IO Figure 1 1 . Log i 1OM3Min KI)
0
VS.
log
H (1.50 X
= -0.500 volt, slope = 0.79 = -0.300 volt, slope = 0.51 Eagp = -0.050 volt, slope = 0.73 EaPP = +O.OlO volt, slope = 0.71
8pp
0 EaPP
A
apparently due to a blockage. For aqueous chloride solutions this effect is not observed a t concentrations less than I m J l in chloride ion ( 5 ) . While it is not strictly established that the mercury halide will behave as an insoluble precipitate in the KKOaN a S 0 3 eutectic mixture a t 250’ C. (see above), analyses of polarograms from solution sess than 1mM in NaCl and KBr do nlot indicate a reversible
formation of the insoluble mercury halide (Figures 2 and 8). The study of current us. height of the mercury column at a potential on the anodic chloride wave where i/imax= 0.72 for various concentrations of NaCl does not indicate a n increase in the extent of surface control as the current density is increased (Figure 6). Similar results were obtained for the variation of current with height of the mercury column at various potentials on the anodic waves of all the halides studied (Figures 5, 9, and 11). I n general then, the present study indicates that the irreversibility of the halide anodic waves in the KNOrNaN03 eutectic mixture is not due simply to a blockage of the electrode surface by the formation of a film of the insoluble mercury halide. The exact nature of the surface controlling phenomena encountered in these studies may only be speculated upon from the data available. However, several interesting possibilities are worth considering. I n the K N O r N a N 0 3 eutectic mixture nitrate ion is present a t a concentration of about 5 5 M . The electrocapillary maximum for the pure nitrate medium is -1.1 volts us. the reference ( 8 ) . All potentials dealt with in the halide studies are, therefore, considerably positive with respect to the so-called point of “0” potential difference between solution and electrode. Consequently the adsorption of both halide and nitrate ion would appear probable. The relative potentials a t which the specific adsorptions of the various halides and nitrate ions occur could partially account for the observed phenomena.
While it does not appear that the formation of a film of mercury halide alone is responsible for the irreversible nature of the observed anodic halide waves, the reaction product cannot be discounted in a discussion of these waves. It is possible that the movement of mercury halide away from the electrode surface, due to its high vapor pressure in this medium, causes a localized stirring near the drop surface. This effect combined with a film formation and/or adsorption might well produce the observed behavior. LITERATURE CITED
( 1 ) Flengas, S. N., Rideal, E., Proc. Royal SOC.A233,443 (1956). (2) “Handbook of Chemistry and Physics,’’ P. 1698, Chemical Rubber Publishing Co., 1958. ( 3 ) Holifield. C. L.. &IS. Thesis. Uni-
tersity
of
Miniesota, Minniapolis,
1965. (4) Kolthoff, I. M., Lingane, J. J.,
“Polarography,” Interscience, New York, 1952. (5) Miller, C. S. Ph.D. Thesis, University of Minnesota, Minneapolis, 1940. (6) Nachtrieb, N., Steinberg, M., J.
Am. Chem. SOC.70,2613 (1948). ( 7 ) Nachtrieb, N., Steinberg, M., Ibid., 72 3558 (1950). (8) &offord, H. S., Ph.D. Thesis, University of Illinois, Urbana, 1962. (9) Swofford, H. S., ANAL. CHEM.37, 610 (1965). (10) Swofford, H. S., Holifield, C. L., [bid., 37, 1509 (1965). (11) Swofford, H. S., Laitinen, H. A,, J. Electrochem. SOC.110, 814 (1963). (12) Tien, H. T., Harrington, G. W., Znorg. Chem. 2 , 369 (1963).
RECEIVED for review June 7 , 1965. Accepted September 1, 1965. Work supported by the School of Chemistry, University of Minnesota, and the Shell Co. (Shell Fellowship in Chemistry, CLH).
Determination of TeIIurium by Cathode-Ray Polarography E.
