The Behavior of Chlorine as a Water Disinfectant

Author(s): Gordon M. Fair, J. Carrell Morris, Shih Lu Chang, Ira Weil and Robert P. Burden
Source: Journal (American Water Works Association), Vol. 40, No. 10 (October 1948), pp. 1051-
1061
Published by: American Water Works Association
Stable URL: http://www.jstor.org/stable/41234959
Accessed: 21-02-2016 10:23 UTC

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The Behavior of Chlorineas a Water Disinfectant
By GordonM. Fair, J. CorreliMorris,Shih Lu Chang,
Ira Weil and RobertP. Burden
A paperpresented on May 5, 1948,at theAnnualConference, Atlantic
City,N.J.,byGordonM. Fair, Dean ofFacultyofEng., Grad.School
of Eng., Harvard Univ.,Cambridge, Mass.; J. CarrellMorris,Asst.
Grad.School of Eng.,HarvardUniv.,Cam-
Prof,of San. Chemistry,
bridge,Mass.; Shih Lu Chang,Asst. Prof,of San. Biology,Grad.
School of Eng. and School of Public Health,Harvard Univ.,Cam-
bridge,Mass.; and Ira Weil and RobertP. Burden,bothof Grad.
Schoolof Eng.,HarvardUniv.,Cambridge, Mass.

the 40 years of its use as be interpreted only when that chem-

a disinfectantfor municipal water istry is known. This statement is es-
supplies in America, chlorine has pecially true for chlorine because of the
proved itself to be so powerful and wide variety of chemical reactions in
versatile that it and certain of its com- which it participates.
pounds are almost the sole chemicals
General Properties
employed for this purpose today. Yet,
despite the accumulation of a large The element chlorine is the second
amount of practical knowledge about member of the seventh column of the
its use, science is only now beginning periodic table. Surrounding its nu-
to unravel the fundamental chemistry cleus, it has an outer shell of seven
and biology of its disinfecting action. electrons, and, since an outer shell of
It is the purpose of this paper to de- eight electrons has great stability,atoms
scribe the recent advances which are of chlorine show a strong tendency to
leading to an understanding of the be- acquire an extra electron to complete
havior of chlorine as a water disinfect- a shell of eight. The tendency to ac-
ant and to indicate brieflythose large quire electrons manifests itself as oxi-
areas of its behavior that still remain dizing activity. Correspondingly, ele-
to be explored. mental chlorine is a powerful oxidizing
agent and in the great majority of
Chemistry of Chlorine in Water its chemical reactions acts as an oxi-
The destructionof bacteria and other dizer. Other properties of elemental
micro-organisms by a substance such chlorine are of importance in connec-
as chlorine is essentially the result of tion with its technical use as a disin-
a chemical reaction between the active fectant. Because it may be liquefied at
agent and some vital substance neces- room temperature,at pressures of 5 to
sary to the functioningof the organism. 10 atmospheres, it is relatively easy to
The characteristic action of the agent transport and store in
necessary quan-
is dependent upon its chemistryand can tities. Because of its considerable solu-
1051

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1052 G. M. FAIRET AL. Jour.AWWA

bilityin water- 7,300 ppm. at 68° F. acid that the major disinfecting action
and 1 atmosphere- littledifficulty
is ex- of chlorinesolutionsis associated.
periencedin administering it to water
in closelycontrolledamounts. Ionizationof HOCl
Hypochlorousacid undergoesa fur-
Hydrolysis therreactionwithwater,one of ioniza-
When chlorineis dissolvedin water tion,whichmay be representedby the
it undergoesa reaction of hydrolysis equation:
whichmaybe represented by thechem- HOC1^±H++OC1- (2)
ical equation:
This is an essentiallyinstantaneous,
C12+H2O^HOC1+H++C1-. .(1) reversibleprocess, so that one may
write an equilibriumexpressionfor it:
In strongsolutions,only a portionof
the chlorinereacts in this way, but at (H+XOC1-) -K
the concentrations (3)
ordinarilyemployed (HOCl)
forwater disinfection the hydrolysisis
virtuallycomplete. Only if the pH of TABLE 2
ofFreeChlorine
Percentage as HOCl
TABLE 1
forHOCl
IonizationConstants
PH
xjrkr>i Free Chlorine
pTcZ PerPP^HOC1
.Temperature K
°c. °f. 7(F8 4 100 1.000
0 32 2.0 5 99.7 1.003
5 41 2.3 6 96.8 . 1.033
10 50 2.6 7 75.2 1.33
15 59 3.0 8 23.3 4.3
20 68 3.3 9 2.9 34
25 77 3.7 10 0.30 331
11 0.030 3,300
thewateris below3, or ifchlorinecon-
centrationsin the neighborhoodof In this expression the parentheses
1,000 ppm. or more are employed,is stand for activities (approximately
any measurablequantityof Cl2 pres- equal to molar concentrations)of the
ent. In addition,it has been shown enclosed substances,and K, called the
(1) that the rate of the reactionis so ionization constant,depends only on
rapid that the hydrolysisis essentially the temperature.Values of K at vari-
completewithina veryfew seconds at ous temperatures,compiledfroma sur-
ordinarytemperatures. vey experimentalresearcheson the
of
Actually, therefore,it is incorrect ionization of HOCl, are shown in
by chlorine,for
to speak of disinfection Table 1.
chlorineitselfis not presentfor more Rearrangement of theionizationcon-
thana fleetinginstantunderconditions stantequationgives:
of practicaldisinfection.However,the
oxidizing capacity of the chlorine is
(oci-)- * (3a)
v '
retained in the hydrolysisproduct, (HOCl) (H+)
HOCl, and it is withthishypochlorous Hence the relative amounts of hypo-

