Spectrophotometric Determination of Palladium with Bromide Precipitation of Palladium with Dithio-oxamide and Extraction of Palladium Phenylthiourea into Amyl Acetate GILBERT H. AYRES AND BARTHOLOMEW L. TUFFLT T h e University of Texas, A u s t i n , Tex. REIGENTS
The spectrophotometric determination of palladium based upon the orange to red color produced I,> treatment H ith excess bromide is more rapid than the graiimetric methods, and is comparable in accurac? The method should be especially suitable for anal) sis of high-palladium al1o)s; the optimum range for the photometric process, by the technique described, is from about 40 to 2OOp.p.m.of palladium, which is onl? slightl? below the range for the dimeth> Iglyoximc praiimetric method. Palladium can he precipitattd yuantitati, el) with dithio-oxamide f r o m solutions of pII 3. i f t e r the precipitate has been dissol, ed in acids, the palladium is easil) determined spectrophotometricallj with bromide. A separation method has been proposed in which palladium phen?lthiourea in dilute hydrochloric acid is quantitatilel? extracted into amyl acetate. The palladium is then determined bj the bromide niethod. In all the separation methods, interference from gold can be preiented b) prior extraction of gold(II1) from h?drochloric acid solution (abont 6 M> into amyl acetate.
Palladium metal powder m-as used for making the standard solutions. Spectrographic t,est of the sample detected only trace amounts of foreign metals. Baker's analyzed hydrobromic acid, 40%, was used as the colordeveloping reagent. Other common chemicals were analytical reagent grade. .4 0.5y0solution of phenylthiourea (Eastman) was made by dissolving 2.5 grams of the solid in 25 ml. of ethyl alcohol and diluting with m-ater to 500 ml. Solutions for the study of interferences from platinum, rhodium, iridium, ruthenium, osmium, and gold were prepared from the metals or their compounds obtained from various sources: spectrographic tests for purity showed no more than trace quantities of foreign materials.
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EXPER131 ENTh L
A
C ' O l I l I O S method for the determination of palladium involves weighing the precipitate produced by treatment of an acid solurion of palladium(I1) with dimethylglyosime ( 8 ) . Other gravimetric methods involve weighing t h e free metal fornird by the action of strong reducing agents. More recently. palladium h w been determined gravimetrically with salicylaldosinie ((j), with 6-furfuraldoxime (Y), wit,h 1,2-cyclohesanedione diosime ( 7 j. and with phenylthiourea, thiophenol, and thiobad)ituric acid i 2 ) . The gravimetric separations and deterniin:itions of palladium are t,ime-consuming, and are likely t.0 he subject t o errors such as incomplete precipitation of the palladiuni and coprecipitation of other elements. . i n emission qectrographic determination of palladium has bern reported recently by =lyres and Berg ( 1 ) . Yoe and Overholwr ( 9 ) detrrniint~tlpalladium colorimetrically with p-nitrosodiphenylamine: faiv other color tests for palladium have been apl)lird t o photometric analysis. M-ith a high concentration of bromide, palladium (11) forms an orange t o red complex which offris promise as a rpect,rophot'ometric method of analysis. I t was the purpose of this investigation to study the palladium bromide system, t o est,ablish t,he best conditions for color development, to cvaluate t,he opt,imuni range and accuracy of the photometric process, t o determine the nature and extent of intrrferences from diverse ions, and t,o separate palladium, espccially from other platinum metals. 4PP4R4TL.S
Transniittancy measurements were made with a Beckinan Model DU quartz spectrophotometer, using 1.OOO-cm. matched silica cells. The instrument was operated a t constant sensitivity, using slit width> corresponding to noniinal band widths of about 2 t o 4 mg.