JUNE MAIENTHAL and JOHN K. TAYLOR
National Bureau o f Standards, Washington, D. C. The determination of trace amounts or greater of tellurium in cartridge brass and in white cast iron may b e conveniently done by cathode-ray polarography. Tellurium is separated from the cartridge brass by ammonium hydroxide precipitation using an iron carrier. Tellurium is separated from the white cast iron by precipitation with sulfur dioxide using a selenium carrier. In 1.5M phosphoric acid, the results by cathodic scan are linear between about 0.05 and 0.7 p.p.m. of tellurium in the final solution. Above this range, nonlinear peak currents are obtained, but by the use of the
1516
ANALYTICAL CHEMISTRY
20234
anodic scan, the limit may b e extended to about 5 p.p.m. The method should also b e applicable to many other materials.
T
of tellurium in many applications requires the development of sensitive and selective methods for its determination. I n the production of white cast irons, the addition of as little as 0.01% of tellurium stabilizes much of the iron carbide; in gray irons, 0.1% of this element causes a complete whitening of the structure. Small amounts of HE INCREASING USE
tellurium also greatly improve the machinability of brasses. Tellurites and hydrogen telluride are even more toxic than the corresponding selenium compounds; therefore, knowledge of the tellurium content is of considerable importance in toxicology and industrial hygiene. Small amounts of tellurium may be determined by such photometic methods as measurement of the absorption of complexes with sodium diethyldithiocarbamate ( I ) , thiourea (6),or of suspensions of the metal (9). However, these methods are subject to many interferences or require complex separations.
a7 v) W
cc $ 6 Q
-u 5 z Iz
-
:4 -
P 3 V
3 2 -
I -
I
2
3
5
4
CONCENTRATION OF TELLURIUM, PPM (p0.x 10 FOR CATHODIC SCAN I
Figure 1 . current
Effect of cathodic and anodic scan on p e a k
Polarography has found limited use in the determination of tellurium. Lingane and Niedrach (‘7) studied the polarography of Te(1V) in the 0.1- to 3-mJ1 range in several supporting electrolytes and found that irregular, poorly shaped waves were often obtained. Bykov and Gorshkova (3) reported a direct method for milligram amounts of tellurium using a n alkaline cyanide supporting electrolyte containing E D T A ; however, iron, lead, cadmium, and manganese interfere. Itsuki, Ide, and Minehira (5) determined tellurium in copper, selenium, or silver by square-wave polarography in dilute phosphoric acid after suitable separations. The present investigation was undertaken to evaluate the applicability of cathode-ray polarography to the determination of trace and greater quantities of tellurium, particularly in some cartridge brass and white cast iron standard reference materials. Investigation of interferences and suitable supporting electrolytes was necessary. The methods developed involve only simple separations and can be applied to a wide range of materials. EXPERIMENTAL
Apparatus. All t h e analytical work was done with a Davis Differential Cathode-Ray Polarotrace, Model A1660, manufactured b y Southern Analytical Ltd., Camberley, England. Cells and electrodes, supplied with t h e instrument, and a mercury pool anode were used in all measurements. Materials. Standard Tellurium Solution. A stock solution of 1 mg./ ml. was prepared b y dissolving 62.5 mg. of high-purity tellurium dioxide (obtained from Spex Industries Inc., South Plains, N. J.) in 20 ml. of 50% nitric acid (v./v.) a n d diluting to 50
ml. All dilute solutions were prepared as needed from t h e stock solution. Standard Selenium Solution. A stock solution was prepared by dissolving 1 gram of high-purity selenium (obtained from the American Smelting and Refining Co.) in 20 ml. of nitric acid and 10 ml. of water, evaporating, just to dryness on a steam bath, dissolving the residue in 2 ml. of hydrochloric acid, and diluting to 100 ml. A11 other chemicals used were reagent grade, and solutions were prepared by conventional methods. INVESTIGATION
