extractant is t h a t it is somewhat soluble in water; however, aqueous solutions can be rapidly and predictably saturated with the solvent. Solutions of all the reagents except hydroquinone are stable for a t least a month; hydroquinone solutions should be prepared fresh daily. All the diverse metal ions studied except cobalt(I1) can be tolerated a t levels above 100 pg. Dichromate interferes and should be converted to chromium(II1) prior t o application of the general procedure. T h e effectiveness of the proposed method in extracting the two chelates from large aqueous volumes should make the
method particularly useful to oceanographers and limnologists. Judicious selection of sample size, extract volume, and cell-path length provides for the analysis of substances with wide and divergent ranges of copper and iron concentrations. Received for review September 26, 1973. Accepted J a n u ary 8, 1974. This research was supported under Grant OH 00324, National Institute for Occupational Safety and Health, Public Health Service, United States Department of Health, Education, and Welfare.
Determination of Palladium by Controlled-Potential Coulometry New Platinum-Working-Electrode Cell for Controlled-Potential Coulometry L. P. Rigdon and J. E. H a r r a r General Chemistry Division. Lawrence Livermore Laboratory. University of Caiifornia. Livermore. Calif. 94550
Palladium can be determined by oxidizing Pd( I I) to Pd(lV) at f0.85 V V S . SCE in a supporting electrolyte of 0 . 2 M NaN3 and 0 . 2 M Na2HP04 at p H 7, then reducing the Pd(lV) to P d ( l l ) at 4-0.125 V. Samples containing 1-10 mg of Pd were analyzed with an accuracy and precision of 0.1%. The electrolysis reaction of the P d ( l V ) / P d ( l l ) couple in this medium is totally irreversible. Five mg of Pd can be determined accurately in the presence of 0.5 mg Ag, 0.1 mg Au or Os, 0.2 rng Ir, 1 0 mg Pt, or >10 m g Rh. Small amounts of Ru, Co, Hg, CN-, and Iinterfere. The procedure was developed using a new platinum-working-electrode cell assembly designed for ease of and moderate cost of construction, as well as good electroanalytical characteristics.
Because of its high accuracy and precision with small quantities of sample, controlled-potential coulometry is ideally suited as an assay technique for the precious metals in alloys and electroplating solutions. Good controlledpotential coulometric assay methods are available for gold, silver, rhodium, iridium, and ruthenium ( I ) ; but no viable procedures have thus far been developed for palladium. Most previous investigations of the determination of palladium by controlled-potential coulometry have been limited to reactions involving t h e electrodeposition of the metal. Takata and Muto ( 2 , 3 ) found that Pd(I1) could be reduced with nearly 100% current efficiency using either an ammoniacal chloride supporting electrolyte and a mercury-working-electrode, or a n acid phosphate electrolyte and a gold electrode. Hydrochloric acid was also tested as a supporting electrolyte; but with t h e acid media and a solid electrode, errors were caused by the absorption of hydrogen in the palladium deposit ( 2 ) . Phillips ( 4 ) has also estimated palladium coulometrically by electrodepoF. E. Beamish and J. C Van Loon, "Recent Advances in the Analytical Chemistry of the Noble Metals," Pergarnon Press, Oxford, 1972, Chap. 6. Y . Takata and G . Muto, B u n s e k i K a g a k u . 1 4 , 259 (1965) Y . Takata and G . Muto, B u n s e k i K a g a k u . 15,862 (1966) G . Phillips, Atomic Energy Research Establishment, Harwell England, personal communication, 1972. A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 6 , M A Y 1 9 7 4
sition on a platinum electrode with sulfuric and hydrochloric acids as supporting electrolytes. Clem and coworkers (5,6) found t h a t azide ion strongly complexes with Pd(IV), lowering the formal potential of the Pd(IV)/Pd(II) couple to a region where this reaction could be utilized for the controlled-potential coulometric determination of palladium (6). For coulometric determinations at solid electrodes, reactions involving soluble products are usually preferred over electrodepositions, which present the possibility of interference by codeposition and problems in the removal of the deposit. Thus, since none of the proposed coulometric procedures suggested for palladium have been subjected to a thorough analytical characterization, and because the Pd(IV)/ Pd(I1) couple in azide medium appeared to be the most promising for further investigation, this reaction has been examined with the goal of developing a useful method. This study was carried out using a new platinum-working-electrode cell assembly t h a t was designed especially for ease of construction, moderate cost, uniformity of electrode potential, and a capability of solution deoxygenation by the gas overflow technique.
