INDUSTRIAL AND ENGINEERING CHEMISTRY
50
TABLE 11. AGREEMENT OF ANALYSES Compound Thiourea Acetone diethyl sulfone Dinitrophenylthiooyanate Benzoyl sulfimide Bensenesulfonamide n-Propyl-p-toluenesulfonate
Sample I 41.96 26.84 13.48 17.31 20.45 14.87
Sample 11 42.05 27.35 13.60
: :a
VOL. 8, NO. 1
In those compounds which are soluble in liquid ammonia and do not form intermediate products which cannot be reduced to inorganic compounds, sulfur can be determined more quickly and just as accurately by this method as by the present lengthy bomb or fusion method.
14.79
The agreement between dudicate analvses of the same sample 2 various sulfur compo&ds is sho& in Table 11.
Summary A method has been Outlined by which a large number
Of
organic sulfur determinations can be carried on simultaneously and without the aid of special apparatus.
Literature Cited (1) Chablay, Ann. chim., (9) 1, 510 (1914) (2) Kraus and White, J. Am. Chem. SOC.,45, 775 (1923). (3) Vaughn and Nieuwland, IND. ENG.CHEM..Anal. Ed., 3, 274 (1931). (4) Williams and Gebauer-Fulnegg,J . Am. Chem. Soc., 53,352 (1931).
RECEIVED July 10, 1935.
A Source of Loss of Ammonia in Kjeldahl Distillations Method of Eliminating This Loss HOKE S. MILLER, Columbia University, New York, N. Y.
I
T H E Kjeldahl distillation process, the first portions of ammonia liberated are so diluted with air as to escape complete absorption in the standard acid. This loss, generally overlooked, is appreciable and may easily be reduced by the use of a proper delivery tube. The purpose of this paper is to show the magnitude of the error and how to reduce it. Moose (1) obviated the loss in the determination of ammonia in ammonium salts by suspending the salt in a small vial in the neck of a Kjeldahl flask and releasing the salt into the alkaline solution, after the solution started to boil. Though the accuracy of his results was one part per thousand, this procedure is not applicable to the usual Kjeldahl process. The usual method of distillation is to make the sulfuric acid solution alkaline with a cold saturated solution of sodium hydroxide, pouring it carefully down the side of the flask so that it does not mix immediately with the acid solution, adding a few pieces of granulated zinc, pumice stone, or glass beads to prevent bumping. The distilling flask is then connected to a Hopkins distilling head which is in turn joined to a condenser, to which is attached a delivery tube dipping into standard sulfuric acid. The distillation is carried out slowly a t first, the total time being about 1 hour. In order to determine the loss of ammonia resulting from the usual method of distillation, a known solution of ammonium chloride was used. ;?j
Twenty-five cubic centimeters of an ammonium chloride solution equal to 42.55 mg. of ammonia (35.00 mg. of nitrogen) were pipetted into an 800-co. Kjeldahl flask to which had been added some glass beads to prevent bumping u on subsequent distillation. Two hundred cubic centimeters oPdistilled water were added. Five cubic centimeters of a cold 41 per cent sodium hydroxide golutfion were carefully poured down the side of the flask, after which the flask was immediately attached to a Hopkins distilling head which by means of rubber tubing had been connected to a block-tin condenser. There was joined to the outlet of the condenser R delivery tube (a calcium chloridetype tube) which dipped under the surface of 50 cc. of standard sulfuric acid, contained in a 400-cc. beaker, into which the ammonia was distilled. The distillation was carried out very slowly for the first 30 minutes, during which time about 15 cc. of distillate were obtained. The total time of distillation was 1.25 hours, a t the end of which 175 cc. of solution had distilled over. The results
obtained from forty-seven 25-cc. samples of the ammonium chloride solution are given in Table I, and show a loss of 1.26 per cent of the total nitrogen, with a deviation from the experimental average of * 1.24 per cent.
TABLE I. NITROGEN Loss Theoretical Nitrogen 35.00
Nitrogen Found
IN
USUALMETHOD
No. of Determinations
34.35 34.58 34.83 34.37 34.85 Av. 34.56 i 0.43
11 11 3 11 11
Deviation from Average 10.26 10.27 10.10 rt0.62 10.18
In order to show that this loss of nitrogen is due to the incomplete absorption of ammonia, at the beginning of the distillation, the 5 cc. of sodium hydroxide were suspended in the neck of the Kjeldahl flask by means of a small glass vial hung to a bent glass rod which led through the stopper of the Kjeldah1 flask to the outside. After the ammonium chloride solution had been boiled to expulsion of the air, the vial of sodium hydroxide was released into the ammonium chloride solution. The distillation from this point onward was identical to that described for the determinations given in Table I. The average amount of nitrogen obtained from nine determinations was found to be 35.01 * 0.01 mg. The average of these nine determinations is 0.03 per cent higher than the theoretical; the average deviation is *0.03 per cent, which may be considered as negligible. Obviously this method of distilling off the ammonia cannot be applied to the sulfuric acid Kjeldahl solutions. As an alternative, it was thought that if the air bubbles were broken up by passing them through very minute openings, the ammonia which they contained would be absorbed by the standard sulfuric acid into which it is distilled. In order to ascertain whether or not the air bubbles could be broken up to such an extent as to prevent the loss of nitrogen from this source, the delivery end of the calcium chloride-type tube was closed and flattened. Ten holes, each 0.08 mm. in diameter, were made in the bottom of the tube. Eight distillations were carried out, except for this modification, identical with those described for the results obtained in Table I. The experimental average of these eight determinations is 34.99 * 0.02
JANUARY 15, 1936
ANALYTICAL EDITION
51
mg. This average is 0.03 per cent less than the theoretical; the average deviation from the mean average is *0.06 per cent. In order to make this last procedure comparable t o a regular Kjeldahl distillation, 20 cc. of concentrated sulfuric acid were added to the ammonium chloride solution plus 180 cc. of distilled water. Fifty-seven cubic centimeters of 41 per cent sodium hydroxide solution were carefully poured down the side of the flask and the distillation carried out as outlined in the previous procedure. The experimental average from eight determinations is 34.98 * 0.04 mg. of nitrogen. This average is 0.06 per cent lower than the theoretical; the deviation from the mean average is *0.12 per cent.
