Arsenit-Ion

The hberated iodine, as the complex triiodide ion, may be titrated with standard thiosulfate solution. A general iodometric assay method for organic peroxides has been pubUshed (253). Some peroxyesters may be determined by ferric ion-catalyzed iodometric analysis or by cupric ion catalysis. The latter has become an ASTM Standard procedure (254). Other reducing agents are ferrous, titanous, chromous, staimous, and arsenite ions triphenylphosphine diphenyl sulfide and triphenjiarsine (255,256). 

Wilson and Dickenson observed no exchange, over a period of three hours at 100 °C, between arsenate and arsenite ions in media ranging from aqueous acid to aqueous alkali. Martin et al have found similar results for the exchange between arsenate and thioarsenite ions in aqueous media. However, in liquid ammonia exchange occurred between ammonium arsenate and arsenic trisulphide. The isotopic method was used -... 

Its solution exhibits amphoteric behavior. It dissolves in aqueous bases to give arsenite ions that have formulas, [AsO(OH)2], [As02(0H)] and [AsOs] . 

Experimental. In order to study the nucleophilic properties of 13 it was necessary to add excess I " to the solutions to prevent precipitation of I2. The rate of formation of CoCCN I-3 was followed spectrophotometrically after the I3 " in aliquots of the solution taken at suitable time intervals was reduced to I by arsenite ion. A typical set of experiments was carried out at 40°C. and unit ionic strength, with all solutions containing 0.5/1/ 1 and variable I3 " at a maximum concentration of 0.28M, the approximate upper limit imposed by solubility restrictions. The results are presented in Figure 3 as a plot of k the symbol used for the pseudo first-order rate constant for this system, vs. l/(lf). It is apparent that 13 is a remarkably efficient nucleophile, with a reaction rate considerably greater than that found for I at comparable concentrations. The points in Figure 3 also show detectable deviation from linearity, despite the limited range of 13 " concentration which was available. 

The reaction between iodate and arsenite ions appears to have contributions from both cubic and quadratic autocatalysis (the autocatalyst is the product, iodide ion). In the previous sections we have treated these two rate laws separately and by different methods. Both methods can be applied to the system in which these processes occur simultaneously, yielding results which, despite not being consistent at first sight, can be resolved by the idea of stability. 

After an extensive study of the adsorption of arsenious oxide by metallic hydroxides,3 Sen concluded that this type of adsorption resembles that of cations by manganese dioxide, and that the chemical affinity between the adsorbent and the substance adsorbed plays an important part, thus differing from adsorption by charcoal. It has been observed that soils having a high absorption capacity for bases also absorb the arsenite ion from solutions of 0-001 to 0-01X concentration.4 The absorption increases with time, without reaching an end-point, and the process follows the normal adsorption equation C1=kC1Jn. The addition of ferric oxide or calcium carbonate to the soil considerably increases the capacity for absorption, but such salts as calcium sulphate or copper sulphate have no effect. 

In spite of the fact that ISEs for more than 60 ions have been described so far, recent findings imply that these ISEs should be re-characterized and re-optimized for trace level applications [19]. The list of ISEs with low-level LODs needs to be expanded either by re-characterization of existing ionophores or by synthesis of new ones. Important ions for which low LODs have yet to be demonstrated are, for example, mercury, chromium, nickel, arsenate and arsenite ions. Hopefully, synthetic chemists will rise to the challenge and new, selective ionophores will be developed that will achieve this goal. 

In reactions where there is no change in the number of charges on the ions the ionic strength does not change and the reaction rates are somewhat simpler. A reaction of interest in this connection is the oxidation of the arsenite ion by the tellurate ion.18 The reaction was chosen because it goes with conveniently measurable velocity at the temperature of boiling water and the reaction can be followed easily by direct titration with iodine. More reactions of this type would undoubtedly be available for study at the higher temperatures if suitable analytical methods could be found. The reaction is... 

The expression for the equilibrium constant given above gives us the clue to the problem one often comes across in qualitative analysis what to do in order to make a reaction complete, in other words, to shift a chemical equilibrium in a desired direction. To examine this problem, let us consider the reaction of arsenate ions with iodide. If solutions of sodium arsenate, potassium iodide, and hydrochloric acid are mixed, the solution turns yellow or brown, owing to the formation of iodine. The reaction proceeds between the various ions present, arsenite ions and water being formed simultaneously, and can be expressed with the equation... 