EXPERIMENTAL Instrumentation. Circuit details and techniques of the operation of the instrumentation for controlled-potential coulometry have been described previously (7). The integrator was calibrated electrically (7) and its readout voltages were measured with a Yon-Linear Systems Model 484-A digital voltmeter. Current-time curves were obtained by the use of a Pacific Measurements Model 1002 logarithmic converter and a n Electro Instruments Model 400 X-Y recorder. Current signals to the logarithmic converter were filtered with a 1-Hz active filter. Electrolysis Cell. The cell used in this work was designed for genera1 application in coulometry with metal-gauze working electrodes. and is shown in Figure 1. The glass container is a Kontes Glass Co. No. K-333000-241, 50-mm-tall weighing bottle. A cylindrical cavity 3-4 m m deep was formed in the bottom of the cell by pressing a glassblower's 3h-in.-diam. graphite rod into the molten glass. and then carefully annealing the glass. This depression holds a Y2 X ?a-in.-diam. Bel-Art Cat KO.F-37125, Teflon Spinfin. The platinum working electrode was made from a 10.5- X 6.5cm piece of 45.mesh gauze which was folded over in quarters, (5) R . G . Clem and E. H Huffman, Anal. C h e m . . 40, 945 (1968) (6) R G . Clem and W W Goldsworthy, A n a l . C h e m 43, 1230 (1967) (7) J. E. Harrar and E. Behrin. A n a l . Chem.. 39, 1230 (1967).
formed around a 30-mm-diam. mandrel, and then flame-welded t o a length of -1-mm-diam. wire. T h e hole in t h e cell cap for this wire was drilled 0.5-1 m m oversize t o vent the inert gas. T h e reference-electrode salt-bridge tube is a n asbestos-fiber-tipped t u b e available from Colemen Instruments as a No. 3-702 reference electrode reservoir. T h e tip can be drawn out, bent, and cut off as shown for better positioning close to t h e working electrode. T h e salt-bridge tube is filled, in most cases, with the supporting electrolyte of the particular electrolysis. The reference electrode is a miniature, pH-measuring type. In the present work, a Fisher Scientific Co. Cat. No. 13-639-210 S C E was used; its potential was checked occasionally against t h a t of a laboratory-prepared SCE. T h e counter electrode is a coiled length of 0.02-in.-diam. platin u m wire. I t is isolated from t h e sample solution by means of a porous-Vycor (Corning Glass KO. 7930)-tipped tube. The lower, porous section has a n outside diameter of 6-8 m m and a length of 1-2 cm; the upper section is No. 7913 Vycor. Preliminary results indicated t h a t a satisfactory counter-electrode tube can also be made from a section of DuPont Nafion-brand perfluorinated ionexchange-membrane tubing (8) in a Kel-F holder. Fritted-glassdisk tubes would also be suitable, provided solution flow is restricted. With a flow of inert gas over the solution a t a rate of 2 liters/ min ( 4 S C F H ) . deoxygenation is complete in < l 5 min for 15 ml of solution and a stirring rate of 1800 rpm, or 20 ml and 2400 r p m . For the present work, t h e solution was stirred a t 2400 rpm using a Scientific Products Co., Tekstir, Cat. No. S8301 magnetic stirrer, calibrated initially using a stroboscopic tachometer. The magnetic stirrer should be centered and touch the bottom of the cell t o ensure good coupling of the magnet to t h e Spinfin. T o ensure complete oxygen removal, it should be made certain t h a t air bubbles are not trapped within the gauze, or between it and t h e cell wall when filling the cell. The sample port is closed with a short length of -3&-in.