Spectrophotometric Determination of PaIla dium(II) with Ti n(II) GILBERT H. AYRES and JOHN H. ALSOP 111' The University o f Texas, Austin, Tex.
,The red color produced b y the reaction of palladium(ll) with tin(ll) and mercury(l1) chlorides, while serving as a very delicate spot test for palladium, was found inadequate for spectrophotometric determination of palladium because of poor reproducibility. Reaction o f palladium(ll) with tin(ll) chloride in hydrochloric and perchloric acids gives a green solution suitable for spectrophotometric application in the range o f about 8 to 32 p.p.m. of palladium. With tin(ll) phosphate in perchloric and phosphoric ucids, palladium produces an intense red color suitable for its spectrophotometric determination. The optimum concentration range a t 1 .OO-cm. optical path i s about 0.5 to 2.5 p.p.m. The tin(ll) phosphate method has a higher sensitivity than most of the existing spectrophotometric methods for palladium.
T
orange-red color produced wlieii palladiurn(I1) chloride solution is treated with tin(I1) and mercury(I1) hlorides in hydrochloric acid was icported bv Pollard (7) as a specific qualitative spot test for palladium. Ayrcs and lleycr ( 2 ) observed that vhen tin(I1) chloride mas added tCJ ~)dladium(II)chloride in hydrochloric acid solution, an intense orange-red color doveloped with the first few d r o p of reagent; on further addition to tin(11)solution the color changed to yellowgreen, then to ne:trly black, and final11 to dark green. When an excess of tin(11) chloridc w i s a d d d rapidly, onl) the dark grwn color was obserwd. Thr. present study 15-ar undertaken to tcst these color reactions for possible applicatioii to the spectrophotomdxic determination of palladium. HE
(
APPARATUS AND REAGENTS
Spectrophotometer. Absorbance measurements werc made with a Heckman Model I>U spectrophotonicter, using matched 1.00-em. cells. For the mole ratio studies, in which solutions of high absorbance were formed, matched silica inserts were
Present address, Pan American Pe-
troleum Corp., Tulsa, Okla.
used in the cells t o cleciease t h e optical path. Platinum Elements. Palladium iiietal and other platinum metals or their salts were obtained from A. D Mackay, Inc. As stated by the supplier, the materials were 99.5y0 pure or better. All samples nere checked spectrographically for impurities; only trace amounts of any other platinum element were found. Standard Palladium Solution. Exactly 1 gram of palladium metal powder mas dissolved in aqua regia; the solution was evaporated twice with hydrochloric acid t o destroy nitric acid and was then diluted t o 1 liter. The palladium content was confirmed by veighing palladium dimethylglyoximate (1) obtained from aliquots of the solution. T'olumetric dilution of the stock solution gave working solutions of qpropriatc concentrations. Chloride-Free Palladium Solution.
500
tion was hcated with 10 ml. of perchloric acid (70 to 72%), 10 ml. of concentrated nitric acid, and sufficient %70phosphoric acid to make the final solution 1.6M in the latter. When the mixture had evaporated to a volume of about 10 ml., :L red-brown precipitate ,~ppeared,but it redissolved on continued heating. After vigorous boiling of the perchloric acid ceased, the solution was cooled and diluted to known volume with distilled Tvater. illiquots of this solution were diluted volumetrically for the working solutions. Tin(I1) Chloride Solution. The stock solution was 2 M in tin(I1) chloride and 3.6M in hydrochloric acid. Volumetric dilutions were prepared as needed. The solutions were protected from air oxidation by storage under carbon dioxide. Tin(I1) Phosphate Solution. About 3 grams of C.P. tin metal were dissolved by boiling with 400 ml. of 85% phosphoric acid. K h e n the solution was cooled and diluted t o 1 liter, a white crystalline solid, presumably tin phosphate (SnHP04) ( b ) , separated leaving a solution that was 0.03M in tin(11) a t 25" C. The solution was protected from air oxidation by storage under carbon dioxide. Frequent standardization against potassium permanganate and against iodine over a period of several weeks shorved the solution was stable. Nore dilute solutions were prepared from the stock by volumetric dilution with water saturated with carhon dioxide. Mercury(I1) Chloride Solution.
