ANALYTICAL CHEMISTRY
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=ANILINE ACETATE METHOD 20-
,bo 2b0 CONCENTRATION OF FURFURAL, y PER ALIQUOT ANALYZED 210
410
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Figure 1. Standard Curves for Furfural Solutions Measured against Reagent Blank with Evelyn Colorimeter at 520 m,u gent are added to each tube and immediately mixed by lateral shaking. After 30 minutes’ standing a t room temperature in the dark, the optical densities are determined with the Evelyn colorimeter using a 520 filter and setting the instrument to zero density with the blank tube. The color reaches a maximum in approximately 30 minutes and remains constant for about 20 minutes at room temperature, then fades slowly. The total concentration of furfural is calculated by means of the customary formula of spectrophotometry and the original data of Reeves and Munro (2).
The range and the accuracy of both methods were established by plotting the absorptancy against logarithm of concentration of furfural, according to the proposal of Ringbom ( 4 ) and Ayres(1). These data are shown in Figure 1. Inspection of the two curves shows the p-bromoaniline method to have an optimum range (0.9 to 6 micrograms) almost twice that of the original aniline acetate method (4 to 14 micrograms). The sensitivity, in turn, is three times greater than the original method. A relative analysis error of 3.2% per 1%absolute photometric error for both methods is established by dividing 230 by the slopes of the respective curves, which is the absorptancy change, in per cent, Qorresponding to a tenfold change in concentration (1). The p-bromoaniline method has been extensively employed in this laboratory for over a year for the analysis of furfural derived from numerous sources. It has proved superior to any other method tried by the author. LITERATURE CITED
(1) Syres, G.H., $NAL. CHEW., 21,652 (1949). (2) Reeves, R. E., and Munro, J., IND.ENQ.CHEM.,ANAL.ED.,12, 551 (1940). (3) Rice, E.W., and Roe, J. H., J . Bid. Chem.. 188,463 (1951). (4) Ringbom, A., Z.anal. Chem., 115,332(1939). (5) Roe, J. H., and Rice, E. W., J . Biol. Chem., 173,507 (1948). January 16, 1951. Work aided, in part, by an Institutional Grant. American Cancer Society, Inc.
RECEIVED
Semimicroanalysis of Organoselenium Compounds EDWIN S. GOULD Brooklyn Polytechnic Institute, Brooklyn, N . Y . the flame combustion method for analysis of orALTHOUGH ganoselenium compounds described by McCullough, Camp-
cases, the sudden formation of a relatively volatile selenide often resulted in too rapid combustion: In attempts to devise a method for the analysis of organoselenium compounds which was rapid, required only small samples, was applicable to compounds containing mercury or bromine or both, and required no extra precautions in analyzing compounds
bell, and Krilanovich ( 3 ) gives highly accurate results when applied to compounds of a number of different types, it is inapplicable to compounds containing bromine, mercury, or sulfur in addition to selenium. For best results with the combustion method, a sample containing 20 to 80 mg. of selenium is desirable, a complete analysis requiring from 60 to 90 minutes in the hands of a skilled operator. It was found that compounds containing a seleTable I. Analysis of Organoselenium Compounds by the niuni-oxygen linkage often burn less smoothly Digestion Method than selenides, diselenides, and selenocyanates. Selenium, % Deviation, Most benzeneseleninic acids and their derivatives Compound Formula, Theory Analysis % decomposed, leaving a nonvolatile residue containDi-p-tolylselenium CidHirSe 30.3 30.1 -0.7 p,p’-Dimethoxydiphenylselenium ClrHirOtSe 26.9 26.9 0.0 ing selenium in t’he burner. Such residues re24.6 24.5 -0.4 ~,p’-Diethoxydiphenylselenium CieHisOzSe mained, even at the hottest telnperature obtainDibenzoselenophene ClzHsSe 34.1 34.1 0.0 CsH7NSe 40.6 40.3 -0.7 able by the heater, and combustion could be com~;$$~~,~~~~enzene ClHdNClSe 36.4 36.5 0.3 p,p’-Dimethoxydiphenylselenoxide ClaHiaOsSe 25.5 25.5 0.0 pleted only by heating the outside of the burner 23.4 23.4 0 0 P p’-Diethoxydiphenylselenoxide CieHisOaSe with the flame from a nlicroburner until ignition Dibensoselenophene oxide CiaHsOSe 31.9 31.7 -0.6 p p’-Dimethoxydiphenylselenone