The value 0.6% was subtracted from the slopes observed for all samples of I to correct for the solubility-product effect, even though that exact value applied only to pure samples. However, this is justified since most samples are >97 % pure, and in addition the impurities consumed essentially the same number of moles of acid as the pure compound. This is shown by titration of the sample with perchloric acid; the equivalent weight of impure samples is the same as the pure compound. It should be noted that any hydrochloride salt present in the sample will not show up as an impurity. Solubility analysis with methanolic HC1 as solvent is useful for measuring the purity of samples of I.HC1. The solubilityproduct correction, of course, is not used in this case. In Figure 3 are shown the analyses obtained on pure I, the impure sample shown in Figure 2, and a synthetic mixture of
I and I1 containing 9.5 weight per cent of 11. This mixture gives a slope of 12.7% after correction for the solubilityproduct effect. This is due to the fact, mentioned above, that the I1 forms a dihydrochloride salt in the system and hence the observed weight of 11is increased. When the slope is corrected for the ratio of molecular weights 216/289, the predicted value of 9.5 % is obtained. The molecular weight correction was not applied in analyses of routine samples because the amounts of each impurity were not known. The molecular weight correction was negligible for essentially pure samples such as those usually assayed. Uncorrected slopes of several per cent might be several tenths of a per cent high. RECEIVED for review April 7,1970. Accepted October 5,1970.
Spectrophotometric Determination and Spectrophotometric Titration of Palladium Formula for the Pd(ll)-Nitroso R Complex Orville W. Rollins and Morris M. Oldham Department of Chemistry, U.S . Naval Academy, Annapolis, Md. 21402 VARIOUSAUTHORS (1-6) have reported procedures for the spectrophotometric determination of palladium utilizing the red colored complex formed by this platinum metal and nitroso R. With the exception of Sangal and Dey (6), who measured absorbance of the solutions at 420 mp and who stated that Beer’s law holds in the pH range 1 to 1 1 , all of the procedures referred to above involved measuring absorbance in the wavelength range 500 to 560 mp, and at pH values in the range 2.0 to 6.8. We have found that the Pd(I1)-nitroso R complex is very stable in solution in the pH range 7 to 12 and have extended the spectrophotometric method to include the basic pH region. In addition, we have developed a rapid and accurate volumetric method for determination of palladium by spectrophotometrically titrating the metal in acidic solution with a solution of nitroso R salt. In the chemical literature, there is disagreement as to the formulation for the complex formed by Pd(I1) ions and nitroso R. As a result of solution studies, Sangal and Dey (6) have reported a molar ratio of 2 to 3 for this chelate, without clearly designating which number corresponds to metal or ligand. Bobtelsky and Mayer (7) have reported a molar ratio of ligand to metal of 3 to 1 for the Pd(I1)-nitroso R complex based on results of a titration procedure termed “heterometry.” (1) J. Shamir and A. Schwartz, Talunru, 8, 330 (1961). (2) F. C. Garcia and M. L. Garrido, An. Real SOC.Espan. Fis. Quim. Ser. B, 58, 397(1962). ( 3 ) S . Nath and R. P. Agarwal, Chim. Anal. (Paris), 47(5), 257
(1965). (4) S . P. Sangal, ibid., 48(6), 351 (1966). (5) F.E.Beamish, Talanru, 12, 743 (1965). ( 6 ) S . P. Sangal and A. K. Dey, 2.Anal. Chem., 202 ( 5 ) , 348 (1964). (7) M. Bobtelsky and B. Mayer, Anal. Chim. Acta, 15, 164 (1956). 262
