The Application of Certain Laws of Physical Chemistry in the Standardization of Disinfectants

Abstract

The idea of standardizing disinfectants was first seriously proposed by Rideal and Walker. I Previous to that .time, despite the fact that the germicidal properties of a great many chemical substances had been thoroughly investigated, no scientific attempt had ever been made to establish a common basis of comparison. The results of any disinfection experiments are fundamentally influenced by such conditions as temperature, character of the organism employed, number of organisms in unit volume, and character of the medium. In the absence of complete data covering these points results are practically worthless, at least for purposes of comparison. Even with such data given it is still impossible, owing to the variable conditions obtaining in practice, to establish any relationship, or order of excellence, among the various disinfectants. At best we can only hope to establish such relationship under specified experimental conditions. Rideal and Walker proposed to establish phenol as a standard. This selection was made because phenol is a substance readily obtainable anywhere in a comparatively pure condition. The effect of phenol and that of any other disinfectant may be determined simultaneously using the same test organism. The germicidal power of the disinfectant under investigation is then expressed as the "carbolic-acid coefficient," this being the ratio of the concentrations of the two disinfectants which will kill all the test organisms in the same time. Rideal and Walker recognized also that the time required to disinfect was dependent upon the number of organisms initially present as well as upon the temperature, a fact previously pointed out by Kronig and Paul. 2 These variable factors were controlled as closely •

The evaluation of bactericides

“…From the values of k at different concentrations and at different temperatures, the concentration exponent n' (Watson, 1908) and the temperature coefficient 0 (Chick, 1908(Chick, , 1910Phelps, 1911) can be estimated; and from n the true reaction velocity (constant at all concentrations) can be calculated from the formula -B KCn't = log1o b' where K is the true reaction velocity, C is the concentration of disinfectant, n' the concentration exponent, t the time necessary for disinfection, B the initial number of organisms, and b the final number of organisms. These characteristics K, n', and 0, Phelps suggests, should be deterinined for each disinfectant and would form a much more accurate and complete estimation of disinfectant activity than any tests based on end-point methods or the calculation of a coefficient from one estimation at one concentration.…”

“…aureus. Phelps's (1911) original suggestion was made on the assumption that the logarithm of the survivors plotted against time always gave a straight line. It is obvious from mote recent work that this is not so (Henderson Smith, 1921, 1923Withell, 1938Withell, , 1942.…”

“…If Phelps's (1911) suggestion is followed and the reaction rates compared, the results in Table 2 show the degree of accuracy attained. As a straight line is not always obtained when the logarithm of survivors is plotted against time, the calculation of relative rates is complicated, for the relative reaction rates vary over the course of the reaction.…”

“…If the temperature coefficient, 0 (Chick, 1908(Chick, , 1910Phelps, 1911) The evaluation of bactericides Apart from the difficulties associated with end-points, the coefficients obtained in the present tests are calculated from one concentration of phenol and one concentration of test bactericide. This is also an arbitrary value, for the relative activity of two bactericides will depend on relative values of the concentration exponent, n, for each bactericide; if both phenol and the test bactericide have identical values of n the coefficients will be the same at all concentrations which are compared, but if values of n are different then different coefficients will be obtained for every different concentration compared.…”

Biofilms and hygiene on dairy farms and in the dairy industry: sanitation chemical products and their effectiveness on biofilms - a review

“…At the same time, they expressed graphically for the first time the correlation between the logarithm of the number of surviving microorganisms and the contact time; this is almost linear. Chick (1908), Watson (1908) and Phelps (1911) continued in this study. They suggested a mathematical model which correlated the concentration of the disinfection agent with the level of the disinfection effect on the tested microorganisms.…”