Colorimetric Determisatior of Palladium with AI pha-Fu riIdioxime OSCAR MENIS
and
T. C. RAINS
Analytical Chemistry Division, O a k Ridge National Laboratory, O a k Ridge, Tenn.
A spectrophotometric method for the determination of microgram quantities of palladium with a-furildioxime was developed. Palladium forms with the reagent in a 0.1 to 1.4N hydrochloric acid medium, a yellow-colored complex which obeys Beer's law at 420 mp. The colored complex is readily extractable into chloroform, in which medium i t adheres to Beer's law at the 380 m p wave length. In the aqueous phase, in the presence of 10% ethyl alcohol, up to 5 y of palladium per ml. of final volume, the colored complex is stable for 20 minutes, after which time the absorbance decreases slowly. In the organic phase the colored complex is stable for at least 24 hours. This reagent is highly specific for palladium. Platinum group metals and gold do not interfere. In the presence of highly colored ions, extraction with chloroform eliminates all interference, except interference of cyanide ions, which prevent the formation of the palladium a-furildioxime complex. The method is very sensitive, as concentrations as low as 0.1 p.p.m. of palladium can be determined in 5-cm. ,cells. The precision at the optimum spectrophotometric range is within 1%.
Solutions of Diverse Ions. Reagent grade salts were employed in the preparation of solutions of inorganic ions. All interfering cations were added as chlorides except thorium nitrate, lead nitrate, nickelous sulfate, manganous sulfate, and zirconium perchlorate. Sodium salts were used for the anion studies. Other Reagents. All other reagents were analytical grade and were used without further purification. COLOR REACTION
The reaction of palladium with a-furildioxime in an acid medium was found to give a yellow color which develops very rapidly. This Bame yellow color was found to be extractible into chloroform.
I . I
\,
-IN
AQUEOUS SOLUTION
0
F T H E several methods generally recommended for the
determination of microgram quantities of palladium, as indicated in a recent article by Beamivh and McBryde ( I ) , the most sensitive reagent is p-nitrosodiphenylamine ( 5 ) . The method in which this reagent is used requires exact control of p H and, furthermore, its utility is decreased somewhat by the interference of high salt concentration, oxidizing agents, and gold. Recently, Yoe and Kirkland ( 4 ) described a method in which palladium is determined with p-nitrosodiphenylamine after the separation of the palladium from most of the platinum group metals by extraction of the diethyldithiocarbamate salts of these metals. Reed and Banks used a-furildioxime ( 3 ) for the gravimetric estimation of palladium and nickel, and also demonstrated that this reagent can be utilized as a very sensitive, qualitative test for these elements. It was the purpose of this investigation to determine the applicability of the a-furildioxime reaction to the quantitative estimation of palladium in an aqueous solution and in an organic solvent extract. APPARATUS AND REAGENTS
Spectrophotometer. Absorbance measurements were made with a Beckman spectrophotometer, Model DU, in 1-em. cells. The absorption spectrum curves were made with the Warren Spectracord attachment to the Beckman spectrophotometer. Standard Palladium Solution. A standardized stock solution was prepared by dissolving palladium chloride in concentrated hydrochloric acid. The palladium content of this stock solution (2.97 mg. of palladium per ml.) was established gravimetrically by the dimethylglyoxime method ( 3 ) . The standard solutions, which were used in this work, u-ere made by appropriate dilution of aliquots of this stock solution. Reagent Solution. A 1% solution of a-furildioxime was prepared by dissolving 1 gram of the purified reagent in 30 ml. of absolute ethyl alcohol, followed by dilution with water to a volume of 100 ml. The purification procedure as outlined by Reed and Banks (3) for the a-furildioxime reagent was followed in its entirety.
$ 'i'
400
440
-. ..
