1827
V O L U M E 2 4 , NO. 11, N O V E M B E R 1 9 5 2 A similar digestion period with additional oxine is also required for the complete recovery of aluminum after the removal of copper, iron, and nickel oxinates by chloroform extraction a t p H 2.8. Concentration of Tartaric Acid. Gentry and Sherrington ( 2 ) stated that the presence of tartrates interfered with the recovery of aluminum. It was desirable in this application to use tartaric acid to complex small quantities of iron. .2 maximum of 0.3 gram of tartaric acid can be tolerated in the 80 nil. of solution prior to the extraction of aluminum without serious loss of aluminum and this quantity of tartaric acid will complex up t o 5.6 micrograms of iron. S o rrror in analysis results when the standard rurve iq prepared in the same manner as samples are analyzed.
Table IT.
100
3
Conditions of Chloroform Extraction
10
10 10
10
2 2
1
1
100
3
aluminum, additional oxine reagent must be added before the second digestion period t o replace losses due t o reagent extraction. Effect of Diverse Ions. The principal constituents in alkali products which would cause interference are iron, copper, nickel, and manganese. By using proper p H adjustments according to information reported by Gentry and Sherrington (3) the iron itnd copper oxinates may be completely extracted with chloroform and the nickel oxinate partially extracted at p H 2.8. After t'he removal of these constituents the aluminum oxinate may be completely recovered a t p H 5.7 without interference from the manganese if this constituent is present. Otherwise, all aluminum extractions were made a t pH 6.6. If the aluminum values to be determined are 25 p.p.ni. of aluminum oxide or higher, usuallJ- the interfering substances in alkalies are small enough to be insignificant and their renioval is unnecessary. Iron will be an iiiterference if, a t the end of the first digestion, the solution is colored greenish black. Reproducibility. Fourteen synthetic samples containing 100 micrograms of aluminum oxide had an arithmetic average of 100.0 micrograms, a maxinium deviation of 0.4 microgram, and a standard deviation of 3 ~ 0 . 2microgram. ACKNOWLEDGMENT
Extraction. Factors considered in the chloroform extraction of aluminum oxinate were: the number of extractions required, the volume of chloroform in each extraction, and the shaking time required for each extraction. From the data in Table 11, it was concluded that the optimum conditions for the complete extraction of aluminum oxinate with chloroform were two 10-ml. portions of chloroform with a 2-minute shaking period for the first extraction and a I-minute shaking period for the second extraction. Where two aeries of extractions are required for the removal of interfering metals and later the determination of
The authors wish to thank G. 1., blurphy and George Oplinger for their assistance. LITERATURE CITED
(1) Alexander, J. IT.,Ph.D. thesis University of Wisconsin. 1941. (2) Gentry, C. H. R.. and Sherrington, L. G.. Analyst, 71, 432 (1946). (3) Ibid., 75, 17 (1950). (4) Merritt, L. L..and Cady, R. T., 11.S thesis, India'na University,
1948. ' 5 ) Moeller, T., ISD. EXG.CHEM., ANAL.ED., 15,346 (1943). (6) Wiberiey, S. E., and Bassett. L. G., A s a ~CHEM., . 21, 609 (1949). RECEIVEDf o r revie%- Soi.eniber 23, 1 9 ~ 1 , Accepted August 15, 1952
Polarographic Studies of Some Organochlorosilanes EARL A. ABRAHAMSON, JR.', AND CHARLES A. REYNOLDS Department of Chemistry, University of Kansas, Lawrence, K a n . RGANOHALOSILANES are particularly important today as intermediates in the field of silicone polymers. However, because of the ease of polymerization of their hydrolysis products, the problem of chemical analysis has been a difficult one, and the analysis has usually consisted of determining the particular organohalosilane in terms of the respective elements present. Since both chloroform and carbon tetrachloride are reducible a t the dropping mercury electrode ( I ) , it was thought that the organohalosilanes might also give polarographic waves which could be used for quantitative estimation of the silanes. An alternative polarographic determination would be possible if the organohalosilane reacted quantitatively with the solvent, producing a substance which was reducible a t the dropping mercury electrode. Since organohalosilanes undergo solvolysis readily in protonated solvents, most of the nonaqueous solvents in which polarographic work has been done wrre eliminated. A solvent had to be chosen in which the silane was soluble without reaction, or with which the silane reacted to give reducible products. Also, enough supporting electrolyte had to be dissolved in the solvent so that a solution of sufficiently low resistance for polarographic work was obtained. In addition, the Polvent itself had t o be nonreducible m-ithin a workable potential span. The solvents which seemed 1
Present address, E. I d u Pont de Kernours 8.z Co., Wilmington. Del.
