Chelation of Platinum Group Metals Complexometric Titration of Palladium WILLIAM M. MacNEVIN
and OWEN H. KRIEGE1
McPherson Chemical Laboratory, The Ohio State University, Columbus 10, Ohio
satisfactory for the control of pH because of the tendency of palladium to form strong complexes with the buffer materials. In general, best results were obtained when the excess of ethylenediaminetetraacetic acid was kept small. In the more concentrated solutions of palladium, the yellow color of the palladium-ethylenediaminetetraacetic acid complex tends to obscure the color of the indicator and a large amount of Eriochrome Black T must be added in order to make the end point visible. At lower concentrations the color change of Eriochrome Black T is blue to pink while at higher concentrations, the color change is from yellow-green to pink. Chloride ion concentrations of 0.01 to 1.0M had no effect upon the titration. The interference of other platinum group metals was studied by adding known amounts of hydrochloric acid solutions of osmium(IV), ruthenium(III), platinum(II), platinum(IV), rhodium(III), and iridium(IV) to the standard palladium solutions. An excess of ethylenediaminetetraacetic acid was added, the pH adjusted to 10, Eriochrome Black T added, and the solutions were immediately back-titrated with the standard zinc solution. In all cases, the solutions containing other platinum metals were
Palladous solutions can be titrated complexometrically by addition of excess ethylenediaminetetraacetic acid The pH with zinc nitrate. followed by back-titration is controlled at 10 ± 1 and Eriochrome Black T is used The titration has been applied to 0.6 as an indicator. to 30 mg. of palladium. Bivalent and quadrivalent platinum do not interfere. Iridium and ruthenium The interfere partially at higher concentrations. titration is not applicable in the presence of rhodium or
osmium.
spectrophotometric determination of palladium has been It depends upon the formation of a yellow complex between ethylenediaminetetraacetic acid and palladous ion. In a continuation of the general study of the chelation of the platinum group metals with ethylenediaminetetraacetic acid, it has been found that the formation of this complex may be used as the basis for a volumetric determination of palladium.' Previous titrations of palladium have involved titration with iodide (1, 7), addition of excess salicylaldoxime followed by oxidimetric titration of excess reagent (4), oxidimetric titration with ferrous sulfate and ceric sulfate (IS), addition of jS-furfuraldoxime followed by oxidimetric back-titration with ceric sulfate (IS), and the acidimetric titration of palladium based on the liberation of hydroxyl ions from the reaction between the palladous cyanide complex, Pd(CN)4--, and excess mercuric oxide (2). Flaschka (3) has also reported the use of ethylenediaminetetraacetic acid in a titration of palladium in which an excess of nickel cyanide is added and the displaced nickel is then titrated with ethylenediaminetetraacetic acid. Direct complexometric titrations for a number of ions using the reagent ethylenediaminetetraacetic acid have been reported by Schwarzenbach and coworkers (9-11). The direct titration of palladous ion with ethylenediaminetetraacetic acid is not applicable in the presence of chloride ions because there is not enough free palladous ion to produce a satisfactory color change with the indicator Eriochrome Black T. In addition, palladous hydroxide precipitates at the pH necessary for the proper functioning of this indicator. Harris and Sweet (S) have modified this procedure by adding an excess of ethylenediaminetetraacetic acid to the metal ion and then back-titrating the excess with zinc nitrate using Eriochrome Black T as indicator. It has been found that the Harris and Sweet modification can be applied to the complexometric titration of palladium. It is applicable to macro and micro quantities of palladium and the precision is better than that claimed by previous authors. Bivalent and quadrivalent platinum do not interfere but iridium and ruthenium interfere at high concentrations. The analytical results obtained in analyses with known amounts of palladium are shown in Table I. The average of the errors in six sets of triplicate determinations ranging from 0.6 to 30 mg. of palladium is 0.2%. The pH of the palladium solution was regulated at 10 ± 1 by the addition of dilute potassium hydroxide so that the indicator Eriochrome Black T would function properly. Buffers were not discussed (8). THE
Present address, Los Alamos Scientific Laboratory, University California, Los Alamos, N. M. 1
Table I.