480
WAVE LENGTH,
* 5hO
?,G
Mp
Figure 1. Molar absorbance index of palladium a-furildioxime
Palladium a-Furildioxime in Aqueous Solution. The yellow complex of palladium a-furildioxime was formed by transferring 59.0 y of palladium (as palladium chloride) to a 25-ml. volumetric flask, and adding successively 1 ml. of concentrated hydrochloric acid, 2.5 ml. of 100% ethyl alcohol, 10 ml. of water, and 1 ml. of reagent solution of a-furildioxime, followed by dilution with water to the calibrated volume. -2s shown in Figure 1, the complex exhibits an absorption peak a t 420 mp, when measured against a reagent blank solution. The purified reagent does not absorb a t this wave length, whereas the unpurified reagent absorbs only slightly. Palradium a-Furildioxime in Organic Solution. The yellow complex of palladium a-furildioxime wae formed by transferring 74.07 of palladium (as palladium chloride) to a 30-ml. separatory funnel, and adding successively 1 ml. of concentrated hydrochloric acid and the requisite amount of a-furildioxime. A 10ml. portion of chloroform is sufficient to remove the palladium complex from the aqueous solution after a 20-second shaking period. The organic layer is removed and collected in a 25-ml. volumetric flask. Approximately 0.5 gram of anhydrous sodium sulfate is added and the extract is diluted to volume with chloroform. The complex, as shown in Figure 1, exhibits an absorption peak a t 380 mp, when measured against chloroform, the reagent blank solution. Stability of Color. To test the stability of the color in the aqueous medium, the colors of solutions containing 15,47, 83, and
1932
1933
V O L U M E 2 7 , NO. 1 2 , D E C E M B E R 1 9 5 5 120 7 of palladium were developed by'the above procedure. Complete color development was reached before measurement of the color intensity could be made, although the absorbance measurements were taken immediately, as well as over a period of several hours. The solutions containing low concentrations of palladium (15 y) were stable for 2 hours, but in solutions containing the higher concentrations of palladium a gradual decrease in absorbance was observed after 20 minutes. The color in the chloroform extract was stable for the 24-hour period tested. Effect of Temperature. The solutions were maintained a t room temperature through the experiment, but it n-as observed that when a test solution was heated to 80" C., the palladium a-furildiosimr precipitated immediately. Effects of Hydrochloric Acid. The effect of hydrochloric acid on the color reaction n n s studied to determine the optimum concentration of this arid. In aqueous solutions it was noted that when the concentration of hydrochloric acid wa8 made greater than 1.45,a marked decrease in absorbance occurred. It was also observed that if the color reagent is added to a strong hydrochloric acid solution, cloudiness occurs. This cloudiness was eliminated by diluting the sample to 20 ml. prior to the addition of the chromogenic reagent. The development of cloudiness in the samples prior to extraction with chloroform did not impair the efficiency of the extraction. If a precipitate of palladium a-furildioxime is formed in the aqueous phase, it is of no significance in the method, since it is readily soluble in the organic phase. Ethyl Alcohol Concentration. I t n-as observed that the colored complex of palladium a-furildioxime precipitates when concentrations of palladium in excess of 30 y were tested. To increase the n-orking range, various concentrations of ethyl alcohol were tried.
Table I. Absorbance Index of Palladium a-Furildioxime Pd, Y
11.8 23.6 35.4 47.2 59.0 74.0
Absorbance Absorbance, Index, A As 142 0.067 145 0.137 0.199 140 141 0.267 0,327 138 138 0.408 AV., 2 141 Std. dev., S 3 0 Coefficient of variation. V 2%
Reagent Concentration. A molar excess of a t least 20 to 1 of reagent over palladium in the aqueous solution is required in order to obtain the maximum absorbance within 3 to 5 minutes. In the presence of a smaller excess of the reagent, the colored complex was slow to develop. Large excess of the reagent, as much as T2M excess, did not affect either the rate or the color intensity of the compound. Similarly, it was found that for extraction of the colored compound with chloroform, a large excess of reagent was required. When solutions were heated on a water bath a t 80" C., however, a 2 to 1M ratio of a-furildioxime t o palladium was sufficient to yield maximum absorbance in the organic phase. EXPERIMENTAL
Calibration Curve for Aqueous Solutions. Aliquots of a standardized, concentrated solution of palladium m-ere transferred into 28-m1. volumetric flasks, to which 1 ml. of hydrochloric acid, 2.5 ml. of ethyl alcohol, and sufficient water to give a 20-ml. volume were added. Then 1 ml. of a-furildioxime solution was added, and the volumetric flask was filled to its calibrated volume with water. The absorbance was measured in a I-em. cuvette within 10 minutes at a wave length of 420 mp against a reagent blank solution. The calibration curve is linear and follows Beer's law. In a range of concentrations from 12 to 71 y of palladium in a volume of 25 ml., as shown in Table I, the coefficient of variation for the absorbance index is 2%.