best to fulfill these three requirements xvere acetone, methyl ethyl ketone, tetrahydrofuran, avetonitrile, formamide, and pyridine. APPARATUS AND REAGENTS
A Sargent Model X X I visible recording polarograph was used for all the work. All values of potential set on the span were checked with an auxiliary potentiometric circuit. Purified methyl ethyl ketone was redistilled and the fraction boiling between 79" and 80" C. was retained. The chloroform used was twice distilled and the fraction boiling between 60' and 62' C. was collected. Analytical grade acetone and for mamide were used without further purification. Analytical grade acetonitrilt~u a s used after drying over phosphorus pentoxide, Tetrahydrofuran was twice distilled, the fraction boiling between 62" and 64" C. being collected and dried over metallic sodium. Analytical grade pyridine was suitable for use after being dried over potassium hydroxide, redistilled, and dried over barium oxide. All the supporting electrolytes used were of analytical grade RESULTS AND DISCUSSION
Direct Reduction of Organochlorosilanes. Although carbonchlorine bonds are knom-n to be reducible a t the dropping mercury electrode, all attempts to reduce the silicon-chlorine bond
1828
ANALYTICAL CHEMISTRY
polarographically failed. Chloroform, carbon tetrachloride, benzene, and dioxane, all of which are good solvents for the organochlorosilanes, could not be used as solvents for polarographic studies, since it was impossible to dissolve enough supporting electrolyte in them. Saturated solutions of many salts were prepared in chloroform but the resulting conductances of such solutions indicated that these solutions would be unsatisfactory as polarographic solvents. Tetrabutylammonium iodide was soluble in chloroform. However, such a solution gave a reduction wave a t -0.3 volt versus the quiet pool of mercury which was not removed upon repeated recrystallization of the salt from ethyl acetate-ethyl alcohol mixtures and therefore could not be used. Reduction of Reaction Products of the Solvent and the Organochlorosilane. An addition compound of tetrahydrofuran and trichlorosilane is known (2), but no reduction was obtained in running such solutions polarographically. Formamide and acetonitrile offered possibilities of forming addition compounds with the organochlorosilanes but also gave negative results. Reduction waves were obtained for organochlorosilanes dissolved in either methyl ethyl ketone or acetone using lithium chloride as supporting electrolyte. The half-wave potentials and diffusion currents so obtained vary with the age of the solution and attempts to standardize conditions with respect to time failed to give reproducible results. A condensation product of the ketone is probably the compound being reduced. Well-defined reduction waves were obtained for solutions of organochlorosilanes in pyridine with lithium chloride as the supporting electrolyte. Trimethylchlorosilane and dimethyldichlorosilane gave half-wave potentials of -0.947 and -0.950 volt, respectively, against the quiet pool of mercury. The diffusion current was found to be proportional to the concentration of the organochlorosilane as predicted by the IlkoviE equation. The addition of water to such solutions shifted the half-wave potentials to more negative values and had no effect on the linear relationship between diffusion current and concentration. Studies were made of nine organochlorosilanes in the 25% by volume solutions of water in pyridine using lithium chloride as supporting electrolyte and in 50% by volume solutions of water in pyridine using potassium chloride as supporting electrolyte. Polarograms M to 0.15 M measured over a concentration range of 6 X showed a linear relationship between diffusion current and concentration within 2% average deviation. The amount of water present in the solutions is not critical as long as the resistance through the polarographic cell is maintained a t a low value. I n Table I a summary of the half-wave potentials of the organochlorosilanes, measured against the saturated calomel electrode, is presented. For comparison, the half-wave potentials of similar solutions of hydrochloric acid in pyridine are presented. It is apparent that in pyridine solutions the half-wave potentials of the individual organochlorosilanes are the same as the half-wave potential of hydrochloric acid, within experimental error. Pyridinium ion, one of the hydrolysis products of the organochlorosilanes in pyridine solution, is obviously the substance being reduced. Table I.
