antimony to the quinquevalent state. On the other hand, thallium causes severe interference in the antimony determination and, if present, w-ould have to be removed (8). The method for the deterniination of antimony recommended above yields somewhat more reproducible results thsn the perchloric acid oxidation method previously ’ recommended (3). In the antimony and thallium methods, i t is permissible to carry a number of samples up through the reduction of antimonj- with hydrazine sulfate, but from this point on the samples should be treated individually and there should be no delay in the manipulation. In this way, air oxidation of trivalent antinion), to the quadrivalent state or bromine oxidation of trivalent antimony to the quinquevalent’state is minimized. The butyl Cellosolvebenzene method ( 3 ) for thc extraction of the rhodamine R compound of antimony or thallium has been recommended because it is as sensitive and more convenient than the chlorobciizene-carbon tetrachloride method ( I ) and the colors of the extracts do not fade as do those of the .isoprop).l ether extracts (4). To obtain
reproducible results in the antimony and thallium determinations, it is necessary to maintain rigid temperature control a t the time of the benzene extraction. In the antimony method the benzene extract can be freed of water droplets by filtration. In contrast, because the thallium-rhodamine B compound appears as a colloidal suspension in ,the benzene, much better reproducibility is obtained if the water is removed by centrifuging in the manner suggested by Onishi ( 5 ) . If the aliquoting technique described is used, it is possible to determine both antimony and thallium on the same sample. Experience has shown that no volatilization losses of either metal occur during expulsion of the hydrochloric acid, if nitric acid is added and if the rate of boiling is not excessive. EXPERIMENTAL
To check the accuracy of the proposed methods, synthetic samples mere prepared and analyzed. Several 1-gram samples of ‘test lead were dissolved in 10-ml. portions of nitric acid (1 3). 1Ieasured amounts of antimony, thal-
+
lium, and other metals in the form of aliquots of standard solutions were added. (Anions that would precipitate lead were avoided.) The mixtures were then analyzed for antimony or thallium. In some instances, after destruction of the filter paper, the sample \vas diluted to 100 ml. with hy9) and an aliquot drochloric acid (1 of this solution was taken for the analysis. Corrections for the small blanks on the reagents and lead were applied. The results obtained are recorded in T:ihle I.
+
LtTERATURE CITED
(1) Culkin, F., Riley, J. P., Analyst 83, 208 (1958).
(2) Luke. C. L.. IND. ENG.CHEM..ANAL. ED. 15: 626 (1943). (3) Luke; C. L., Campbell, M. E., ANAL. CHEM.25, 1592 (1953). (4) Maren, T. H., Ibid., 19, 487 (194i). (5) Onishi, H., Bull. Chem. SOC.Japan 30, \
,
5 6 i (1957). (6) Ssndell, E:. B. “Colorimetric Determinatiori of Traces of Metals.” D. 556, Intcrsrience, New York, 1950:
RECEIVED for review December 22, 1958. Accepted 1Iny 8, 1959.
Dete rmination of Residua I Acrylonitrile in Polymeric Systems
Pola rogra phic
GEORGE C. CLAVER and MARY E. MURPHY Plastics Division, Monsanto Chemical Co.,Springfield, Mass..
b Residual acrylonitrile monomer in styrene-acrylonitrile copolymer is determined polarographically in a nonaqueous system. Polarographic analysis, run directly on a N,N’-dimethylformamide solution of the polymer, avoids indirect measurement based on extraction or reprecipitation procedures. Sensitivity of the dropping mercury electrode for small amounts of acrylonitrile enables the determination of residual monomer in these topolymers. Styrene monomer does not interfere with the polarographic determination of acrylonitrile in N,N’-dimethylformamide. of trace quantities of residual monomers in polyIii”r1c sJ.stems has long been a problem f o r the analyst. The polarographic method for the determination of residual a(-rylonitriie in styrene-acrylonitrile copolymers set forth here is unique in two respects. I t is capable of accurately measuring acrylonitrile levels (parts
per million) in polymers without using combustive or separative techniques, and the solvent medium is organic rather than aqueous. The latter point is most outstanding. The system involved requires that a polymer and an electrolyte be mutually solvated. The total solution must tolerate a small amount of water (5%) which is added to increase electrical conductivity. The solvent found best suited for this application vias N , N’dimethylformamide. Tetrabutylammonium iodide (0.1izi) is used as the electrolyte. In this system acrylonitrile levels in the range of 30 to 200 p.p.m. are measured with a relative accuracy of +3.6%.
EAsuRmiENT
1682
J
ANALYTICAL CHEMISTRY
EXPERlMENTAl
Apparatus. The polarograph was the Sargent Model XXI. A voltohmmeter was used t o measure voltages and cell resistance, In accompaniment with 30-ml. straight mercury-pool cells (Sargent, 5-29385). Because of the Iarge temperature
coefficient of diffusion currents in dimethylfoimiamide, the cells were thermostated a t 25’ + 0.1’ C. The m * W 6 value of the capillary used was 1.42 mg.2i3s e ~ . -a~t -1.6 / ~ volts. Materials. Acrylonitrile, 99.8% commercial grade. N, AT’-Dimethylformamide, technical grade. TetrabutJdanimonium iodide, polarographic grade. Styrene, caomniercial grade. Styrene, acrJ-lonitrilecopolymer, commercial grade. Nitrogen, ivater pumped. PROCEDURE
Preparation of Solvent. Distillation procedures were of no value (I, 4) in preparing N, “-dimethylformamide for polarography. However, scrubbing with nitrogen for 30 to 40 minutes produced a satisfactory solvent. Preparation of Standard Copolymer. The copolymer used for blanks was made u p of standard solutions and was freed of residual acrylonitrile by reprecipitation from methyl ethyl ke-
Table I. Comparison 3f Diffusion Currenis of Acrylonitrile in N,N'Dimethylformamide-Water, StyreneAcrylonitrile Copolymer Solution Acrylunitriie, E' *, \701rs P.,'F ?,I , irr, pa. V8. EIg I>IJrJl
L'
0-1.
-15
-1,6
-11,
i -118
-;9
4;
l'oly me I 6.41
-
-20
VOLTS
Figure 1.
Typical polarogram
CONCLUSIONS
Thc method has been used successfully to detrwnine acrglonitrilr monomer in production samples of styrcne,ic*ryionitriiv copolymers. The poiarographic procedure in dinwthyiforni:imide provides a fast and accurate inalysis for rpsidual nionomcr in thew vnpolymrrs :\.hich cannot hr analyzed . i i i i 1 ~ 11)).~ other instrummtal method^.
v.iTERATURE CITED
!tadin,
I.,ANAL.
( 4 ) Findies, .%.! r)c Vriea, T., Zbid., 28, '09 f 1956). (5) Hiime, 9. X . , XjeFord, i). D., Cave, G. C. B., J . .ha. Cherr. ? h c . 73, 5323 (1951 ). RECEIYED for review January 2i;, 1959. Accepted May 11, 1959. VOL. 31, NO. 10, OCTOBER 1959
* I683