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October1948 CHLORINE BEHAVIOR 1053

chlorous acid and hypochloriteion failureto carryout thetestsat thesame

presentin a solutionof "freechlorine"* pH value. The addition of chlorine
are a functionof the hydrogenion ac- tends to lower the initial pH value,
tivity,or pH. Analysesfor"freechlo- whereas the addition of hypochlorite
rine" always determinethe sum of the tendsto raise it.
two species. Table 2 shows the per-
Chloramines*
centageof the freechlorinepresentas
undissociated HOC1 at various pH The disinfectingaction of chlorine
values at 20°C. The tablealso records solutionsis furthermodifiedby the re-
the amount of free chlorinerequired action of the hypochlorousacid with
to give 1 ppm.of HOC1. certain substancesof frequentoccur-
Since, as will be shown,the HOC1 rence in natural waters. Ammonia
is the principaldisinfectant
in chlorine and manyorganicaminescombinewith
solutions,the efficiencyof these solu- hypochlorousacid to give the chlo-
tionsis a functionof pH and varies in ramines. Again, as in the ionization
much the same way as the percentage of the hypochlorous acid, the oxidizing
of HOC1. capacityof the chlorineis retainedby
the chloramines,so that starch-iodide
Hypochlorites and standard o-tolidine tests record
Hypochlorites,such as chloride of chlorine combined with nitrogen as
lime, calcium hypochlorite and bleach well as "freechlorine." Since the mo-
solution,establishthis same ionization lecularnatureof the chlorinehas been
equilibriumin water. For example, modified, however, the disinfecting
whencalciumhypochlorite is dissolved poweris also changed.
in water, it ionizes accordingto the The most importantof the reactions
equation: of chlorinewithnitrogencompoundsis
that of hypochlorousacid with am-
Ca(OCl)2->Ca+++2 OC1". . . (4)
monia itself. Although it is known
The hypochloriteions then combine thatthreecompounds,monochloramine
withhydrogenions fromthe water,as (NH2C1), dichloramine(NHC12), and
shownby the reaction: nitrogentrichloride(NC13), may be
H++OC1-^±HOC1 (2a) formed,very little has been reported
about the fundamentalcharacter of
This is just the reverseof the pre- these reactionsor about the interrela-
viousionizationequation(Eq. 2). The
tionshipsamong them. Investigations
same equilibriumexpressionand con- initiatedin the Laboratoriesof Sani-
stanthold,and the relativeamountsof
taryEngineeringat Harvard f on the
hypochlorous acid and hypochloriteion acid reactions
ammonia-hypochlorous
at a givenpH are exactlythe same as have so far led to these conclusions:
fora chlorinesolution. Hence, at the * See editorial
same pH, chlorinesolutionsand hypo- noteon p. 1061.
fThe resultsquotedas issuingfromthese
chloritesolutionsmust have the same laboratories
wereobtainedin researchessup-
disinfectingefficiencies.Reporteddif- portedin partby the Officeof the Quarter-
ferencesin efficiencyshould be attrib- master General, in part by the Engineer
uted eitherto experimental erroror to Researchand Development Laboratoriesand
in partby the Wallace & TiernanResearch
* See editorialnote on p. 1061. Fund in Chlorination.

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1054 G. M. FAIR ET AL. Jour.AWW'A

1. The formationof the chloramines chloramineshouldbe affectednot only

is a stepwiseprocessforwhichthe suc- by the pH, but also by the excess am-
cessive reactions monia present. This has been con-
firmedby spectrophotometric measure-
NH3+HOC1->NH2C1+H2O. .(5) ments,whichalso give a value of 6.7 X
NH2Cl+HOCl-*NHCl2+H2O. .(6) 105fortheequilibriumconstant,where:

NHC12+HOC1-+NC13 +H2O . . (7) _(NH4+)(NHC12) "