Summary of Half-Wave Potentials for Organochlorosilanes
Organochlorosilane Trimethylchlorosilane Dimethyldichlorosilane Methyltrichlorosilane Triethylchlorosilane Diethyldichlorosilane Ethyltrichlorosilane Diphenyldichlorosilane Phenyltrichlorosilane Vinyltrichlorosilane Average Hydrochloric acid
E112 in 7525 PyridineWater Mixtures -1,340 1 0 . 0 0 3 -1.347 1 0 . 0 0 3 -1.337 f 0 . 0 0 6 -1.358 1 0 . 0 0 9 -1,350 10.007 -1.344 f 0 . 0 0 5 - 1.359 f 0.007 -1.346 f 0 . 0 0 3 -1.322 f 0 . 0 0 4 -1.329 f 0 . 0 0 3 -1,331 1 0 . 0 0 3 -1.342 f O 009 -1.362 10.006
E l / r in 5050 PyridineWater Mixtures -1.406 f 0 . 0 0 2 -1,415 f 0 . 0 0 5 -1,398 1 0 . 0 0 5 -1.420 1 0 , 0 0 4 -1.410 f 0 . 0 0 4 -1.408 f O . 0 0 1 -1.400 zk0.004 -1.429 i O . 0 0 5
.............
-1.412 -1.398 -1.410 -1,419
iO.005 f0.003 f0.008 f 0.005
Table 11. Summary of Diffusion Constants for Organochlorosilanes I n 75-25 PyridineWater Mixtures with LiCl as S.E. Organochlorosilane Trimethylchlorosilane Dimethyldichlorosilane
I n 50-50 PyridineWater Mixtures with KC1 as S.E.
id id id id ~-~~ C4m2' s t l l a Cbm2/ Carn2latll8 Cbm2/ati /b 3tl/6
0.81 0.86 1.82 1.96 2.78 0.92
Methyltrichlorosilane Triethylchlorosilane Diethvldichlorosilane Et hylirichlorosilane 2:43 Diphensldichlorosilane 1.71 Phensltrichlorosilane 2.25 Vinyl tric hlorosils ne 2.25 Average Average deviation, % ' Hydrochloric acid 0.98 Ca refers to millimolar concentration of Cb refers t o millimolar concentration hydrolysis.
0.81 0.86 0.91 0.98 0.93 0.92
1.06
1.05
2:39
i:ig
3151 1.14 2.44 3.57
i.'i7 1.14 1.22 1.19
o:i1 0.86 0.75 3:47 i:is 0.75 3.47 1.16 0.87 1.16 5.7 2.48 0.98 1.13 1.13 organochlorosilane. of hydrochloric acid formed o n
A summary of the diffusion constants for the organochlorosilanes is presented in Table 11. Diffusion constants were calculated based both on the concentration of the organochlorosilane and on the concentration of hydrochloric acid resulting from complete hydrolysis of the organochlorosilanes in order that a comparison could be made with the diffusion constant of pyridinium ion in similar solutions. These results further confirm that pyridinium ion is the substance being reduced. -I 400
X
Figure 1.
Water by
Volume
Effect of Pyridine-Water Ratio on Half-Wave Potential
Greater self-consistency of half-wave potentials and of diffusion constants is observed in the 50% by volume pyridine solutions. I n order to determine the effect of the water concentration on the half-wave potentials and on the diffusion constant, measurements were made on a 4.408 mM hydrochloric acid solution in which the water content was varied from zero to 50% by volume. The concentration of supporting electrolyte, lithium chloride, was maintained constant a t 0.8 M . The results of these measurements are presented in Figures 1 and 2. I t is found that both the half-wave potential and the diffusion constant change more rapidly with water concentration in the region of 25y0 water by volume than in the region of 50% water by voluhe. Any fluctuations in water content which might occur in preparation of the solutions due to
V O L U M E 24, NO. 11, N O V E M B E R 1 9 5 2
1829
the hygroscopicity of lithium chloride and pyridine would have a greater effect in the solutions containing 25% water by volume. The inconsistencies in diffusion constants are not serious if the purity of the organochlorosilane is considered. The purity of the organochlorosilanes ranged from 90 to 99% and the impurities consisted of other organochlorosilanes. This would tend to give either low or high results depending upon the nature of the orgnnochlorosilane present as impurity-Le., whether or not it was a monovhlorosilane or trichlorosilane.
can be accomplished by weighing the organochlorosilane in a sealed glass bulb and breaking the bulb under pyridine in a closed flask. When the substance is completely dissolved in pyridine, transfer to a volumetric flask and dilute to volume with pyridine. From this solution remove an aliquot, such that when it is diluted to 50 ml. i t will fall in the concentration range of 6 X 10-3 &Ifto 0.15 M , and place it in a 50-ml. volumetric flask. Add sufficient pyridine to bring the volume to 25 ml. Add 5 ml. of 3 M potassium chloride and dilute to 50 ml. with water. Place the solution in a polarographic cell and obtain the polarogram by conventional means. Measure the diffusion current and compare the value obtained with a ealibration curve obtained by treating known amounts of hydrochloric acid dissolved in pyridine in the same manner. The calibration curve is a plot of the observed diffusion current against the concentration of chloride ion and is a straight line within the limits of experimental error. The accuracy of this procedure corresponds to an average deviation of 2%. CONCLU SION s
8
0
I6
24
32
40
48
56
7 . Wafer by Volume
Figure 2.