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Figure 1. Absorption spectra of solutions at pH = 4.0 Upper curve. Pd(I1)-nitroso R complex Lower curve. Nitroso R blank
In an attempt to resolve the above mentioned discrepancy, we have studied both acidic and basic solutions containing the Pd(I1)-nitroso R complex. We have also isolated and analyzed the sodium derivative of this complex. All of these studies corroborate one another and show clearly that the ratio of ligand to metal in this red colored complex is 2 to 1 . EXPERIMENTAL
Apparatus. The absorption spectra shown in Figure 1 were obtained with a Cary Model 15 Spectrophotometer. All other absorbance measurements were made with a Beckman Model DU-2 Spectrophotometer. A Sargent portable pH meter equipped with a combination electrode was used to measure pH. Reagents. A standard palladium(1I) solution was prepared by dissolving 0.46792 gram of the metal (obtained from
ANALYTICAL CHEMISTRY, VOL. 43, NO. 2, FEBRUARY 1971
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Foreign ion Pt(IV) Ru(II1) Rh(II1) Ir(I1I) Os(VII1) Au(II1) Mn(I1) Ce(1V) Ce(II1) Y(II1) Zr(IV) MOW) Zn(I1) Cd(I1) Ga(II1) In(II1) Al(II1) M004'w04'-
Table I. Effect of Foreign Ions Added Final Concentration, ppm as PH No Significant Interference 8.0 1.9 HzPtCle 3.1 RuCla 7.8 1.0 Rhz(SO4)a 7.7 8.8 IrCh 8.1 OS04 8.0 18.7 1.1 AuCla 7.6 MnClz 7.6 3.3 Ce(S03~ 8.0 11.5 Ce(NO& 8.1 15.6 Y(NOa)a 7.7 16.5 Zr(S04h 8.0 9.1 4.9 Mg(N0dz 7.8 Zn(N0dz 8.0 6.5 13.5 CdSO4 7.7 Ga(NOd3 7.8 4.0 In(N03) 3 8.1 17.5 1.4 Al(NOa)3 7.5 NaZMoO4 8.0 20.0 NazW04 7.7 16.7
Significant Interferencea Co(I1) COClZ 8.1 1.1 Cu(I1) Cu(N03)~ 7.7 1.8 NiS04 8.0 1.2 Ni(I1) Cr(II1) alum 7.9 1.2 Cr(II1) Fe(II1) alum 8.1 1 .o Fe(II1) The criterion for significant interference was an absorbance reading which varied more than 2.5 from the expected value. 0
Electronic Space Products, Inc., Los Angeles, Calif., having an assay of 99.995% Pd) in nitric acid and diluting to a volume of 100 ml. Other standard solutions were prepared by appropriate dilutions of the aforementioned solution. The concentration of a diluted solution was verified by titration with standard EDTA (ethylenediamine tetraacetic acid) solution according to the method of MacNevin and Kriege (8). The EDTA solution was standardized against a solution of Zn(I1) prepared from ZnO which was assayed gravimetrically (9). The nitroso R salt which was used was obtained from K & K Laboratories, Inc. with a certified purity of 99% minimum. This was verified by potentiometric titration with standard NaOH solution. In a similar titration with a sample of Fisher Certified nitroso R salt, 80% of the calculated acidic hydrogen ions were titrated. The following solutions were used to obtain the desired pH values: 33% sodium acetate, 6M acetic acid, 6 M nitric acid, and dilute sodium hydroxide. Deionized water was used throughout this study. Effect of pH on Formation of the Pd(I1)-Nitroso R Complex. Two milliliter samples from a Pd(II)-HN03 solution, 2.197 X 10-3M with the metal, were pipeted into 100-ml beakers, exactly 5-ml samples of a nitroso R solution (0.5020 gram per 200 ml) were added, the pH was adjusted, and the resulting solutions, after being diluted to approximately 50 ml, were boiled on a hot plate for approximately 2 minutes. The solutions were then cooled to room temperature, diluted to 100 ml, and the absorbance was measured at 500 mp in I-cm quartz cells against water as the reference. The results are depicted as Figure 2. (8) W. M. MacNevin and 0. H. Kriege, ANAL.CHEM.,27, 535 (1955). (9) 0.W.Rollins, Doctoral Dissertation, Georgetown University, 1965: University Microfilm No.66-6056,Ann Arbor, Mich.
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