-diam. Teflon rod. T h e cell is emptied by inserting a small plastic tube connected to a suction flask into the solution. T h e speed of electrolysis attainable with this cell, and its electrical characteristics, are nearly identical to those of a cell described previously (9, Figure 6). The electrolytic rate constant is -0.02 s e c - l for the reduction of Fe(II1) in 0.5M HzS04 a t +0.24 u s . S C E (electrolysis 99.9% complete in 6 m i n ) . Typical cell transfer-functions are given in Ref. 9. The coaxial placement of t h e counter-electrode tube minimizes variations in the working electrode potential due to geometrical effects, but there still is some nonuniformity of potential because of t h e unsymmetrical stirring ( 1 0 ) .However, the reference electrode tip is placed in the region where t h e rate of mass transfer is lowest, thus the control voltage should not be exceeded a t any point on the working electrode. Reagents. Standard solutions of palladium were prepared from Materials Research Corporation Marz-grade, zone-refined palladium wire. Analysis of this material by emission spectrography, spark-source mass spectrography, vacuum-fusion analysis, and combustion analysis showed t h a t the total impurity level did not exceed 0.0570, with the major impurity being platinum a t -200 p p m . Approximately 0.1- to 1-gram portions of the palladium were first cleaned in hot 6M HC1, rinsed with water, and air dried. Samples were then weighed and dissolved in a mixture of 3-10 ml of concentrated HC1 and 1-3 ml of concentrated H S O 3 on a warm hotplate. After dissolution, 25-50 ml of water was added, and t h e solutions were gently boiled to expel oxides of nitrogen. The solutions were cooled a n d transferred with water to certified volumetric flasks. Calibrated micropipets were used to take aliquots of these standard solutions for coulometric analysis. Supporting electrolyte solutions were prepared from reagent grade N a 2 H P 0 4 and H3P04. and from four different samples of NaN3: Fisher Chemicals "purified": J. T . Baker "practical": M a theson, Coleman and Bell "practical"; and B D H Chemicals Ltd. No. 30111 99%. An investigation showed t h a t the source of the N a S 3 had no effect on the analysis of pure solutions of palladium. T h e chemicals used for interference testing were of reagent grade or comparable quality. "High-purity" grade nitrogen (typically containing 10
Ag (1) As(II1) Au(II1) Bi (111) Br Ce(1II) Ce(1V) CN -
-0.2 >10 -0.5 -0.5 >20
Amount t o cause 0.5% re1 error, mg
1.o 0.1 > O .5
-2
Cr(II1) Cr(V1)
-0.5 -1
Cu(I1) Fe(II1) H g (11)
-0.5
>O . 5
-0.5
>2.5 0.05
I-
>20
0.025 1
0.045
IrC13 IrCls2M n in H3P04
>10
0.30
>10 -2
0.20
Mo(V1)
M o in H?S04
>10
5
Ni(1I) NO?-
NiS04 NaNOr
>20
3
Os(II1) Os(IV) Os(VII1) Pt (11) Pt(1VI Pb(t1) Rh(II1) Ru(II1) Ru(IV)~ Ru(VII1) Sb(II1)
OSCli H?OsCls Os04 in N a O H PtCl? Na?PtC16 P b (NOa)? RhC1sC RuC13 RuC16'RuOa in N a O H ShClj
>10 >10 >10 >20
5
Se(1V) Sn(1V) Te(IV) Ti(1V) V(IV)
H2SeO3 SnC14 Na?SOb HLTeOl T i in HZSO4 V in H?SO1
V(V) w (VI)
V in H2SOIC W in H2S04
SO," -
>20 -0.5 >10 >1
>1 >5 -5 >20 -20 >250 >15 -0.1 >10 >10
>15
Negative bias, prevents Pd(I1) oxidation Quantitative interference, Co(I1) Co(II1)
*
>0.5
Ir(II1) Ir(IV)b MntII)
-4
Negative bias
15 >O .5 >O . 5 0.02
Co(I1)
-5
Remarks
-0.5
0.05
Negative bias, poisons electrode
-
Quantitative interference, Hg(W Hg(O) Quantitative interference, I - tr 1 '?I2 Negative bias, poisons electrode Higher tolerance at E +0.16 V
>4
0.1 0.1 10 10 >O .5 >10 0.01 0.008 0.01
0.2
Negative bias if not removed on oxidation
Electrolysis rate increased Electrolysis rate increased Negative bias, Negative bias, Negative bias, Negative bias, electrode
see text see text see text poisons
>10 >10
>250 >10
>o .1
>10
Initial ppt that slowly dissolves
>10 10
Higher tolerance at E $0.16
=
J\
5 mg Pd, supporting electrolyte: 0.2M NaNs, 0.2M Na?HPOa, pH 7.0. Prepared according to Reference 1, p 39. Dissolved by fusion with K.S.O.. pared by electrooxidation of Ruc1.1a t 1.0 V os. SCE in 2hZ HC1 11,5).Species is possibly a dimer (16).e Oxidized with S?Os?-,Ag.
