of The Firestone Tire and Rubber Co.
T a b l e IV.
Turbidimetric Determination of Zinc O x i d e in Compounded Rubber (Beckman Model B Spectrophotometer) Xatural Tread, 1.85’% ZnO GR-S Tread, 1.45% ZnO ZnO Av. f mean Dev. from ZnO Av. =k mean Dev. from dev., % theory, % found, % dev., % theory, % Analyst found, % 1 1.89,1.86” 1.32, 1.33, 1.85 f 0.03 1.39 1.35 f 0.03 1.78,1.86” 0.00 -0.10 2 1.89, 1.91, 1.36,1.43“ 1.85, 1.89 f 0.02 1.89 $0.04 1.38, 1.36,’ 1.41 & 0.04 1.48, 1.43 -0.04
i
a
Different runs by the same analyst.
ble material in normal black tire stocks is usually insignificant because of the small sample size. It is corrected for b y the sample blank absorbance measurement after addition of ammonia. I n the case of white-wall tire stock, the acid-insoluble titanium dioxide forms a turbidity which adds to the absorbance of the zinc present. However, only about 3 mg. of white sidewall sample containing 20 to 40y0 zinc oxide is taken for 100 ml. of final solution. The turbidity due to titanium dioxide is relatively uniform and so low t h a t i t is corrected for by blank absorbance. This method for zinc cannot be applied to determining trace concentrations in materials which form insoluble carbonates. Calcium and barium cations may be present a t concentrations somewhat above five times the amount of zinc present without causing interference. However, an attempt to determine traces of zinc in calcium carbonate after solution in acid yielded a dense calcium carbonate precipitate on neutralization; this made a turbidimetric determination impossible. ACCURACY A N D PRECISION
The absorptiometric method follows Beer’s law u p t o the limit of accurate and essentially linpar instrument response between about 0.1 and 1.8 absorbance units. I n the 40-ml. ether solution the concentration range is about 0.2 to 3.5 p.p.m. of zinc for the absorbance at 262 mp. Concentrations below 0.05 p.p.m. can be determined by reducing the ether volume to 5 or 10 ml. The precision and accuracy are probably as good as those of the conventional ASTM volumetic method for the amount of zinc oxide normally found in black tire stocks (1 to 4%). TabIe I1 bears this out, though the data are not representative of the best obtainable by either the volumetric or absorptiometric methods. 440
ANALYTICAL CHEMISTRY
Colorimetry is usually applied t o the determination of trace amounts of metals below about O.Ol%, and seldom above 1% as a minor constituent (14, 18). T o obtain ultimate accuracy a t higher concentrations, a differential method might be used, but this was not investigated. The turbidimetric method is capable of yielding satisfactory results on a routine basis (Table IV). The accuracy is as good as or better than that obtained by the volumetric method. A mean deviation from the average ranging from zt0.02 to +0.05% was obtained by three different analysts. COMPARISON OF METHODS
Both methods give satisfactory results for all normal rubber products, and both offer a definite improvement over present volumetric methods. The convenience and safety of using ‘water as a solvent, plus measurement in the visible region makes the turbidimetric method a good procedure for normal rubber products, particularly where the Beckman Model B or a filter photometer is available. However, the ether extraction procedure is more of a universal absorptiometric method for zinc because of its greater sensitivity and the ease of detecting and correcting for possible interference of other elements forming carbamates. The ether absorptiometric method is relatively free of interference from all types of turbidity found in the aqueous medium. ACKNOWLEDGMENT
The author would like t o acknowledge the assistance of J. Z. Falcon, L. W. Dannemiller, and Borivoj Pavlovich. Thanks are due to G. H. Wallace for supplying all standard stocks and for his encouragement and assistance in preparation of this report. Permission
to publish this work is gratefully acknowledged.
LITERATURE CITED
(1) Am. SOC.Testing Materials, Philadelphia, Pa., Tentative Methods of Chemical Analysis of Rubber Products, Designation D 297-50T, ASTM Standards, Part 6, 1952. (2) Ibid., Designation D 297-551‘. (3) Atkins, W. R. G., Analyst 80, 400 (1935). (4) Chilton, J. M., ANAL. CHEM. 25, 1274 (1953). (5) Ibid., 2 6 , 940 (1954). (6) Cowling, H.,Miller, E. J., IND. ENG.CHEM.,ANAL. ED. 13, 145 (1941). (7) Dasler, W., Bauer, C. D., Ibid., 18, 52 (1946). (8) Du Pont de Nemours, E. I., & Co., “Rubber Chemicals,” Rept. 50-2, 1R.w
(9) HGiebkd, W. F., Lundell, G. E. F., “Applied Inorganic Analysis,” Wiley, Sew York, 1929. (10) Jacobs, 11. B., “Analytical Chemistry of Industrial Poisons, Haaards, and Solvents,” Interscience, Kew York, 1944. (11) Kress, K. E., ANAL. CHEM.23, 313 (1951). (12) Kress, K. E., A p p ! . Spectroscopy 6 . No. 4. 19 f1952l (13) Krek, K. E., hI‘ees, F. G. S , ANAL. CHEW27, 528 (1955). (14) Kruse, J. RI., Brandt, W.W.,Ibid., 24,1306 (1952). (15) Lankenau, R . 0. A., Firestone Tire & Rubber Co., Akron, Ohio, personal communication. Rogers, S. S;;“Vanderbilt Rubber Handbook, R. T. Vanderbilt Co., K e a York, 1948. Rush, R. M,, Yoe, J. H.,AN.~L. CHEM.26, 1345 (1954). Sandell, .E. B., “Colorimetric Determination of Traces of Metals,” Interscience, New York, 1950. Smith, G. F., “Mixed Perchloric, Sulfuric, and Phosphoric ilcids and Their Application in Analysis,” G. Frederick Smith Chemical Co., Columbus, Ohio, 1935. Tyler, W. P., IXD.ENG. CHEM., ANAL.ED. 14, 114 (1942). RECEIVEDfor review May 31, 1956. Accepted August 30,1957.
Corrections I n the article on ‘‘Ultramicromethod for Molecular Weight Determination” [Guerrant, G. O., ANAL. CHEM.30, 143 (1958)l Figure 1 was printed upside down. I n the article on “Tracer Techniques in Fiber Research” [White, H. J., Jr., ANAL. CHEM. 29, 1744 (1957)] references t o (14) and (15) were interchanged in the text. Reference (15) should have been cited at the bottom of the first column on page 1745, and (14) a t the bottom of the third column on page 1745.