OF
CONDITIONS
Alkaline or ammoniacal supporting electrolytes are usually recommended for the determination of tellurium (’7). Checks of some of these supporting electrolytes showed them to be unsuitable for the amounts of tellurium in question, even in the absence of interferences. A LV sodium hydroxide supporting electrolyte gave erratic, poorly-shaped peaks; a n oxalic acidammonium chloride-ammonium hydroxide mixture as used by Bush ( 2 ) gave nonlinear results in the 1- to 2p.p.m. range. Moreover the classical method of separation of tellurium b y precipitation with ammonium hydroxide using a n iron carrier, which seemed most applicable to the brasses, would make the more basic supporting electrolytes inadvisable unless the iron is separated prior to polarography. T o investigate 1.551 phosphoric acid as a supporting electrolyte and also the toleration of the cathode-ray polarograph for the iron carrier, solutions containing 20 mg. of iron and from 0.1 to 5 p.p.m. of tellurium in a final volume of 100 ml. were measured by a cathodic sweep. Well-defined peaks at -1.2 volts showed a linear currentr concentration relationship between 0.05 and 0.5 or 0.7 p.p.m. The upper region of linearity varied JTith capillary
characteristics, being extended somewhat with increase of the mt values, This region of linearity allowed adequate coverage of the tellurium range of the brasses, b u t extension of the upper limits was desirable. The problem of the nonlinearity was next investigated. I n earlier work, use of anodic potential sweep for 0.1 to 0.5 p.p.m. of tellurium in 1.5X phosphoric acid resulted in small, ill-defined waves which were not proportional to concentration. When an anodic sweep was later used on larger amounts, well defined waves which were linear between about l and 5 p.p.m. were obtained. The results are shown in Figure 1. Next the effect of interfering ions was studied. Possible interferenies remaining in the brasses after a n ammonia separation were lead, tin, and any copper which was not completely separated. These ions and also bismuth, cobalt, cadmium, chromium, manganese, nickel, and vanadium were possible sources of error in cast irons and other samples. Checks of their peak potentials showed t h a t none except vanadium was close enough to tellurium to provide direct positive interference. Negative interference might occur, however, through the formation of insoluble tellurides a t the electrode. One-half to one-tenth milligram of the ions mentioned was added to 20-1g portions of tellurium and the solutions were measured polarographically in 100 ml. of 1.5V phosphoric acid. The results, shown in Table I, indicate the greatest interferences to be positive from vanadium and negative from chromium, It was shown above that 20 mg. of iron caused no interference, but the maximum amount permissible or the effect of varying amounts was not known. Accordingly, solutions con-
Table
1.
Interferences in Tellurium Determinations
Added Bismuth Cadmium Chromium Manganese Cobalt Nickel Vanadium Copper Lead Tin
Te recovered, % 101.0 98.8 84.9 98.8 97.6 100 0
109.0 95.4 97.6 95.4
Table II.
Effect of Iron in Tellurium Reduction
Te, p.p.m.
Sensitivity (p.p.m. Te/pa.) Fe, mg./100 ml. . 0 10 20 50 0.29 0.26 0.26 0.27 0.27 0.26 0.26 0.25 0.27 0.26 0.26 0.27 0.24 0.23 0.24 0.23
0.2 0.3 0.4
1.0
VOL. 37, NO. 12, NOVEMBER 1965
1517
Table 111. Tellurium in White Cast Irons by Butyl Acetate-Hexone Extraction
Specimen A B C D
Table IV.
No. of detns. 12 11 6 5
Te found,
Std. dev.,
0.072
0.011 0.011
%
%
0.069
0.002 0.002
0,008 0.008
Tellurium in Cartridge Brass Standards Tellurium found. % I
i
_
SpectroPolaroActivachemicalb graphic tion" 0.0045 0.0O4Oc 0.0032 0,0047 0.0030 0.0032 0.002 1101 0.0017 0.0011 0.0018 CllOl 0.0018 0.0011 0.002 0.0019 By D. A. Becker and G: W. Smith, Radiochemical Analysis Section. * By R. A. Alvarez, Spectrochemical Analysis Section. c 0.1-gram sample.