Effect of Pyridine-Water Ratio o n Diffusion Current Constant
Plots of log i / ( i d - 1) against the applied voltage were made for the reduction waves of hydrochloric acid dissolved in 50% by volume pyridine solutions and a straight line with a slope of 0.055 was obtained indicating a reversible reduction involving one electron. Similar plots for the organochlorosilanes also indicated a reversible one electron reduction. PROCEDURE
The reduction of pyridinium ion produced by the hydrolysis of organochlorosilanes in the presence of pyridine can be adapted to the analysis of these substances in terms of their chloride content. Mix a weighed amount of the substance t.0 be determined with dry pyridine in a closed vessel so that the salt formed does not escape as a smoke but dissolves in the excess pyridine. This
Perhaps the greatest limitation of this method is that one organochlorosilane cannot be determined in the presence of others. Only the total chloride content can be determined. If the method is to be used as a purity determination on one organochlorosilane, the presence of other organochlorosilanes or the presence of any substance which hydrolyzes to give an acid will interfere and cause the results to be high. The method can be adapted to the analysis of two organochlorosilanes in the presence of each other, if no other impurities are present and the two organochlorosilanes are knoivn. ACKNOWLEDGMENT
The authors gratefully acknowledge the aid, in the form of a fellowship to one of them (E.A.A.), furnished by the Eastman Kodak Co. LITERATURE CITED
(1) Kolthoff, I. AI., Lee, T. S.,Stocesova, D., and Parry, E. P., A s a ~ CHEM., . 22,521 (1950). (2) Sisler, H. H., Schilling, E. E., and Groves, W. C., J . Am. Chem. Soc., 73,426 (1951). RECEIVED for review March 19. 1952. Accepted August 7, 1952. From a thesis submitted by Earl A. Abrahamson, Jr., in partial fulfillment of the requirements for the degree of doctor of philosophy In chemistry in the Graduate School of the Cniversity of Kansas, 1951.
Modified Procedure for Determination of Protein-Bound Iodine in Serum HARRY SOBEL AND S . SAPSIN Department of Biochemistry, Division of Laboratories, Cedars of Lebanon Hospital, Los .4ngeles 29, Calif.
protein-bound iodine involves precipitation, digestion, and distillation carried out in separate containers. Recently Conner et al. ( 3 )described an apparatus in which both precipitation and digestion could be carried out. The present report presents a further simplification whereby precipitation, digestion, and distillation are carried out in the same tube, resulting in reduction of the time required for analysis.
Zinc sulfate (ZnSOa.’iH,O), 1.25%, A.C.S. specifications (Baker and ildamson). Potassium iodide, C.P. (Baker). The apparatus (Greiner Glassblowing Laboratory, 3604 East bledford St., Los Bngeles, Calif. j , Figure 1, consists of a digestion tube (22 X 140 mm.) containing a 24/40 standard taper joint and a delivery U-tube (22 X 230 mm. total length and 35-mni. inner distance) containing a standard taper 24/40 male joint a t one end and a 1.25- to 1.75-mm. capillary tube 200 mm. long at the other end.
REAGENTS AND APPARATUS
PROCEDURE
Sulfuric acid, 70% by weight, A.C.S. specifications (Baker and Adamson j. Chromic acid, 60% by weight, 4.C.S. specifications (Merck). Ceric ammonium sulfate, 0.1 X, reagent grade (G. Frederick Smith Co.), in 3.5 N sulfuric acid. Sodium hydroxide, 1% and 0.75 iV, A.C.S. specifications (Merck). Phosphorous acid, 50% by weight, C.F. (Baker and Adamson). .4rsenic trioxide, 0.15 N , C.P. (Baker), in 1.5 N sulfuric acid.
To 0.5 ml. of serum in the digestion tube are added 4.0 ml. of zinc sulfate solution and 0.5 ml. of 0.75 N sodium hydroxide. After thorough mixing the tube is centrifuged for 7 minutes at 1800 r.p.m. and the supernatant is discarded. The contents of the tube are washed with 10 ml. of water and recentrifuged. Only one washing is used in the modified and the Chaney procedures in the authors’ laboratory. The washed precipitate is dissolved in 4.0 ml. of 70% sulfuric acid and 0.5 ml. of chromic acid solution is added. The con-
HE widely used Chaney ( 2 ) procedure for the determination
