Spectrophotometric Determination of Pa I ladium with Qui noxa line-2,3-dithiol GILBERT H. AYRES and HARVEY F. JANOTA Department of Chemistry, The University of Texas, Austin, Tex.
b Palladium gives an orange-red color with quinoxaline-2,3-dithiol in N,N-dimethylformamide solution in the presence of a small amount of hydrochloric acid. The color develops rapidly and is stable for several hours. The absorbance is not highly sensitive to the concentration of reagent or of hydrochloric acid. The system conforms to Beer's law, and is suitable for the determination of palladium in the concentration range 0.4 to 3 p.p.m. Platinum, osmium, iron(lll), cobalt, and nickel interfere and require separation from palladium. When palladium is present in excess, a yellow 1 to 1 complex (absorbance maximum at 466 mp) is formed; when quinoxaline-2,3dithiol is in excess, a red 1 to 2 palladium-reagent complex is formed, having maximum absorbance at 548 ml.ci
T
preparation of quinoxaline-2,3dithiol was described in 1956 by Morrison and Furst ( 5 ) , who observed that it formed chelates with several of the transition elements, notably with nickel ion in ammoniacal solution. Skoog, Lai, and Furst (9) reported in detail the spectrophotometric determination of nickel in ammoniacal solution using this reagent. They studied the effect of various other ions on the nickel determination, but the tests did not include the platinum elements. The presence of the thiol groups in the /N-CSH reagent, CBHd 1 , and its re\N=c-SH action with ions of elements such as nickel, cobalt, copper, and silver, suggested that it might be applicable to the determination of some of the platinum elements, HE
REAGENTS AND APPARATUS
Quinoxaline-2,3-dithiol. The reagent (Eastman Yo. 5870) was obtained from Distillation Products Industries, and was used as received. Morrison and Furst ( 5 ) reported that the reagent could not be recrystallized from any solvent tried, but was purified by repeated dissolution in alkali and reprecipitation by acetic acid. Solubility tests showed the reagent to be insoluble in water, carbon tetrachloride,
chloroform, benzene, and heptane, The reagent is very sparingly soluble in ethyl alcohol, isopropyl alcohol, ethyl acetate, dioxane, and Ar,N-dimethylformamide. Preliminary tests with solutions of the reagent in these solvents showed that the solution in dimethylformamide was the most sensitive for development of the red color with palladium. For use in this work, a 0.1% solution was prepared by dissolving 50 mg. of quinoxaline-2,3-dithiol in 50 ml. of dimethylformamide. The reagent solution does not keep well, but it is stable for use for about 2 weeks, provided it is protected from light; the reagent bottle was shielded by wrapping with aluminum foil. N,N-Dimethylformamide. The solvent was obtained from Distillation Products Industries (Eastman No. 7217), and was used as received. Standard Palladium Solution. The preparation of standard palladium solution from the metal has been described (1). Working solutions of appropriate concentration were prepared as needed from a stock solution containing 1003 p.p.m. of palladium. Spectrophotometers. Spectral curves and other measurements a t varying wave lengths were made on a Beckman Model DK-1 recording spectrophotometer. Analytical measurements a t fixed wave length were made on a Beckman Model D U spectrophotometer. Stoppered silica cells of 1.00-cm. optical path were used for all measurements. RECOMMENDED PROCEDURE
Into a 25-ml. volumetric flask measure a volume of standard solution or sample for analysis up to 8 ml. containing an amount of palladium to give 0.4 to 3 p.p.m. in the final solution; add water a s necessary to give a total volume of 8 ml. of aqueous solution. Add 1 ml. of 1 M hydrochloric acid and 10 ml. of dimethylformamide, then cool the mixture to room temperature. Add 2 ml. of 0.1% quinoxaline-2,3-dithiol solution and mix well; allow 10 minutes for development of the red color, then dilute to 25-ml. volume ith dimethylformamide. Simultaneously prepare a corresponding blank solution. Measure the absorbance of the sample solution against the blank, a t 548 mfi. The spectral curve of a 1.60-p.p.ni. palladium solution, prepared and measured as described above, is shown in Figure 1, curve A . Curve B shons
Figure 1. A. B. C.
