ANALYTICAL CHEMISTRY
1768 grinding the sample in a slurry of potassium bromide and volatile solvent, one obtains spectra of excellent quality which can be used for quantitative as well as qualitative analysis.
(4) (Jones,S . ,J . Am. Chem. Soc., 74,2681 (1952). (5) Perkin-Elmer Corp., Instrumeut S e w s , 4, S o . 3, 1 (1953). (SI Scheidt, U., and Reinwein, H , Z. .\‘aturforsch., 76, 270
LITERATURE CITED
(7) Stunson.. 11..\I., and Levinske, P. N.. Paper 79. Pittsburgh Conference on -1nalytical Chemistry and Applied Spectroscopy,
(1952). ~~
(1) Inderson, D. H., and hliller, 0. E., J . Opt. SOC.Anzer., 43, 777
(1953). (2) Anderson, D. H., and Woodall. N. B., ANAL. CHEM.,25, 1906
(1953). (3) Harp, W. R., Jr., Stone, H., and Otvos, J. W., Paper 77, P i t h burgh Conference on dnalytical Chemistry and Applied Spectroscopy, 1954.
1954. ( 8 ) Stirnson, .\I. X., and O’Donnell, 11, J., J . Am. Chem. Soc., 74, 1805 (1952). RECEIVED for review M a y 10, 1954. Accepted July 26, 1954. Presented a t t h e Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy. March 1954.
Chelation of Platinum Group Metals Spectrophotometric Determination of Palladium with Ethylenediaminetetraacetic Acid WILLIAM M. MAcNEVINand OWEN H. KRIEGE McPherron Chemical Laboratory, The O h i o State University, Columbus 10, O h i o
Ethylenediaminetetraacetic acid has been shown to form a complex with palladous ion which may be used for the photometric determination of palladium. The ratio A A / A pH (change in absorbance per pH unit) has been found equal to 0.012 in the pH range 5 to 9. A shorter plateau has also been found in the pII range 1.6 to 2.0. Beer’s law is obeyed in both pII ranges. Palladium concentrations from 9 to 180 p.p.m. can be measured. Other platinum metals interfere when present in more than minor amounts.
I
S A general study of the chelation of platinum group metals with ethylenediaminetetraacetic acid (EDTA), it has been found that the yellow complex formed between this reagent and palladous ion is useful in the photometric determination of the palladous ion. Hynes, Yanowski, and Ransford (8) list palladium as one of 32 cations that give “no visible reaction” with ethylenedianiinetetraacetic acid. Pfibil and coworkers (10) state that pall:i(ious ion disturbs the reaction in the reduction of silver ion t)y the ferrous complex of ethylenediaminetetraacetic acid, but do not elaborate on this statement. Tomicek and associates ( 1 3 ) and Beran and associates ( 5 ) have each reported that ethylenediaminetetraacetic acid does not affect the half-wave potential of the polarographic reduction of palladous ion and conclude that complexing does not occur. Since the solutions used by these authors also contained a 50-fold excess of ethylenediamine tartrate, the effect of ethylenediaminetetraacetic acid could easily have been obscured. The only other work relating palladous ion and ethylenediaminetetraacetic acid is that of Flaschka ( 6 ) , who reports the displacement of nickel ions from the cyanide complex by the addition of palladous ion8 and the subsequent titration of the nickel ions with ethylenediaminetetraacetic acid. This laboratory has shown that palladous ion forms a stable chelate with ethylenediaminetetraacetic acid which has masimum absorbance in the near ultraviolet. The ratio of 1 to 1 for palladium to ethylenediaminetetraacetic acid in the complev has been pstablished The ethylenediaminetetraacetic acid complex has also been found more stable than PdCla-- and Pd(OH)a--. A detailed study of this romplex and its stability is the subject of another paper (9). I n the development of the proposed method for the photometric determination of palladlum, studies have been made on the effect of pH, the effect of the concentration of ethylenediamine-
tetraacetic acid, adherence to Beer’s law, the effect of concenti,:ition of palladium, and tlic effect of interfering ions, particularlJt,hosrx of the platinum gi’oup. Figure I shows the absorbance curves for solutions containing 115 p.p.ni. of palladium, added as palladous chloride, and an equivalent arnount of cthylenediaminetetraacetic acid as thr disodium salt. This (*oncentration was chosen because preliminary experiments rhowed that the absorbance with 1 - ( ~ 1 . cells lay in the range of maximum sensitivity. The solutions w r e also adjusted to the pH values shown with potassium hydroside and hydrochloric acid. The ionic strength was finally adjusted to 0.2 by addit.ion of sodium perchlorate. These curves indicate an equilibrium system with an isosbest’ic point at X i 3 nip. Figure 2 shows absorbance curves for the same system in basic solutions. I n the p H range 5 to 9, the absorbance a t its maximum value is constant within 0.05 unit as shown in Figure 3. Also in the p H range 5 to 9, the absorbance masimum occur$ :it a constant wave length, 337 mK. Figure 3 also s h o w that a short plat.eau occurs in the absorption curve in the p H range 1.6 to 2.0. This makes possible t.he measurement of absorbance in acid solution which is advantageous when osmium, ruthenium, iridium, or rhodium is present (Table I).