an extent that part of it escapes absorption by the standard sulfuric acid through which it passes. The average loss of nitrogen resulting from forty-seven determinations with ammonium chloride solution has been shown to be 1.26 per cent, the average deviation being * 1.24 per cent. The usual Kjeldahl procedure has been improved by the use of a delivery tube containing holes, each 0.08 mm. in diameter, which cause the air bubbles resulting during the first few minutes of distillation to be broken up to such an extent that the average loss of nitrogen resulting from eight determinations is 0.06 per cent, with an average deviation of kO.12 per cent.
Summary Loss of ammonia by the usual Kjeldahl distillation procedure has been ascribed to the fact that some ammonia during the first few minutes of distillation is diluted with air to such
Literature Cited Moose,
F.,~
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~colurnbia ~ University, ~ ~ 1935. ~
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RECEIVED October 1% 1935.
A Rapid Method for the Volumetric Determination of Indium HENRY B. HOPE, MADELINE ROSS,
AND
J. F. SKELLY, Cooper Union Institute of Technology, New York, N. Y.
THE
1 recent commercial availability of indium has revealed the need for rapid analytical methods for its determination. The method usually employed in commercial laboratories a t the present time consists of precipitation of the indium as the hydroxide and ignition to the oxide (4). This procedure is unsatisfactory because of the unavoidable interference of iron, and because of the excessive time required for routine determinations. A potentiometric method has been devised (1) using potassium ferrocyanide and the usual potentiometric equipment. This equipment is not always available in commercial laboratories. The method to be described involves the titration of an indium acetate solution with potassium ferrocyanide in the presence of diphenylbenzidine as an internal oxidationreduction indicator.
Reagents Diphenylbenzidine, 2 grams in 100 cc. of concentrated sulfuric acid (sp. gr. 1.84). Potassium fluoride, 10 grams of salt in 100 cc. of water. Potassium ferrocyanide, 2.5 grams of trihydrate in a liter of water plus 0.2 gram of potassium ferricyanide. The sulfuric acid used t o make up the indicator solution should be free from nitrates and nitrites. Heating the acid until fumes of sulfur trioxide are evolved will eliminate these radicals.
Procedure Weigh out a sample containing approximately 10 to 15 mg. of indium and dissolve in a suitable acid such as nitric acid or aqua regia. Remove the metals of Groups I and I1 with hydrogen sulfide. Make alkaline with ammonia in slight excess and digest on a steam bath or boil gently on a hot plate until faintly ammoniacal, Filter through a tight filter paper and wash sparingly with water. Dissolve the precipitate in about 15 cc. of concentrated warm acetic acid (glacial) by repeatedly pouring the acid through the filter. Wash the filter with 10 additional cc. of the acetic acid and then with three 5-cc. portions of hot water, uniting both acid and washings. The resulting solution will contain both the indium and whatever iron is present as acetates. If iron is present, as indicated by the tawny color of the hydroxides, add 0.5 gram of potassium fluoride dissolved in water as described above. The resulting solution should be about 60 per cent by volume of glacial acetic acid.
Cool the indium solution if necessary and add 2 drops of indicator, Titrate with standard ferrocyanide solution in a small cone flask, rotating the flask steadily until the end point is reached. The color change at the end point depends on the presence or absence of iron. If iron is absent and no fluoride has been added, the color change is sharp from slate blue to pea green which persists for 10 secondswith shaking. If iron is present and fluoride has been added, the end point is a sharp change from dull green to bright blue, the blue to persist for at least 10 seconds.
Discussion One of the major applications of indium a t the present is in dental gold alloys, which usually contain (in addition to indium) gold, silver, platinum metals, copper, and zinc. The indium can be separated from the other metals in such an alloy by means of a sulfide precipitation. Dissolve the sample in aqua regia, add 5 to 10 cc. of sulfuric acid, take to fumes of sulfur trioxide, add enough hydrochloric acid t o make about 1 N in total acidity (to prevent precipitation of indium sulfide), heat to boiling, and pass in a rapid stream of hydrogen sulfide for 30 minutes on a hot-plate. Filter off the sulfides without dela , boil the filtrate to expel hydrogen sulfide, and make slightly algaline with ammonia. Digest, filter off the precipitated indium hydroxide, and treat as previously described. Acetic acid must be used to dissolve the precipitated hydroxide. For small percentages of indium no reprecipitation of the sulfides is necessary. The potassium ferrocyanide solut'ion is most conveniently standardized by titration against a solution of known indium content, best prepared by dissolving the pure metal in dilute nitric acid and proceeding as with an unknown sample. Should the supposedly pure indium contain tin, its presence will be indicated by a white residue insoluble in nitric acid. The freshly precipitated indium hydroxide is very gelatinous and difficult to filter (6). Aging or digestion remedies this condition ( 2 ) . The sulfuric acid used to make the indicator solution should be fumed to sulfur trioxide to insure its being nitrite-free, as the presence of this ion causes a permanent blue in the indicator. The titrating solution must be cooled to room temperature before the addition of the indicator ( 2 ) .
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