C. SURASITI AND E. B. SANDELL
890
Vol. 63
KINETICS OF THE RUTHENIUM-CATALYZED ARSENIC (111)-CERIUM(1V) REACTION' BY C. SURASITI AND E. B. SANDELL School of Chemistry, University of Minnesota, Minneapolis, Minn. Received February 19, 1869
The rate of the ruthenium-catalyzed reaction of cerium(1V) with arsenic(II1) in sulfuric acid solution is independent of arsenic(III), arsenic(V) cerium(II1) and hydrogen-ion concentrations. The rate expression (25") is -d[Ce(IV)] /dt = (4.0 X lOlo[R~][Ce(IV')]~.6~/(1 2.1 X 103[Ce(IV)]1J}, in which concentrations are given in moles per liter and time in minutes.
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The very slow reaction between arsenic(II1) and cerium(1V) in sulfuric acid solution is strongly catalyzed by iodine, osmium and ruthenium. The catalysis by iodine and osmium has already received attention especially because of its importance in the determination of traces of these elements. The study summarized here dealt with the kinetics but not the mechanism of the ruthenium-catalyzed reaction. Experimental Reagents. Arsenious Oxide, 0.2000 N Solution.-Reagent grade As203 (9.892 9.) was dissolved in 30 ml. of 1 M sodium hydroxide and the solution was diluted to 1 liter after acidification with sulfuric acid. Ceric Ammonium Sulfate Solution, 0.2000 N.-A solution obtained by dissolving 126 g. of the dihydrate in 2 M sulfuric acid and diluting to 1 liter was allowed to stand for two weeks, filtered and standardized against arsenious oxide. This solution was diluted with 2 ill.sulfuric acid to make the ceric concentration 0.2000 N , and the normality was verified by rest,andardization. Ruthenium(1V) Sulfate.-A solution of potassium ruthenate, obtained by fusing metallic ruthenium with a mixture of potassium hydroxide and sodium peroxide. in a gold crucible, was added slowly with vigorous stirring to 1 M sulfuric acid. The resulting solution was treated with 30% hydrogen peroxide and evaporated to half its volume. The evaporation was repeated several times after addition of water and hydrogen peroxide. This treatment served to remove any osmium present. Ruthenium tetroxide was distilled from the resulting solution after addition of ceric ammonium sulfate and absorbed in ice-cooled sulfuric acid solution containing hydrogen peroxide. This solution was heated overnight to destroy hydrogen peroxide and filtered through sintered glass. The ruthenium concentration of the solution was obtained by hydrolytic precipitation of hydrous ruthenium(IV) oxide (sodium bicarbonate as neutralizing agent), followed by ignition of the preci itate to ruthenium metal in hydrogen and nitrogen. T f e metal was leached with hot water to remove any soluble salts present. The stock ruthenium sulfate solution was suitably diluted to obtain working solutions. Solutions as dilute as 0.01 7 Ru per ml. (in 2 M sulfuric acid) showed no change in concentration in a year's time. Ruthenium Tetroxide.-The tetroxide, obtained as described above, was swept through anhydrous .magnesium perchlorate in a stream of nitrogen and trapped in a U-tube immersed in Dry Iceacetone. The collected tetroxide was distilled into another cooled trap. Solutions of ruthenium tetroxide were prepared by dissolving weighed amounts in 1 M Rulfuric acid. Water.-Deionized water was douhly distilled, fist from alkaline permanganate in a tin still, then from alkaline permanganate in a Pyrex still with a 40 cm. Vigreux column to trap spray. Rate Measurements.-The rate of the catalyzed reaction As(II1) 2Ce(IV) + 2Ce(III) As(V) was obtained by determining spectrophotometrically the concentration of cerium(1V) in the reaction mixture as a function of time. The ceric ammonium sulfate solution,
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(1) From the Ph.D. thesis of C. Surasiti, University of Minnesota,
1957.
t80 which ruthenium had been added as the sulfate(1V) or tetroxide, was added by means of a hypodermic syringe to the magnetically stirred arsenious oxide solution. A B-ckman DU spectrophotometer was used in conjunction with a percentage transmittance-time recorder (Varian graphic recorder model G-10) for the photometric measure; ments. All solutions were thermostated at 25 0 f 0.1 before mixing and during the transmittance measurements. The percentage transmittance-time curve was transformed into a concentration-time curve with the aid of standard curves prepared from cerium( IV) solutions containing appropriate amounts of arsenic(II1) and arsenic(V), which affect the absorbance of cerium(1V). The concentration-time curve was extrapolated to zero time and the initial rate was obtained by finding the slope of the curve at zero time. When the initial reaction rate was not needed, the "reaction time" was measured, this being the time required for a given fraction of cerium(1V) to react. Usually the time required for one-half of the original amount of cerium(IV) to be reduced was taken as the reaction time.
