Differential Potentiometric Determination of Selenium and TeIIurium in Refined Selenium Products SlLVlO BARABAS and PETER W. BENNETT Canadian Copper Refiners, ltd., Montreal East, Quebec, Canada
b A precise and accurate method involving a one-vessel technique has been developed for the determination of selenium and tellurium in refined selenium products. It i s based on controlled separation of approximately 97% of the selenium expected and final simultaneous potentiometric titration of the remaining selenium and all tellurium originally present. This reduces the original high seleniumtellurium ratio of approximately 1000 to approximately 20. Hydrazine sulfate has been used for the controlled, incomplete reduction o f selenium in a 3.6N sulfuric acid solution. The remaining selenium and tellurium was titrated b y stannous chloride in 9N hydrochloric acid solution. Although the end points are located b y the derivative method, the term “differential” i s given here a new meaning analogous to that used in differential spectrophotometry. The error in selenium analysis i s less than 0.0570; tellurium results compare favorably with those obtained b y longer conventional methods.
N
procedures pertaining to the determination of trace elements in high grade materials have been reported. Advanced techniques allow accurate determination of elements in the parts per million and even parts per billion range. Little progress has been reported in the determination of the main constituents of refined materials; yet the need for such procedures in industry has been great. Analysts have often eluded the problem by determining knoivn impurities in refined materials and then assuming that the remainder is the main constituent. However, a strictly inorganic product may have been contaminated by organic matter of n-hich the inorganic chemist is completely unaware. I n this instance, the “analysis” by difference of the main constituent will be misleading and grave error may be committed. UMEROUS
CONVENTIONAL VI. DIFFERENTIAL PROCEDURES
Specifications for refined selenium vary with its uses from 99 to 99.9% selenium. Accurate determination of selenium in this relatively narrow
range is essential. An error of as little as 0.1% could disqualify a lot of refined selenium. Gravimetric analysis is not satisfactory, as some moisture is retained by selenium even after prolonged drying, causing high results. Errors as high as 1% were produced in most careful gravimetric analysis. Barabas and Cooper (1) have devised a permanganometric procedure for selenium determination, whereby the excess permanganate used in selenium oxidation is back-titrated with ferrous ammonium sulfate. By using this procedure the error was reduced to 0.25%. The present work was undertaken to improve the accuracy of selenium determination. A great limitation of all known procedures for selenium in high grade materials was the necessity of using small samples. A relatively small error in n-eighing or titration was magnified many times. I t v a s thought that this difficulty could be overcome by employing a technique which we call differential, by analogy to a technique originally applied by Bastian (9) and Hiskey (3) to spectrophotometry and known as differential or precision spectrophotometry. The theory and applications of differential spectrophotometry were discussed in detail by O’Laughlin and Banks (6). The concept of differential titrimetry, introduced here for the first time, is basically the same as that of differential spectrophotometry. The two differential techniques employ similar, although not necessarily identical, devices aimed a t reducing the relative error in concentration mcasurernent. I n differential spectrophotometry, the use of a standard sample of known concentration is essential; in differential titrimetry, the reference to a standard may be convenient, but it is not indispensable as long as accurately standardized titrants are available. In this investigation, differential titrimetry involved separating from a sizable sample approximately 97% of selenium by reaction with a calculated deficiency of hydrazine sulfate and then accurately titrating the remaining selenium with stannous chloride. Any error in titration was minimized, as it affected only a small portion of the original sample. Thus an error as
