ANALYTICAL EDITION
NOVEMBER 15, 1935
vias observed recently during the analysis of some soils that were unusually high in titanium (2 to 12 per cent). In making the determination of manganese by the persulfate method no color developed at first, although there was good reason for believing that the samples contained some manganese. When more reagents were added, to an excess of four or five times the usual amount, the color finally appeared. In order to verify the interference of titanium, which also reacts with persulfates, a sulfuric acid solution containing 54 mg. of titanium oxide and 0.2 mg. of manganese oxide was treated in the usual manner, using 1 gram of ammonium persulfate; no color developed. It should be noted here that ammonium persulfate, with or without silver nitrate, produces the characteristic brown color with titanium only in concentrated solutions strongly acid with sulfuric acid. When it was found that titanium interfered seriously with the ammonium persulfate method, an attempt was made to eliminate the titanium t o avoid its interference. Certain textbooks state that titanium is volatilized by evaporation with hydrofluoric acid unless sulfuric acid or some other acid is present. This statement is not entirely accurate, as shown by the following experiments: A solution containing 28 mg. of titanium oxide was precipitated by ammonia, filtered, and washed; the precipitate was then evaporated twice to dryness with 20 cc. of hydrofluoric acid. The residue was redissolved in acid, and the titanium determined gravimetrically; 24.2 mg. of titanium oxide were found. One gram of a soil colloid sample which contained 4.51 per
409
cent of titanium oxide (determined by the color method) was evaporated twice to dryness with 25 cc. of hydrofluoric acid. The residue was then fused with potassium yrosulfate, and titanium determined gravimetrically by the sulfurous acid method; 4.63 per cent of titanium oxide was found. One hundred milligrams of finely ground rutile were evaporated three times to dryness with 25 cc. of hydrofluoric acid. The residue was then fused with potassium pyrosulfate, and the titanium determined as above; 100 mg. of titanium oxide were found. Finally, a given weight of pure potassium titanium fluoride containing 70 mg. of titanium oxide was taken, and evaporated twice to dryness with 20 cc. of hydrofluoric acid. When the residue was analyzed gravimetrically, 68.5 mg. of titanium oxide were found. These tests show that while hydrofluoric acid alone may have a tendency to volatilize titanium, the effect is so slight, even on pure, freshly precipitated material, that it would be useless as a method of eliminating titanium for the purpose of preventing its interference in the persulfate color method. It is recommended that potassium periodate be used as the oxidizing agent in the presence of much titanium. Sodium bismuthate can also be used successfully, as there is no interference with the development of the manganese color.
Literature Cited (1) Hillebrand, W. F., U.8. Geol. Survey Bull. 305 (1906). (2) Robinson, W.O., U.8. Dept. Agr. Circ. 139 (1930). RECEIVED August
2, 1935.
Determination of Selenium in Organic Matter K. T. WILLIAMS AND H. W. LAKIN Bureau of Chemistry and Soils, U. S. Department of Agriculture, Washington, D. C.
N 1934 Robinson, Dudley, Williams, and Byers pubIquantities lished a series of methods for accurate estimation of small of selenium, which included modification sug(2)
a
gested by Van Kleeck for vegetation. This modification has been found generally satisfactory and perhaps the best hitherto available. Several thousand determinations have been made by its use (1). The method provides for the preparation of the sample by digestion with concentrated sulfuric acid as in the Kjeldahl method for nitrogen and subsequent distillation with concentrated hydrobromic acid and bromine. There are two unsatisfactory features in this procedure: In order to estimate quantities of selenium as low as 1 to 2 parts per million it is necessary to digest 10 grams of the air-dry vegetation, and even with the aid of mercuric oxide as a catalyzer this requires 6 to 8 hours for completion. Also, unless the digestion is carefully regulated the evolution of gases-carbon dioxide, sulfur dioxide, and water vaporis so rapid that there is danger of loss of small quantities of selenium which are not absorbed by the bromine trap. The modification described below has been found to be much more rapid in execution, lends itself to routine work more readily, and is, when properly executed, free from all danger of selenium loss. To prepare a sample of air-dry vegetation it is first ground to pass a 2-mm. mesh sieve, then mixed and quartered. A weighed sample, usually 10 grams, is stirred into a mixture of 50 CC. of concentrated sulfuric acid and 100 cc. of nitric acid in a 600-CC. Pyrex beaker. The mixture is stirred with a thermometer until it becomes homogeneous, after the first few minutes with gentle heating, without allowing the temperature t o rise above 100" C. After all frothing has ceased, the temperature of the mixture is raised t o a maximum of 120" C. until all evolution of nitrogen peroxide has ceased. The end of the operation is marked also by an incipient carbonization of the mixture, although longer
TABLE I. SELENIUM CONTENT OF VEGETATION AS DETERMINED BY KJELDAHL AND WET-COMBUITION PROCEDURE8 Lab. No. B14447
Material
Selenium Content Kjeldahl Wet oomdigestion bustion P. p . rn. P. p . m.
Astragalus grayii
14448
Mixed vegetation
14481
Corn
14482 14796A 14863 14733A 147188 16187 16190 16193 16197 16199
Corn Tium racemosum Stanleya bipinnata Tium racemosum Tium racemomm Diholcos bisulcatus Vegetation, not identi6ed Sunflower Wild aster Corn
5
450 460 45 150 370 7 3 220 4
..
