DISINFECTION 227
situation since all other factors are held constant. Often, it is
the variation in response of the system to changes in experi-
mental conditions which is of particular interest. This varia-
tion in response may be monitored by constructing families
of survivor curves, one curve for each level of the factor
being varied. Comparison of curves within these families
indicates the nature of the change in response.
The two factors most prominent in the regulation of
disinfection processes are temperature and concentration of
disinfectant. For both of these factors, it is found that their
influence on the disinfection process varies in a regular
manner over a fairly wide range of conditions. As a result,
empirical parameters have been derived which enable the
influence of these two factors to be characterized in a con-
venient form.Temperature CoefficientIn general, it is found that the activity of a disinfectant varies
directly with the temperature, i.e., the higher the tempera-
ture the greater the activity. This change in activity may be
quantified by expressing the disinfection rates observed at
two different temperatures as a ratio. The value of this ratio
is found to remain reasonably constant over a wide range of
temperature. This may be expressed mathematically:uTT (^21) kk
21
/
where k 1 and k 2 are the rate constants at temperatures T 1 °C
and T 2 °C, respectively, with T 2 greater than T 1. The con-
stant, u, is termed the temperature coefficient, and assumes
a numerical value which is characteristic of the agent and,
to a certain extent, of the organism, employed. The super-
script ( T 2 − T 1 )°C is necessary to indicate the temperature
difference. Since the disinfection rate is inversely related to
killing or extinction time, the latter may be used instead. The
expression then becomes
u
TT (^21) tt
12
/
where t 1 and t 2 are the extinction times at temperatures T 1 °C
and T 2 °C, respectively. At high value for u indicates that the
process is relatively sensitive to temperature changes.
It is usual to find two versions of the temperature coeffi-
cient in most common use: the coefficient for a 1°C tempera-
ture change, u, when the superscript is usually omitted; and
the coefficient for a 10°C temperature change, u 10 , which
may be sometimes expressed as Q 10. The popularity of the
10°C coefficient follows from its use to characterize changes
in reaction rates in chemical systems. This enables interest-
ing comparisons to be made between disinfection processes
and non-living chemical reactions. Attempts have been made
to deduce modes of death of the organisms by such compari-
sons. However, subsequent biochemical investigations have
tended to disagree with these deductions.
Dilution Coefficient
The activity of disinfectants under otherwise constant condi-
tions is found to vary directly with the concentration of dis-
infectant employed over a considerable concentration range.
As before, killing or extinction times are usually employed
as a measure of disinfection rate. The effect of concentration
may be expressed mathematically as:
c^ ht a constant
or:
h log c log t a constant
where
c concentration of disinfectant
t extinction time
h dilution coefficient.
A high value for the dilution coefficient indicates that the
process is relatively sensitive to changes in concentration of
disinfectant. The dilution coefficient is sometimes referred
to as the concentration exponent.
Other Factors Influencing Activity
The antimicrobial activity of several disinfectants is influenced
to a considerable extent by changes in pH. For example, a rise
in pH results in a decrease in the activity of phenols (Bennett,
1959), organic, acids, compounds liberating chlorine, benzoic
acid and iodine, although iodine is less affected by acidity
than is chlorine. An increase in pH increases the dissociation
of phenols and benzoic acid (Wedderbern, 1964); chlorine in
water forms HClO, which is dissociated with a rise in pH with
a concomitant loss of activity. In contrast to the above, how-
ever, there is an increased microbicidal activity of the quater-
nary ammonium compounds (QACs) and of acridines (Foster
and Russell, 1971); in the case of QACs, the effect of pH is
considered by Salton (1957) to be on the cell rather than the
disinfectant molecule, since the number of negatively-charged
groups on the bacterial surface will be increased as the pH
rises, thus influencing the number of positively charged mol-
ecules which can be attached.
Another factor which influences the activity of certain
antimicrobial agents is organic matter, e.g., the presence of
blood, serum, pus, etc. In general terms, the more chemically
reactive a compound, the greater the effect of organic matter
on its activity. This is particularly true with the hypochlorites.
other examples are provided under individual compounds
later.
Mathematical Models
A considerable number of mathematical models have been
derived at various times, in attempts to reconstruct the disinfec-
tion process. These have utilized deterministic, probabilistic,
and thermodynamic approaches to the problem, and have been
reviewed in detail by Prokop and Humphrey (1970).
In general, these models are based on attempts to recon-
struct the survivor curves obtained, even though, as already
discussed, these are liable to be changed by changes in
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