high as 2% in the potentiometric titration of the remaining 3% selenium meant an over-all error of only 0.06Y0,. STOICHIOMETRY OF HYDRAZINE SULFATE AND STANNOUS CHLORIDE REACTIONS WITH SELENIUM AND TELLURIUM
Reducing properties of hydrazine sulfate and stannous chloride vs. selenium and tellurium have been known for years (4, 6, 7 , 9). There is no record, however, that the two reducing substances have ever been used for the direct, stoichiometric determination of either selenium or tellurium. Usually, hydrazine sulfate or stannous chloride is added in excess to separate elemental selenium and/or tellurium as ai1 intermediate step in the procedure, in the same \yay as sulfur dioxide or sodium hypophosphite is used. The final estimation is then made gravimetrically, or T-olumetrically. colorimetrically, Suseela ( 8 ) established the stoichiometry of the reaction HpSeOa
-
+ N2HsHS04 Se + Nz + 3 H 1 0 + HzSO4
in 2 to 611’hydrochloric acid by adding a measured excess of hydrazine sulfate to pure solutions of selenious acid and back-titrating the excess with potassium iodate. Hydrazine sulfate consumed was stoichiometrically equivalent to precipitated selenium determined gravimetrically. Suseela does not mention, however, that in the presence of tellurium, selenium could not be evaluated, as erratic amounts of tellurium would also have separated from hydrochloric acid solution upon addition of hydrazine sulfate. I n the course of this investigation, it was established that no tellurium will precipitate in the presence of as much as 1000 times its amount of selenium from a dilute sulfuric acid solution (3.6N) when a small deficiency of hydrazine sulfate, related to the selenium content, is added. In this rvay it was possible to reduce the original selenium-tellurium ratio of refined selenium materials from approximately 1000 to 1 to 20 to 1 and make simultaneous potentiometric determination of the two elements practical. The stoichiometry of the reactions of VOL. 35, NO. 2, FEBRUARY 1963
135
selenium(1V) and tellurium(1V) with stannous chloride
+ + 4HC1+ Se + 2SnCb + ~ H z O H2Te03 + 2SnCl~+ 4HC1--* Te + 2SnCll + ~ H z O
H~Se03 2SnC12
was established by potentiometrically standardizing two stannous chloride solutions prepared to approximate 1 ml. = 1 mg. of Se and 1 ml. = 1 mg. of Te, respectively, with potassium iodate. It took 10.57 nil. (average of five titrations) of the first solution against the calculated 10.48ml. to titrate 10 mg. of selenium(1V) and 10.37 ml. (average of five titrations) of the second against the calculated 10.27 ml. to titrate 10 mg. of tellurium(1V). The precision of the standardization titrations being within *0.02 ml., the difference between the observed and calculated values was attributed t o a slight divergence of the potentiometric end points from the equivalence points for the given rate of titration.
100 mv
EXPERIMENTAL
Apparatus. Titrations were performed using the first derivative setting of a Metrohm recording potentiograph Model E 336 (Metrohm, Ltd., Herisaw, Switzerland), with platinum and saturated calomel electrodes. Reagents. The stannous chloride solution was prepared by dissolving 7.18 grams of tin metal (analytical reagent grade) in 2.5 liters of hydrochloric acid against the pressure of a zinc-hydrochloric acid hydrogen generator. The container was protected from light and a n atmosphere of hydrogen always maintained. One milliliter reduced approximately 0.93 mg. of selenium(1V) and 1.5 mg. of tellurium(1V). Aqueous hydrazine sulfate solution, 20 grams per liter, was freshly prepared from analytical reagent grade material. Standard selenium solution, 0.5 mg. per milliliter in 12A' sulfuric acid, was prepared from high purity selenium. Sulfuric-nitric acid mixture was prepared by mixing equal volumes of water, sulfuric, and nitric acids, Methanol solution was a 15 volume % ' aqueous solution of analytical reagent grade methanol. Procedure. Weigh accurately into a 400-ml. beaker 612 to 615 mg. of refined selenium powder containing 99+% selenium, or proportionally more for various selenium salts. I n the case of selenium powder, add 20 ml. of the sulfuric-nitric acid mixture, cover, and digest a t a temperature not exceeding 80" C. to complete solution. Overnight digestion is usually convenient. Remove the cover and evaporate the remaining nitric acid, a t a maximum temperature of 50" C., until nitrogen oxide odor is barely perceptible. Dilute with 10 ml. of water, mix well, and cool. Add 20 ml. of methanol 136