A
600
700
WAVE LENGTH, rnp
A measured voluinc of the stock soluFigure
.
Absorbance curves
0.05% aqueous solution was prepared from reagent grade mercury(I1) chloride. PALLADIUM(II)-TIN(I1)-MERCURY(l1)-CHLORIDE SYSTEM
Dropwise addition of tin(I1) chloride solution t o palladium chloride solutioii first formed a yellow-orange solution, which changed through brown, purple, blue-green, olive green, and finally t o almost colorless with a large excess of tin(I1). Addition of 1 drop of 0.05yo mercury(I1) chloride caused development of an orange color, which increased in intensity as more mercury(I1) was :itlded up t o about 0.5 ml. Larger amounts of mercury(I1) produced a turbidity of mercury(1) chloride. Ylaximum color developed in about 2 minutes. The absorption maximum occurred at 465 mp (Figure 1, curve 1); the absorbance of a 2 pap.m. palladium solution was about 0.5. Gradual fading of the color was retarded by addition of perchloric acid ( 3 ) ; although the color was stable for a t least an hour, reproducibility was poor. Wide variations in conditions failed t o give satisfactory precision. PALLADIUM(ll)-TIN(l1)-CHLORIDE SYSTEM
Procedure.
After a study of the
VOL. 31, NO. 7, JULY 1959
1135
O.4Y6 (absolute) different from that of a color-developed solution containing 20 p.p.m. of palladium alone. The tolerance for a number of foreign ions, To an aliquot of standard pallaadded in the oxidation states given, are dium(I1) chloride solution in a 25-ml. shown in Table I. Nitrate and sulfate volumetric flask, 10 ml. of mixed acid (2.4M in hydrochloric acid and 2.3M could be present in very large amounts 8 in perchloric acid) were added. The without causing interference. 2 green color was developed by addition Extraction with Organic Solvents. 20 of 2 ml. of 0.5M tin(I1) chloride solu2 There was no evidence of eytraction tion which was 1M in hydrochloric acid. 0.8 from color-developed solutions that The mixture was diluted to volume, and 0.1 contained perchlorate ion. I n the 140 after 30 minutes its absorbance was of perchlorate ion, extiaction absence 150 measured. 16 with ethyl ether, ethyl acetate, amyl acetate, and octyl alcohol (methyl-nThe spectral curve for this system is hexylcarbinol) gave organic layers that shown in Figure 1, curve 2 ; the absorpwere yellow to red, but there remained color development of a fixed amount of tion peak a t 635 mp vias used for in the aqueous phase some green palladium was 0.92M. Lower conmeasurement. The system conforms to product n-hich faded rapidly to yellow. centrations gave low values for the Beer's law; absorbance measurements The octyl alcohol extract was deep absorbance; higher ones gave constant of solutions of 10 different concentrared, but it became turbid after a few absorbance values. tions, ranging from 4.0 t o 40.0 p.p.m. ORDEROF ADDITIOKOF REAGCSTS. minutes. Chloroform, carbon tetraof palladium, gave a specific absorptivity chloride, and xylene did not extract any The reagents were added in the order of 0.0001. per p.p.m.-cm. of 0.0228 colored material. palladium, dilute mixed acids, and The optimum range for measurement in Mole Ratio Tests. Attempts, by tin(I1). Erratic, lorn results m-ere 1.00-cm. cells is about 8 to 32 p.p.m. of the usual spectrophotometric methalways obtained when the palladium palladium. ods, t o determine thP mole ratio of tin solution was added to the tin(I1) Reproducibility. The average abt o palladium in the color reaction were chloride. Turbid samples resulted from sorbance of 14 samples, each containunsuccessful; ratios oT tin to pallathe addition of tin(I1) chloride t o the ing 20.0 p.p.m. of palladium, was dium of less than about 100 t o 1 gave palladium solution if insufficient acid 0.457, with an average deviation of a fleeting green color. only was present. The total volume of 0.003. solution to which the tin(I1) chloride Rate of Color Development. The PALLADIUM(II)-TIN(I1)-PHOSPHATE SYSTEM was added was found t o be important. green color, developed a t room temFor a final volume of 25 ml., addition perature, reached maximum intensity During the study