Arsenous acid rate constants

Arsenous acid is a weak acid, known to exist only in solution. Its molecule has three -OH groups attached to the arsenic atom. The dissociation constant of this acid is 8.0 x ICRi at 25°C. It is produced by hydrolysis of arsenic trioxide in water. The trioxide is sparingly soluble in water and the rate of... 

The predicted arsenate vs time curves for demethylation of cacodylic acid are shown Tn Figure 5. Both curves are for first order decay with a rate constant of 0.04 day" but with different amounts of adsorbent present. Adsorption of arsenate results in dissolved arsenate concentrations that are lower than total arsenate. An s-shaped curve results because arsenate produced during the first few days is almost entirely adsorbed. 

Demethylation of mpnomethylarson c acid to produce arsenate with the two species competing for adsorption sites is depicted in Figure 8. Adsorption of MMAA results in slower conversion rates and the apparent rate constant in this case is 0.03 day-1. Since arsenate is adsorbed more strongly than MMAA the final arsenate concentration is less than the initial MMAA concentration. 

The rates of these reactions have been measured, and also the equilibrium constant has been determined experimentally for the equilibrium mixture obtained either by mixing arsenious acid and triiodide ion or arsenic acid and iodide ion. It was found that the equilibrium constant has the value expected from the rates of the reactions when the pure reactants are mixed. 

The rate of this reaction and the extent to which it proceeds to the right can be readily judged by observing the orange-red color of the triiodide ion IJ. (The other participants in the reaction are colorless.) If, for example, 1 mmol of arsenic acid, H3ASO4, is added to 100 mL of a solution containing 3 mmol of potassium iodide, the red color of the triiodide ion appears almost immediately. Within a few seconds, the intensity of the color becomes constant, which shows that the triiodide concentration has become constant (see color plates lb and 2b). 

A model that reproduces the homogeneous dynamics of a chemical reaction should, when combined with the appropriate diffusion coefficients, also correctly predict front velocities and front profiles as functions of concentrations. The ideal case is a system like the arsenous acid-iodate reaction described in section 6.2, where we have exact expressions for the velocity and concentration profile of the wave. However, one can use experiments on wave behavior to measure rate constants and test mechanisms even in cases where the complexity of the kinetics permits only numerical integration of the rate equations. 

Dill KR, Adams ER, O Connor RJ, McGown EL (1989) Structure and dynamics of a lipoic acid-arsenical adduct. Chem Res Toxicol 2 181-185 Dill KR, Huang L, Bearden DW, McGown EL, O Connor RJ (1991) Activation energies and formation rate constants for organic arsenical-antidote adducts as determined by dynamic NMR spectroscopy. Chem Res Toxicol 4 295-299... 

The kinetics of sorption of arsenite and arsenate in the presence of sorbed silicic acid have been only recently examined (Waltham and Eick 2002). These authors demonstrated that the sorption of silicic acid (added 60 h before arsenic) decreased the rate and the total amount of arsenic sorbed. The amount of arsenite sorbed decreased as the surface concentration of silicic acid increased. Furthermore, the inhibition of arsenite sorbed ranged from about 4% at a pH of 6 and 0.1 mM silicic acid up to 40% at a pH of 8 and 1 mol IT1 silicic acid. In contrast, silicic acid reduced the rate of arsenate sorption which decreased by increasing pH and silicic acid concentration, but the total quantity of arsenate sorbed remained nearly constant, indicating that arsenate was able to replace silicate. 

Ferm, V.H. and D.P. Hanlon. 1986. Arsenate-induced neural tube defects not influenced by constant rate administration of folic acid. Pediatric Res. 20 761-762. 

In the oxidation of arsenic(iii) in perchlorate media the reaction order of less than unity with respect to reductant is interpreted in terms of the formation of an unstable 1 1 intermediate. Activation parameters are essentially constant (Ea = 9.3 0.5 kcal mol ) at both high (excess) and low [Cr " ] and the rate is increased on addition of sodium hydrogen phosphate. This latter observation may reflect an acid-catalytic route described earlier. The mechanism at low [Cr T with reductant in excess is considered to involve the rate-determining decomposition of the 1 1 complex to give Cr and an arsonium species of the type... 

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