Specimen (31100
Q
taining varying amounts of iron and tellurium were prepared and measured polarographically in 1.5M phosphoric acid. With the use of 0 to 4oy0 of the slope compensation of the cathode-ray polarograph, a linear current-concentration relationship was obtained as shown in Table 11. The change in sensitivity at the 1-p.p.m. level is independent of the iron concentration and will be discussed later. I n view of these results, 1.5N phosphoric acid is a suitable supporting electrolyte although some separations are needed. A simple ammonium hydroxide precipitation seemed applicable to the brasses. T o determine the completeness of separation of traces of tellurium from a large amount of copper, 6 synthetics containing copper, iron, and from 5 to 35 pg, of tellurium, respectively, were taken through an ammonia precipitation. Organic material was destroyed b y acid treatment and the solutions were reduced by cathodic sweep in 1.5M phosphoric acid in a final volume of 100 ml. The recovery was quantitative, showing a n average deviation of *0.8 pg. from the amount added. For the cast irons, solvent extraction a t first seemed preferable. The feasibility of carbamate extractions for separation of tellurium was first tested with solutions containing 250 and 500 of tellurium, respectively, and 0.5 gram of NBS standard 55d at a p H of 8.8 in the ,presence of ethylenediaminetetraacetate, potassium cyanide, and sodium citrate. Almost no recovery of tellurium was obtained, probably because of the excess of sodium citrate added to prevent precipitation of iron. I n another experiment, a cupferron separation of the iron was made prior 1518
ANALYTICAL CHEMISTRY
to the carbamate extraction; however, only about 50% of the tellurium was recovered. Claassen and Bastings (4) found in their studies of the extraction of ferric chloride with methyl isobutyl ketone and amyl acetate that 97% of Te(1V) could also be extracted. Uzumasa, Hayashi, and I t o (IO) found that iron could be extracted from Te(V1) with butyl acetate. Tellurium could then be reduced to the quadrivalent state and extracted with methyl isobutyl ketone. Although iron up to a t least 500 p.p.m. may be tolerated in the measurement by cathode-ray polarography, large amounts of iron salts would make the destruction of organic material more difficult owing to bumping and splattering. Hexone extraction of Te(1V) after a butyl acetate separation of iron was next investigated. The oxidation of tellurium to the sexivalent state with potassium dichromate prevents its extraction by butyl acetate, provided the organic layer is backwashed about three times. The excess dichromate and tellurium were then reduced with hydrogen peroxide and the tellurium was extracted with hexone from 6AV hydrochloric acid. The organic material was destroyed with nitric acid and sulfuric acid, and the tellurium was measured polarographically in 1.5M phosphoric acid. Hexonebutyl acetate extractions of synthetic mixtures showed that 94 to 103% recovery of the tellurium was obtained. Results of the analyses of samples of the white cast irons are shown in Table 111. I n view of the large standard deviation, probably caused by partial, and hence erratic, extraction of vanadium and also by some chromium contamination resulting from the dichromate oxidation, a more satisfactory separation was needed. Precipitation of metallic tellurium with sulfur dioxide using a carrier, such as gold or selenium, appeared to offer a shorter and simpler means of separation. Tellurium was then precipitated with a selenium carrier using sulfur dioxide, ascorbic acid, and hydrazine hydrochloride, The bulk of the selenium was volatilized prior to polarography. Of all the methods studied, this proved to be the simplest separation, giving the most complete recovery. An average recovery of 97y0 was obtained for synthetics ranging from 20 to 200 pg. of tellurium. A preliminary investigation indicated that gold would also be suitable as a carrier; however, it offers no advantage over selenium. PROCEDURE
Brasses. Dissolve 0.5 t o 1 gram of sample in 15 ml. of %Yo nitric acid (v./v.). Add 20 mg. of F e ( I I I ) , boil briefly, dilute t o 50 ml., neutralize with ammonium hydroxide, and add 5 ml. in excess. Digest about 5 minutes on a steam bath, and filter through a medium porosity filter
-
