The Disinfection Concentration Exponent *

It is of course well recognized that the length of time required by a germicide to kill a given number of bacteria varies with the concentration of the substance. The relation between concentration and time can usually be expressed by the formula Ct k, often written in logarithmic form log t = log kn log c for convenience The obvious value of the concentration exponent n is that it gives direct information as to the effect of dilution upon the speed with which a disinfectant can kill. Silver nitrate and iodine, with n values of 1, can be diluted considerably without undue effect upon the time, whereas phenol, with a value of 6, rapidly loses its practical efficacy. Values for the concentration exponents have been determined for relatively few germicides, and the results obtained by different workers have often been discordant. It is always found, however, that the substances which have large n values belong to compounds which are surface active in character. In a recent papers Tilley has reported numerous values for concentration exponents, some of which, for alcohols, were as high as 12. Some attempt has been made to account for the existence of these high powers. Watson' has suggested that they may indicate chemical reactions of high order, but Buchanan and Fulmer' state that "such a value as is secured for n (if a constant is secured) should not be regarded as related directly to the order of the reaction, but as an arbitrary constant useful for comparative purposes." It has been mentioned that the disinfectants which have high concentration exponents possess surface-active properties. We know, too, that in any homologous series of substances, such as the alcohols, the surface tensions parallel thegermicidal properties. Presumably, therefore, the interfacial tension between germicide and cell also parallels the killing activity, and, if surface forces play any part in disinfection, will probably be the important factor. Attempts have been made to invoke such forces as being the active agents in killing, but the direct action has never been demonstrated and seems improbable.