of the palladiumof the tin(I1) chloride reagent to 17 ml. in about 20 minutes. It was constant tin chloride system it I\-as observed or less of the palladium and acid mixture for more than 30 minutes longer bethat palladium(I1) chloride gave a gave constant, reproducible absorbfore gradual fading began. red-violet color when treated with If the volume of palladium Effect of Variables. HYDROGEN ances. tin(I1) chloride in 85% phosphoric and acid mixture was more than 17 ml. IONCONCENTRATION. Total acid in acid, The reaction is a very sensitive when the tin(I1) reagent was added, the range of 1.5 to 2.1M gave absorbspot test for palladium; a single drop low and erratic absorbance values were ance values in agreement with the of 0.1-p.p.m. palladium solution is obtained. standardized method. Lower coneasily detected by addition of 1 drop of RATE O F ADDITIOX O F TIN(II) CHLOcentrations of acid caused the color 0.1M tin(I1) in phosphoric acid. The RIDE. The rate of addition of the to be more unstable, while higher conred color is not developed in the absence tin(I1) chloride was not a critical varcentrations extended the time required of phosphate. iable. Constant, reproducible results for maximum color development beyond Preliminary tests for applying this were obtained by adding the tin(I1) 30 minutes. reaction to spectrophotometric determichloride as slowly as 1 drop per second, PERCHLORATE I O N COXCEXTRATION. nation of palladium showed that the or more rapidly by normal gravity flow The minimum concentration of perred color was reproducible only when from a buret or pipet, provided the chlorate ion to give absorbance in chloride ion was absent and perchlorate solution in the receiving flask was agreement with the standardized proion was present. diluted to volume within 2 minutes cedure was 0.5M. At lower concentraProcedure. A study of the effects after addition of the tin(I1) reagent. tions the green color faded rapidly to of varying concentrations of reagents Solutions not diluted to final volume yellow; higher concentrations extended led t o adoption of the following prowithin this time gave low and erratic the time for full color development ceduie. absorbances. beyond 30 minutes, although the final An aliquot of the chloride-free pallaTEMPERATURE. IT'orking temperabsorbance attained was constant for a dium solution in phosphoric acid was 28' to 35' C. had atures ranging from given amount of palladium. transferred to a 25-nil. volumetric flask. Trx(1I) CHLORIDECOSCESTRATION. no effect on the measured absorbance. Dilute perchloric acid and dilute phosHigh temperature could not be used to Constant absorbance values were obphoric acid nere added, and the color accelerate the color developnient; the tained for tin(I1) chloride concentrations was developed by adding 2.0 ml. of green color changed rapidly to yellow tin(I1) phosphate solution which was of 0.02 to 0.04M in the final solution. 0.0065M in tin(I1) and 1.6M in phosa t elevated temperatures. Lower tin(I1) chloride concentration phoric acid. After the solution was Effect of Foreign Ions. Cations as did not develop the maximum potendiluted to volume, it &-as1.16M in pertheir chlorides, and anions as their tial color for a fixed amount of pallachloric acid and 0 . i i M in phosphoric sodium salts, were added t o samples dium; concentrations in excess of acid. The red color developed t o maxiof 20 p.p.m. (final concentration) of 0.04.V retarded the rate of color developmum intensity within 10 minutes a t palladium, and the standard method ment. room temperature, and was stable for \vas followed to develop and measure CHLORIDE10sCONCENTRATIOX.. In 1.5 hours, after which the solution faded the color. Interference was taken as to yellow and finally to colorless in the absence of chloride ion, no green about 4 hours. The color could be rethe amount of foreign ion which gave color was produced. The minimum stored partially, although not coma transmittance which was more than chloride ion concentration for full
controllable variables, the follon-ing standardized procedure was adoptpd.