Spectral curves
Palladium, 1.5 X 10-6M, 1.60 p.p.m. ahd excess quinoxafine-2;3-dithiol,4.1 X 10-'M Quinoxaline.2,3-dithiol, 4.1 X 1 O-SM Palladium, 3.0 X 1 O-sM and quinoxaline2,3-dithiol, 3.0 X 1 O-6M
Table 1.
Palladium Concn., P.P.M. 0.40 0.80 1.60 2.40 3.20
Effect of Water-Dimethylformamide Ratio
Absorbance at 548 20 %
hfp
36%
water
water
0.128 0.245 0.503 0.777 1.06
0.119 0.242 0.483 0.733 0.996
t h a t the absorbance of the blank solution is negligible a t 548 mp. The system conforms to Beer's law. STUDY OF THE VARIABLES
Effect of Water - Dimethylformamide Ratio. T h e insolubility of quinoxaline-2,3-dithiol in water demands the use of a considerablc. amount of water-miscible organic. solvent t o prevent precipitation of the reagent in the reaction mixture. Dimethylformamide was found t o be most suitable for this purpose. Solutions containing diffcrent amounts of palladium were prepared by the recommended procedure, except that the ratio of water to dimethylformamide in thr final solution was varied. Table I shows absorbance measurements of several such solutions. When the amount of water exceeded about 40% by VOL. 3 1 , NO. 12, DECEMBER 1959
1985
4
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Figure 2.
Mole ratio plot
Quinoxaline-2,3-dithiol constant, 3.0 X 1 0-6 M and palladium concentration varied
b
+
///
Figure 3.
548rn"
~
1.00 1.2s M O L E S PALLADIUM/MDLE REAGENT
volume, some quinoxaline-2,3-dithiol iTas precipitated. Effect of Amount of Hydrochloric Acid. I n t h e absence of acid, very little red color was developed. Addition of a s little as 0.05 ml. of 1M hydrochloric acid per 25 ml. of final solution caused a pronounced increase in absorbance (Table 11). Effect of Reagent Concentration. The d a t a in Table I1 show t h a t wide variations in t h e reagent concentra-
Table II. Effects of Hydrochloric Acid and Reagent Concentration (Palladium concentration, 1.60 p.p.m.) lfl. of 1M HCl Absorbance a t per 25 hll. 538 Mp 0.0 0.05 0.10 0.I 5 0.25 0 50 1.00 2 00
0,205 0.446 0.484 0,495 0.497 0.503 0.499 0.498
M1. of 0.1yc Reagent per 25 All. 0.500 0.501 0.502 0.507 0.504 0.505
4 6 8 12
Table 111. Color Stability (Palladium concentration, 1.60 p.p.m.) Time after Preparation, Absorbance at 548 M p Hours 0 0 0 1 1 2 3 4 13 16 19 21 27 40
1986
25 50 80 0 6 0 0 0
0 50i
0 514 0.514 0.514 0.514 0.514 0,515 0.518 0.514 0 514 0.504 0.498 0,484 0.438
ANALYTICAL CHEMISTRY
1.50
Mole ratio plot
Palladium concentration constant, 1.5 X 1 O-SM and quinoxaline-2,3dithiol concentration varied
0-
tion did not affect the absorbance, if t h e same amount of reagent was used in t h e blank as in t h e sample solution. Stability of Reagent. The reagent solution gradually became less sensitive for t h e development of t h e palladium color a n d a yellow solid deposited on the walls of the container. Deterioration of t h e reagent solution was retarded by protection from light. Solutions stored in glass-stoppered bottles wrapped with aluminum foil were generally suitable for use for about 2 weeks. Color Stability. Full development of t h e red color was attained in less than 10 minutes; the absorbance was then constant for about 16 hours (when fresh reagent solution was used), after which a turbidity developed in both t h e blank and in t h e sample solutions. -4s t h e age of t h e reagent solution increased, the turbidity in t h e blank a n d in t h e sample solutions occurred in a shorter time. Data on color stability are shown in Table 111. Precision, Range, and Sensitivity. l h e d a t a shown in Table I V were used for construction of t h e calibration curve. Each absorbance entry in the table is the average of five separate preparations a t the concentration given. The 40 samples represented in Table I V gave an average specific absorptivity of 0.306 =t 0.003
Table IV. Palladium Concn., P.P.M. 0 40 0 80 1 20 1 60 2 00 2 41 2 81 3 21
Calibrat:on and Sensitivity Data Specfic -4bsorp-
Absorbance tivity per at 548 h . 1 ~ P.P.M.-Cm. 0 0 0 0 0 0 0 0
124 246 365 492 613 729 850 981
0 0 0 0 0 0 0 0 Av. 0
310 308 304 308 30i 303 303 306 306
0
os
10
I S
20
2 5
3 0
MOLES R r A G E N T / MOLE PALLADIUM