paper. Wash with 5y0 ammonium hydroxide (v./v.), and return the paper and precipitate to the original beaker. Add 5 ml. of concentrated sulfuric acid and 25 ml. of concentrated nitric acid, and evaporate to sulfuric fumes to destroy organic material. If organic material is still present, repeat the fuming with additional nitric acid. Fume to dryness, dissolve the residue in 10 ml. of phosphoric acid and 30 ml. of water, and dilute to 100 ml. Reduce polarographically, measuring the tellurium peak at -1.2 volts. Carry synthetics containing equivalent amounts of copper, iron, and tellurium through the procedure for the construction of a calibration curve. Cast Irons. Dissolve a 0.250-gram sample in a mixture of 15 ml. of hydrochloric acid, 10 ml. of nitric acid, and 30 ml. of water, or dissolve a larger sample and take a n equivalent aliquot. Evaporate to dryness on a steam bath, add a few milliliters of 50% hydrochloric acid (v./v.) and take t o dryness again. Dissolve salts in 25 ml. of 4Oy0 hydrochloric acid (v./v.) and add 10 mg. of selenium. Add approximately 0.5 gram of ascorbic acid, and dilute to 50 ml. Heat to boiling, add 25 ml. of water saturated with sulfur dioxide and 15 ml. of 10% (w./w.) hydrazine hydrochloride, boil, and then add 15 ml. more of water saturated with sulfur dioxide, and boil for a t least 5 minutes. Remove from the heat and gas with sulfur dioxide while cooling in ice for approximately 5 minutes. Let settle, filter, and wash with 2% hydrochloric acid (v./v.) saturated with sulfur dioxide, then with water containing sulfur dioxide. Return the paper and precipitate to the original beaker, add 10 ml. of concentrated sulfuric acid and 25 ml. of concentrated nitric acid and evaporate to sulfuric fumes to destroy organic material and volatilize the bulk of the selenium. Repeat the fumings with additional portions of nitric acid if necessary, and evaporate to dryness. Dissolve the residue in 10 ml. of concentrated phosphoric acid and 30 ml. of water and dilute to 100 ml. Measure polarographically the tellurium peak at -1.2 volts, For tellurium concentrations less than about 1 p.p.m., use a cathodic potential sweep; for greater than 1 p.p.m., use a n anodic sweep. The exact region of linearity for both the cathodic and anodic sweep must be established for each capillary. Carry standards containing equivalent amounts of tellurium and selenium for establishment of a calibration curve. RESULTS
Tellurium in 0.5-gram samples of cartridge brass was determined by the procedure given. The results are shown in Table IV and are in good agreement with values obtained concurrently by two other techniques-activation and spectrochemical analysis. These samples were later certified at 0.0035, 0.0015, and 0,001570 tellurium, respect ively .
T h e cast irons were analyzed by the procedure given. The results, as well as those obtained by activation analysis, are shown in Table V. Samples A and B were swept anodically and samples C and D, cathodically. DISCUSSION
The tellurium peak in 1.5M phosphoric acid obtained by the cathodic sweep is very steep and sharp. When measured by conventional polarography, a peak resembling a maximum is obtained. This apparent maximum cannot be eliminated with sodium carboxymethyl cellulose or Triton X-100. Lingane and Niedrach (7) observed, as did Schwaer and Suchy ( 8 ) , the presence of a large maximum, which was relatively unaffected by suppressors, in the middle of the Te(1V)Te(0) diffusion-current plateau in several supporting electrolytes. Lingane and Niedrach attributed this peak to the further reduction of Te(0) to Te(-11) and reported that the reduction ceased as the potential became more negative, the current dropping back to the plateau corresponding to the 4electron reduction. T o confirm that the tellurium reduction in 1.5X phosphoric acid was not diffusion controlled at these levels, conventional polarograms of a 1.5p.p.m. solution using a dropping mercury electrode were made with varying heights of the mercury column. The wave height was virtually independent of the column height. Varying concentrations of tellurium in 1.5M phosphoric acid were measured on the cathode-ray polarograph at 28' and 37°C. For tellurium concentrations between 0.1 and 0.9 p,p,m,, the temperature coefficient for the cathodic sweep was that of a diffusion-controlled wave, but a t concentrations of 3 , 4 , and 5 p.p.m., the peak actually decreased 25 to 45%. The peak currents for the anodic sweep in the same region showed only the normal increase. The anodic sweep would seem to be more reliable for solutions of more than one 1i.p.m. of tellurium. On pure synthetics of higher tellurium content, by bracketing their range closely with calibration points, very reproducible results may be obtained. With actual samples, the presence of many foreign ions may cause the peak heights to be quite unreliable a t higher concentrations. The effects of hydrochloric acid, sulfuric acid, and varying concentrations of phosphoric acid were in-
Table V.