1 136
e
ANALYTICAL CHEMISTRY
Table 1. Tolerance of 20.0 P.P.M. of Palladium for Foreign Ions [Tin(II) chloride method] Tolerance, Foreign Ion P.P.51.
pletely, by addition of more tin(I1) reagent. The spectral curve of the colordeveloped solution containing 2.0 p.p.m. of palladium is shown in Figure 1, curve 3. The narrow absorption band has its maximum absorbance a t 487 mp. The system conforms to Beer's lam-. The measured absorbances of solutions containing from 0.5 to 4.0 p.p.m. of palladium varied linearly with concentration, from 0.154 to 1.25. The average specific absorptivity, per p.p.m.-cm., nas0.313 i 0.004. Theoptimum range formeasurementa t 1.00-em. optical path is about 0.5 t o 2.5 p.p.m. of palladium; in this range the results are reliable to about 0.6% relative error. Reproducibility. At the 2.0-p.p.1~1. concentration level (A. = 0.622), the absoibance of 12 solutions was reproducible t o a n ayerage deviation of 0.001 and a range of 0.006. Effect of Variables. ORDER O F MIXINGREAGENTS.Addition of the palladium solution t o the tin(I1) solution, lather t h a n the reverse, consistently gave lower absorbance and poor reproducibility. REAGENT CoxcEwRATIoN. The absorbance for a fixed amount of palladium was found to be dependent upon the concentration of tin(II), perchloric acid, and phosphoric acid. The effects were studied by varying the concentration of one reagent while maintaining the other two constant a t the concentration specified in the color-developing procedure described earlier. The results are shown in Table 11. At the lolTer concentrations of tin(I1) reagent, the color m-as stable for about 30 minutes; with increasing concentration of tin(I1) the stability was increased, but somcwhat a t the expense of sensitivity. Perchloric acid concentrations belon 0.5M and phosphoric acid concentrations greater than 2.V produced unstable solutions. Thtb color system n as not sensitive to moderate changes in reagent concentration around the values selected for the standardized procedure. TEhfPERATURE. 50 significant difference in absorbance mas produced by temperature variations in the range of 25' to 35' C. At elevated temperatures the red solution changed t o yellon, more rapidly as the temperature n as increased; at 80' C. the change from red to yellow occurred within 5 minutes. Effect of Foreign Ions. ChloIidcfree solutions of t h e metal ions, in 1-11 perchloric acid, mere taken along n i t h palladium (1.0 p.p.m. in t h e final solution), and t h e color was developed by the standardized procedure; nbsorbancc was measured at 487 nip. The tolerance (not more than 0.4% absolute transmittance difference) of the palladium syskm for various ions,
Effect of Reagent Concentration on Absorbance (at 487 Mp) Developed with 2.0 P.P.M. Palladium HC104 &PO* Tin( 11) Concn., Concn., Concn., M Absorbance M Absorbance M X lo3 Absorhanw
Table II.
0.46 0.93 1.16" 1.39 1.86 2.32 a
0.475 0.552 0.558 0.565 0.587 0.596
0.51 0.64 0.77" 1.41 2.06
0.546 0.553 0.558 0.589 0.336
0.41 0.54 0.68" 1.36 2 .04
0.595
0.590 0.558 0.503 0.471
Conditions chosen for standardized procedure.
