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
Literature Cited
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
Moose,
F.,~
i
~colurnbia ~ University, ~ ~ 1935. ~
~
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 ) .
~
i
~
INDUSTRIAL AND ENGINEERING CHEMISTRY
52
TABLEI. RESULTS
a
Weight of Sample Grams 0.0226 0.0226 0.0452 0.0192 0.0113 0.0141 0.0168 1.5047a 1. 5970a 1.5493= Dental gold alloys,
Indium Indium Present Found Gram Gram 0.0226 0.0224 0,0226 0,0228 0.0452 0.0449 0.0192 0.0192 0.0114 0.0113 0.0142 0.0141 0.0169 0.0168 0.0089 0.0089 0.0091 0.0094 0.0090 0.0087 C. A. Wamser, analyst, J. F. Jelenko
Error Gram -0.0002 $0.0002 -0.0003 0.0000 +0.0001 +o. 0001 0.0001 0.0000 -0.0003 -0.0003 & Co.
+
The end point cannot be obtained in the presence of chlorides. If no fluorides have been added, the titrating solution should be made up to contain about 40 per cent by volume of glacial acetic acid. If fluoride is used, the final concentration should be about 60 per cent acid. The fluoride reduces the ferric-ion concentration by the formation of the -~ preventing the formation of Pruscomplex [ F ~ F G ]thereby sian blue on the addition of potassium ferrocyanide. In this way the laborious separation of iron by 1-nitroso-2-naphthol (8) is entirely avoided.
VOL. 8, NO. 1
The end point is not permanent, as in the case of the permanganate titration of iron in the presence of chloride ion, and it is therefore necesmry to standardize the ferrocyanide under exactly the same conditions as prevail in the routine analysis. This volumetric method requires considerably less time than the usual gravimetric analysis.
Aclrnowledgmen t The authors wish to thank J. F. Jelenko for his kindness in providing pure indium and several indium alloys. They also wish to thank C. A. Wasmer and R. W. Weyand for aid in the investigation.
Literature Cited (1) Bray and Kirohmann, J. Am. Chem. SOC.,49, 2739 (1927). (2) Cone and Cady, Ibid., 49, 556 (1927). (3) Mathers, Ibid., 30, 209 (1908). (4) Thiel and Koelsch, 2.anorg. Chem., 66, 288 (1910). (5) Thiel and Luchman, 2.anorg. allgem. Chem., 172, 353-71 (1928). RECEIVED October 19, 1935.
Lignin in Douglas Fir Composition of the Middle Lamella A. J. BAILEY, College of Forestry, University of Washington, Seattle, Wash.
T
HE middle lamella has been subjected to extensive research, yet our knowledge of its structure and composition is far from complete. The exact relationship of this membrane to the various physical properties of wood is still a matter of conjecture. The nature of the so-called matrix surrounding each cell in wood is also an extremely important consideration in the impregnation of wood with preservatives. Furthermore, dissolution of the middle lamella is a goal of chemical pulping, yet even its approximate composition is unknown. Previous Work Initial investigations of the middle lamella were made by botanists who, on the basis of anabolic processes, stains, and solvents, concurred in the main in the belief that pectin or a derivative of it comprised most, if not all, of the middle lamella. This botanical concept was supported by the work of Fremy, Treub, Strasburger, Mangin, Timberlake, Allen, and others (1) who relied largely on supposedly specific pectic stains and solvents. It was demonstrated that in certain tissues the middle lamella could be stained with ruthenium red or completely dissolved by pectic solvents; in nearly all tissue this staining effect. was observed prior but not subsequent to treatment with a pectic solvent; hence, the conclusion that the middle lamella of most tissue was pectic. Botanists, however, studied tissues exceedingly diverse in kind, function, and age, and some recognized differences in the middle lamella which they attributed to differences in physical structure or chemical composition. While chemists, on the other hand, have disagreed almost unanimously with the botanical concept of the composition of the middle lamella, the two schools of thought were not always controverting the same thing, since studies by chemists were confined chiefly to mature xylem. As neither proponent completely or conclu-
sively disproved the other, it appears probable that both are partially correct. At any rate, each has made valuable contributions, while the controversy has stimulated progress. In view of the present knowledge of the combination and adsorption of stains, it seems probable that the early botanical investigators overestimated the significance of parallelism in staining effects, since Ritter, Harlow, Scarth, et al., demonstrated the unreliability of specific stains and solvents (10). These and other investigators found the lamella of mature xylem to be largely soluble in lignin solvents and insoluble in cellulosic solvents, to exhibit staining effects with supposedly specific lignin stains, and to possess optical properties attributive to lignin (3). Although the evidence in favor of chemists’ conception of the composition of the lamella proponderates, there are no data indicative of the complete absence of pectic compounds or of an integral lignin content.
Experimental Procedure In view of the fact that investigation of the middle lamella by the usual technic, in situ, did not make quantitative results possible, a method was developed to isolate mechanically a quantity sufficient for analysis. With the aid of a micromanipulator of the Janse and PBterfi type (Figure l), it was found possible t o remove the lamella1 in virtually a pure state. After experimenting with various types of glass microinstruments, a satisfactory tool was ‘devised by grinding high-grade sewing needles to a chisel-like point on a silicon carbide stone, maintaining a constant grinding angle by mounting the needle in a small block of wood much as a blade is mounted in a carpenter’s plane, and rotating the needle through 1 Ritter’s definition of the middle lamella (a), “the ieotropic peripheral layer of oell wall, including the irregular massea of isotropic material commonly formed when three or more oelle adjoin,” describe6 the isolated material rather accurately.
