The variation of potential of the cerium(IV)-cerium(lll) couple with the type of anion present is reviewed. The spectrophotometric data on sulfate complex formation and on hydrolysis correlate nicely with the potentiometric data, yielding a value of -
From the measurements a value for the molal potential for the half cell reaction 4H+ f Mn-OZ(S) + 2e-= Mn++ + 2Hz0 of 1.230 volts a t 25' has been calculated. This value is 6 mv. less than that computed by Brown and Liebhafskyl from data obtained in a similar study.Trivalent manganese has been shown to oxidize oxalate through the disproportionation of oxalate manganate complexes.' I n order t o further test the postulate that specific oxidations of glycols and related compounds generally proceed through the disproportionation of coordination complexes, i t was considered desirable to study as different an oxidation as possible which fits into the same category of specificity. The oxidation of glycol by periodate fulfills these conditions. Previous work on the kinetics of the periodate oxidation of g l y c 0 1~~~ is very sketchy, presumably because of the rapidity of the reaction at ordinary temperatures. Price3 concluded that the reaction is second order in periodate and glycol and suggested a cyclic ester as intermediate, the slow reaction being taken as the ester formation.
TheoryThe coordination intermediate should be formed by the reaciion The kinetics, according to the theory,l would be expressed by the equation where [A] is total periodate, k the rate constant and K the equilibrium constant for reaction (1) as written ([H20] taken as unity), and [C] is the Concentration of uncoordinated glycol. Equation ( 2 ) may be subjected to experimental verification by measurin.g the rate of disappearance of periodate from solutions containing sufficient glycol to validate the assumption that [C] remains constant throughout the portion of the reaction studied4 Under these conditions, a series oi firstorder (in oxidant) pseudo-constants may be obtained, the pseudo-constant k being identified with the bracketed portion of equation ( 3 ) .(1) Duke, THIS JOURNAL, 69, 2885 (194i).(2) Criegee, Kraft and R a n k , A n n . , 607, 159 (1933). (3) Price a n d Kroll, THIS JOURNAL, 60, ?726 (1938). (4) T h e stoichiometry of t h e oxidation is t h e same as t h a t 2f equation (1). Thus, "sufficient glycol" concentration depends upon t h e magnitude of IC. If K is very large, t h e uucooudinated g1y:ol concentration will remain essentially constant even though [ C ] : s of t h e same order of magnitude as [A]. If K is small [C] shoulc exceed [A] by ten-fold or more. Experimental The constant temperature baths used in the study were melting ice and melting benzene to obtain temperatures of 0 * O.Ojoand 5.1 =t 0.05',respectively. Thesolidphasein each case was placed into the corresponding liquid in a 2-liter wide-mouth Dewar flask, and the contents stirred. The temper'rture of the melting benzene is somewhat below the reported value for the melting point (5.48') presumably because the benzene was saturated with water. Fifty-ml. volumetric flasks with an additional 48 ml. calibration line were used for reaction vessels. The amount of 1.0 JI glycol to give the desired final concentration, and 5.0 ml. of 2 AI potassium nitrate were pla...
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