V O L U M E 2 5 , NO. 7, J U L Y 1 9 5 3 respondinglg, a bathochromic shift in the visible band is brought about. Also with increasing tellurium concentration, the band becomes sharper and the maximum optical density per unit concentration becomes greater. The spectral effects resulting from variation in tellurium concentration are discussed in greater detail ( 7 ) . STATISTICAL SU’\IMARY OF REAGENT EFFECTS
For the study of reductmt, gum arabic, and acid effects, H t’tictorial design was used. -4n analysis of variance derived therefrom for the visible band is given in Table I1 for wave length characteristics. -411 three variables show a highly significant effect,, with the reductant decidedly showing the greatest deet tmd acid the least effect. Only one significant interaction is observed-namely, reagent-gum arabic. Because first-order interactions are so small, it is assumed that the second-order interact,ion is a valid estimate of error n.ithin replications. ACKNOWLEDG.MENT
The author wishes to acknowledge the assistance of Albert E. \-nt,ter, Jr., who prepared the electron micrographs and made measurenient.~on them.
1017 LITERATURE CITED
(1) .Auerhach, R., Kulloid-Z., 38, 343 (1926), ( 2 ) C‘rossley, P. B., Analyst, 69, 206 (1944).
(3) DeAIeio, R. H., . k . i L . C H E Y . , 20, 488 (1948). (4) Gans, R., Ann. Physik, 62, 331 (1920). (5) Gribnau, B., Kolloid-Z., 77, 289 (1936); 82, 15 (1938). (6) Gutbier. A.. 2. unoru. Chem.. 32. 91 11902): 42. 174 (1904). , , (7j Johnson’,R: A , , Kwan, F. P:, and Westlake, D.’, ANAL. CHEM.. 25, l O l i (1953). (8) Krishnan. R.S.,Kolloid-Z., 84, 2 (1938). (9) Krishnan, R. S., Proc. I n d i a n Acad. Sci., A5, 407 (193i). (10) Kruyt, H. R.. “Colloid Science,” Vol. I. D. 34. .InistcrcIam. Elsevier Press, 1952. (11) LaRler, V. K., and Dinegar, R. H., J . Am. Cheni. Soc., 72, 4847 (1950). (12) Mie, G., Ann. Physik,25, 377 (1908). (13) Ostwald, Wo., Kolloidchem. Beih., 2, 409 (1911). (14) Rayleigh, Phil.J f u g . , 47, 375 (1899). (15) So:thern, H. K., in Report BR-606, May 31, 1945; cited in Analytical Chemistry of the Manhattan Project,”S e w Tork. AIcGraw-Hill Book Co., 1950. (16) Volkov, S. T., Zacodskaya Lab., 5, 1429 (1936). (17) Zemel, V. S., Ibid., 1433 (1936). RECEIVED for review January 9, 1953. hrcepted .4pril 18, 1953. Prpiented before the Division of Colloid Chemistry at the 123rd Xeeting of t h e A M E M C A Y CHEMIC.AL SOCIETY, Los Angeles. Calif.
Spectrophotometric Determination of Tellurium as Hydrosol IZiLl’II i.JOHNSON, FIZiNCIS P. ICWiN’, AND DON WESTLAKE Department of C h e m i s t r y , C7niz.ersitj of Illinois, Urbana, I l l . ‘l’elluriuni(I\) m a ) he deterniined spectrophotometrically in trace concentrations upon formation of the colored hjdrosol b) reduction to TeO. Sols so produced may exhibit a variet) of colors, according to the conditions of their formation. The color variation arises because t h e absorption band in the tisible is nonspecific!; in particular, i t is due to conservative absorption, simple scattering, which is dependent upon particle dimensions and geometry. Choices of conditions of sol formation and \*ate length for spectrophotometric measurement are iniplicitlj related. Because of variations i n the spectra with changing tellurium concentration, special consideration is demanded for t h e selection of wate length for Beer’s law measurements. The sols are formed b? reduction with hypophosphite and stabilized with gum arabic. By varying t h e amount of hypophosphite, prototype sols of blue, purple, and red areobtained. Red sols prove superior in linearity of Beer’s law relationship, reproducibilitj, and stabilit) against agglomeration. For the blue sols there are marked deviationb from Beer’s law, and the results are more susceptible to small variations in conditions of formation. Purple sols are intermediate i n behavior. Blue or purple sols may be indicated for spectrophotometrj in t h e presence of interferences absorbing in the blue.
