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A Note on the Variation of the Rate of Disinfection with Change in the Concentration of the Disinfectant

Published online by Cambridge University Press:  15 May 2009

Herbert Edmeston Watson
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In a recent paper by Miss Chick on “The Laws of Disinfection”, it was pointed out that disinfection of bacteria is strictly analogous to a chemical reaction in which individual bacteria play the part of molecules. Thus, if n be the number of bacteria present at any time t during dis-infection, , where K is a constant. Also, if K1, K2 are these constants for two different temperatures is also constant, i.e. Arrhenius' formula for the temperature coefficient of chemical reactions holds good in the case of bacteria as well. In addition to this, it was found that the relation between the concentration of the disinfectant and the time of disinfection (that is, the time required to reduce the original number of bacteria by a given percentage) might abe approximately expressed by the empirical law

where C is the concentration at time t.

Type
Research Article
Information
Journal of Hygiene , Volume 8 , Issue 4 , September 1908 , pp. 536 - 542
Copyright
Copyright © Cambridge University Press 1908

References

page 536 note 1 This Journal, vol. III. p. 92, 1908.Google Scholar

page 537 note 1 It may perhaps be here mentioned that in cases of this kind, a graphical method is greatly superior to the usual methods of calculation, provided that the quantities plotted are so chosen that the resulting curve extends well across the paper, and, if possible, approximates to a straight line. Actually in the present case, the advantage is not very great, but in formulae such as , employed by Madsen, and Nyman, , Zeitschr. f. Hygiene, vol. LVII. p. 388, 1907CrossRefGoogle Scholar, and Chick, H., this Journal, vol. VIII. p. 92, 1908Google Scholar, to express the reaction velcity of disinfection, the calculated value of K may lead to quite erroneous results, firstly because the values t and n may be inaccurate if taken from a single experimental value, as is usually the practice, and secondly, because the effect of a given experimental error on the value of K is greatly exaggerated when t−t0 is small, while the same error when t−t0 is great is almost inappreciable. The magnitude of these errors is, however, at once seen from a suitably drawn curve. Consequently, before making any deductions from values calculated by means of a formula, it is always advisable to examine the graphical solution to see if they are justified.