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Preparation of Standard Sample. Two grams of palladium metal powder were trrated wit.h nitric acid-hydrochloric acid mixture ( 3 to I) and evaporated almost' to dryness. Perchloric acid was added and the misture was heated t o dense white fumes: several treatment's Tvith perchloric acid were necessary for complete dissolution. Dilution t o 1 liter v i t h 5% perchloric acid gave a solution containing approximat,ely 2000 p,p.m. of palladium(II1, from which working standards were prepared by appropriate dilution. The palladium stock solution x-as standardized by reducing the palladium(I1) in a 25-ml. aliquot n-ith formic. acid. The mixture was boiled for 5 minutes, and digested for 30 minutes, and the cooled solut'ion was filtered t'hrough \\-hatman S o . 42 paper. hfter being washed with 0.1% hydrochloric acid, the paper and metal were ignited with a high-temperature burner. B e h e e n successive ignitions t o bring t o constant weight, several drops of formic acid were added t o ensure complete reduction to metal. The solution x a s also standardized by treating a 25-ml. aliquot of the stock palladium solution with 25 ml. of 2Y0 tlimethylglyoxime; the precipitate was filt'ered, washed, dried, and weighed as palladium dimethylglyoximate. The average of' several determinations by the two methods was 2.090 nig. of' palladium per ml., the average deviation being 3~0.003. Development of Color. I n low concentration, bromide produced with palladium (11) a precipitate of PdBr?, soluble iii excess bromide t o give a n orange to red color. For spec.ti.ophotometric esamination a suitable aliquot of the stock st,antiard solution was treated n-ith a n excess of hydrobromic acid and tliW e d to 100 ml. Transniittancy measurements were matic. against a corresponding blank containing the reagent distilled wat'er was shown to be a suitable blank under most circumstances, the use of a reagents blank has the advantag? of compensating for any free brominc that might be present in 01' formed from the hydrobromic acid used. The spectral curves, for the region 400 t o 650 mp, for several concentrations of palladium are shown in Figure l. Below 400 mp the curves continue t o decrease sharply, the solution being almost opaque at 330 mp! followed by a rapid increase in transmit,tancy a t still shorter wave lengths. The transmittancy minimum a t 505 mp is in some respects more practical than the one a t 330 mg, in that t.he former does not. require a n ultraviolet source nor silica absorption cells, and would be suitable for use with a filter phot.omet,er. The syst,em conforms t o Beer's law up to a t leaPt 200 p.p.m. of palladium. All measurements in the experiment: reported below were made a t 505 mp.
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ANALYTICAL CHEMISTRY
Efiect of Reagent Concentration. For a given concentration of palladium, constant transmittancy was obtained when the amount of hydrobromic acid varied from 1 to 25 ml. of 40% hydrobromic acid per 100 ml. of h a 1 solution; use of the same volumes of a 5070 solution of sodium bromide instead of hydrobromic acid gave the same constant transmittancy. Effect of Acidity. The acidity was varied from 0.1 to 8 M in perchloric acid; for a given amount of palladium, constant transmittancy was observed in all cases.
Table I.
Effect of Diverse Ions
(All solutione, 100 p.p.m. of palladium) Ion Tolerance, P.P.M. Platinum(1V) 100 Iridium(1V) 20 Rhodium(II1) 5 Ruthenium(II1) 0.4 Osmium(1V) 10 Gold 111) 1 Iron&I) 100
500 160 1500 5
: : ! : % \ I i ? Nickel(I1) Chromium(V1) Chromium(II1)
150 ~
~~
Table 11. Determination of Palladium after Dimethylglyoxime Separation Gold Added, Mg.
Palladium Taken,
0 0 3.4 3.4
20.0
5.1
Mg. 20.0 10.0 10.0 10.0
Palladium Found, Mg.