T”
LLURIL-31 may be determined spertrophotometrically as the highly colored, soluble iodotellurite complex ( 5 )or as the rlementary tellurium hydrosol. The latter is formed by reduction of tellurite and the method has been applied to tellurium in i
1 Piesent addresa, Department of Biocheniiitr\ l l r d i c a l School Chicago 11, TI1
S o r t h w w t e r n Tnivei,it>
ores (10, 1 2 ). in steels ( b ) , in industrial atmospheres ( 3 , 6, 9 ) , and in biological materials (3, 6). The sol methods have also been described by Shakhov ( 7 ) and Southern (8). The optical properties of tellurium sols vary greatly according to the size anti geometry of the sol particles; thus they are dependent upon the conditions of formation of the hydrosol. The conditions for forming a variety of tellurium hydrosols have been presented and the relationships between particle characteristics and optical properties discussed ( 4 ) . It is the purpose of the prePent investigation to produce sols of different spectral t1q)es ant! to study the suitability of each type for spectrophotomrtry, with particular reference to the Beer’s law relationship, stability, and interferences. The reducing agent used is hypophosphorous arid and the protective colloid is gum arabic. FORMATION OF HYDROSOLS
T o simplify and systematize the study, conditions are chosen to form prototypes with the following colors: blue, purple, and red. Amber sols may also be produced, but they are not suitaljle for quantitative work because their visible and ultraviolet ban(l. are not separate and exhibit anomalous optical density behavior. A s many conditions as possible arc’ held constant for the production of all sols: Preliminary dilution is made so that the volume in which the sol develops is always 40 ml. The reaction mixture containing all reagents but the reducing agent is heated to the boiling point. The reagent is then added rapidly from a pipet with vigorous stirring and the mixture is allowed to digest for 15 minutes a t or very near the boiling point, then cooled in a water bath a t room temperature for 15 minutes before being diluted to final volume (50 ml.). I n all cases, the reaction mixture is made to contain 0,3y0gum arabic. This amount achieves a high degree of stability of the sol, but avoids excessive foam production found when larger nmounts of gum arabic are used.
ANALYTICAL CHEMISTRY
1018 I n order to maintain the acidity a t a definite level in all tests, the hypophosphite reagent is added as a buffer of p H 2 (1 to 1 mole ratio of hypophosphorous acid to hypophosphite). At p H 2, there is a tendency for the sols to fade after formation, a tendency which becomes more pronounced a i t h decrease in tellurium concentration and with increase in excess reagent (hence, greater in red than in blue sols). T o eliminate this effect, all sols, after cooling, are acidified with sulfuric acid to about 0.2 S [ H + ] . Properly acidified, the sols are very stable.
wave length, using a blank prepared as above but omittlng tellurium. The standard curve is prepared, using 2 to 14 ml. of 50 p.p.111. tellurium standard. VISIBLE BAND
Sols with maximum optical density between 550 and 375 nip were tested for Beer’s law relationship. For bluer sols-Le., maxima above 550 mp-the reproducibility falls off rapidly because the optical properties are more sensitive to small variations in conditions of formation and in particle size. The relatively large particles of blue sols are more easily agglomerated. For amber sols-Le., maxima below 375 mp-there is overlapping of the visible and ultraviolet bands with anomalous variation in optical density. I n Figure 1 is shonn a series of spectra from sols of varying tellurium concentration. For the visible band, three notable features are apparent:
14
L
I Mx)
Figure 1.
-I I
I
550 500 WAVE LENGTH M,U
I
450
I
400
Effect of Tellurium Concentration on Visible Band of Purple Sols
The single variable used for the attainment of different prototype sols is the amount of hypophosphite added. Blue sols are produced by the sloiv reaction taking place when smaller amounts of reducing agent are added. Purple and brown sols are produced by adding greater amounts of hypophosphite. Specifically, the three prototype sols treated in Table I are formed in media of approximately the following hypophosphite concentrations: Blue Purple Red
0.06 M
0.12 M 0.25 M
INSTRUMENTS USED
Cary Model 11 recording spect’rophotometer, and Beckman Model DU spectrophotometer.