Complexometric Titration of Palladium
Pd
Error in Mean Mg. %
Pd Found
Mean
Mg. 30.67
Mg. 30.58 30.65 30.53
30.59
-0.08
0.3
21.42
21.32 21.37 21.51
21.40
-0.02
0.1
15.21
15.25
15.25
+ 0.04
0.3
15.27 15.23
6.128
3.064
0.620
6.136 6.132 6.136
6.135
+ 0.007
0.1
3.063 3.061 3.059
3.061
-0.003
0.1
0,621 0.622 0.621
0.621
+ 0.001
0.2
Average
Table II.
0.2%
error
Effect of Other Platinum Metals
on
Complexometric Titration of Palladium
Taken, Mg. 3.064
Added, Mg. 4.7 (Ptw)
Pd Found Mean
Mg. 3.057 3.059 3.062
-0.004
0.1
3.063 3.060 3.059
3.061
-0.003
0.1
3.064
2.0 (IriV)
3.099 3.090 3.094
3.094
+ 0.030
1.0
3.064
1.0 (Rum)
3.146 3.144
3.139
+0.075
2.4
3.134
535
3.060
4.o rpt«)
3.064
of
Error in Mean Mg. %
ANALYTICAL
536
treated in a manner identical with that used for those containing only palladium. The results are shown in Table II. The chloride complexes of bivalent and quadrivalent platinum are sufficiently stable to prevent the reaction of these ions with ethylenediaminetetraacetic acid under these conditions, hence, platinum does not interfere with the complexometric determination of palladium. However, if the titration is made at room temperature, and as rapidly as possible after the ethylenediaminetetraacetic acid is added, ruthenium or iridium does not interfere seriously. The rhodium chloride complex is sufficiently strong to prevent the formation of a rhodium-ethylenediaminetetraacetic acid complex in chloride solution. However, the presence of rhodium seriously interferes in another way with the complexometric titration of palladium. The effect of the rhodium is to increase the amount of zinc solution required for a back-titration, thereby decreasing the apparent concentration of palladium by as much The nature of the reaction is not known. as 50%. Chloride solutions of palladium containing even small amounts of osmium inactivate and decolorize several milliliters of Eriochrome Black T, making the end point almost impossible to detect. It was found that palladium solutions containing more than a trace of osmium could not be satisfactorily titrated complexometrically using Eriochrome Black T as an indicator. RECOMMENDED
PROCEDURE
Add a slight excess of a standard solution of the disodium salt of ethylenediaminetetraacetic acid to a chloride solution of bivalent palladium. Regulate the pH to 10 ± 1 with O.liV potassium hydroxide. Add 5 drops of the Eriochrome Black T indicator solution and titrate with a standard zinc solution until the equivalence point is reached, as shown by the color change from blue or green to pink. REAGENTS
Standard Palladium Solution. A 2.5-gram sample of paliadous chloride, from Coleman and Bell Co., was dissolved in 500 ml. of 0.2M hydrochloric acid. Spectrographic investigation showed only traces of platinum present. This solution was standardized by precipitation of the palladium with dimethylglyoxime followed bv drying at 110° C. (6).
