Improvement in Spectrophotometric Determination of Palladium with Excess Iodide SIR: In an investigation of the solubility of palladium iodide, a convenient and reliable method was sought to analyze for the total Pd(I1) concentration in aqueous solutions. The method of Fraser et al. whereby an excess of potassium icidide produces the characteristic red color attributed to the PdI4+ species seemed most adaptable to our study (1). In an evaluation of this method, our findings differed in one respect from that reported by Fraser et al. In addition, an abstract of this method as given by Sandell seems to be somewhat in error ( 2 ) . It is the purpose of this note to clear up some of the things we found confusing in the literature pertaining to this method and to present a system that is rapid and capable of good results in determining palladium colorimetrically.
5.00M was prepared by dissolving the salt in distilled water through which nitrogen has been bubbled for 15 minutes. None of our sodium iodide solution exhibited the behavior noted by Fraser et al. for some of their potassium iodide solutions. Ascorbic acid, 5.0% was prepared freshly every two weeks. Procedure. Pipet a suitable aliauot of the standard oalladium solution to give a final cbncentration of from 0.1 to 1.0 mg. P d per liter into a 50-ml. volumetric flask. Add by pipet 5.0 ml. of 10M HC101, 10.0 ml. of 5.00M sodium iodide solution, and 10.0 ml. of 5.0yoascorbic acid. Dilute t o volume and measure the absorbance in a 10.0-cm. cell a t 408 mp us. a blank of the reagents. The range may be extended to 1.0 mg. to 10 mg. Pd per liter with the same concentration of reagents by measuring the absorbance in 1-em. cells. RESULTS AND DISCUSSIONS
EXPERIMENTAL
Apparatus and Reagents. A Beckman model DU Spectrophotometer with 1.00-cm. and 10.0-cm. matched cells was employed fcir all absorbance measurements. Standard Pd(I1) perchlorate solution 0.0100M was prepared from palladium powder, 80mesh J. Bishop and Co. Platinum Works, Malvern, Pa., according to the method described by Sundaram and Sandell ( 3 ) . Sodium iodide solution
I n this work it was found that ascorbic acid was superior to sodium sulfite as a reducing agent for iodine in an acid medium. Fraser and his collaborators recommend 0.03% sodium sulfite to prevent air oxidation of iodide in approximately 2N HzS04. Attempts to reproduce this in our laboratory were not successful. Too little sulfite will not totally reduce the yellow colored triiodide ion; and, if present in excess,
0.7,
I
A
sulfite forms a yellow complex with iodide. It was our finding that when ascorbic acid is present at 0.1% for each 0.005 equivalent of acid in 50 ml., air oxidation of iodide is prevented. Sandell, in his monograph reporting on a recommendation of Fraser et aE., suggests that a solution containing from 0.1 to 5 p.p.m. Pd be made 1% in K I to develop maximum color. This corresponds to 0.06M iodide and is somewhat in disagreement with our data. Figure 1 shows the effect of iodide concentration on the intensity of color developed for a 1-em. light path. No change in the uppermost curve was observed as the iodide concentration was increased from 0.60 through 1.OM. All solutions in Figure 1 were 6.4 p.p,m. in Pd, 10M in HC1O4, and 1.0% in ascorbic acid and the ionic strength was maintained constant a t 2.OM as iodide varied by adding a suitable amount of sodium perchlorate. In subsequent work it was found convenient to work with solutions whose final concentration was 1.OM in iodide over the entire range from 0.1 to 10 p.p.m. Pd. It was observed that palladium must be complexed by iodide before ascorbic acid is added. Addition of the reducing solution first results in the formation of a fine black precipitate, presumably palladium metal. When solutions were treated as outlined above, samples repeatedly gave reproducible readings a t 2 minutes and 50 minutes after the addition of iodide. Our absorptivity a t 408 mp is 1.10 X 104 liter per cm. mole slightly higher than the 1.01 X lo4 liter per em. mole reported by Fraser and coworkers. The method, although not extremely sensitive, is precise and is recommended as convenient for the spectrophotometric determination of palladium in the absence of interfering ions. LITERATURE CITED
(1) Fraser, J. G., Beamish, F. E., MacBryde, W. A. E., ANAL.CHEM.26, 495 (1954). (2) Sandell, E. B., “Colorimetric Determination of Traces of Metals,” 3rd ed., p. 717, Interscience, New York, 1959. (3) Sundaram, A. K., Sandell, E. B., J.Am. Chem. SOC.77,855 (1953). JAMESJ. R’IORROW~ JAMESJ. MARKHAM Chemistry Department Villanova University Villanova, Pa.
WAVELENGTH, mp
Figure 1.
Absorption spectra of 6.4 p.p.m. Pd as iodide concentration varies I. 0.06MI-.
II. O.lOMI-.
111.
0.60MI;
1 Present address, Fischer 6 : Porter, Warminster, Pa.
VOL. 36, NO. 6, MAY 1964
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