Results Effect of Initial Oxidation State of Ruthenium.The results obtained will be summarized without presenting the data. Provided that ruthenium is added to the ceric solution before this is mixed with the arsenite solution, the rate of the catalyzed reaction is the same whether ruthenium(1V) sulfate or ruthenium tetroxide is used. Ruthenium(IV) is completely oxidized to ruthenium(VII1) by ceric sulfate within 10 seconds. However, if ruthenium(1V) sulfate is added first to the arsenite solution, the reaction rate is markedly lower. If ruthenium tetroxide solution is added with good stirring to excess arsenite solution acidified with sulfuric acid, the reaction rate is almost the same as when the tetroxide is added to the ceric solution and, moreover, does not depend on the time elapsing before the arsenite-ruthenium solution is mixed with the ceric solution. But if the mixing is poor so that there is an appreciable local excess of the tetroxide, the reaction rate decreases and is irreproducible. It was observed that treatment of a ruthenium tetroxide solution with arsenious oxide in the amount needed t o reduce ruthenium(VII1) to ruthenium(1V) gave a reddish brown solution (the color of Ru(S0.J2), which did not change on subsequent addition of more arsenious oxide. On the other hand, if a large excess of arsenious oxide was added to well-stirred ruthenium tetroxide solution, a straw-yellow color characteristic of ruthenium(II1) was produced. The conclusion may be drawn that, once formed, ruthenium(1V) is stable, and can be obtained in a form which is not very active catalytically. Further, ruthenium(111) in arsenious oxide solution shows a catalytic
June, 1959
KINETICSOF
THE
Ru-CATALYZED As(III)-CE(IV) REACTION
activity much the same as that of ruthenium(VIH) originally present in ceric solution. Effect of Arsenic(II1) Concen tration.-The initial rate of the catalyzed reaction as a function of the arsenious oxide concentration was determined a t a constant cerium concentration of 0.0250 N and a constant ruthenium concentration of 2.46 X 10-7 M in a 2.0 M sulfuric acid solution at 25.0'. The rate was found to be independent of the arsenic(111) concentration. An average value of 0.0286 mole l.-lmin.-l was found for (-d [ A S ( I I I ) ] / ~ ~ ) ~ over the range 0.01 - 0.08 M As(III), the lowest and highest values of eight measurements being 0.0274 and 0.0296. The ionic strength was maintained constant in this series with sodium sulfate. Effect of Arsenic(V) and Cerium(II1) Concentrations and Other Factors.-The effect of these on the rate was tested in a mixture which was 0.030 N in As(III), 0.020 N in Ce(1V) and 2.0 M in sulfuric acid, and which contained 0.02 y Ru/ml. (1.96 X lo-' M ) . Sodium perchlorate was added to keep the ionic strength constant. The rate remained constant over the range of concentrations examined: As(V), 0-0.025 M ; Ce(III), 0-0.04 M . Variation in ionic strength has but a small effect on the rate of the catalyzed reaction (Table I), the rate increasing slightly as the ionic strength is raised. TABLE I RECIPROCAL O F HALF-REACTION TIME5 AS FUNCTION OF IONIC STRENGTH (0.0075 N As(III), 0.005 N Ce(IV), 0.020 y Ru/ml., 0.5 M HtSOa) Ionicstrength 0.5 1 . 1 1.7 2.3 2.9 3 . 5 l/tL/*, rnin.-' 8.56 7.80 7.11 7.40 6.67 7.11 a The half-reaction time, t l / z , is defined as the time required for one-half of the Ce(1V) to be reduced.
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t ~
2 3 4 5 [ C e ( I V ) ] P X 108. Fig. 1.-Plot of [ C ~ ( I V ) O ] ~ ~vs. ~ / R[Ce(IV)]1.6. O Ro is the initial rate of the catalyzed reaction (mole 1.-1 min.-I). 1
3.00 Irs
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Variation in the concentration of sulfuric acid from 0.5 to 2.0 M in a mixture of 0.0050 N cerium+ 2.00 (IV) and 0.0075 N arsenic(II1) containing 0.04 y G Ru/ml., in which the ionic strength was maintained 0 constant with sodium bisulfate, produced no change in reaction rate. Chloride reduces the reaction rate markedly. 1.oo Thus, in a mixture which was 0.020 N in Ce(IV), 0.025 N in As(II1) and 2.0 M in sulfuric acid, and which contained 0.0060 y Ru/ml., the half-reaction times (minutes) varied as follows with the indicated sodium chloride concentrations: 0.0000 M , 1.05; 0.0005 M , 1.73; 0.0010 M , 3.49; 0.0020 M , 3.1 3.2 3.3 3.4 3.5 10.5; 0.0050 M , 82.5. In these experiments the (l/T)-l x 108. sodium chloride was added to the ceric solution Fig. 2.-Plot of In R, (initial rate of catalyzed reaction) before mixing with arsenite. vs. reciprocal of absolute temperature (0.020 N Ce(IV!, Initial Rate of Catalyzed Reaction as a Function 0.03 N As(III), 2 M sulfuric acid, 0.020 y Ru/ml.). of the Ce(1V) and Ruthenium Concentrations.In this series of experiments, the ionic strength was maintained constant by the addition of ammonium When kz[Ce(IV)]1.5>>l,this equation reduces to sulfate. The results are given in Table 11. The rate is first order with respect t o ruthenium - d[Ce(lV)l = [Ru] [Ce(IV)] over the whole range investigated (to concentradt tions as low as 5 X lo-* M or 5 y Ru/liter). The rate is not a simple function of the cerium- and when Icz[Ce(IV)]*J