Table 111. Tolerance of 1.0 P.P.M. of Palladium for Foreign Ions [Tin(II) phosphate method] Tolerance, Foreign Ion P.P.M. Platinum(IV) Rhodium(111) Iridium(IV) Gold(111) Cobalt( 11) Nickel( 11) Iron( 111) Copper( 11) Chromium(VI) Sulfate
Table IV.
per mole of palladium. In the range of 5 to 6 moles of tin(I1) per mole of palladium the absorbance increased linearly, then broke sharply a t the 6 to 1 ratio to a constant value as the tin-topalladium ratio was further increased.
4 14
SUMMARY
2 0.06 5 440 4 70 0.2 650
Comparison of
Reagent p-Nitrosodimethylaniline 3-Hydroxy-l-p-sulfonatophenyl-3-
The red product obtained by reaction of palladium(I1) with tin(I1) phosphate in perchloric acid and phosphoric acid solution (and absence of chloride ion) has a very narrow absorption band with a sharp peak at 487 mp. Maximum color devclop within 10
Spectrophotometric Methods for Palladium Wave Length of Max. Optimum Specific Absorbance, Range, Absorptivity," Ref. P.P.M. P.P.hl.-Cm.
m
(11)
525
0 25-1.0
(IO) 420,430 0 2-jb phenyltriazine 2-Nitroso-1-naphthol (toluene 370 1-5 extraction) 2.4-6 (4) 520 p-Bromaniline (8 9) 430 2-15 2-Mercapto-4,5-dimethyl thiazole Dimethvlglyoxime (6) 360-380 10-50 (C"& extraction) 635 8-32 Tin(I1) chloride 487 0.5-2,5 Tin(I1) phosphate In some cases, absorbance data not given; values estimated from graphs. * Best accuracy not attainable over this entire range.
added in the oxidation states given, is s h o m in Table 111. Mole Ratio Study. The usual spectrophotometric method for determining the mole ratio of reactants could not be applied directly t o this system. Table I1 shons t h a t for t h e concentrations of perchloric acid and phosphoric acid used in the standardized procedure, the absorbance for a fixed amount of palladium decreased slightly with increasing concentration of tin(I1). Furthermore, a t tin-topalladium ratios of less than about 5 t o 1, the color \\as so unstable (about 1 minute) that reliable absorbance measurements could not be made. The mole ratio study was therefore made on solutions of constant tin(I1) concentration and varying palladium concentration; the measured absorbance of each solution was calculated to absorbance
0 75 0 28 0.20 0.10
0.07 0.02 0.023 0.32
minutes at room temperature and is stable for 1.5 hours. The absorbance, for a given amount of palladium, is only slightly sensitive t o large changes in reagent concentration. The optimum concentration range for measurement at 1.00-em. optical path is about 0.5 to 2.5 p.p.m. of palladium. Treatment of palladium(I1) solution with tin(I1) chloride in hydrochloric acid and perchloric acid solution gives a green color (absorption maximum a t e35 mp). The optimum concentration range a t 1.00-em. optical path is about 8 t o 32 p.p.m. of palladium, or a n order of magnitude higher than with the tin(11) phosphate method. The t n o methods are complementary in regard to their application t o solutions of different palladium concentration. A summary of the range and sensitivity of several spectrophotometric VOL. 31, NO. 7,JULY 1959
1137
methods for palladium is given in Table IV; the sensitivity of the tin(II) p h o s p b t e method reported herein is relatively high. LITERANRE CITED
(1) Ayres, G.
H., Berg, E. W., ANAL.
CEEM.25,980 (1953).
(2) Ayres, G. H., Meyer, A. S., Jr., Ibid.. 23,299 (1951). (3) Ayres, G. H., Tutliy, B. L., Ibid., 24, 949 (1952).
4) Cheng,K. L.,Zbid., 26, 1894x1954). !5).Mel10r7 J. “Comprehensive Treatise on Inorganic and Theoretioal Chemistry;’ vel. VII, p. 482, hogmans, Green, h n d o n , 1947. (6) Nielsch, W., 2. anal. Chem. 142, 30 (1954). (7) Pollard, W. B., Analyst 67, 184
w.,
(1942). (8) Rice, E. W., ANAL.CHEM.24, 1995 (1952). 9) Ryan, D. E., Analyst 76, 310 (1951). [lo) Sogani, N. C., Rhattacharyya, S. C., ANAL.CHEV 29, 397 (1957).