CHEMISTRY
Standard Zinc Solution. A 1.8-gram sample of Mallinckrodt analytical reagent grade zinc oxide was dried for 2 hours at 100° C., dissolved in a minimum amount of 1 to 1 nitric acid, and diluted to 1 liter with distilled water. This solution was standardized by precipitating with ammonium phosphate and weighing the zinc as the pyrophosphate according to the procedure of Vance and Borup (If. Standard Ethylenediaminetetraacetic Acid Solution. A 5.5gram sample of the disodium salt of ethylenediaminetetraacetic acid, analytical reagent grade from Versenes, Inc., was dissolved in 1 liter of distilled water. The molarity of this solution was obtained by a complexometric titration using Eriochrome Black T and the standard zinc solution. Indicator Solution. A 0.1-gram sample of Eriochrome Black T from W. H. and L. D. Betz Co. was dissolved in 50 ml. of distilled water which had been made slightly basic with 3 drops of LV potassium hydroxide. Other Platinum Metals. Reagent grade perosmic acid, ruthenium chloride, platinous chloride, platinic chloride, rhodium chloride, and iridium chloride were used for the study of interferences. Spectrographic analysis showed less than significant amounts of impurities. All solutions were standardized, using modifications of the Gilchrist-Wichers scheme (6). LITERATURE
CITED
(1) Atkinson, R. H., Analyst, 79, 368 (1954). (2) Burriel, F., and Pino Pérez, F., Anales real soc. españ. fís. y quim. (Madrid), 47B, 261 (1951). (3) Flaschka, H., Mikrochim. Acta, 1953, 226. (4) Gabide, M., Bull. soc. chim. Belg., 45, 9 (1936). (5) Harris, W. F., and Sweet, T. R., Anal. Chem., 26, 1649 (1954). (6) Hillebrand, W. F., Lundell, G. E. F., Bright, H. A., and Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., pp. 33883, Wiley, New York, 1953. (7) Kersting, R,, Ann. Chem. Justus Liebigs, 87, 25 (1853). (8) MacNevin, W. M., and Kriege, O. H., Anal. Chem., 26, 1768
(1954). (9) Schwarzenbach, G., and Ackermann, H., Helv. Chim. Acta, 31, 1029 (1948). (10) Schwarzenbach, G., and Biedermann, W., Ibid., 31, 459 (1948). (11) Schwarzenbach, G., Biedermann, W., and Bangerter, F., Ibid., 29, 811 (1946). (12) Syrokomskii, V. S., and Gubel’bank, S. M., Zhur. Anal. Khim.,
4, 146 (1949). (13) Ibid., p. 203. (14) Vance, J. E., and Borup, R. E., Anal. Chem., 25, 610 (1953). Received
for review August 9, 1954.
Accepted December 7, 1954.
Action of TV-Bromosuocinimide on Ascorbic Acid New Titrimetric Method for Estimation of Vitamin C MOHAMED ZAKI BARAKAT, MOHAMED FATHY ABD EL-WAHAB, MOHAMED MAHMOUD EL-SADR
and
Biochemistry Department, Faculty of Medicine, Abbassia, Cairo, Egypt
Estimation
of ascorbic acid with 2,6-dichlorophenolby the presence of interfering acid. substances such as reductones and reductic Slow reducing substances that may be present in biological fluids render the end point less distinct. This defect induced the authors to investigate the estimation of ascorbic acid by jV-bromosuccinimide, the oxidizing action of which might be selective. The fact that ascorbic acid is selectively oxidized by Nbefore other reducing substances bromosuccinimide that may be present provides a reliable titrimetric method of extensive applicability. The proposed method is simple, rapid, and sufficiently sensitive to determine a concentration as low as 7.04 y of ascorbic acid per milliliter. The experimental error does not exceed ±2%.
indophenol is limited
A
VARIETY of bioassay and chemical methods for the determination of vitamin C activity or ascorbic acid content has been developed (4, 11, 12, 17). Although bioassays have the advantage of measuring the summation of chemical entities that possess vitamin C activity, they are time-consuming and expensive, and leave much to be desired in precision. Consequently, bioassays are now confined to use in comparative studies to establish the specificity of chemical methods for determining ascorbic acid in individual products. Chemical methods for the determination of vitamin C are based mainly upon the reducing properties of the vitamin. These methods include titration of an acid extract with iodine, methylene blue, ferricyanide, and 2,6-dichlorophenolindophenol (9, 19, 20). Oxidation of ascorbic acid with the dye 2,6-dichlorophenolindophenol is the most satisfactory and extensively used method (5, 6).
However, the value of the 2,6-dichlorophenol-
