980
ANALYTICAL CHEMISTRY
beam as the null point. I t is convenient to turn off the sweep (horizontal) amplifier of the oscilloscope during measurement. With the above procedure, it is possible to measure polarographic circuit resistances with different cell arrangements with a precision of 2 2 0 ohms. These visual measurements have the added advantage that they are not subject to the individual experimentor’s sensitivity towards fluctuating sound signals of low amplitude and to noise in the laboratory. They are, therefore,
Table I. Resistance of a Polarographic Circuit Containing Dropping Electrode, Agar Bridge, and Saturated Calomel Electrode (Test solution: 2 millimolar Cd ion in 0.1 N KCl solution, airfree. No maximum suppressor. Temp., 25 =k 0.1’ C. Applied e.m.f.: - 0.600 y. versus S.C.E. Mass of mercury flow 1.47 mg./sec., drop time 5.30 sec. In circuit not connected t o W>eatstpne_ b_ri.ge. AmpIitu@- of. signal across dropping eiecwoae--s.L.fi. in ai1 cases u.8 V.J Frequency of Signal, Resistance, Drop Time, Cycles/SPc. Ohms See. 150 1930 5.13 2 00 1830 5.22 400 1630 5.28 1570 600 2.30 1000 1560 J 28 1500 1550 5.28
obtained in a few minutes time. The data in Table I are typical for a set of resistance measurements over a frequency range of 150 to 1500 cycles per second. The minimum values determined have been adjusted to the average resistances in the life of the mercury drops by multiplying by 4/3, as usual. Table I1 exhibits results obtained in the same manner before and after the introduction of an “ohmite” resistance of 450 ohms into the circuits of two cells. In this case, the actually found minimal resistances are reported. The data show that at frequencies between 600 and E00 cycles per second, the resistance measured is stable -within 1 2 0 ohms and that the drop time likewise is stable and unaffected as compared with the drop time determined under the same conditions of experiment but without the input of an alternating signal. Resistances of known magnitude are accurately indicated by the method when introduced in polarographic cell circuits. The significant deviations in drop time ae well as resistance observed a t frequencies lower than 400 cycles per second confirm the generally known fact that resistance values obtained with the Wheatstone bridge method become inaccurate a t very low frequencies due to the polarization of the dropping mercury electrode by the signal fed into the system. In a separate series of measurements a t a constant frequency of 1000 cycle8 per qecond both the drop time and the resistance remained constant when
Table 11. Recovery of Known Resistance Included i n Circuit of Two Polarographic Cells (Test solution: 2 millimolar Cd ion in 0.1 N KCl solution, airfree. N o maximum suppressor. Applied e.m.f.: 0.600 v. versus S.C.E. Added resistance 450 ohms. Amplitude of signal across dropping electrode-S.C.E. in all cases 0.8 v , ) Frequency of signal, cycles/sec. 150 200 400 600 1000 1500 Cell I Original resistance, ohms 730a 670 620 610 600 Drop time, sec. 4 15 4.30 4.44 4.43 4.43 4.47 Resistor added Resistance, ohms ... . .. 1130 1070 1050 1030 Recovery, ohms ... . .. 460 450 440 430 Cell I1 Original resistance, ohms 1300“ 1060b 950 940 940 930 Drop time, sec. 4.62 4.78 4.90 4.87 4.86 4.86 Rpsistor added Resistance, ohms . . . 1500 1380 1380 1380 1380 Recovery, ohms ... 440 430 440 440 450 ’ Difficult t o measure due to irregularity of wave. b Uncertain t o 2 ~ 3 0 ohms.
-
varying the amplitude of the signal between 0.5 and 1.2 v., measured across the dropping electrode and the saturated calomel electrode. I t was ascertained that the Leeds and Northrup bridge is satisfactory for measurements under the experimental conditions given. A t higher frequencies and for still more precise R values, an impedance bridge would have to be used. ACKNOWLEDGMEKT
The authors are indebted to Otto H. 1Iuller for his helpful suggestions. LITERATURE CITED (1) F u r m a n , N. H . , a n d Biicker, C . E., J . A m . Chem. SOC.,64, 660 (1942). (2) H e y r o v s k y , J., “ P o l a r o g r a p h y , ” Wien, Springer T’erlstg, 1941. (3) IlkoviE, D., Collection Czechoslor. Chem. Conzmuns., 4, 480 (1932). (4) Kolthoff, I. hl., a n d Lingane, J. J., “ P o l a r o g r a p h y , ” S e w York, Interscience Publishers, I n c . , 1941. (5) Lingane, J. J., J . Am. Chem. SOC.,61, 2099 (1939). (6) Meites, J., Ihid.,72, 2293 (1950). (7) Muller, 0. H . , “ P o l a r o g r a p h y ” in Weissberger, A. “Physical M e t h o d s of Organic C h e m i s t r y , ” Vol. I. P a r t 11. pp. 1877-8, N e w York, Interscience Publishers, Inc.. 1949. (8) hliiller, 0. H., “ T h e Polarographic M e t h o d of d n a l y s i s , ” E a s t o n , P a . , Chemical E d u c a t i o n P u b l i s h i n g C o . , 1951. RECEIVED for review Septemher 23, 1952.
