with heated chambers is highly sensitive, and any decrease of signal size observed does not make analytical determinations severely difficult. Decrease of signal in the presence of high concentrations of sodium chloride can be interpreted as a consequence of (a) a change of the diffusion rate of the analyte in the flame, when transported to the flame in particles nearly dried-due to the partial evaporation of the solvent in the heated spray chamber -and (b) a shifting of the evaporation equilibria of the particles reaching the top of the burner. These circumstances decrease the efficiency of the atomization process. In the spraying process, the presence of sodium chloride in high concentration can also change the droplet size in spite of the fact that the aspiration rate was not changed, as it was experimentally tested, by comparison of aspiration rate in the absence of sodium chloride and in the presence of variable concentrations of this salt. Practically all analytes tested can be determined with the laminar flow burner, hot operation, in t h e presence of high concentration of sodium chloride at low interference ratios
(lower than the limiting interference ratio). These elements may be measured with the help of a series of standards without compensation. At high interference ratios (higher than the limiting interference ratios), standards should be compensated with equivalent sodium concentrations expected (or previously measured) in the samples. Whenever it is possible, samples should be diluted down t o lower concentrations. It is true that such a dilution does not change the ratio between ppm of concomitant and ppm of analyte, but by diluting the solutions the total solid content is decreased; the hot operation allows enough sensitivity to work a t lower analyte concentration ranges; and determinations can be done easily in the lower linear portions of the calibration curves; the burner is then less exposed t o deposits and clogging, and also the flame is less concentrated with respect t o entities liberated in it, resulting in better conditions to achieve the proper diffusion and evaporation equilibria.
RECEIVED for review July 30, 1969. Accepted January 19, 1970.
lodometric Determination of Tellurium(V1) in the Presence of Tellurium(IV) and Selenium(V1) Richard Beyak1 and Bruno Jaselskis Department of Chemistry, Loyola University, Chicago, Ill.
A RAPID METHOD for the determination of tellurium (VI) in the presence of tellurium(1V) and selenium(V1) has been developed. The method depends on the quantitative reduction of tellurium(V1) to tellurium(1V) by iodide ion and on the unreactivity of tellurium(1V) and selenium(V1). These species are not reduced at the experimental conditions described in our procedure. Although there are many methods and reviews of tellurium analyses reported, relatively few of these are for tellurium(V1) (1-3, especially in the presence of mixtures containing tellurium(1V) and/or selenium(V1). Tikhomirova (6) and Chavdarova and Sheytanov (7) have reported tellurium(V1) determinations in the presence of tellurium(1V). Their methods suffer the practical deficiency of requiring two determinations per sample with the amount of tellurium(V1) being obtained as the difference between a n initial tellurium(1V) assay and a total tellurium(1V) assay following the reduction of tellurium(V1) present in the sample. 1 Present address, Alberto-Culver Co., 2525 Armitage, Melrose Park, Ill. 60160
(1) G. Charlot and D. Bezier, “Quantitative Inorganic Analysis,” John Wiley and Sons, Inc., New York, N. Y., 1957. (2) T. E. Green and M. Turley in “Treatise on Analytical Chemistry,” I. M. Kolthoff, P. J. Elving, and E. B. Sandell, Eds., Part 11, Vol. 7, Interscience Publishers, New York, N. Y.,1961. (3) J. Dolezal, E. Lukshyte, V. Rybacek, and J. Zyka, CoNect. Czech. Chem. Commun., 29, 2597 (1964). (4) J. G. Lanese and B. Jaselskis, ANAL.CHEM., 35, 1878 (1963). (5) J. 0. Edwards and A. L. LaFerriere, Chemist-Analyst, 45(1),
12(1956). (6) N. P. Tikhomirova, Metody Anal. Khim. Reuctivov Prep., 12, 90 (1966). (7) R. Chavdarova and Ch. Sheytanov, Bulgarska Academia Na Naukite, Sofia, Doklady, C. R. Acud. Bulg. Sci., 20, 565 (1967). 518
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4,APRIL 1970
In the usual case of mixtures containing tellurium(V1) and selenium(VI), a preliminary separation procedure is required prior to the quantitative determination and the gravimetric and distillation separation procedures (8,9) are time-consuming, laborious, and subject to experimental error. The separation of selenium(V1) from tellurium(V1) in the proposed method is not necessary. However, the reduction of selenium(1V) by iodide is well established (IO), and its presence must be avoided. This paper reports the experimental conditions established for the quantitative analysis of tellurium(V1) in the presence of tellurium(1V) and selenium (VI). EXPERIMENTAL
Reagents and Materials. Telluric acid and sodium selenate were purchased from A. D. MacKay. The telluric acid was recrystallized four times before use while sodium selenate was purified by the method of Feher (11). High purity tellurium dioxide was prepared from tellurium metal (11)and the purity checked by the method of Gardels and Cornwell (12). Primary standard grade potassium iodate, for the standardization of the sodium thiosulfate, was obtained from Anachemia and vacuum dried at 80 “C for two hours. Thyodene starch indicator was purchased from Fisher Scientific Co. All other chemicals were analytical grade reagents. The dis(8) H. Bode, 2.Anal. Clzem., 144,90 (1955). (9) P. W. Bennett and S. Barabas, ANAL.CHEM.,35, 139 (1963). (10) J. S. McNulty, E. J. Center, and R. M. Macintosh, ibid., 23, 123 (1951). (11) G. Brauer, “Handbook of Inorganic Preparations,” Academic Press, New York, 1965, pp 433, 447-8. (12) M. C. Gardels and J. C. Cornwell, ANAL. CHEM.,38, 774 (1966).