I
I
250
300
I
I
350 403 450 W A V E LENGTH IN MILLIMICRONS
54x
Figure 1. .4bsorbance Curves for PalladiumEthylenediaminetetraacetic Acid Complex pH range, 0.4 to 6.3
550
1769
V O L U M E 26, NO. 1 1 , N O V E M B E R 1 9 5 4
a less favorable pH range than 5 to 9 unless interfering ions require the use of the acid solution. Interference by other platinum group metals was studied by observing the change in absorbance of standard palladiumethylenediaminetetraacetic acid solutions caused by the addition of known amounts of hydrochloric acid solutions of osmium(IT'), ruthenium( 111), platinum(II), platinurn(IV), rllodium(III), iridium(III), and iridium(IV), prior to the addition of ethylenediaminetetraaceBic acid. I n all cases the salt of the platinum group metal was dissolved in dilute hydrochloric acid and aged for a t least one week One set of solutions ~ m adjusted s t'o pH 6.5 =t 0.5 and another to 1.87 f 0.02. Table I shows the amounts of other platinum group metals that cause not more than a 5% variation in absorbance in the photometric mtmurement of palladium. Ayrcs and Berg ( 2 ) have recently enumerated the optimum conditions for t.he separation of palladium from other platinuni group metals by precipitation with dimethylglyoxime. Ayres and Tuffly ( 3 ) have also used the dimethylglyoxime method to separate palladium prior to colorimetric determination as the
Figure 2. Absorbance Curves for PalladiumEthylenediaminetetraacetic Acid Complex pH range, 9.0 to 13.4
Table I.
-1
Effect of Other Platinum Metals on Colorimetric Determination of Palladium Metal
(Pprmissible weight in milligrani8, giving not more than 5 % error in tlie detection of 1 mg of palladium) I n Acid S o h I n Neutral Soln a t 337 hlr tlt 377 IIfi 1 n 1.0" Osmium ( I V ) 0 02 Ruthenium(II1) 0 03a 1 0 2.0 Platinum(I1) 0 2 0.3 Platinum(1V) 0 05 0.04 Rhodium(II1) 0 2 0.v Iridium(II1) 0.4b 4.0b Iridium (1V) p A precipitate forms in neutral or slightly basic solution on extended aging. T o avoid formation of this precipitate, adjust p H t o region 5 t o 6.5 and complete photometric measurement within 0.5 hour after preparation of solution. b Provided t h a t sufficient ethylenediaminetetraacetic acid is present. ____. .~
.-
~
~~~~~
~-
. .
~
.