The band is broad and flat a t the lowest concentrations and becomes narrower and sharper as the tellurium concentration is increased. At the wave length of maximum optical density, the extinction coefficient becomes slightly greater as the concentration is increased. The band undergoes a bathochromic shift with increasing tellurium concentration. This combination of effects demands special attention for the selection of wave length for Beer’s law measurements. Best compensation of the second effect is achieved if measurements are made a t a wave length intermediate for the maxima-e.g., for the 10 p.p.m. maximum. -4t this wave length the extinction coefficients for the greater concentrations are somewhat less than their maximum values. Because the bandE of the smaller concentrations are relatively broad and flat, their optical densities are not much affected by small shifts in wave length. Extinction coefficients measured a t the intermediate wave length are thus brought into rather good agreement with each other. With regard to the critical selection of the wave length for Beer’s law measurements, it is emphasized that sols obtained in different laboratories apparently using the same procedure may
REAGENTS USED
Hypophosphite buffer, 4 M , is made by dissolving 21 grams of Baker’s C.P. sodium hypophosphite monohydrate and diluting with 21 ml. of Mallinckrodt purified 50% (9.5 M ) hypophosphorus acid and water to 100 ml.; 40 ml. of USP 30% ( 5 M ) hypophosphorous acid may be substituted for the purified reagent. Gum arabic, 4% in water, is made by dissolving 4 grams of gum arabic powder (Schaar and Co.) in 100 ml. of hot water and centrifuging to remove large particulate matter. Sulfuric acid, 2 N. Tellurium standard, 50 p.p.m. of tellurium in 0.2 -17 hydrochloric acid. Potassium tellurium hexabromide (0.537 gram) is prepared and purified as directed by Archibald ( I ) , dissolved in 100 ml. of 4 A- hydrochloric acid, and diluted to 2 liters. PROCEDURE
The follomkg procedure produces red sols and is recommended for tellurium determinations. Experimental sols of other colors described in this paper were prepared by suitably modifying the amount of hypophosphite added as indicated above.
To a solution containing 0.1 to 0.7 mg. of quadrivalent tellurium in an Erlenmeyer flask, add 3 ml. of 4% gum arabic and sufficient distilled water to make the volume, after addition of hypophosphite reagent, 40 ml. Heat to boiling. While rapidly swirling the mixture, add rapidly from a pipet 3 ml. of 4 M hypophosphite buffer, Allow to digest near the boiling point for 15 minutes, cool in a bath of tap water for 15 minutes, and add 5 ml. of 2 N sulfuric acid. (In less exact application of the method, 4 M hypophosphorous acid may be substituted for the hypophos hite buffer, in which case it is not necessary to acidify with s u h r i c acid.) Transfer to a 50-ml. volumetric flask and dilute to the mark. Read the optical density a t an appropriately determined
P.P. M. TELLURIUM
Figure 2.
Beer’s Law
Blue eo1 measured at 580 m p Red sol measured at 400 mp
V O L U M E 25, NO. 7, J U L Y 1 9 5 3 present small, but significant, differences in wave lengths for the mean about which the concentration series is grouped. The variations arise in differences in techniques and in the use of different lots of reagents, especially hypophosphorous acid, having different actual concentrations although bearing the same conwntration label. Hence, it is recommended that certain orientation runs be made preliminary to choosing the wave length for the standard working curve. Three possibilities are: Spectra in the vicinity of the maxima may be determined and treated as discussed above. The maximum for the 10 p.p.m. sol may be determined and wed as a very good approximation to the “mean value.” Calibration curves may be run a t a series of wave lengths in the vicainity of the maxima and the best one chosen. J2‘ith the xave length for measurement’ appropriately chosen. the data shown in Figure 2 are obtained. Equations representing the data are also given, as det,ermined from linear regression treatmerit. Marked deviations from linearity are observed a t the highest concentrations for blue sols. Deviations from linearity a t the lowest concentrations for blue sols are indicated by the displacement of the zero intercept-Le., a significant constant ttwn in the equation. The red sol appears to be superior with rrspect to adherence to Beer’s law and precision of replications. The slope given for each curve is derived from statistical regression treatment. (Data for 2 p.p.m. of tellurium are not included.) The trend toward decreasing extinction coefficient with shift of sol color from blue to red is not,ed. -4 statistical summary of the precision of the method nnii adherence to Beer’s law is given in Table I. The data are treated as a linear regression of optical density on tellurium concentration, and an analysis of variance is made. Two requirements for t,his analysis are met-h0mogeneit.y of variance of optical density replication exists and the variance of tellurium concentration i.4 The agreement iiegligible relative to that of optical density. within replications (individuals) is strikingly superior for red sols. The deviation from linearity (about regression) is greatest for the blue sols; however, the poor agreement of replications desensitizes the statistical method so that the deviat,ion does not appear at a “significant” level. I n contrast, statistically “highly significant” deviations from linearity are indicated for red sols in spite of a lower “mean square about regression.” I t may be inferred that a linear curve does not do justice to the superior reproducibility of red sols and a calibration line curved to fit the data would bring about significant reduction of the “total” Table I.