Accepted February 9, 1933.
Separation of Palladium from Platinum, Iridium, and Rhodium with Dimethylglyoxime GILBERT H. ATRES AND EUGENE W. BERG’ The Uniaersity of Texas, Austin, Tex. HE spectrographic determination of palladium, platinum, T i r i d i u m , and rhodium, previously reported by the authors ( I ) , was preliminary to a detailed study of the sharpness of separation of these four platinum metals by conventional precipitation methods. The present report concerns the separation of palladium from platinum, iridium, and rhodium. The separation of palladium from osmium and ruthenium was not included, because the separation of the latter two from the other platinum metals, by distillation of their volatile tetroxides, was shown by Gilchrist and Wichers (2) to be strictly quantitative; spectrographic tests 1 Present address, Department of Chemistry, Louisiana State University, Baton Rouge, La.
on the residue from distillation showed neither osmium nor ruthenium. Precipitation by diniethglglyoxime from hydrochloric acid solution is virtually specific for palladium. The method has enjoyed widespread use, but apparently little work has been done toward determining the completeness of the separation. 41though literature reports ( 2 ; 3, p. 278; 4 ) indicate that the precipitation of palladium by dimethylg1yoxime is complete, in most cases the procedures involved separation of palladium from only one or two of the other platinum metals, and only seldom were separations effected from solutions containing all four of these platinum metals. In the cases reported, convincing gravimetric
V O L U M E 2 5 , NO. 6, J U N E 1 9 5 3
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A very small amount of each palladium dimethylglyoximate precipitate was taken for a direct current arc examination, to indicate the presence or absence of platinum, rhodium, or iridAverage % ium; in only the first series of palladium precipitates (first line of Completeness of Separation Weight Taken, M g . Table I ) was none of the three metals detected. The remainder Palla- Plati- RhoPalladium nuin dium Iridium dium Platinum Rhodium Iridium of the palladium dimethylglyoximate precipitates within a given series was dissolved and combined by placing the filtering cruci4 50 10 25 99.50 100.0 100.0 100.0 50 10 25 99.95 99.9049 9 . 7 5 f 99.75 50 bles containing the precipitates in hot aqua regia until dissolution 25 99.97 99.9O-l 100 50 10 99.75+ 99,75+ of the precipitates was complete. After removing and washing the crucibles, the solution was evaporated nearly to dryness then diluted successively to 10, 20, or 30 ml., etc. At each difution stage a portion of the solution was analyzed spectrogra hically data were given; however, either no endeavor was made or inby the porous cup electrode method. The dilution at w&ch the sufficiently accurate methods were applied to examine the pallaspectral lines of the metals (platinum, iridium, and rhodium) last dium dimethylglyoximate [bis(2,3-butanedionedioxomo-N,N’)- appeared was noted, and the amount of each metal was calculated from its known lower concentration limit of detection. The main palladium( IT)] for coprecipitation of the other platinum metals. constituent, palladium, was determined spectrographically by Similarly, the completeness of precipitation of the palladium was the method given previously (1). checked only by the visual observation for no further precipitation when a slight excess of the precipitant was added. In graviRESULTS 4YD DISCUSSION metric precipitation separations it should always be borne in mind The completeness of separation of palladium from the other that close agreement of the amount of constituent found with that elements was calculated from the amount of each element found taken could result from a compensation of solubility losses by in the precipitate and the amount of each element found in the coprecipitation of other substances from solution. filtrate. The results are shown in Table I. The intent of the present work was to analyze all precipitates In the work previously reported ( I ) , the relative analysis error and all filtrates after a separation was performed, in order to arrive in the determinations of the four platinum metals was found to a t a more accurate evaluation of the quantitative nature of the be about 2.8%; hence, the spectrographic analyses for these eleseparation, particularly in the separation of rather small amounts ments as major constituents (palladium in the precipitate, and (5 to 100 mg.) of the metals. platinum, iridium, and rhodium in the filtrate) mould not justify The reagents used, the general technique followed, and calibrathe accuracy given in Table I. However, use of the data from tion curves for the four platinum metals were given previously (1). the lower concentration limit of detection of minor constituents EXPERIMEYTAL PROCEDURE (palladium in the filtrate, and platinum, iridium, and rhodium in the solution of the precipitate) permitted a reliable calculation The procedure used for the precipitation of palladium dimethylglyoximate was essentially that described by Hillebrand