Polarographic Determination of Palladium RAY F. WILSON and ROBERT C. DANIELS Department
of Chemistry, Texas Southern University, Houston, Tex.
This study was undertaken to investigate the possibility of determining palladium(I1) in ammoniacal medium, and to evaluate optimum conditions for carrying out the determination. A n analytical method has been developed for the determination of palladium in ammonia-ammonium chloride buffer. The precision of the method is less than 1%, when the concentration of palladium is greater than 0.35 millimolar. Palladium was determined after its separation, using dimethylglyoxime, from platinum, rhodium, and iridium. A study was made of the possible interference of certain associated base metals; in general, the polarographic method showed a fair degree of tolerance for these ions. The method, without great loss of accuracy when compared with gravimetric methods, saves time and permits the determination of low concentrations of palladium.
diluted to 500 ml. This solution was standardized by treating 10-ml. aliquots of the stock palladium solution with dimethylglyoxime ( 6 ) ; the precipitate was filtered, washed, dried, and weighed as palladium dimethylglyoximate. The average of triplicate determinations of palladium was 2.31 mg. per ml., the average deviation being 2Z0.003. Test solutions of other platinum elements were prepared a? described by Ayres and Wells (4). Analytical reagent grade chemicals were employed for the preparation of all other test solutions. Triple distilled C.P. grade mercury \vas used in the dropping mercury electrode. A 0 2% gelatin solution for use as a maximum suppressor was prepared by dissolving 0.200 gram of powdered gelatin in 100 ml. of freshly boiled distilled water cooled to 50" C. Two drops of toluene were added as a preservative. Fresh gelatin solutions were prepared weeklr 20
P
P
P TO the present, few polarographic or amperometric studies ( 7 , 1 0 )have been carried out using solutions of the platinum elements. Although colorimetric and emission spectrographic methods (1, 3 ) have been reported for the determination of palladium, the most generally accepted methods now in use are gravimetric. It seemed desirable to investigate the polarography of palladium(II), to establish optimum conditions for the best precision of the method, and to determine the nature of interferences from certain diverse ions on the system. -4graphical method for the precise evaluation of diffusion currents, which are proportional to the concentration of palladium, is presented. Although the precision of this method is not quite comparable to that of the gravimetric methods for the determination of palladium, there is a considerable saving in the time required for the determination. The quantitative determination of the platinum elements has been considered among the more difficult and tedious analytical procedures. Gilchrist and Wichers ( 5 )made a detailed study of the separation of the platinum elements from each other and of their subsequent determination. I n this procedure palladium was precipitated with dimethylglyoxime, and the precipitate was weighed as such, or ignited and weighed as the metal. The timeconsuming technique of gravimetric analysis of this element involves added difficulties if low concentrations are to be employed. Willis (11) carried out preliminary investigations on the polarography of the platinum elements and observed that palladium(I1) produced a well-defined reduction wave in ammonia-ammonium chloride solution using methyl red as a maximum suppressor. In this study the palladium elstem was found to be irreversible and the half-wave potential n as found to be slightly dependent upon the concentration of palladium. The diffusion current was observed to be proportional to the concentration of palladium. I t was the purpose of the present investigation to make a detailed study of the polarographv of palladium(JI), in ammoniacal solution, and to evaluate optimum conditions for carrying out the determination.
I
0
-1.0
-0.5
E.d.aVs.
4.5
-2
S.C.E.,VOLTS
Figure 1. Current-voltage curves 1. 0.0018M palladium(I1) in 0.4M a m m o n i u m chloride buffer
2.
0.0018M palladium(I1) i n 0.4.W acetate buffer
Apparatus. -4 Sargent hfodel XI1 polarograph was used to obtain both manual and photographic recorded data. The H-type polarographic cell used in this study is described b?Kolthoff and Lingane ( 8 ) . This cell contained a saturated calomel electrode and a potassium chloride-agar-fritted-glass disk salt bridge; the entire H-cell was jacketed in water a t 25' 2Z 0.1" C. 411 measurements were made and are reported versus the saturated calomel electrode a t 25' C. Oxygen was removed from the solution with a stream of oxygen-free nitrogen which had b e e n conditioned by passage through a blank solution. The characteristics of the capillary used in this work wit,h a mercury height of 50.8 em. were: in ammonia-ammonium chloride buffer on the horizontal portion of the current-voltage curve, a t - 1.10 volts m = 1.847 mg. set.-', t = 3.70 seconds, and m 2 / 3 t 1 ' ' 3 = 1.8i2 mg.2'3 s e c . ~ ~ . ~ . DEVELOPMENT OF ANALYTICAL PROCEDURE
Polarographic Behavior of Palladium. The polarography of palladium was examined in hydrochloric acid-potassium chloride, sodium hydroxide-disodium hydrogen phosphate, acetic acidsodium acetate, and ammonia-ammonium chloride buffers to determine the applicability of the method for the determination of this element. Preliminary experiments indicated that only the acetate and the ammonium chloride buffers would be suitable for further study. The polarography of palladium was studied in more detail in the ammonium buffer than in the acetate buffer.