160 550 18 7 3 10 4
heating at or below 120' C. does little harm. After the mixture is cooled it is transferred to an all-gIass distilling flask, 100 cc. of hydrobromic acid and 1 cc. of bromine are added, and 75 cc. of distillate are collected. Care is taken that the first portion of the distillate contains a smalI excess of bromine. The further procedure-filtration t o remove wax, ,precipitation with sulfur dioxide and hydroxylamine hydrochloride, and subsequent estimation by color or weighing-% as previously described. The only important precaution to be observed is t o remove the nitric acid, completely, by digestion. If this is not done the distillate will contain nitroxyl tribromide, which is not decolorized by sulfur dioxide and requires excessive quantities of hydroxylamine hydrochloride. If the temperature be allowed to rise until fumes of sulfur trioxide are evolved, certain loss of selenium will ensue. In Table I are given the results obtained upon identical samples by the procedure suggested and by the Kjeldahl digestion.
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INDUSTRIAL AND ENGINEERING CHEMISTRY
TABLE11. SELENIUM CONTENT OB VEQETATION (Comparison by treatment with nitric and sulfuric acids to incipient carbonization after removal of nitric acid at 120' c. and by subsequent heating for half an hour.) Selenium Content b Wet Combustion Metzod Incipient Heated 0.5 carbonihr. after carLab. No. Material zation bonization P. p . m. P. p . m. B15942 Wheat gram 10 5 2 15944 Side oats grama 2 15946 Wild aster 210 195 15948 Wild aster 8 6 15950 Little blue stem 1 1
In Table I1 are given five determinations made by wet digestion heated to incipient carbonization and for half an hour after carbonization has occurred but without allowing the temperature to rise above 120' C. It will be observed that while a rather minor loss of selenium is produced by heating beyond carbonization, such practice is inadvisable. By contrast when standard sample B14447, Table I, was heated with nitric and sulfuric acid until white fumes of sulfur
VOL. 7, NO. 6
trioxide were evolved, the quantities of selenium found were, in two samples, 770 and 710 parts per million. It is therefore essential that during the wet-combustion process the temperature be kept low. Small as is the modification suggested, it is of importance if large numbers of determinations are to be made, since the time required to prepare a sample for distillation is but 2 hours as against 6 to 8 for the Kjeldahl digestion. It is also more accurate, particularly for large quantities of selenium. By the use of this procedure an experienced operator with suitable equipment may readily effect distillation of twentyfour samples in an 8-hour day.
Literature Cited (1) Byers, H. G., U. S. Dept. Agr. Tech. Bull. 482 (1935). (2) Robinson, W. O., Dudley, H. C., Williams, K. T., and Byers, H. G., IND.ENQ.CHRIM., Anal. Ed., 6, 274-6 (1934). RECEIVED September 2, 1935,
Quantitative Analysis of Solutions by Spectrographic Means 0. S. DUFFENDACK, F. H. WILEY,' A N D J. S. OWENS,a University of Michigan, Ann Arbor, Mich.
M
,
has been worked out for the deETHODS for quantitative A method for determining the amounts termination of s o d i u m , potasanalysis by spectroscopic of metallic elements in a solution of their sium, magnesium, and calcium in means have been developed to salts is described. A n uncondensed spark solutions of mixtures of their such a degree of reliability that in air between a suitably chosen solid salts over the ranges of abunit seemed desirable to investigate dance in which these elements electrode and the solution under analysis the feasibility of applying them occur in urine. to the determination of the inis photographed, and, from measurements organic bases in biological mateon the relative intensities of chosen spectral Spectroscopic Source rials. The Department of Inlines, the analysis is made by reference to A n u m b e r of spectroscopic ternal Medicine of the university previotlsly determined working curves. An sources were tested before one became interested in the problem internal standard is employed. Two methsatisfactory for the purpose was and it has been carried out as a , joint investigation by this de- ods for correcting for the influence of the found. It is essential that the solution under test be vaporized partment and the Department presence, in varying amounts, of one elein such a way that the vapor of Physics. I n this paper, the ment on the intensity of the spectral lines shall contain t h e e l e m e n t s in general technic for making quanof another are described. The method has amounts representative of their titative analyses f o r m e t a l l i c concentration in the s o l u t i o n , been applied in the determination of soelements in a mixture of their Hence the vaporization of the sosalts will be d e s c r i b e d . The dium, potassium, magnesium, and calcium lution m u s t n o t change t h e fundamental method employed in solutions of mixtures of their salts in nature of the solution not yet is the same as that described by concentrations varying over the range of used, and the solution must be Duffendack, Wolfe, and Smith their occurrence in human urine. The fed into the spectroscopic source (I), except that a different speccontinuously, any part of it not accuracy of analysis of an unknown is, in troscopic source was demanded. v o l a t i l i z e d being immediately The particular problem that general, such as to give an error of about removed. had to be solved was that of mak2 per cent of the amount of the test element The source found to be most ing correct determinations of the present. satisfactory is an uncondensed several metallic elements in the mark in air between a suitable presence of each other when the solid electrode and an electrodeconsisting of the solution itself: amounts of the elements present in different solutions of the The material of the solid electrode should be one that permits same salts varied over considerable ranges. As the presence of ready and complete cleaning and has no spectral lines close to one element or ion oftentimes markedly affects the intensities those used for the analyses. Silver was used in the present inof the spectral lines of another, corrections had to be found for vestigation. The solution is volatilized by the electric dissuch of these effects as appeared. Two methods were devised charge. for making corrections for the effect of variations in the abundance of one element on the analysis for another. The technic The sparking apparatus, Figure 1, is a modification of one * NOWa t Haskell Laboratory of Industrial Toxicology, Wilmington, Del. described by Twyman and Hitchen (8). S is the solid electrode. * Now at The Dow Chemical Co., Midland, Mich. The solution is fed from the reservoir, E, through & aapillary, C,
