128 DISINFECTION 



been known to be highly germicidal. Kronig and Paul (1897) compared the dis- 

 infectant activity of certain oxidizing agents with their oxidative capacity, as 

 measured by the method of electrical oxidation chains. According to this method, 

 oxidizing agents are arranged in order of decreasing oxidizing capacity thus : 

 HNO3, dichromic acid, chloric acid, CI2, HaSaOg, and permanganic acid. This 

 order was, with the exception of chlorine, the same as that of the germicidal action 

 of these substances. Chlorine, bromine and iodine were found to be germicidal 

 in inverse order to their atomic weight. Their action appears to depend on the 

 liberation of nascent oxygen. Ozone is another powerful oxidizing agent ; like- 

 wise H2O2, a 3 per cent, solution of which kills anthrax spores in an hour. One 

 of the most commonly used of this group of chemical substances is KjMnjOs ; 

 like (NH4)2S208 its action is increased by the presence of HCl. Kronig and Paul 

 prepared a mixture containing 1 per cent. KaMugOs and 1-1 per cent. HCl dis- 

 solved in water, and found that it would kill anthrax spores in 30 seconds. A 

 similar mixture containing 3-7 per cent. (NH4)2S208, and 1-1 per cent. HCl was 

 found by Andrewes and Orton (1904) to exercise an effecji; very nearly as power- 

 ful. Both these are extremely potent, but even more potent is HOCl, which in 

 a concentration of 0-01 per cent, kills anthrax spores in 30 seconds. So far as 

 activity is concerned, this is one of the most powerful germicides we know. 

 Bleaching powder acts by virtue of its ability, when acted upon by weak acids 

 such as H2CO3, to yield nascent oxygen, which then combines to form HOCl. 

 HOCl combines with organic substances containing the =NH group, to form 

 chloramines. 



Ri Ri 



>NH + HOCl -> >NC1 + H2O 

 R2 R2 



It is found that all bodies containing the NCI group are strongly antiseptic (Dakin 

 1915). 



(3) Reduction. — Certain salts, such as the sulphites and the ferrous compounds, 

 appear to act by virtue of their reducing power. Apart from such salts there are 

 other substances that act mainly as reducing agents — sulphurous acid and form- 

 aldehyde. A 5 per cent, solution of formaldehyde, i.e. a 1-8 dilution of the 

 commercial formalin, kills anthrax spores in between 1 and 2 hours. 



(4) Molecular Action. — In a previous section we saw that certain acids, such 

 as acetic and benzoic, and in fact most of the organic acids, act not by virtue 

 of their H-ion concentration but by virtue of the undissociated molecule. Benzoic 

 acid is dissociated very slightly, and its strong disinfectant power must therefore 

 be attributed to the benzoate anion or to the undissociated molecule. Probably 

 the same explanation will account for the action of the salts of the organic acids. 

 The combination of mercury salts with the — SH compounds of bacteria described 

 by Fildes (see p. 122) is regarded by Albert (1942) as an example of molecular 

 action. 



(5) Ionic Action. — It is clear that salts which are freely dissociated in solution 

 owe their germicidal power to the action of the ions into which they are dis- 

 sociated. The way in which these ions act is a matter for speculation. Bayliss 

 (1924) points out that there are three ways in which electrolytes may exert their 

 influence on living matter, (o) They may produce effects through the electrical 

 charges that they bear ; this is specially marked with ions with valencies above 