EXPERIMENTAL
Reagents and Solutions. Palladium( 11) chloride was obtained from the American Platinum Works. The stock solution of palladium was prepared by dissolving 2 grams of palladium(II1 chloride in distilled water. The solution was treated with 2 ml. of concentrated hydrochloric acid t o prevent hydrolysis and then 904
905
V O L U M E 2 7 , NO. 6, J U N E 1 9 5 5 -
precision, or higher deviations, were observed when the concentration of Std. Pd, palladium was less than 0.35 mW. id, Corrected, pa. Alean Dev. id/C m -M Other platinum elements examined 7.10 L25 1.35 1.25 1.29 0.067 1.25 1.40 0.182 1.25 n i t h the exception of platinum(1Vj did 2.55 2.58 2.55 2.56 0.021 7.05 2 54 2.53 0.363 2.58 3.80 3.83 3.87 3.83 0.029 7.03 3 79 3.83 0.545 3.83 not give polarographic waves under the 5.12 0.033 7.05 5.17 5.08 5.11 5.11 5.15 0.726 6.11 7.00 6.36 0.016 6.37 6.37 6.36 6.37 6.37 0.908 6.33 same conditions employed for the deter7.04 12.77 0.053 12.75 12.79 12.75 12.76 12.72 1.815 12.87 mination of palladium Platinum(1V) 19.11 18 97 0.076 6 97 18.92 18.91 18.98 18.99 2.723 18.92 gave fairly reproducible waves which start a t or near zero volts. Iridium formed a grayish green precipitate in ammonium chloride buffer. Presence of iridium, rhodium, or Typical current-voltage curves, corrected for the residual platinum, in moderate amounts in test solutions which contained current, for 1.82 m X palladium in ammonium chloride and palladium had a marked effect on the diffusion current of palacetate buffers are shown in Figure 1. The half-wave potential ladium. Hence, in ammonia-ammonium chloride buffer palof palladium in 0 . 4 N ammonium buffer is -0.82 volt. When ladium cannot be determined accurately in the presence of platithe concentration of ammonia-ammonium chloride was varied num, iridium, and rhodium. The interferences of rutheniun from 0.08 to 0.2M the half-wave potential changes from -0.78 and osmium were not investigated, since in the standard proto -0.82 volt. The half-wave potential was essentially indecedure for separating the platinum elements ( 6 ) these two elependent of the concentration of ammonium buffer when the conments are quantitatively separated as their volatile tetroxides. centration of ammonia-ammonium chloride exceeded 0.2M; also above this concentration the diffusion current was very little affected by moderate changes in the buffer concentration. These polarographic R aves 11 ere 11 ell defined and reproducible Table 11. Polarographic Determination of Palladium after Dimethylglyoxime Separation when the concentration of gelatin exceeded 0.005%. Measurement of Diffusion Currents. After a preliminary Palladium, Millimoles Taken Found study of the variables involved the following procedure was o 9oa 0.908 adopted for the polarographic determination of palladium. o goa 0 907
Table I.
Diffusion Currents of Palladium in Ammonium Chloride Buffer
Appropriate aliquots of the stock standard solution, to give the final concentration desired, were added to a 25-ml. flask; after the addition of 2 ml. of a 0.2% gelatin solution, 5 ml. of 2,M ammoniaammonium chloride solution were added, then the solutions were diluted t o volume and oxygen-free nitrogen was passed through for 15 minutes. A series of replicate determinations was made, starting n i t h known amounts of palladium ranging from 0.18 to 2.i2 mM. Polarograms were recorded for the above solutions, and the diffusion,currents were measured graphically a t - 1.10 volts and were corrected for the residual current.
20
I:
I .