ANALYTICAL CHEMISTRY
Figure 1. Effect of hydrochloric acid concentration on reduction of selenium and tellurium b y stannous chloride 1 2 mg. of Se and 12 mg. of Te added in each case
solution and heat a t 80" to 100" C. for 1 hour. I n the case of watersoluble selenium salts, dissolve in 40 ml. of 20 volume % sulfuric acid solution. To the hot solution, now free from nitric acid, add 50 ml. of hydrazine sulfate reagent and boil until coagulation of elemental selenium is complete. Cool, dilute with 300 ml. of concentrated HCl, and titrate the remaining unreacted selenium potentiometrically with stannous chloride solution. To determine the tellurium content, continue the titration beyond the selenium end point to the second potential break. Simultaneously run standards of high purity selenium to determine the selenium equivalent of the hydrazine sulfate solution. Establish the selenium equivalent of the standard stannous chloride solution by direct potentiometric titration of 20-ml. aliquots of the standard selenium solution. DISCUSSION
OF PROCEDURE
Removal of Nitric Acid. As the determination of selenium is based on an oxidation-reduction reaction, the absence of interfering reducing or oxidizing substances from the selenium solution is essential. Any organic matter t h a t might be present in refined selenium is destroyed by digestion with the mixed sulfuric-nitric acid. The subsequent removal of the residual nitric acid is the most critical part of the procedure. The acid digestion cannot be carried out to sulfuric
acid fumes, as some selenium would inevitably volatilize. The procedure therefore requires overnight acid digestion at a moderate temperature and subsequent destruction of the residual trace amounts of nitric acid by treatment with methanol. Effect of Hydrochloric Acid Concentration on Reduction Potential of Selenium and Tellurium. The reduction potential of both selenium(1V) and tellurium(1V) is considerably affected by variations in the strength of hydrochloric acid a t the time of titration. Thus the reduction potential of selenium increases, while t h a t of tellurium decreases with increasing acid concentration. For hydrochloric acid concentrations below 3N the selenium-tellurium titration curve is not well defined. The titration curves obtained in 3, 5, 7, and 9N HC1 are shown in Figure 1. The sharpest definition of the titration curve is obtained in 9N HC1 with the total potential span between the selenium and tellurium end points of about 300 mv. In particular, the potential change for selenium occurs in the range 0.52 to 0.33 volt and that for tellurium in the range 0.21 to 0.05 volt us. saturated calomel electrodes. I n 7-V HCl the potential change has been reduced to about 150 mv. and in 5N HC1 t o about 25 mv. because of the characteristic shifting of the midpoint potentials
toward lower values for selenium and higher values for tellurium. Finally, in 3N HCl selenium and tellurium titration curves coalesce into one, with selenium showing an inverted inflection. RTith decreasing acid concentration the total potential change at the end points is reduced for selenium only, while that due t o tellurium remains unaffected. I n all cases considered the total titration of both selenium and tellurium remains practically constant, regardless of HC1 concentration. In experimenting with stannous chloride titrations of selenium alone in hydrochloric acid solutions of various concentrations, it was established that the selenium titration curve follows about the same pattern noted in the simultaneous titrations of the two elements (Figure 2). This indicates clearly that the shape of the selenium titration curve is practically unaffected by the presence of tellurium. Accuracy and Precision. Varying amounts of accurately weighed high purity selenium were dissolved in the rnised acid. Approximately 97% of the total selenium was then precipitated by adding from a n automatic pipet 50 nil. of a 20 grams per liter hydrazine sulfate solution t o each sample solution. The remaining selenium \Vas titrated potentiometrically. The amount of selenium precipitated by hydrazine sulfate mas calculated by difference. The higher the precision of the procedure, the smaller is the deviation from the calculated average for selenium precipitated by hydrazine sulfate.