I t is apparent that in the p H range 5 to '3, the rate of change of A (absorbance) with pH is about 0.012, and that in the p H range 1.6 to 2.0, the rate of change of A with p H is 0.032. This means that p H must be controlled in earh region according to t.hc accuracy desired in the measurement. Although the ionic strength TYas controlled a t 0.2 in these experiments, no appreciable change in absorbance occurred over a range of ionic strength from 0.05 to 0.4. I n separate experiments, an excess of ethylenediaminetetraacetic acid up to 10 t,imes the equivalent amount wae found to have no effect on the absorbance a t 337 and 373 mp. From this it was concluded that. ethylenediaminetetraacetic acid does not take part in additional complesing with palladium beyond the 1 to 1 ratio. Adherence to Beer's law was tested for neutral solutions containing 6.3 to 98.2 p.p.ni. of palladium. Sufficient ethylenediaminetetraacetic acid was added to each solution to provide an excess of 20% over that needed for the maximum palladium added and the solution x a s then adjusted to p H 5 to 9. Absorbances measured a t 337 mp are plotted in Figure 4, curve 1. When a phosphate buffer was used to control the pH a t 7 , the absorbances were slightly less and are shown in Figure 4, curve 2. A similar set of observations was made a t a p H of 1.87 & 0.02 over a concentration range of 6.3 to 254 p.p.m., and absorbances were measured a t 377 mfi. Data are plotted in Figure 4, curve 3. Good adherence to Beer's law ia apparent in all three cases, except in acidic palladium solutions exceeding 220 p.p.m. as shown in Figure 4,curve 3. The molar absorptivities for the complex in neutral unbuffered, in neutral phosphate buffered, and in acid solutions are 1.26 X 103, 1.20 X IO3, and0.60 X 103, respectively. The sensitivity of absorbance to pI1 in the acid range makes this
0
1
2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3 ^ Y
Figure 3 . Absorbance Maxima for I'alladium-Ethylenediaminetetraacetic Acid Complex as a Function of pH 14
/
b fONCF'TO&TION
OF PALLAQUM IN P P M
Figure 4. Adherence of Palladium-Ethylenediaminetetraacetic Acid Complex to Beer's Law 1. Unbuffered solution, pH 5 to 9 2. Buffered solution, pH 5 to 9 3. Acid solution, pH 1.6 to 2.0
1770
ANALYTICAL CHEMISTRY
Table 11. Spectrophotometric Determination of Palladium after Dimethylglyoxime Separation P d Taken, Pd Found, hlg.
Mg.
1.00 1.00 1.00 1.50 1.50 1.50
0.99 0.98 1.00 1.48 1.48 1.49
bromide complex. Digestion of the precipitate with aqua regia followed by destruction of the remaining organic matter with fuming perchloric acid preceded the colorimetric measurement. Application of this method in this laboratory shows that palladium separated in this manner from large amounts of other platinum metals can be satisfactorily determined by the photometric method proposed in this paper without treatment with perchloric acid to destroy remaining organic matter following the aqua regia treatment. Table I1 shows data obtained in the determination of palladium by this method RECOMMENDED PROCEDURE
Absence of Other Platinum Group Metals. Add an excess of the disodium salt of ethylenediaminetetraacetic acid to a solution containing palladium in chloride solution. Adjust the pH to 7 =!= 1 or to 1.8 zk 0.2 with potassium hydroxide and hydrochloric acid. If neutral solutions are used, adjust the volume so that the concentration of palladium is 9 to 90 p,p,m. If acidic solutions are used, adjust the concentration of palladium to 18 to 180 p.p.m. Measure the absorbance in 1-cm. cells. For acid solutions use 377 mp; for neutral solutions use 337 mp. Presence of Other Platinum Group Metals. Precipitate the palladium with dimethylglyoxime according to the method of Ayres and Berg ( 2 ) . Filter through an unglazed porcelain crucible and dissolve the precipitate in aqua regia, evaporate to dryness, and take up the residue in 0.1M hydrochloric acid. Apply the previous procedure for the determination of palladium. EXPERIMENTAL
Apparatus. Absorbance measurements were made with a Reckman uartz spectrophotometer, Model DU, with 1.000-cm. matched s%ca cells. A constant sensitivity was maintained by use of variable slit widths. p H measurements were made with a Reckman Model H battery-operated meter. In basic solutions a blue-tipped glass electrode was used. Reagents. Palladous chloride was obtained from Coleman and Bell Co., Norwood, Ohio. Spectrographic investigation showed no traces of other platinum metals. Reagent grade perosmic acid, ruthenium chloride, platinous chloride, platinic chloride, rhodium chloride, and iridium chloride mere used for the study of interferences. All solutions were prepared by dissolving the salt in dilute hydrochloric acid and were standardized using procedures from the Gilchrist-Wichers ( 7 ) method. Spectrographic analyses indicated less than significant amounts of impurities. Ethdenediaminetetraacetic acid was obtained as the disodium salt in analytical reagent grade from the F. W. Bersworth Co., Framingham, Mass. Mallinckrodt’s analytical reagent grade tribasic sodium phosphate was used to buffer the palladium solutions in the p H range 5 to 9. Discussion. Existing colorimetric methods for palladium have been discussed critically by Beamish and McBryde ( 4 ) . Yoe and Overholser ( 1 4 ) recommend p-nitrosodiphenylamine for 0.5 to 5 p,p.m. of palladium. pH control a t 2.1 is necessary and oxidizing agents, high salt concentrations, gold, silver, and platinum interfere. Ryan (fa)uses 2-mercapto-4,5-dimethylthiazole for 0.5 to 8 p.p.m., although platinum seriously interferes A41varez ( 1 ) proposed l-nitroso-2-naphtholate, but the color fades and excess reagent must be removed. 6-Furfuraldehyde has been suggested by Rice (11). A heating period is required to develop the color and the absorbance decreases on aging. Gold and excess reagent interfere. Ayres and Tuffly ( 3 ) develop an orange-red product with hydrobromic acid which is useful for 40 to 360 p.p.m.