PALLADIUM, M I L L I M O L E S PER L I T E R
Figure 2. Calibration curve for polarographic determination of palladium These corrected diffusion currents are shown in Table I. A calibration curve for the determination of palladium is shown in Figure 2; the average diffusion current for each concentration has been plotted, so that any scatter of points on the curve represents a measure of the precision of the method. The precision of the method is better indicated by the deviation values (Table I). The precision is of the order expected for inorganic polarographic methods, ranging from about 1 part in 100 to 0.3 parts in 100 over a tenfold change in concentration. Lower
0.908 0 908
0 908 0 908
Interference of associated base metals on the polarographic determination of palladium in ammonia-ammonium chloride buffer was studied by adding equal molar concentrations of the diverse ions, individually, to 0.908 mill solutions of palladium (in the final concentration); these solutions were made up in the usual way. The metallic ions selected for this study were gold(III), silver(1j, copper(II), chromium(III), iron( 111), nickel(II), and titanium(II1). Of these ions silver, chromium, copper, and nickel had no effect on the height or position of the reduction wave for palladium. The half-wave potentials or apparent half-wave potentials of these four ions, with the exception of nickel, were well separated from the half-wave potential of palladium. I n determining the diffusion current of palladium in the presence of nickel it was necessary to make the measurement a t - 1.0 volt rather than - 1.1 volts, because the half-wave potentials of these two elements are about 0.3 volt apart. Equal molar concentrations of either gold or iron had the effect of reducing the height of the diffusion current of palladium by about 15%. In the presence of titanium a dark gray precipitate was produced, and a reduction wave for palladium was not observed. This proposed polarographic method was applied to the determination of palladium after this metal had been separated from large amounts of platinum, rhodium, and iridium using the dimethylglyoxime procedure ( 2 ) . MacNevin and Kriege (9) have determined palladium, after its separation from large amounts of other platinum elements using dimethylglyoxime, satisfactorily employing the photometric method. These authors pointed out that in their method it was necessary only to filter palladium dimethylglyoximate precipitate, dissolve in aqua regia, evaporate t o dryness, and take'up the residue in 0.l.U hydrochloric acid. When palladium was separated from other platinum elements using dimethylglyoxime, it was obswved in this laboratory that palladium could be determined polarographically after the precipitate had been filtered, dissolved in aqua regia, and evaporated to dryness, and the residue taken up in 0.LV hydrochloric acid. This solution was diluted to a known volume with water and appropriate aliquots were taken and made up according to the usual procedure. The data ob-
ANALYTICAL CHEMISTRY
906
LITERATURE CITED
tained from the polarographic determination of palladium after the separation of this metal, using dimethylglyoxime, from platinum, rhodium, and iridium are shown in Table 11.
Ayres, G. H., and Berg, E. W., ANAL.CHEM.,24, 465 (1952).
Ibid.,25, 980 (1953). Ayres, G. H., and Tuffly, B. L., Ibid.,24, 949 (1952). Ayres, G. H., and Wells, W. N., Ihid., 22, 317 (1950). Gilchrist, R., and Wichers, E., J . Am. Chem. SOC.,57, 2565
DISCUSSION
A study of the polarography of palladium in ammonia-ammonium chloride solutions indicates that palladium can be determined accurately over a moderate concentration range The precision of the method is less than 1%, when the concentration of palladium is greater than 0.35 mM. This method can be used to determine palladium after this metal has been separated, using dimethylglyoxime, from platinum, rhodium, and iridium. A brief study of the polarography of palladium in acetic acidacetate buffer showed that the diffusion currents of these waves were proportional to the concentration of palladium and could be used for analytical purposes. The diffusion current constant ( I ) value ( I = id/Cm*/3t*/6) for palladium in 0.431 ammonium chloride buffer a t 25” C. is 3.76.
(1935).
Hillebrand. W. F., and Lundell, G. E. F., “Applied Inorganic Analysis,” p. 278, Tlriley, New York, 1929. Kolthoff, I. AI., and Langer, -I,, J . Am. Chem. Soc., 62, 3172 (1940).
Kolthoff, I. ll.,and Lingane, J. J., “Polarography,” 2nd ed., Vol. 1, p. 362, Interscience, S e w York, 1952. MacNevin, W.ll.,and Kriege, 0. H., .$TAL. CHEM.,26, 1763 (1954).
Toropova, V. F., and Yakovleva, G. S.,Zhur. -4naZ.Khim., 1 , 290 (1946).
Willis, J. B., J . Am. Chem. SOC.,67, 547 (1945). RECEIVED for review July 13, 1954. Accepted January 28, 195.5. vestigation supported by a grant from the Research Corp.