Precision of the hfethod in an Analysis of Variance
The error “about reeression” reoresents deviations from Beer’s law (linearity). The error”within repiications” represents errors not attributable t o changes in tellurium concentration-e.g.. errors in pipetting or reading optical density. The “total” error includes both of the aforementioned errors and best represents the error in a determination in which a linear relationship (Beer’s law) is assumed for the data. The mean squares are variance estimates, Sz or cz, for optical densities. The F values compare deviations from linearity with deviations within replications, a high P value indicating nonlinearity. I n the last column. estimates of standard error, S or c, are given for earh classification in units of p p.m. Te. Mean Standard Sum of Degrees of Square Error, Error Squares Freedom (O.D.) F P.P.RI.T e Blue sols About regression 0.00133 4 0.000333 0.30 1.70 Within replications 0.00117 6 0.000196 0.23 Total 0.00250 10 0,000250 0.26 Purple sols 0.37 About regression 0.000242 4 O.OOM)58 0.14 Within replication8 0.000949 6 0.000158 0.25 Total 0.00119 10 0.000119 0.20 Red sols About regression 0.000526 4 0.000107 0.17 Within replica13.5Q tions 0.000048 6 0.000008 0.05 Total 0.000552 10 0.000055 0.12 Statistically highly significant.
1019 error. The “total” precision improves in the shift of sol color from blue to red. The general trends and behavior outlined above are supported by similar sets of data for a purple sol measured a t 475 mp and a red sol measured a t 375 mp. Minor variations in degree of linearit. can doubtless be brought about by varying the wave length a t which readings are made. All indications are, however, that red sols offer the best ultimate all-round precision. STABILITY
-UI sols in approximately 0.2 S sulfuric acid show no significant change up to 10 hours’ standing, a t the end of 15 hours a slight change, about 1% decrease, is observed; after 24 hours the decrease is of the order of 5%. The instability a t pH 2 has been noted in the discussion of acid effects. Larger particles, such as those found in the bluest sols, are more easily agglomerated than stnaller particles. INTERFERENCES
Interferences may be classified as follows: Substances also producing sols with hypophosphite-e.g., selenium, gold, platinum. Oxidizing agents which attack the hypophosphite or telluriume g., halogens, ferric or cupric ion-, and especially nitric acid and higher oxides of nitrogen. Complexing agents. Iodide and thiosulfate are noted particularly as ions complexing tellurium. Chloride has a very weak effect, but when present in larger concentrations, i t adds an extra degree of variability to the determination. Indifferent electrolytes. T o test this effect, potassium, zinc, and aluminum salts of chloride and sulfate were used a t 0.5 M concentrations. Recoveries on red and on purple sols were within the limits of error of the method for all salts tested. The errors were somewhat greater for blue sols, especially for the aluminum qalts. Soluble colored substances must be considered as in all spectrophotometric methods. In the tellurium sol method, this dificulty is met in an unusual way. The spectra of the colored interference is surveyed to determine the region of maximum tranenuttance. Conditions for sol formation are then chosen to produce a sol with maximum absorption in that region. I t is probable that a preliminaq- separation of tellurium will be tiecessarj in most applications of the method @,S, 5-11 ). ULTRAVIOLET BAND
The ultraviolet band has been studied simultaneously with the visible band. I n the ultraviolet, the band is flat, so that choice of Rave length for Beer’s law measurements is not so critical. Contrary to the behavior found for the visible band, the ultraviolet band shifts only slightly with changing conditions, and the extinction coefficient increases, rather than decreases, as the sol color shifts from blue to red. Some excellent Beer’s law curves have been obtained from this band. Some anomalies have also been encountered and a more complete investigation is suggested. LITERATURE CITED
.kchibald, E. H., “Preparation of Pure Inorganic Compounds,” Sew York, John Wiley 8- Sons, 1932. Crossley, P. B., Analyst, 69,206 (1944). Delleio, R. H., -4N.IL. C H E Y . , 20,488 (1948). Johnson, R. A , , Ibid., 25, 1013 (1953). Johnson, R. A , and Kwan, F. P., Ibid., 23, 651 (1951). Kronenberg, M . H., and Setterlind, A. K., What’s h‘ew in I n d . Hyg., 4, KO,2, 11 (1947). Shakhov, A. S., Zavodskaya Lab., 11, 893 (1945). Southern, H. K., R e p t . BR-606,May 31, 1945, cited in “Analytical Chemistry of the Manhattan Project,” c. J. Rodden, editor-in-chief, Sew York, McGraw-Hill Book Co., 1950. Steinberg, H. H., Rlassari, S.C., Miner, A. C., and Rink, R., J . Ind. Hyg. Toricol., 24, 183 (1942). Volkov, S. T., Zavodskaya Lab., 5 , 1429 (1936). Zemel, V. S., Ibid., 5 , 1433 (1936). RECEIVEDfor review January 9, 1953. Accepted April 18,1953. Presented before t h e Division of Analytical Chemistry a t the 123rd Meeting of the -4MERrCAh- CHEMICAL SOCIETY, Los Angeles, Calif.