Residual organic matter must be dest,royed by fuming perchloric acid. Gold and other metals interfere and a blank determination is required to allow for accidental liberation of trace amounts of bromine from the excess bromide present. The use of ethylenediaminetetraacetic acid in the photometric determination of palladium has five advantages over one or another of the previous methods.
It is applicable over a concentration range from 9 to 180 p.p.m. Therefore, this method is accurate in the concentration range (10 to 40 p.p.m.) where other reagents are less sensitive. Organic matter remaining after aqua regia digestion of the dimethylglyoxime precipitate need not be destroyed prior to the development of color. The color develops immediately and does not fade on standing. The concentration of ethylenediaminetetraacetic acid is not critical and the excess may be as large as threefold. Only one reagent is needed for the development of color after the regulation of pH. Beer’s law is obeyed over the whole range of 5 to 200 p.p.m. Limitations to the use of ethylenediaminetetraacetic acid as a developing agent are that the pH must be adjusted and kept within limits based on the accuracy desired in the analysis. Figure 3 shows that the least change of absorbance with p H AA occurs in the regions 1.6 to 2.0 and 5 to 9. The relationship A PH is equal to 0.012 in the neutral range and to 0.032 in the p H region 1.6 to 2.0. Therefore, by regulating the pH it is possible t’o make accurate det,erminations of palladium hp this method. A second limit,ation is that other platinum group metals interfere \\-hen they are present in appreciable quantities. The extent of interference of t.hese metals is shown in Table I. I n the presence of large quantities of interfering plat,inum group metals it, is necessary to separate the palladium by precipitation with dimethylglyoxime prior to the development of color with ethylenediaminetetraacetic acid. This method is primarily suggested for the rapid, accurate determination of chloride solutions of palladium containing only traces of other platinum metals.
-
LITERATURE CITED
(1) Alvarer, E. R., Anales direc. gen. ofic. quim. nacl. ( B u a o s Aires),
2,88 01949). (2) Ayres, G . H., and Berg, E. W., A s . 4 ~CHEM., . 25, 980 (1953). (3) Ayres, G. H., and Tuffly, B. L., Ibid.,24,949 (1952). (4) Beamish, F. E., and McBryde, W. A. E., Anal. Chini. Acta, 9. 349 (1953). (5) Beran, P., Cihalik, J., Dolesal, J., Simon, V.. and Zyka, J., Chem. Listy. 47,1315 (1953). (6) Flaschka, H., Mikrochim. Acta, 1953,226. (7) Hillebrand, W. F., Lundell, G. E. F., Bright, H. d.,and Hoffman, J. I., “Applied Inorganic Analysis,” 2nd ed., pp. 33983, Sew York, John Wiley Br Sons, 1953. ( 8 ) Hynes, W.A,, Yanowski, L. K., and Ransford, J. E., ibfikl-ochemie ver. Mikrochim. Acta, 35, 160 (1950). (9) AIacSevin, W. AI., and Kriege, 0. H., unpublished data. (10) PTibil, R., Dolezal, J., and Simon, V., Chem. L i s t y , 47, 88 (1953). (11) Rice, E. W., ASAL.CHEM.,24, 1905 (1952). (12) Ryan, D. E., Analyst, 76, 167 (1951). (13) Tornicek, O., Cihalik, J., Dolesal. J.. Simon, V.. and Zyka, J., Chem. Listy. 46,710 (1952). (14) Yoe, J. H., and Overholser, L. G., J . A m . C h e n ~ Soc.. . 61, 2058 (1939). RECEIVED for review March 19, 19.54. .iccepted J u n e 21, 1954.
Colorimetric Determination of FluorideCorrection I n the article on “Colorimetric Determination of Fluoride” [Nichols, M. L., and Condo, -4.C., Jr., ANAL. CHEY., 26, TO3 (1954)], the ninth line in the paragraph headed Experimental in the second column should read: “and the standard fluoride solution of 204 p.p.m. of fluoride ion war prepared from reagent grade sodium fluoride.”