CiaHidOaSe 24.3 24.1 -0.8 of the residue in the stream of oxygen occurred. p:p-Diethoxydiphenylselenone C16HlsOPSe 22.5 22.4 -0.4 p-Tolueneseleninio acid CiHsOzSe 38.9 38.7 -0.5 It was essential that the latter operation be carried m-Bromobenzeneseleninic acid CeHaOzBrSe 29.4 29.2 -0.7 out with great care, for too rapid heating resulted m-Nitrobeneeneseleninic acid CeHsOaNSe 33.7 33.5 -0.6 ClrHloSez 50.6 50.3 -0.6 in a semiexplosive cornbustion or decomposition ~-’,,;$$~~$d;iselenide CzrHisSez 34.3 34.0 -0.9 p,p-Diethoxydiphenylselenium of the residue and the escape of some of the seleClsHiaO~ClzSe 20.1 20.0 -0.5 dichloride niuin dioride. If the residue exploded without Diphenylselenium &bromide CnHioBrzSe 20.1 20.0 -0.6 complete combustion, elementary selenium and ~-(o-BiphenylylsBleno) acid propionic Cd’hcOtSe 25.9 25.5 -1.6 sizable amounts of soot were carried into the abp,p’-Dimethoxydiphenyl selenoxide-mercury(I1) chloride CiaHlaOsSeHgClz 1 3 . 6 13.5 -0.7 sorption tube, necessitating a thorough rinsing of p,p,-Diethoxydiphenyl selenoxidemercury(I1) bromide Ci6HisOsSeHgBrz 1 1 . 3 11.3 0.0 the apparatus with chromic acid-sulfuric acid Dibenzoselenophene oxidesolution. Many selenoxides decompose to the mercury(I1) bromide CuHeOSeHgBrz 13.1 13.0 -0.8 parent selenides at, their melting points; in such
V O L U M E 23, NO. 10, O C T O B E R 1951 having selenium-oxygen linkages, it was found that the selenium in a number of different types of compounds is converted epsentially quantitatively to selenious acid by the method of digestion described by Banks and Hamilton ( 1 ) . The selenious acid thus formed may then be titrated by the method of van der ll(5ulen ( 4 ) as modified by McCullough and coworkers ( 3 ) . In such a "ay, samples containing 2 to 5 mg. of selenium may be analyzed, a single analysis requiring from 10 to 15 minutes. Mercury or bromine does not interfere with the analysis, ap 1 the digeation of selenoxides, selenones, or seleninic acids proceeds without difficulty. PROCEDURE
A sample of 10 to 20 mg. is weighed out into a small platinum boat, and the boat and contents are dropped into a 100-ml. round-bottomed flask. Three milliliters of concentrated sulfuric acid are added, and the mixture is heated over the low flame of the Bunsen burner until it is well charred and appreciable fuming occurs. Fuming nitric acid is added dropwise until the mixture becomes yellow in color, and heating is continued for about 15 seconds more, decomposing some, but not all, of the nitric acid added. If no additional charring occurs, the digestion is halted; if the mixture darkens again, 0.5 ml. more of fuming nitric acid is added, and the mixture is reheated until the yellow color is restored. Overdigestion must be avoided; and no attempt should be made to boil off all the nitric acid, lest appreciable quantities of selenious acid be lost by volatilization a t the higher temperatures. After digestion, the mixture is cooled under a stream of runnin water and carefully diluted with 25 ml. of distilled water whict has been boiled to remove dissolved oxygen. The cool mixture, before and after dilution, should be perfectly colorless; a tinge of yellow color indicates incomplete digestion and almost infallibly predicts low results in the subsequent titration. To the solution are then added 3 rams of urea (to destroy any nitrous acid present), 5 ml. of 2# starch solution, a few small pieces of dry ice, and 1 gram of potassium iodide. The solution is swirled, allowed to stand 15 seconds, and titrated with approximately 0.05 i V sodium thiosulfate as previously described ( 3 ) . In digesting com ounds containing chlorine or bromine, 1 ml. of fuming nitric aci! is added to the sulfuric acid before initiating the heating; this has been found to prevent escape of the volatile selenium tetrahalide during digestion, Diselenides are best di-
1503 gested by addition of 1 ml. of nitric acid before the sulfuric acid, thus convertin the diselenide to the corresponding seleninic acid hydronitrate wlich is digested more smoothly. RESULTS AND DISCUSSION