Table 111. Effect of Certain Cations on Determination of Palladium in Aqueous Solution by a-Furildioxime Method Duplicate determinations. -111 solutions contained 59.0 y of palladium. ?jot in order determined-arranged in order Amount Pd, Present, Found, Element IIg. Y 1 ~ + 4 0.06 59.0 Pt + 4 0.7 59.0 Fe+T+ 2.0 59.0 Cd:4+ 2.1 59.0 os 3.3 59.0 cu++ 3.7 59.0 Sa 3.8 59.0 lJ +6 8.4 59.0 T h +4 25.0 59.0 Rh+'+ 0 8 58.1 58.1 ZrTd 1.9 Pb-2.3 58.1 hlo +6 4.0 59.4 Cr++4.3 59.4 31n 1.0 59.9 Si+1.1 59.9 .ill 2.5 61.6 cot+ 2.5 60.8
of differences.
Differencea Y
7i
0.0
0.0
+
+
-0.9 -0 9 -0.9
-1.6
-1.6 6
-1
+
+ + +
Table 11. Extraction of Palladium cy-Furildioxime in Chloroforma Pd. Y
14.8 29.6 44.4 59.2 74.0
a
Absorbance, Index, Absorbance, A AS 0.133 226 0.265 223 222 0 394 222 0.525 220 0 650 AV., 2 223 Std. dev., S 2 3 Coefficient of variation, V 1%
For method of calculation see Table I.
It was observed that in a 10 (v./v.) 7 0 solution of ethyl alcohol, concentrations as high as 120 y of palladium in a final volume of 28 ml. could be analyzed without the occurrence of precipitation, but higher concentrations of ethyl alcohol only delayed the color formation and did not increase the stability of the color. The presence of ethyl alcohol in the aqueous solution, however, in the extraction procedure is detrimental. Its presence in excess of 1% by volume reduced the efficiencyof the extraction.
z
Ar., 59.2 Std. dev., S 0.3 Coefficient of variation, I: 0 . 6 % a Differences are not statistically significant a t t h e concentration level shown above.
Purified a-furildioxime was used as the chromogenic reagent for the aqueous solution. It was noted that when the purified reagent is used the sensitivity is increased by 5% and does not introduce the necessity for a correction for the absorbance of the reagent blank.
A.
=
A Fb
-where A , = absorbance index A = absorbance b = length of absorbing layer of material, cm. C = concentration in grams/1000 grams of solution Calibration Curve for Organic Solution. Bliquots of the standardized concentrated solution of palladium were transferred into
1934
ANALYTICAL CHEMISTRY
a 30-ml. separatory funnel to which 1 ml. of hydrochloric acid, 1 ml. of a-furildioxime, and 10 ml. of chloroform were added. A chloroform-insoluble stopcock grease was used, prepared from starch and glycerol ( 2 ) . The solutions were shaken for 20 seconds, following which the chloroform phase was removed and collected in a 25-ml. volumetric flask. With 10 ml. of chloroform droplets of chloroform were removed from the aqueous layer and combined with the first extract. The organic phase was freed of traces of water by the addition of 0.5 gram of anhydrous sodium sulfate. The flask was filled to its calibrated volume with chloroform, following which the absorbance was measured in a 1-cm. curvette a t a wave length of 380 mp against a chloroform blank solution. The calibration curve, although not shown, is linear and follows Beer’s law. The data from which this curve can be constructed are shown in Table 11. The coefficient of variation for the absorbance index in the concentration range from 15 to 74 y of palladium is 1%.
photosensitive detector increased. This imposes an upper limit on the quantities of colored ions which can be handled successfully. The palladium complex can be extracted readily from the aqueous phase with chloroform. The absorbance of this compound obeys Beer’s law when measured a t a wave length of 380 mp. The quantity of reagent may be reduced to a 1 to 2 ratio of palladium to a-furildioxime, but then the solution must be heated to allow the complex to form and precipitate prior to the ,extraction. This would indicate that the compound extracted is Pd(C10H704K\T2)2 as reported by Read and Banks ( 3 )in the gravimetric method for estimating palladium. The advantage of the extraction procedure over the aqueous color system is that all interferences, due to the absorbance of colored ions which may be present in the solution, is thus eliminated.
EFFECT O F DIVERSE IONS
To the extent that the effects of other ions were examined (Tables 111, IV, and V), no serious interference \Tas observed.
