CHEMICAL PROPERTIES 603 



rett et al., 1940), but it is doubtful if these polymers occur in sufficient 

 concentrations near neutrality to be biologically important. 



Aqueous solutions of arsenious acid are quite stable in air up to at least 

 pH 10-11. However, if certain oxidants or catalysts are present, oxidation 

 to arsenate may occur rapidly. Arsenite can be oxidized by Fe+++ and, 

 if a further system is present whereby the Fe++ can be reoxidized, arsen- 

 ate will be formed when only catalytic amounts of Fe+++ occur (Wieland 

 and Franke, 1928). Such reactions, furthermore, occur at neutrality. 

 The redox system: 



HAsO, + 2 H2O ^ H3ASO4 + 2 H+ + 2e~ 



has an ^0 = + 0-559 v, so that Eq (pH = 7) would be near - 0.27 v. 

 Arsenite could thus be oxidized by a variety of substances present in tissues, 

 and this will be discussed in greater detail in the section on the metabolism 

 of arsenite (page 785). 



A point often totally neglected, and occasionally of importance in work 

 with enzymes and tissues, is the formation of poorly dissociated or relatively 

 insoluble salts between arsenite and the alkaline earth or heavy metal 

 ions. For example, if dilute CaClg solution is added to a solution of potas- 

 sium arsenite, a precipitate of calcium arsenite, Ca(As02)2, appears, this 

 being soluble to the extent of only 0.04% (around 1.8 mM). Thus the 

 presence of these metal ions may decrease the free arsenite concentration 

 and, conversely, arsenite in some cases may exert a secondary effect on 

 the system studied by reducing the concentrations of enzymically or 

 physiologically active cations, in a manner similar to that for malonate 

 or fluoride. 



It may be noted that the sodium and potassium arsenites available com- 

 mercially are not always pure. They may contain mixtures of the meta- 

 and orthoarsenites and, hence, for quantitative work the arsenite concen- 

 tration should be determined. In addition, some preparations are 95% 

 or less pure and are apt to contain active impurities. A pure sample of 

 arsenic trioxide produced much less gastrointestinal damage than did 

 the usual impure preparation (Harrisson et al., 1958). Since the impure 

 preparation contained 1.2% SbgOg and small amounts of heavy metals 

 it was suggested that the tissue damage might have been caused by these 

 rather than the arsenical. Similar problems could, of course, arise in enzyme 

 work. 



Organic Arsenicals 



The earliest therapeutic arsenicals were usually the pentavalent arsonic 

 acids, but it was recognized by Ehrhch that reduction to the trivalent 

 forms is probably necessary for activity, inasmuch as substances such 

 as atoxyl are generally inactive in vitro, and become active upon reduction. 