In-
Polarography with Platinum Microelectrodes in Fused Salts EDWARD D. BLACK’ and THOMAS DE VRlES Department o f Chemistry, Purdue University, Lafayette,
Conditions for obtaining polarograms in fused salts with platinum microelectrodes were established. Automatic recording was employed in most experiments. In the eutectic mixture, lithium chloride-potassium chloride between 380” and 450’ C., waves were obtained with a microcathode and a platinum coil anode. The theoretical shape of the cobalt and nickel w-ates was expressible by a linear relation between log ( i d - i) and applied potential. A linear relation between wate height and mole fraction was obtained for dilute solutions of cadmium chloride, cobalt chloride, nickel chloride, lead chloride, zinc chloride, and potassium chromate in the alkali halide melt. Half-wave and decomposition potentials of cobalt chloride and nickel chloride shifted in the expected direction and magnitude. The effect of variation in polarization rate, area of microelectrode, speed of rotation of the electrode, and temperature was examined with respect to polarograms of nickel chloride. The temperature coefficient and energy of activation for the limiting digusion currents of nickel have been determined. In a melt of lithium nitrate, sodium nitrate, and potassium nitratc as solvent, with rotating spherical microcathodes of platinum or silver, a linear relationship between wave height and concentration of copper sulfate was observed.
I
S R E C E 3 T years an increasing interest has been shoan in the study of polarography with fused inorganic salts as solvent media. Sachtrieb and Steinberg, employing the dropping mercury electrode, verified the application of the Ilkovii. equntion to the reduction of nickel(I1) ions in the ternary eutectic of lithium nitrate-ammonium nitrate-ammonium chloride at 125’ C. (6) and to the reduction of cadmium, lead, nickel, and zinc in the eutectic mixture of lithium nitrate-sodium nitratepotassium nitrate a t 160” C. ( 7 ) . A dipping platinum electrode has been adapted by Lyalikov and Karmazin ( 6 ) to the polarographic determination of cadmium, copper, and nickel in potassium nitrate fusions a t 360” C. I n extensions of this work, 1
Present address, Quartermaster Research and Derelopinent Laboratories,
Katick, M a s s .
Ind. Lyalikov ( 4 ) has reported on the analytical use of polarography for a number of cations and anions rvith various fused salts as solvents. The latter included the following salts or mixtures: potassium nitrate, potassium hydrogen sulfate, potassium nitrate with potassium hydrogen sulfate, and potassium nitrate with potassium chloride a t temperatures in the range 360” to 520” C. il measure of success was attained a t temperatures as high as 1000” C. in obtaining polarographic waves for copper in a mixed silicate melt. Also results were claimed by Lyalikov for the use of molten lithium chloride-potassium chloride as solvent, but details in these instances were not given. The ultimate objectives of the adaptation of polarography to fused salt media concern the analysis and/or study of metallurgical slags, molten rocks, and glasses as well as the kinetics of the decomposition of substances a t high temperature. The polarographic technique also offers an approach to the in situ study of corrosion in the presence of molten electrolytes as ne11 as possible means of elucidating the nature of complexes in fused salt media. Since most of the progress to this time has been with relatively simple salt systems, the application of polarography to many of the more complex problems cited above a~ sits further development. EQUIPVENT AND PROCEDURES
The mixture lithium chloride-potassium chloride a t a mole ratio of 3 to 2 has a eutectic point of 352” C. Mixtures in this proportion (31.8 grams of lithium chloride, 37.9 grams of potassium chloride) were prepared by weighing the dry salts, stirring rvell, and immediately placing them in the preheated furnace The container and polarographic cell was a 100-ml. borosilicate glass beaker open to the atmosphere. Prolonged contact with the molten salts resulted in considerable etching and weakening of the glass. The cell holder consisted of a steel cup, 3 inches in diameter and 5 inches deep, in which the beaker v a s suspended by means of a nickel \Tire triangle. Electrical grounding of this steel beaker provided necessary shielding for the electrolysis cell. The platinum microcathodes viere constructed by sealing the wire into a small diameter bulb a t the end of a length of 6-mm. glass tubing. The wire, which was perpendicular to the axis of the tubing. was cut off flush with the glass surface and buffed smooth. Wire sizes employed were Iioe. 24. 27, and 30 (B. and S. gage). I n the studies nith rotating electrodes, the electrical connection was effected with a commutator of platinum foil pressing against a spiral of platinum wire wound on the glass tubing of the electrode. The anode consisted of a platinum wire spiral made from 5 inches of KO.24 wire. The separation of anode and cath-