Table V. Effect of Certain Highly Colored Cations on Determination of Palladium in Chloroform Extracts by or-Furildioxime Method (All solutions contained 74.0 y of palladium)
Table IV. Effect of Certain Anions on Determination of Palladium in Aqueous Solutions by a-Furildioxime Method
Amount Present, bIg. 7.31
Elementn
(All solutions contained 59.0 y of palladium)
Bnion F-
so,-NO,-
c104CN-
.4mount Present, Mg. 7.15 6.90 7.29 7.15 4.0
Pd, Found,
Differencea
Y
Y
58 1 58.8 58.6 58.1 0
0.0 0.2 0.4 0.9 59.0
z
Av., Std. dev., 8 Coefficient of variation,
Pd, Found,
V
0.7%
% ’
1
0.0 0.3 0.7 1.5 100
AV.,
z
Std. dev., S Coefficient of variation,
V
Differevce
Y
Y
%
74.0 5.0 2.7 5.3 1.8
0.0 +10 -1 3 +I 3 -2.2
0.0
73.8 1.4 2%
N o attempt was made to check any ion t h a t whs not highly colored.
Ex6kc!ing
a Except in the case of C I T - , t h e differences are not statistically significant s t the concentration tested.
The precision of the method over the range of palladium concentrations, yielding for spectrophotometric measurement, a minimum relative error is 1 to 2%.
The platinum group metals, including gold, do not form a complex with the reagent. The chlorides of ruthenium, rhodium, iridium, platinum, and gold do, however, form deeply colored solutions, and in their presence a sample blank must be used as a reference solution. The interference due to the presence of highly colored ions is best eliminated by extraction with chloroform, as shown by the data presented in Table V. As shown in Table IV, large quantities of anions can be tolerated with the exception of the cyanide ion.
CONCLUSION
DISCUSSION
In a dilute hydrochloric acid medium, microgram quantities of palladium form a yellow-colored complex with a-furildioxime which obeys Beer’s law when absorbance measurements are made a t 420 mp. In aqueous solutions containing 10% by volume of ethyl alcohol, with 1 to 5 y of palladium per ml., a colored complex is formed which is stable for a t least 20 minutes. After that period a gradual decrease in absorbance is noted. A large excess of the reagent is required for the rapid formation of color. In the case of purified reagents, this large excess can be tolerated, because it does not absorb in the wave-length region around 420 mp. The reagent, under the conditions of this method, is specific for palladium, except that colored ions absorbing a t this wave length present some minor difficulties. The absorbance of the interfering substances can be compensated for by incorporating them in reference solutions. To achieve initial instrument balance, however, either the slit must be widened, the intensity of the light source increased, or the sensitivity of the
a-Furildioxime can be utilized as the reagent in a highly selective method for the spectrophotometric estimation of palladium. Cyanide interferes seriously and must be removed prior to the addition of the a-furildioxime. At relatively low concentrations of substances which yield colored ions in solution, no significant interference occurs. On the other hand, in the chloroform extraction procedure none of the commonly encountered ions interfere even xhen present in relatively high concentrations. The palladium complex is stable for a t least 24 hours in the chloroform extract, whereas in aqueous solutions a gradual decrease in stability occurs after 20 minutes. The precision of the method under the conditions reported herein is within 1%. LITERATURE CITED
(1) Beamish, F. E., and McBryde, W. A. 349 (1953).
E., Anal. Chin. Acta, 9,
(2) Herrington, B. L., and Starr, 11. P., IWDEKG.C H m f . . AXAL. ED.,14, 6 2 (1942). (3) Reed, S. d.,and Banks, C. V., Proc. Iowa Acad. Sci., 5 5 , 267 (1948). (4) Yoe, J. H., and Kirkland, J. M,, ; ~ N L L . CHEM., 26, 1335 (1954). (5) Yoe, J. H., and Overholser, L. G., J . Am. Chem. Soc.. 61, 2058 (1939). RECEIVED for review M a y 24, 1955. Accepted August 26, 1955. Division of Analytical Chemistry, 127th Meeting, ACS, Cincinnati, Ohio, hlarchApril 1955. T h e Oak Ridge h-ational Laboratory is operated b y the Carbide and Carbon Chemicals Co., a Division of Union Carbide and Carbon Corp.. for t h e Atomic Energy Commission. Work carried out under Contract No. TV-7405-eng-26.