and of the values shown, and the differences between lOO.O%, 99.90%, Lundell (3, p. 292). Solutions containing the several platinum and 99.i5% are significant. The figures marked in the metal chlorides, in different proportions, were made approxitable represent cases in which these elements, as minor conmately 3% by volume in concentrated hydrochloric acid; the stituents in the precipitate, were not detectable in the solution palladium concentration was taken so as not to exceed 0.1 gram of palladium per 250 ml. of solution. Each series of tests conmade by dissolving the precipitate, but were detected and estitaining a given amount of palladium (represented by each line in mated by a direct current arc examination of the precipitate; Table I ) was run in quadruplicate. A calculated 10% excess of actual values are therefore somewhat higher than the figures 1% dimethylglyoxime in 95% ethyl alcohol was added a t room given. temperature, the mixture was stirred well and allowed to stand for exactly 1 hour. The precipitate was collected on a mediumThe quantity of palladium found in the combined filtrates of porosity sintered glass filter by suction, and washed four times each series was 0.1 mg. of palladium per liter, which corresponded with cold 3% hydrochloric acid. to approximately 1 X 10-6 mole per liter for the solubility of palPreliminary tests showed that the amount of palladium unpreladium dimethylglyoximate under the conditions of the expericipitated, as well as the amount of the other platinum metals coprecipitated, was so small as to be below the range for the most ment. accurate spectrographic determination by the porous cup elecFrom this study it was concluded that the loss of palladium trode technique ( 1 ) . For this reason, it became necessary to because of the solubility of the dimethylglyoximate precipitate make the examination of the precipitate and of the filtrate for was negligible, except when very small quantities of palladium these small amounts of elements on the basis of a lower limit of detection. For each element, this limit was determined by makwere precipitated. Platinum, rhodium, and iridium appeared ing successive dilutions of a standard solution, and taking a specnot to be coprecipitated with small amounts of palladium, altrogram of each solution by the porous cup electrode method, though with larger amounts of palladium some coprecipitation of noting the concentration at which the characteristic line of the given element nas just detectable. (The spectral lines used were the other elements occurred. There is some indication that a the same as reported previously.) In this way, the lower conslight compensation of errors did exist in the separation that would centration limits of detection for palladium, platinum, rhodium, tend to make gravimetric results appear of somewhat better and iridium were found to be 5,20,10, and 25 p.p.m., respectively. accuracy than that with which the actual separations really were The lower limit of detection was highly reproducible when the same technique, excitation conditions, etc., were used, and was accomplished. sufficiently sensitive to correspond to about 0.05% in terms of ACKNOWLEDGhl EYT “completeness of separation” as listed in Table I. 4 s a further This irivestigation was supported jointly by The University of aid in detecting the small amounts of elements involved, the precipitates from the quadruplicate runs at each concentration Texas and the United States Atomic Energy Commission, under were combined, and the corresponding filtrates were combined, the terms of Contract No. ;2T-(40-1)-1037. for making the spectrographic examinations. The combined filtrates from quadruplicate runs in a series were LITERATURE CITED evaporated nearly to dryness, and then diluted successively to 10, 20, 30 nil., etc. At each dilution stage a portion of the solu(1) Ayres, G. H.. a n d B e r g , E. TV., d x . 4 ~CHEhr., . 2 4 , 465 (1962). tion was analyzed spectrographically by the porous cup electrode (2) Gilchrist, R., a n d Wichers, E., J . A m . C h e m Soc., 57, 2565 method, to determine the volume at which palladium lines were (1936). ju-t detectable. The amount of palladium not precipitated was (3) H i l l e b r a n d . W. F., a n d Lundell. G. E. F., “A%pplied I n o r g a n i c determined by multiplying the volume of solution in which palAnalysis,” New York, J o h n Wiley & Sons, 1929. ladium was just detectable by the lower concentration limit of (4) W u n d e r , AI., a n d T h u r i n g e r , V., 2. anal. Chem., 5 2 , 101. 660 detection, expressed in appropriate units. It was significant that (1913). the same quantity of palladium, within the limits of experimental error, was found in all of the filtrates. The main constituents of RECEIVED for review December 23, 1952. Accepted February 27, 1953. the filtrate (platinum, iridium, and rhodium) were determined Presented before the Division of Analytical Chemistry a t the 123rd Meeting spectrographically by the method reported previously ( 1 ). of the AxrERrcar C H E M I C ASOCIETT, L Loa Angeles. Calif.
Table I.
Separation of Palladium Dimethylglyoximate
“+”