11 1 : mv
-
Table 1. Accuracy and Precision of Differential Separation of Selenium (50 ml. of a 20 gram per liter hydrazine sulfate solution added to each sample solution) Weight of Se Se pptd. by Deviation v/fl H.P. Se, mg. titrated, mg. hydrazine, mg. Mg. 619.0 18.55 600.45 -0.05 0,008 614.2 13.83 600.37 -0.13 0,022 615.3 14.96 600.34 -0.16 0.027 617.2 16.32 600.88 4-0.38 0.063 619.8 19.34 600.46 -0.04 0.007 -4v. 600.50 0.15 0,025 St. dev. 0.037
Ideally, identical results should be obtained, as the same volume of hydrazine sulfate was added to each sample solution (Table I). The reproducibility of selenium precipitation by hydrazine sulfate is remarkable. The standard deviation of 0.037% for the five samples is indicative of a precision that compares favorably with that of the most accurate procedures known, such as those based on electrogravimetry. Interferences. As shown in Table 11,the composition of refined selenium is such that only two impurities, tellurium and oxygen, are present in concentrations high enough t o have a n y effect on selenium analysis, and they do not interfere. Disregarding silicon, presumably present as inert silica, the combined remaining impurities amount in most instances to less than 0.03%. Even if they all had a one-sided, cumulative effect on selenium analysis, this should hardly be a reason for concern. HoFvever, because of the possible
I
1
1-
Figure 2. Effect of hydrochloric acid concentration on shape of selenium titration curves in absence of tellurium 12.3 mg. of Se present in each case
Table II. Per Cent Composition of Typical Refined Selenium Powder
Se
Te Fe Pb
cu
Hg
99.76
Sb Sn
0.14
o oio
0 005 0.0002 0 0023
Total
Ni
Mn Si 0 99.995
0.0001
0.0002 0.0003 0 0006 0.01 0 060
Selenium determined by differential potentiometric method. Impurities, except osygen, determined epectrographically . Oxygen determined by conductometric measurements in inert atmosphere.
Table 111. Effect of Foreign Ions on Differential Potentiometric Titration of Selenium
(Controlled reduction with hydrazine sulfate and titration with stannous chloride. All solutions contained 674.4 mg. of Set4 10 mg. of foreign ion) Se found, Se recovered, Foreign ('7 ion mg . io 671.09 99.51 Sb+3 671.99 99.64 Bi f 3 AE +a 669.29 99.24 675.72 100.19 -4g + 672.26 99.68 Xi + 2 673.11 99.81 Cd +2 674.59 100.03 Pb+2 676,31 100.13 Znf2
+-
applicability of the potcntionietric titb-ation method, either directly or following the differential separation of selenium, to other than refined selenium materials, an investigation was carried out to establish interferences froin some common foreign ions (Table 111). Each inipurity added approximated 1.5% of the selenium present. As the differential procedure was intended for the analysis of higher grade selenium materials, the combined impurities should not exceed 1.5%. Silver, lead, and zinc did not interfere. The effect of other impurities varied from 0.19% loss in selenium in the presence of cadmium to 0.76% loss in the presence of arsenic. Although errors of this VOL 35, NO. 2, FEBRUARY 1963
137
RESULTS
Table IV. Effect of Foreign Ions on Direct Potentiometric Titration of Selenium with Stannous Chloride
(Nohydrazine sulfate added. All solutions contained 12.28 mg. of SeC4 10 mg. of
foreign ion) Foreign ion None Sb+3 Bi+S Asi3
SnC12, ml. 13.48“ 13.23 13.19 13.00 13 17 kf:2 13:27 Cd+2 13.43 Pb+* 13.39 Zn+2 13.42 Au+S 16.30 C U + ~ 18.85 Fe+3 KO end point Pd+4 13.91 Pt+4 14.38
+
Deviation 70 M1. 0.04 0.30 0.25 1.86 0.29 2.15 0.48 3.56 0.31 2.30 0.21 1.56 0.05 0.37 0.09 0.67 0.06 0.45 3.18 23.59 5.37 39.84 0.43 0.90
3.19 6.68
Average of 5 titrations.
