Diagram I — Effect of Colour on Absorption of Chlorine by Water
Absorption of Chlorine
by water at 63° F.
Value of K calculated from
K = Log (N1/N2) When t1 = 0
t2 - t1
Time of
Contact
in
Minutes
Colour of Water
Time of
Contact
in
Minutes
Colour
3
25
40
3
25
40
Nil
10.00
10.00
10.00
5
9.62
7.70
6.50
5
0.0033
0.0227
0.0374
10
9.41
7.03
5.91
10
0.0026
0.0153
0.0228
20
9.17
6.40
5.18
20
0.0018
0.0096
0.0190
40
8.95
5.82
4.47
40
0.0012
0.0057
0.0087
60
8.85
5.63
3.90
60
0.0008
0.0041
0.0068
80
8.80
5.58
3.65
80
0.0007
0.0032
0.0056
The shape of the curve obtained with a colour of 40 p.p.m. somewhat resembled that of a mono-molecular reaction and the results were calculated accordingly. The mathematical expression of this law is dN/dt = KN where N is the concentration of the available chlorine in parts per million. Integrating between t1 and t2 the formula K = log(N1/N2)/(t2 - t1) is obtained. If the compound absorbing the chlorine were simple in character, and the chlorine were present in large excess, the value of K would be constant. In the experiments recorded, K constantly decreases, due to the decreasing concentrations of the reacting substances and the complex nature of the organic matter.
The results show the effect of organic matter on the reduction of the chlorine concentration available for germicidal action and also the importance of avoiding a local excess of chlorine (vide p. 41).
An effort has been made by some observers to find a quantitative relation between the organic matter, expressed as oxygen absorbed in parts per million, and the chlorine required for oxidation, but without definite result. Some of the results obtained are given in Table VII.
Table VII — Oxygen to Chlorine Ratio
Observer.
| Oxygen Absorbed | |
| Ratio | ————————. |
| Chlorine Absorbed |
Rouquette
1
Bonjean
0
.5
Orticoni
Less than 1
Valeski and Elmanovitsch
0
.4
Race
0
.4
Theoretical
0
.22
The value of 0.4 (0.39) obtained by the author is the average of over one hundred determinations covering a period of two years. The experiments of Zaleski and Elmanovitsch were made with the water of the Neva River.
The divergence in the ratios affords additional evidence in favor of reaction (2) mentioned on page 28 and also shows that the chlorinated compounds are less readily oxidized than those from which they are produced. Heise8 has found that the amount of chlorine consumed is usually proportional to the concentration in which it is added though not necessarily a function of the concentration of the organic matter.
Temperature. The evidence regarding the effect of temperature upon the dosage required is somewhat conflicting. Ellms (vide supra) found that the velocity of the germicidal action varied directly with the temperature and this has also been the author’s experience with laboratory experiments. Typical examples of these are given in Tables VIII and IX.
Table VIII — Effect of TemperatureB
| Available Chlorine 0.4 Part Per Million | |||
| Contact Period. | Temperature, degrees, Fahrenheit. | ||
| 36 | 70 | 98 | |
| Nil | 424 | 424 | 424 |
| 5 minutes | 320 | 280 | 240 |
| 1.5 hours | 148 | 76 | 12 |
| 4.5 hours | 38 | 14 | 3 |
| 24 hours | 2 | 0 | 0 |
| 48 hours | 2 | 0 | 0 |
| B Results are B. coli per 10 c.cms. | |||
Table IX — Effect of TemperatureC
| Available Chlorine 0.2 Parts Per Million | ||||
| Contact Period. | Temperature, degrees, Fahrenheit. | |||
| 36 | 70 | 98 | ||
| Nil | 240 | 240 | 240 | |
| 5 | minutes | 240 | 250 | 235 |
| 1 | hour | 245 | 235 | 195 |
| 4 | hours | 215 | 190 | 170 |
| 24 | hours | 143 | 130 | 115 |
| 48 | hours | 130 | 59 | 19 |
| 72 | hours | ... | 28 | ... |
| 96 | hours | ... | 16 | ... |
| 120 | hours | ... | 6 | ... |
| C Results are B. coli per 10 c.cms. | ||||
The reaction velocity of a germicide is proportional to the temperature9 and the influence of temperature may be mathematically expressed by the formula K1/K2 = θ(T2 - T1), in which K1 and K2 are the constants of the reaction at temperatures T2 and T1, respectively, and θ is the temperature coefficient. From the value of θ, the velocity constant of a germicide for any temperature may be calculated from the equation KT = K20° × θ(T - T20°). K1 and K2 are obtained from the formula KT = log(N1/N2)/(t2 - t1) in which N1 - N2 is the number of bacteria destroyed in the interval t2 - t1.
A reduction of temperature also lowers the oxidizing activity of the chlorine so that a greater concentration is available for germicidal action. This is shown by the results plotted in Diagram II.