per p.p.m.-cm., with a standard deviation of 0.004. A plot of absorbance against concentration is a straight line which passes through the origin; conformity to Beer's law is shown also by the constant value for the specific absorptivit'y. The optimum concentration range for measurement a t 1.00-cni. optical path is about 0.4 to 3 p.p.m. of palladium; in this concentration range the results are reliable to a relative error of less than 1%. Khile the present method is only about 40Yo as sensitive as the p-nitrosodimethylaniline method of Yoe and Kirkland ( I S ) , it has about the same sensit'ivity as the tin(I1) phosphate method of Ayres and Alsop ( I ) , and it is considerably more sensitive than several other methods for palladium (3, 6-8: 10). Effect of Foreign Ions. 1 I r t a l ions for these tests were used in the form of readily available solublc salts,, usually the chlorides. Anions were added in the form of sodium salts. Interference tests were made a t the 0.4- and 1.60-p.p.m. level of palladium by adding the foreign ion in large excess relative to t h e palladium (20 p.p.m. for cations, and 100 p.p.m. for anions), and developing the color by the recommended procedure. From the measured absorbance, the amount of foreign ion producing an absorbance difference of 0.01 compared to that of palladium, was calculated. This absorbance difference is 2.5 times the standard deviation in the palladium measurement. For cases in which the interference was extensive, the trsts were repeated with successively smaller amounts of foreign ion. If the first test quantity (20 p.p.ni. of cation or 100 p.p.ni. of anion) gave a n absorbance within 0.01 of that of the palladium alone, tests with larger amounts were not made. The results, summarized in Table \-. s h o x that platinum, osmium, nickel, cobalt, and iron(II1) interfere seriously with the determination of palladium, and would require separation. Osmium can be separated from the other elements by volatilization as osmium
tetroxide (4,11). Precipitation with dimethylglyoxime in acid solution will separate palladium froni platinum (2, 4 ) and from most other elements, including nickel. Additional work is needed to test the reliability of the separation of small amounts of palladium from larger amounts of nickel, cobalt, and iron. Composition of the Complexes. Application of t h e mole ratio method (12) shorvcd the existence of two complexes: a red complex, n i t h absorbance maximum a t 548 nig and a side plateau a t 517 mu (curve A of Figure I ) , in which the palladium to reagent ratio is 1 to 2, and a yellow complex, Rith absoibance maximum a t 466 mp (curve C of Figure l ) , in n hich the palladium to reagent ratio is 1 to 1. Data for the mole ratio plots were taken in two series. I n one series the concentration of the reagent was held constant (3.0 X 10 4.M in the final solution) and the palladium concentration n as varird; absorbance measurements were made a t several wave lengths, including the LT ave length of the absorption peaks of the two species (Figure 2). In the other series the palladium concentration vas held constant (1.5 X 10-6JI in the final solution) and the concentration of quinoxaline-2.3-dithiol n as varied (Figure 3). The evidence for the 1 to 1 and the 1 to 2 complexes iq clearly shown; the
slight deviations in the positions of some of t h r breaks in the curves from the integral mole ratios could be caused by uncertainty of the exact purity of the solid quinoxaline-2,3-dithiol reagent.
Table V.
Tolerance for Foreign tons
(Palladium concentration, 0.4 and 1.60 p.P.m.) Tolerance, Foreign Ion P.P.hI. Platinum( IV) 0.3 Rhodium( 111) 8 “ RutheniumlITI’i Osrnium(1V) ’ 0.2 Iridium( IV) 10 Sickel(11) 0.05 Cobalt( 11) 0.02 Iron(I1j 4 Iron(II1) 1 GoldlIII) 10 10 >20 8
>20
15 10
17 70
200
50 > 100 60
Acetate Sulfate 50 Phosphate a Sitrite caused decrease in absorbance, probably through complex formation with palladium.
ACKNO WLEDGMENl
Preliminary n ork on thr palladium quinoxaline-2,3-dithiol color reaction was performed in this laboratory by Harold T). Powell in 1957, while cmployed on Sational Science Foundation Grant SSF GI889 to the senior author. LITERATURE CITED
(1) Ayres, G. H., Alsop, J . H., ANAL.