kind might be tolerated in the analysis of selenium refinery intermediate products such as crude selenium, additional work is planned to devise means of suppressing or at least reducing the effect of interfering ions. A greater variety of interfering ions present in concentrations as high as 45% was considered in evaluating the applicability of the direct potentiometric procedure for selenium (Table IV). Such a procedure would be useful in the analysis of solids and liquids containing moderate amounts of selenium in the presence of a number of other ions, each approximating selenium concentration. With the exception of iron, in the presence of which the selenium end point could not be detected, and gold and copper, which caused a positive error of 20 to 40%, interferences could be tolerated in the analysis of a number of selenium refinery intermediate materials. However, additional work is planned to devise suitable complexing or masking agents for interfering ions.
138
ANALYTICAL CHEMISTRY
Selenium in Refined Selenium M a terials. T h e great reliability of the
differential procedure for selenium has been proved in daily routine analysis of hundreds of refined selenium materials. The results from duplicate determinations generally agree within 0.05%. Some typical duplicate results for refined selenium, selenium dioxide, and sodium selenite are reported in Table V. The average deviation in duplicate analyses for eight of the 12 lots considered was less than 0.025%. Tellurium in Refined Selenium Materials. The differential technique of analysis does not apply t o tellurium, which is usually present in relatively low concentrations. Consequently all tellurium is titrated potentiometrically and simultaneously with selenium. The results obtained by potentiometric titration were in good agreement with those obtained by other methods of analysis (Table VI). The results obtained by potentiometric titration were originally compared against spectrographic results. As only four of the seven assays compared agreed within 0.0275, there v a s uncertainty as to the accuracy of the potentiometric procedure. Consequently it was decided to establish the true tellurium content by the most careful chemical analysis involving the preliminary separation of selenium, and subsequent titration of tellurium by a standardized potassium permanganate solution. The agreement between the potentiometric and chemical assays was most satisfactory; three results were identical, three agreed within O.Ol%, and one within 0.02%. LITERATURE CITED
(1) Barabas, Silvio, Cooper, W.C., ANAL. CHEM.28, 129 (1956). (2) Bastian, R., Ibid., 21,972 (1949). (3) Hiskey, C. F., Ibid., 21, 1440 (1949). (4) Noakes, F. D. L., Analyst 76, 542 (1951). (5) O’Laughlin, J. W., Banks, C. V., in “Encyclopedia of Spectroscopy,” G.
Table
V. Typical Analyses of Refined Selenium Products
Material Refined selenium
Lot 583
584 585
586 587 588 589 590 Selenium dioxide Sodium selenite
54 5501 69 70
%
yo Se deviation 99.55 99.51 99.51 99.63 99.85 99.79 99.74 99.87 99.64 99.68 99.81 99.83 99.76 99.77 99.36 99.36 70.30 70.39 71.73 71.77 45.38 45.40 ~. . -. 45.37 45.39
Av . Std. dev.
0.02 0.06 0.03 0.065 0.02
0.01 0.005 0.00
0.045 0.02 0.01
0.01 0.025 0.033
Table VI. Comparative Results b y Potentiometric Titration and Other Methods
Tellurium, yo Potentiometric 0.17 0.17 0.21 0.20 0.18
0.22 0.17
Chemical 0.17 0.17 0.20 0.18 0.18 0.21 0.16
Spectrographic 0.24 0.20 0.21 0.20 0.21 0.24 0.16
L. Clark, ed., p. 19, Reinhold, New York, 1960. (6) Schoeller, W-. R., Analyst 64, 318 (1939). (7) Strecker, W.,Schartow, L., 2. anal. Chem. 64, 218 (1924). ( 8 ) Suseela, B., Ibid., 147, 13 (1955). (9) Yannasch, P., Muller, hf., Ber. deut. Chem. Ges. 31, 2393 (1898). RECEIVEDfor review July 23, 1962. .4ccepted November 20,1962. Thirteenth Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 1962.