At pH = 6 the potential of a solution containing arsenate and arsenite ions at equal concentrations decreases to +0-20 V. Under such circumstances therefore the opposite reaction will occur ... 

The dissolution of sulphides in ammonium polysulphide can be regarded as the formation of thiosalts from anhydrous thioacids. Thus the dissolution of arsenic(III) sulphide (anhydrous thioacid) in ammonium sulphide (anhydrous thiobase), yields the formation of ammonium- and thio-arsenite ions (ammonium thioarsenite a thiosalt) ... 

Two series of compounds of arsenic are common that of arsenic(III) and arsenic(V). Arsenic(III) compounds can be derived from the amphoteric arsenic trioxide As203, which yields salts both with strong acids (e.g. arsenic(III) chloride, AsC13), and with strong bases (e.g. sodium arsenite, Na3As03). In strongly acidic solutions therefore the arsenic(III) ion As3+ is stable. In strongly basic solutions the arsenite ion, AsO is the stable one. Arsenic(V) compounds... 

Potassium tri-iodide (solution of iodine in potassium iodide) oxidizes arsenite ions while becoming decolourized ... 

Raman spectra show that in acid solutions of As406 the only detectable species is the pyramidal As(OH)3. In basic solutions ([OH ]/[Asin] ratios of 3.5-15) the four pyramidal species As(OH)3, As(0H)20", As(0H)02 , and As03 appear to be present. In solid salts the arsenite ion is known in the As03 form, as well as in more complex ones. Alkali metal arsenites are very soluble in water while those of heavy metals are more or less insoluble. 

I X( cpi for arsenate ion, and possibly arsenite ion, the anions are hydroxide complexes as indicated. 

This lack of rearrangement 69 has been explained by assuming that the arsenite ion, which is produced by oxidative addition, recombines with the w-allyl group by its oxygen atom again instead of by the arsenic atom (equation 70) ... 

C(3j. This indicated that, after forming the 7t-allyl palladium complex, the arsenite ion OAs attacks at both positions and with the same probability and gives the deuterium-scrambled recoupling product according to equation 71. 

Arsenite ion has been used 2i according to equation (2), but certain difficulties were noted with simple aUcyl azides. If acid treatment quantitatively liberates hydrazoic acid such as in acid azides. 

It follows from Equations 19 and 20 that kj, may be increased by adding catalysts, which increase the chemical reaction rate constant K. A well-known example is the eflEect of arsenite ion on the rate of absorption of COo into alkaline carbonate solutions (27),... 

A strong association between As and Fe-oxides in river and lake sediments has been reported (eg., Aggett and Roberts, 1986 Belzile and Tessier, 1990 Brannon and Patrick, 1987). This association is attributed to the adsorption of arsenate onto the Fe-oxide coatings on sediment particles. Arsenite ion, although also adsorbed, is not as rapidly or completely adsorbed onto Fe-oxide as arsenate (Belzile and Tessier, 1990 Pierce and Moore, 1982 Swedlund and Webster, 1999). This observation has important implications for the re-release of adsorbed As under anoxic conditions. Microbial reduction of arsenate can occur even when As is bound to Fe-oxide (Langner and Inskeep, 2000), and reduction or methylation of sedimentary arsenate under anoxic conditions is the principal mechanism for re-release of As into the water column (Aggett and... 

Arsenic chemistry is complex, involving a variety of oxidation states, both as anionic and cationic species, and both inorganic and organometallic compounds. Of these, III and V are the most common oxidation states. The oxidation states of arsenic change easily and reversibly. As(III) is commonly encountered as the arsenite ion, H2ASO3 . Arsenious acid is a weak acid, p/Cai = 9.2, pK. 2 = 13. 

Allicin (in Zollner, 1989) Arsenite ion ATP Butyric add, 2-3-epoxy 3GA-destran Estradiol, 17-B Fatty acids Glycerate Butynoic acids Lactic acid L-maleic acid Mandelic acid ... 

Interestingly, small amounts of arsenic have been used to put back the spring in the step, and the shine on the coat, of older and decrepit horses, as Ben Green writes in Horse Tradin . Neither Jain (1982) nor Zollner (1993) include arsenic as an enzyme inhibitor per se, but list enzymes inhibited by arsenate and arsenite ions, that is, by compounds of arsenic.)... 

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