CHEX 31, 1135 (1959).
R., Ibid., 25, 980 (1953). (3) Cheng, K. L., I b i d . , 26, 1894 (1954). (4) Hillebrand, W.F., Lundell, G. E. F., Bright, H. A,, Hoffman, J. I., “Applied Inorganic Analysis,” 2nd Pd., pp. 356, 372, )$?ley, Sew York, 1953. ( 5 ) Morrison, D. C., Furst, A , , J . Org. Chem. 21, 470 (1956). (6) Sielsch, W., 2. anal. Chem. 142, 30 (1954). ( 7 ) Rice, E. FY.,AX.kL. CHEM. 24, 1995 (1952). (8) Ryan, D. E., Analyst 76, 310 (1951). (9) Skoog, D. .4.,Lai, hI., Furst, A . , ANAL.CHEM.30,365 (1958). (10) Sogani, N. C., Bhattacharyya, S. C., I b i d . , 29,397 (1957). (11) Steele, E. L., Yoe, J. H., Ibid., 29, 1622 119571. (12) Yoe, J. H., Jones, H. L., ISD. ENG. CHEM.,ANAL.ED. 16, 11 (1944). (13) Yoe, J. H., Kirkland, J. J . , ANAL. CHEJI.26,1335 (1954)
( 2 ) Ayres, G. H., Berg, E .
RECEIVEDfor review July 24, 1959. ilccepted August 31, 1959.
Reactivity of Oxidizing Agents with Potassium Iodide Reagent AUBREY P. ALTSHULLER, CAROL M. SCHWAB, and MARIBEL BARE Robert A. Tuft Sanitary Engineering Center, Public Health Service, U. S. Department o f Health, Education, and Welfare, Cincinnati 26, Ohio
b The reactions of iodide ion with peracetic acid, succinic acid peroxide, cumene hydroperoxide, di-tert-butyl peroxide, hydrogen peroxide, ozone, nitrite ion, and tert-butyl nitrite in the 10+ to 10-sM range have been investigated at various pH values. The effect of adding sulfamic acid along with the phosphoric acid used for acidification was studied also. Reaction rate curves were obtained for most of the reactions studied.
T
HE PRESCWE of oxidizing materials in the atmosphere has been detected in man\- localities by the use of iodide solutions (4,10-13, 1 5 ) . The iodide ion in solution is readily oxidized by many oxidizing agents; consequently, the term “oxidant” 13 used t o represent the
net result of atmospheric reactivity with iodidc. I n the Los Angeles area, the major part of the oxidant usually is ozone, but the nature of the remaining oxidizing matcrial is not dcfinitely knonn ( 7 , 1 1 , 12). Although somr work has bwn tione on the reactivity of srveral organic- reroxidcs in the niicrogram rang(. nitli iodide ion ( S ) , it was f P l t that it noiild be useful to investigate thrl reaction of 6 X 10-’M (1% by weight) iodide ion with ozone, hydrogen peroxide, peracetic acid, succinic acid peroxide, cumene hydroperoxide, di-tert-butyl peroxide, nitrite ion, and tert-butyl nitrite in the to lo-5.V range undcr varying conditions of pH. As sulfamic acid has a decided effect in quenching the reactivity of nitrogen dioxide with iodide, particularly in 1M sodium hydroxide solution
( d ) , the effect of sulfamic a d \vas drtermined on all of the oxidizing materials studied. Reaction rate data were obtained to decide the conditions under which the determinations would br least deprndrnt on time of standing befow analysis. EXPERIMENTAL DETAILS
The cumenc hydroperoxide, peracetic acid, succinic acid peroxide, di-tertbutyl peroxide, hytlrogrn prroxide, tert-butyl nitrite, and inorganic nitrite were all diluted to about 0.5 pmolt per ml., and 0.25 to 2 ml. of these solutions were diluted up to 10 ml. with l.007, potassium iodide solutions. Consequently, t,he concentrations of reactants were 10-5 to 10-4.11 in prroxides and nitrites and 6.0 X 10-*.\1 in iodide ion. The ozone was produced by a small ultraviolet ozone generator and was passed a t VOL. 31, NO. 12, DECEMBER 1 9 5 9
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1987