Tellurium in White Cast Iron Standards
Tellurium found. ?Z Polarographic, Se Specimen coprecipitation Activation0 A 0.071, 0.071, 0.071, 0.071, 0.079 0.070, 0.072, 0.071 Av. 0.071 0.075 B 0.069. 0,069. 0,067. 0.073, 0.075, 0.068, 0.@?0,0.069, 0.&7 0.080, 0.072 0.074 Av. 0.069 C 0.009& 0.0094, 0.0086 0.0091, 0.0092 Av. 0.0091 0.0092 D 0.0084, 0.0087, 0.0089 0.0092, 0.0096 Av. 0.0087 0.0094 By D. A. Becker and G. W. Smith, Radiochemical Analysis Section. vestigated. The cathodic sweep peak for 1.5 p.p.m. of tellurium decreased by almost 40%, and the peak potential was displaced from -1.2 to -0.7 volts by the presence of 10% of hydrochloric acid, while the peak resulting from anodic sweep was practically unchanged. The presence of 1 to 10% of sulfuric acid depressed the cathodic sweep peak several per cent with no change in the peak potential and practically no change in the anodic peak height or potential. The cathodic and anodic peaks were virtually unchanged by variations of between 5 and 20% in phosphoric acid concentration. A t 1 and 25%, a pronounced decrease occurred. CONCLUSIONS
The results of this investigation demonstrate the applicability of cathoderay polarography to the determination of tellurium in metallurgical materials such as brasses and white cast irons. The determination is best done in a supporting electrolyte of 1.551 phosphoric acid. A cathodic sweep is recommended for solutions with concentrations of less than about 1 p.p.m. while an anodic sweep should be used for more concentrated solutions. While the procedure described has been developed for materials containing tellurium in the range of 0.002 to 0.075%',, it may be adapted to ultratrace analysis by the use of smaller volumes and/or larger samples. The use of larger samples in the case of brass or highly alloyed metals might raise the level of interferences if only an ammonia separation is used. However, a subsequent separation with sulfur dioxide should reduce interferences to an acceptable level. Because of the relatively high tolerance for a number of elements, i t is possible to determine tellurium directly in low-alloyed samples or in simple matrices without recourse to
separation. This situation is demonstrated in the case of the white cast irons when small samples are used. Thus, direct analysis of 50-mg. samples of these metals gave results that were only slightly lower than those obtained with the recommended procedure. Actually, it should be possible to analyze biological material directly after oxidation of the organic material. The procedure described is also adaptable to conventional polarographic determinations. The method of anodic sweep is not applicable in this case; hence, the concentration of tellurium in the final solution should be less than about 1 p.p.m. The practical limit of sensitivity would appear to be about 0.1 pqp.m. using this technique. LITERATURE CITED
(1) Bode, H., 2. Anal. Chem. 144, 90
(19a5). (2) Bush, E. L., Analyst 88, 614 (1963). (3) Bykov, I. E., Gorshkova, L. S., Zauodsk. Lab. 25, 674 (1959). (4) Claassen, A., Bastings, L., Z . Anal. Chem. 160, 403 (1958). (5) Itsuki, K., Ide, A., Minehira, A,, Japan Analyst 9, 840 (1960). (6) Jilek, A., \-restal, J., Havir, J., Chem. Zvesti 10, 110 (1956). (7) Lingane, J. J., Niedrach, L. W., J . Am. Chem. SOC.71, 196 (1949). (8) Schwaer, L., Suchy, K., Collection Czech. Chem. Commun. 7, 25 (1935). (9) Shakov, A. S.,Zavodsk. Lab. 11, 893 (1945). (10) Uzumasa, Y., Hayashi, K., Ito, S., Bull. Chem. SOC.Japan 36, 301 (1963). RECEIVEDfor review July 29, 1965. Accepted September 1, 1965. Presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1965. Certain commercial equipment and materials are identified in this paper in order to adequately specify the experimental pi-ocedure. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the equipment or material identified is necessarily the best available for the purpose.
VOL. 37, NO. 12, NOVEMBER 1965
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