen is found in the reagents used in the gunning method. In only one case was more nitrogen reported in a blank by this method; in all the others the amount averaged considerably less.
196. The Official Gunning Method.—In a digestion flask holding from 250 to 500 cubic centimeters place from seven-tenths to two and eight-tenths grams of the substance to be analyzed, according to its proportion of nitrogen. Then add ten grams of powdered potassium sulfate and from fifteen to twenty-five cubic centimeters (ordinarily about twenty cubic centimeters) of concentrated sulfuric acid. Conduct the digestion as in the kjeldahl process, starting with a temperature below boiling-point and increasing the heat gradually until frothing ceases. Digest until colorless or nearly so. Do not add either potassium permanganate or potassium sulfid. Dilute, neutralize, and distil as in the kjeldahl method. In neutralizing, it is convenient to add a few drops of phenolphthalein indicator, by which one can tell when the acid is completely neutralized, remembering that the pink color, which indicates an alkaline reaction, is destroyed by a considerable excess of strong fixed alkali. The distillation and titration are conducted as in the kjeldahl method. In distilling, the use of zinc or of any substance to prevent bumping or foaming is generally unnecessary, if too great an excess of fixed alkali be avoided. The amount of sulfuric acid recommended by Gunning is two grams for each gram of potassium sulfate; but Voorhees has found that this mixture is so viscous as to cause troublesome foaming frequently, and after cooling it cakes in a hard mass, which may be difficult to redissolve.[167] To avoid foaming and caking, he has found it an effective means to increase the amount of sulfuric acid used, taking instead of two grams to one of potassium sulfate three or four grams of acid to one of potassium sulfate. It is, therefore, suggested in carrying out the work, to use from five to twenty-five cubic centimeters (ordinarily about twenty cubic centimeters) of sulfuric acid for ten grams of potassium sulfate. In case the potassium sulfate is not free from nitrogen compounds, one or two recrystallizations will make it pure.
197. Gunning Method Adapted to Nitrates.—The essential features of this modification are due to Winton and Voorhees.[168] The modifications of the kjeldahl method, for similar purposes, furnished the material details for the gunning modified process. Winton reports good results from digesting for two hours, from half a gram to a gram of the sample with thirty cubic centimeters of sulfuric containing two grams of salicylic acid, in a flask of half a liter capacity. Two grams of zinc dust are then slowly added, with constant shaking, and the flask heated, at first gently, until, after a few minutes boiling, dense fumes are no longer emitted. Three grams of potassium sulfate are next added and the boiling continued until the solution is colorless, or if iron be present, until a light straw color is produced. On cooling, when the mixture begins to solidify, water is added with caution, and afterwards sodium hydroxid, and the ammonia is obtained by distillation.