I10
100
90
-
-51 el
80
70
I-
>
E > 2
60
50 I-
z
y
40
a w
30
20
10
0
10
20 30 40 50 60 70 REACTION T I M E I N M I N U T E S
BO
90
Figure 1. Effect of pH on recovery of tellurium(V1) in citratephosphate buffers Temperature
=
80 "CyTe(V1) = 3 X 103M, and KI = 0.2.44
tilled water was de-aerated either by boiling or purging with nitrogen gas. The reaction flasks were 250-ml iodine flasks and a 10-ml class A buret was used for titrations. For routine analytical determinations, a laboratory drying oven capable of maintaining 80 i 2 "C served as the reaction environment while an electric hot plate was used to pre-heat the sample solutions. A Beckman Expandomatic p H meter was used for all p H measurements. General Procedure. Ten milliliter aliquots of de-aerated tellurium(VI), containing 4 to 80 mg Te, and 0.5M citric acid buffer solutions were added to an iodine flask. The contents were rapidly heated to approximately 80 "Cyremoved from the hot plate, and 10 ml of a de-aerated solution containing 2.5 g potassium iodide were quickly added. The flask was immediately purged with nitrogen for ten seconds; stoppered securely, and placed in an 80 "C oven for 30 minutes. Before opening, the sample flask was cooled in an ice bath to room temperature, and the resulting iodine titrated with 0.02N sodium thiosulfate solution to the starch end point.
R E A C T I O N T I M E I N MINUTES
Figure 2. Recovery of tellurium(V1) as a function of temperature The dashed lines indicate tests run without pre-heating the sample solutions, and solid lines with pre-heating. [Te (VI) = 3 X 10-aM, KI = 0.5M and pH = 2.01 IO0
90
80 I
!
70
I-
&
60
W
>
0
a
50
I-
z
w 40 V
a W n 30 20
RESULTS AND DISCUSSION
The iodometric reduction of tellurium(V1) in a citric acid buffer may be expressed by the followjng equation: HeTeOe
+ 31- + 3H+
+
TeOOH+
+ + 4H20. 13-
The resulting triiodide ion concentration depends upon at least five variables; time, temperature, pH, iodide, and tellurate ion concentrations. Nernstian calculations based upon oxidation potentials for 13--31-, Te042--Te032-, and Se042--Se033- couples predict the reductions of tellurium (VI) and selenium(V1) by iodide to be spontaneous. The dependence of tellurium(V1) reductions on p H are presented in Figure 1. The optimum p H range is 2 t o 3. At higher acid concentrations the formation of tellurium metal has been observed as predicted by Tolstikov (13). Reduction (13) V. P.Tolstikov, Zh. Obshch. Khim., 36,987 (1966).