The accuracy of the method is indicated by the results given in Table I for the analysis of 22 carefully purified compounds of differing types. These examples indicate that the method yields good results for compounds containing selenium together with carbon, hydrogen, nitrogen, oxygen, chlorine, bromine, and mercury. The method is not applicable to compounds containing iodine, as this element is partially converted during the digestion to iodate which also oxidizes iodide in acid solution. Although no compounds containing sulfur were tested, no difficulty is expected in their analysis, as any sulfur dioxide not volatilized during the digestion procedure should be oxidized to sulfate, which does not interfere with the titration. The digestion method has an advantage over other methods in being more rapid, requiring less skill on the part of the analyst, and calling for no special equipment. It has been found by Gabriel and Wenberg ( 2 ) that this method is also applicable to the analysis of inorganic selenides. Selenides both soluble and insoluble in water have been so analyzed. ACKNOWLEDGMENT
The author wishes to thank J. D. McCullough of the Uriiversity of California for his help in the preliminary experiments leading to this method and for his assistance in preparing the manuscript. LITERATURE CITED
(1) Banks, C. K., and Hamilton, C. S., J. Am. Chem. SOC.,61, 2306 (1939). (2) Gabriel, H., and Wenberg, H., private communication. (3) McCullough, J. D., Campbell, T. W., and Krilanovioh, N. J.,
IND.ENG.CEIEM., ANAL.ED.,18,638 (1946). (4) Meulen, J. H. van der, Chem. Weekblad, 31,333-5 (1934). RECEIVED March 7, 1961.
Spectrographic Determination of Impurities in Gallium Chloride Solutions WILLIAM M. SALTMAN' AND NORMAN H. NACHTRIEB Institute f o r the Study of Metals, University of Chicago, Chicago, I l l .
N T H E course of electrochemical experiments with gallium Iquantities metal and gallium salts it was necessary to determine the and types of metallic impurities present in the metal and salts. As the spectrographic method lends itself readily to the simultaneous determination of a number of impurities, it was applied to the problem. A prior chemical separation of the gallium from the impurities was desirable for several reasons. An increase in the sensitivity of detection of the impurities was to be expected. As the working curves would be based upon standards which were free of the major constituent, they could be used for the analysis of other substances by similar methods. Furthermore, there would be no problem of providing a quantity of highly pure matrix material or of applying residual impurity corrections. Of the various methods of chemical separation-electrolysis, distillation, precipitation, chromatography, etc.+xtraction is most attractive from the standpoint of completeness. The degree of separation of elements by solvent extraction methods is not subject to errors of coprecipitation or codeposition and can be relied upon as a characteristic property of the extraction system. Extraction methods offer the further advantage of simple techniques. 1 Present address, Physics Laboratory, Julius Hyman Co., Denver, Colo.
In the present work advahtage was taken of the partition of chlorogallic acid (HGaCI,) between aqueous hydrochloric acid and an ether. The quantities of nonestractable impurities left behind in the aqueous phase were then determined spectrographically. The method was adapted from the procedure of Wolfe and Fowler (IO), who determined trace impurities in steels after extraction of the iron with isopropyl ether. Conditions for the extraction were established from the work of Swift (9) and Fryxell and Sachtrieb (3). The copper spark method ( 2 ) was used for the determination of impurities. The method fails for those impurities-e.g., gold and iron-which are extracted into the ether along with gallium. EQUIPMENT
The flat-topped electrodes were commercial hard copper rod 0.25 inch in diameter. Their preparation has been described ( 2 ) . .Syringe-controlled micropipets (0.100 ml.) were convenient for dispensing exact volumes of solutions upon the electrodes. Solutions were evaporated upon the electrodes by means of electrically heated Nichrome coils. A low-inductance high-voltage spark discharge produced by a commercial power source of conventional design was used for electrode excitation. Excitation conditions are not critical; once selected, however, they should be reproduced as closely as possible.