(11) Yoe, J. H., Kirkland, J. J., Zbid. 26, 1335 (1954).
RECEIVEDfor review July 28, 1958. Accepted February 24, 1959. Condensed from dissertation submitted by John H. Alnop I l l to the graduate school of The University of Texas in partial fulfillment of the requirements for the dcgree of doctor of philosophy, May 1957. Work s u p ported in part by U. S. -4tornic Energy Commission under terms of Contract No. AT-(40-1)-1037 with the University of Texan.
Modified Sargent-Malmstadt Automatic Titrator for Remote Control Use with Plutonium Solutions GLENN R. WATERBURY Universiiy of California, Lor Alamos Scientific Laboratory, Los Alomos, N. M. bSeveral modifications to a d a p t the commercially available, differentiolpotentiometric automatic titrator for use with plutonium solutions, and to improve parts of it for specific purposes include replacing t h e gravity-flow buret with a motor-driven syringe buret, replacing the stirrer with a magnetic stirrer, rewiring the titrator for remote operation inside a plutonium dry box o r hood, and adding a microammeter to indicate potential changes. For trial titrations of cerium(lV) and chromiumlVl) with iron(ll), standard deviations of less than 0.01 relative 7 0 were obtoined by using large samples and weight burets with the modified titrator.
T
EB u8e of an automatic potentiometric . titrator . for the determination of plutonium seemed advantageous, if a commercial model mere available with certain features. The titrator should permit the use of corrosive oxidizing titrants such as cerium(1V) sulfate, accurate and rapid indication of the equivalence point for solutions of various concentrations and of different starting potentials, rapid indication of a slight excess of oxidizing or reducing agent prior to a titration, and remote operation from within a plutonium dry box t o prevent radioactive c o n t a m h e tion of the main electronic components of the titrator which would be kept outside the dry box. Because the backtitration technique, in which only a small excess of an oxidant or reductant is titrated, is often advantageous, t h r third feature is particularly important. Several automatic potentiometric titrators have been described (f-10). In general, they operate by recording the entire potentiometric titration curve
1138
ANALYTICAL CHEMISTRY
( 1 , 5, 4,8),or by stopping the addition of the titrant either at the inflection point (6) or at some preset value of the potential (4, 7 , 9 , 10). The titrant may he added at a constant rate ( 1 , 5) or added slowly or interrupted near the end point to avoid an excess (4,7, 8). No commercial model possesses all the required features. The advantages of end point detection with the differential automatic titrator (6) made the SargentMalmstadt instrument preferable. This paper discusses the modifications made to include desirable features and gives data for trial titrations. MODIFICATIONS OF TITRATOR
A complete description of the SargentMalmstadt automatic titration is given by Malmstadt and Fett (6) and in the operating manual for the instrummt. The changes made include: replacing the gravity-flow burrt with a motor-
Figure I .
drivel, Gilinont buret, replacing the stirrer with a magnetic stirrer, rewiring the titrator for remote operation, and adding a microammctw to indicate potmtial changes. Mixing-Delivery Assembly. The original assembly consisted of a stirring propeller, a delivery tip attached to a gravity-flow buret by rubber tubing, and a solenoid-actuated plunger operating on the rubber tube to control the flow of titrant. The stirring motor, propeller, solenoid, and electrodes are mounted in a housing above the titration beaker. This buret system has %vera1 disadvantages: Special overhead extensions of the usual plutonium dry boxes are required for use with the tall gravity-flow buret, solutions such as eerium(1V) sulfate react with the robber tubing connector, and the components of the titrator mounted directly over the titration vessel increase corrosion mid contamination problems and reduec neccss to the equipment. For
Modifications a d d e d to Sargent-Malmstodt titrotor