19.9 20.0
9.7 9.7 9.7
of the palladium as a separation from other platinum metals, followed by dissolution of the precipitate and determination of the palladium spectrophotometrically with bromide, might be feasible. Interference from gold might be expected, due to reduction of its compounde to the metal by the reagent (4). Samples of the palladium(I1) solution, acidified with hydrochloric acid, were precipitated with dimethylglyoxime, and the preci itate was filtered and washed. The preci itate was decomposecf with hot nitric and hydrochloric acids, Fumed down with perchloric acid, and diluted to known volume, and appropriate aliquots were taken for color development with hydrobromic acid, as described reviously. Other samples containing both palladium(I1) an$ gold(II1) were treated similarly. From the measured transmittancy, the concentration of palladium was determined by reference to the calibration curve (Figure 2 ) . The presence of gold resulted in low recoveries of palladium. Figure 1. Spectral Curves for Palladium(I1) with Bromide
Effect of Temperature. Between 20' and 30" C . the transmittancy for a given amount of palladium was independent of temperature; hence no attempt was made t o maintain constant temperature within these limits during the transmittancy measurement. Range and Accuracy. Figure 2, in which per cent absorptancy (100 % transmittancy) is plotted against log concentration, shows that the optimum concentration range for the conditions used is about 40 to 200 p.p.m. of palladium; in this range the relative analysis error is 2.7yG per 1% (absolute) photometric error, or about 0.5% relative analysis error for a precision of 0.2% in measuring the transmittancy. Effect of Diverse Ions. Table I shows the tolerance of 100 p.p.m. of palladium for various foreign ions. For this purpose, tolerance is defined as the amount of the foreign substance which will produce a transmittancy not more than 0.4% (absolute) different from the transrnittancy of the palladium system alone. High concentrations of chloride ion increased the transmittancy somewhat, but the effect was eliminated by the use of a larger amount of hydrobromic acid. I n order to ensure maximum color development, the molar ratio of bromide to chloride should be a t least 20 to 1. Separation of Palladium. The rather extensive interference by the other platinum metals and gold indicated the necessity for a separation of these elements from palladium prior to its determination by bromide. PRECIPITATION WITH DIMETHYLGLYOXIME. Palladium is the only platinum metal precipitated by dimethylglyoxime ( 4 ) ; therefore, it appeared likely that dimethylglyoxime precipitation
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Representative data are shon n in Table 11. PRECIPITATION WITH DITHIO-OXAMIDE. Dithio-oxamide produces a precipitate with palladium salts; the precipitation is markedly influenced by the pH of the solution. In order to establish the optimum pH for the precipitation, solutions containing 10.0 mg. of palladium were treated with dithio-oxamide solution and the pH was adjusted to various values. The precipitate was removed, and dissolved as in the dimethylglyoxime method, and the palladium was determined s ectrophotometrically with bromide. The results are shown in l ! able 111.
Table 111. Effect of pH on Precipitation of Palladium with Dithio-oxamide Palladium Taken,
Mg. 10.0 10.0 10.0 10.0 10.0
PH Of Solution 0 2.95
3.0 4.3 10.8
Palladium Found, Mg. 8.2
10.0 10.0 9.8 (Negligible ppt.)
Standard alladium(I1) solutions were treated with 10 ml. of 6 M hydrochyoric acid and 15 ml. of 0.2% dithio-oxamide (solution in glacial acetic acid), and diluted with 25 ml. of water. broug t wise addition of 6 M sodium hydroxide the solution wasBy to pH 3.0 (tested with p H meter). The yellow precipitate was digested for 30 minutes, then filtered and washed with 0.001 M hydrochloric acid. The precipitate was decomposed with the acids as before, and the palladium was determined spectrophotometrically with bromide. In triplicate samples containing 10.0 mg. of palladium, the recovery was 10.0 mg. in each case.
droR
Under the conditions used for the precipitation of palladium, the reagent forms precipitates with platinum, gold, iron, copper,
V O L U M E 2 4 , NO. 6, J U N E 1 9 5 2
95 1
and chromium (111), but not with cobalt, nickel, or the other platinum metals. EXTRACTION OF PALLADIUM-PHENYLTHIOUREA. Young (IO) has shown that small amounts of palladium dimethyglyoxime can be extracted into chloroform. With the amounts of palladium used in the present study, attempts to extract the palladium dimethylglyoxime and the palladium dithio-oxamide into chloroform or amyl acetate resulted in collection of most of the precipitate a t the liquid interface. It waa observed that the slightly soluble palladium-phenylthiourea complex could be extracted quantitatively, from slightly acidic solution, into ethyl acetate or amyl acetate; the latter has the advantage of lower solubility in water.
90t
Extraction of Palladium-Phenylthiourea with Amyl Acetate
P d Taken, Mg.
P d Extracted, Mg./25 M1.
P d Unextracted,
20.92 20.92 20.92 10.46 10.46 10.46
20.5 20.3 19.6 10.3 10.3 10.2
0.16
PAL L A D l U M BROMIDE
Mg./100
MI.
% Extracted 99.8 99.1 99.0 99.4 99.5 99.2
0.71 0.73 0.24 0.18 0.36
Table V. Effect of Diverse Ions in Extraction of Phenylthiourea Mixtures into Amyl Acetate Ion Platinum(1V) Iridium(1V) Rhodium(II1) Ruthenium(II1) Osmium(1V) Gold(II1) Iron(II1) Cobalt(I1) Xickel(I1) Coppe:(II) Chromium(V1) b
+
'
Table IV.
% Extracted (in .4bsence of P d j
Tolerance of 100 P.P.M. Pd, P.P.M.
40