+
(15) N . I . Stenina. Yu. A. Krylov, and P. K. Agasyan, J . A n a / . Chem. USSR. 21, 1174 (1966)
=
Pre-
(16) L. W . Potts, Thesis, University of Minnesota, Minneapoiis, Minn., 1972; N u d S o . A b s t r . . 27, No. 69 (1973) A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 6, M A Y 1 9 7 4
699
-
-''A
result is a negative bias in t h e coulometry; all but very small amounts of ruthenium should therefore be removed prior to the determination of the palladium. Variation of C u r r e n t US. T i m e d u r i n g Electrolysis. As illustrated in Figure 3, t h e logarithmic graph of the electrolysis current of the reduction of Pd(1V) exhibits a n abnormal downward curvature and, in addition, shows a n unusual behavior a t the beginning of the electrolysis. During a normal determination, when the palladium is oxidized and reduced in the same cell, the reduction. current rises slightly from the initial value to a peak -30 sec after the start of the electrolysis, and then falls a t a n increasing rate. The peak in the current is absent (see Curve B ) when the Pd(1V) is reduced a t an electrode previously equilibrated a t the reduction potential. The peak thus appears to be caused by the platinum oxide layer t h a t forms at the oxidation potential, inhibits the reduction, and is removed as the reduction proceeds. No explanation can be advanced for the increase in the apparent electrolytic rate constant during the reduction. Curvature of the log plots was evident even at the 100-pg level of palladium, and neither the shape nor the magnitude of the curve was changed upon interruption of the electrolysis current. The predominant Pd(I1) species is Pd(N3)4*- a t the relative concentrations of azide and palladium dealt with here (18-21); a t lower ratios of N3-:Pd, a dimeric species, P d ~ ( N 3 ) 6 ~ -has , been identified (12, 21). There is no information in the literature (22, 23) concerning the identity of the Pd(1V)-azide species t h a t are present, and these would have to be known before a n interpretation of the electrolysis behavior could be given, The fact t h a t the overall electrolytic rate constant changes markedly during the electrolysis precludes the use of predictive coulometry (24) as a means of decreasing the time required for a determination; a better approach for this type of determination would be to use a higherspeed cell such as that devised by Clem ( 2 5 ) .T h e new cell used here, however, has proved to be quite convenient, reliable, and accurate; and it should be even better with less complicated electrochemical systems. ACKNOWLEDGMENT Impurity analyses of the palladium metal were carried out by J. W. Fischer. E. G. Walter, and J. R. Stevens. Discussions with Ray G. Clem during the course of this work were very helpful. Received for review September 20, 1973. Accepted December 10, 1973. This work was performed under the auspices of the L.S. Atomic Energy Commission. Reference to a company or product name does not imply approval or recommendation of the product by the University of California or t h e U.S. Atomic Energy Commission to the exclusion of others t h a t may be suitable. (16) F. G. Sherif and K . F Michail, J . inorg. Nucl. Chem.. 2 5 , 99 (1963). (19) R. G. Clem and E . H . Huffman, J. Inorg. Nucl. Chem.. 27, 365 (1965) (20) R. G. Clem and E. H. Huffman, Ana/. Chem.. 37, 86 (1965). (21) W . Beck, W . P. Fehlhammer. P. Poellmann, E. Schuierer, and K . Feldl. Chem 8er . 100, 2335 (1967). (22) F. R . Hartley, "The Chemistry of Platinum and Palladium." Applied Science Publishers. London, 1973, pp 264-5. (23) W. Beck and W. P. Fehlhammer, in "MTP International Review of Science, Vol. 2, Main Group Elements, Groups V and V I , " C. C. Addison and D. B Sowerby, Ed., University Park Press, Baltimore, Md., 1972, pp 253-300. (24) F. B. Stephens, F. Jakob. L. P. Rigdon, and J. E. Harrar, Anal. Chem., 42, 764 (1970) (25) R. G. Clem, Anal. Chem.. 43, 1853 (1971)
700
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 6, MAY 1974