IO
REACTION TIME IN M I N U T E S
Figure 3. Effect of iodide ion concentration on recovery of tellurium(V1) Temperature = 80' C, Te(V1) = 3 X 103M and pH = 2.0
of tellurium(V1) is also quantitative in 0.01N perchloric and sulfuric acids, and the resulting tellurium dioxide will precipitate in these media but in citrate buffers no solid TeOn separates. There is a marked temperature effect on the iodometric reduction of tellurium(V1) which is illustrated in Figure 2. These studies have been carried out in iodine flasks at temperatures up to 80 'C and in 50-ml sealed ampoules at 100 "c. ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
519
Table I. Titration Results of Tellurium(V1) in Presence and Absence of Tellurium(1V) Iodometric Method" Micromoles Micromoles taken found Te(V1) Te(V1) Te(IV) 13- or Te(V1) recovered 35.69 99.78% 0.00 35.77 87.65 100.07% 0.00 87.59 93.75 100.11% 0.00 93.65 134.16 99.91% 0.00 134.29 358.2 99.82% 358.8 0.00 693.0 99.42z 0.00 697.1 0.00
0.00
No. of detmns 3
5 5 3
3 3 3
...
0.20
Av 99.93%
Stddev 87.48 87.48 87.48 0.00
81.24 203.1 406.2 406.2
87.54 87.53 87.52 0.20
0.20% 100.07z 100.06% 100.04 Z
3 3 3 3
...
Av 100.05% Std dev 0.14% Alkalimetric Titration Methodb 0.00 658.5 100.02% 3 658.4 677.5 0.00 678.4 100.13% 3 a Recoveries taken after 30 minutes at pH 2.0 and 80 "C on pre-heated buffered sample solutions. * Method of Edwards and LaFerriere (5). Table 11. Titration Results of Tellurium(V1) by Iodometric Method in Presence of Selenium(V1). Micromoles taken Te(V1) Se(V1) 86.21 90.0 0.00 90.0 86.21 264.0 900.0 86.21 0.00 900.0
Micromoles foundb I or Te(V1) 86.19 0.39 86.05 85.91 3.68
Te(V1) recovered 99.97%
detmns 3
99.81% 99.65%
3 3 3
...
No. of
... 3 Av99.8lz Stddev 0.22% a Recoveries taken after 30 minutes at 80 OC in pH 2.5 citratephosphate buffer solution. 6 Corrected for blanks shown.
During the first fifteen minutes of the reaction, the increase in the reduction rate of tellurium(V1) at 100 "Cis not significantly different from that at 80 "C, but when pre-heated sample solutions are used a t 80 "C, the reaction is quantitative within 20 minutes. This is shown in Figure 2. To avoid air
520
ANALYTICAL CHEMISTRY, VOL. 42, NO. 4, APRIL 1970
oxidation in the practical case, it is recommended that only the sample and not the iodide solutions be pre-heated. The iodide ion concentration effect o n the reduction rate of tellurium(V1) is illustrated in Figure 3. The optimum iodide concentration is 0.5 molar. The blank titration had been minimized by two precautionary steps: (1) all reagent solutions are prepared with freshly boiled or purged water, and (2) the sample flask is purged with nitrogen gas just before it is stoppered for high temperature incubation. Blank titrations are in the order of 0.4 microequivalents. Mixtures of telluric acid and tellurium dioxide dissolved in citrate buffer solution have been used for the recovery of tellurium(V1) in the presence of tellurium(1V) and the results are tabulated in Table I. The recoveries encompass a range of 4 to 80 mg of tellurium. The extension of the upper and lower ranges of total tellurium(V1) to 1 mg and 130 mg, respectively, resulted in quantitative recoveries within 1 % of the content. Tellurium(V1) in the presence of selenium(V1) was determined by analyzing prepared mixtures of telluric acid and sodium selenate. Initial high recoveries of triiodide for these mixtures have indicated the presence of selenium(1V) as a n impurity. This has been confirmed by assaying for selenium(1V) (14), The original selenate salt had 0.21 % of selenium(1V) as impurity while the purified salt had only 0.08 %. The blank of triiodide liberated is reduced by approximately threefold when using the purified salt. This correlation appears to confirm that the iodide oxidation by selenate is due to the selenium(1V) impurity. The blank titration due to selenium(1V) was further reduced by increasing the pH of the reaction solution to 2.5 which is still within the optimum range for the quantitative reduction of tellurium(V1). The results of mixtures containing tellurium(V1) and selenium(V1) are summarized in Table 11. Chloride ion in molar quantities up to 100 times that of tellurium(V1) does not interfere. Nitrate ion a t equimolar quantities does not interfere. Interference due to small amounts (