eq "K }
(H+)(NH2C1)2
may be written.
2. The rate of the firststep,the for- Table 3, calculated from this con-
mation of monochloramine,is very stant, shows the relative percentages
much dependent on the pH of the of monochloramineand dichloramine
solution. The maximum rate occurs at various pH values in solutionswith
at a pH of 8.3 and decreasesrapidlyat a chlorineto ammonia-nitrogen weight
ratio of 5:1.
higherand lower pH values. For ex-
ample,at 25°C, with0.8 ppm. of chlo- Just as the disinfecting efficiencyof
rine and 0.32 ppm. of ammonia-nitro- solutionsof freechlorinewill varywith
gen, it requiresabout one minutefor
the reactionof 99 per cent of the chlo- TABLE 3
rine,whereas at pH 5 it requiresap- EquilibriumChloramine Distribution
proximately 210 minutesand at pH 11 u Chlorineas
P
approximately50 minutes. The re- Dichloramine
actionhas been shownto be bimolecu-
percent
lar in accordance with the equation 5 84
presented,and the variation in rate 6 62
withpH can be calculatedpreciselyon 7 35
the basis of the solutionequilibriathat 8 15
9 6
occur. The rate of the reactionalso
varies greatly with the temperature.
pH because of a change in the HOC1 :
Q10values {see p. 1059) of 2.0-2.5, de- OC1" ratio,thatof chloraminesolutions
pendingon thepH, have been obtained. will
3. The generalstatementthatdichlor- vary dependingon the NHC12:
NH2C1 ratio. Consequently,any fun-
amine is formedpredominantly at low
damentalapproach to the problemof
pH values (5-6.5) and monochlor- disinfectionby chlorine or its com-
amineat highpH values (greaterthan musthave as a backgroundthe
7.5) can be interpreted in termsof the pounds
typesof basic chemicaldata that have
equilibriumreaction: been presented.
2 NH2C1+H+^NH4++NHC12. .(8) ChlorineDemand
An excess of hydrogen ions should Other reactionsthatlead to a loss of
displace this reaction to the right, oxidizing chlorine also occur. Since
yielding greater amounts of dichlor- HOC1 is a strongoxidizingagent, its
amineat lowerpH values. This equa- reactionwithreducingsubstancesis to
tion also predicts that the relative be expectedand resultsin the so-called
proportionsof monochloramine and di- "chlorinedemand." In the reactions

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October 1948 CHLORINE BEHAVIOR 1055

includedunderthis heading,the chlo- tive chlorine is probably reduced to

rine atom manifestsits great tendency chlorideand the stoichiometry of the
to gain electronsand is therebychanged process indicates that ammonia-nitro-
into the chlorideion or organicchlo- gen is oxidizedto the + 1 valencestate,
ride. In this form,the chlorineatom corresponding to N2O, butthismayre-
no longer has oxidizing properties sult froma combinationof products,
or disinfecting power. The substances suchas N2 and NO3~, ratherthanfrom
responsiblefor the reductionof chlo- any single product.
rine include inorganic Fef+, Mn^, Experimentsnow in progress have
NO2-, and H2S, along withthe greater so far establishedseveral points:
part of the organic material in the 1. The rateofthereactionis strongly
water. The reactionof the inorganic
substancesis generallyrapid and stoi- dependenton pH. A maximumrate
is. obtained at about pH 7.5 and de-
chiometric;thatof theorganicmaterial creases rapidly at high and low pH
is generallyslow, and its extent de- values.
pendsupon the excess concentration of
2. One of the substancesconcerned
available chlorinepresent. Since the in the break-pointreactions is prob-
amountof organicmaterialin natural
watersis usuallycloselyrelatedto the ably dichloramine.
3. The rate of the reactionis only
color or stain,the organicchlorinede-
mand can oftenbe approximately esti- slightlydependenton the temperature.
4. The rate of the reactionis very
matedfromthedepthof color. of
The occurrenceof these reactionsis dependenton the totalconcentration
dissolvedsalts.
a disadvantageto theuse of chlorineas
a disinfectant, for one must provide The complexitiesof the break-point
sufficient chlorineto take care of these reactionsare so greatthatconsiderable-
side reactionsbeforea reliable disin- additionalworkmustbe done beforea
fectingactioncan be assured. Yet the consistenttheoreticaltreatmentcan be
occurrenceoftheseside reactionsstems achieved.
from the same basic propertythat
makes chlorine such a potent disin- Biological Reactions of Chlorine
- its high oxidizing activity
fectant -The way in which the remarkable
and thebad musttherefore be accepted
disinfectingpowerof chlorinesolutions
with the good in order to secure theis achieved has been the subject of
activity. speculationever since the time of the
firstuse of chlorineas a disinfectant.
Break-PointReaction But the researchnecessaryto provide
Ammonia or organic amines may an experimental foundationfortheories
also exert chlorine demand in the of the mechanismof the disinfecting
"break-pointreaction." This is an- processwas begunonlya fewyearsago.
otherphenomenonwhich,like the for-
mationof chloramines,is of great im- Mechanismof Cell Destruction
portancein disinfectionbut of which Early ideas thattheactionofchlorine
virtuallynoneofthefundamental chem- was due to nascentoxygenor to a com-
istryis known. Even the productsof plete oxidative destructionof the or-
the reactionsare uncertain. The ac- ganismssoon had to be abandonedbe-

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1056 G. M. FAIRET AL. Jour.AWWA

cause of the small concentrationsof take place with - SH groups of the

hypochlorous acid requiredand because enzyme, which are oxidized by the
of the failureof otheroxidizingagents chlorine. This theoryprovidesan ex-
to worksimilarly. It was apparentthat planationof the extremesensitivityof
there was some selectiveattack on a organisms to chlorine,since enzymes
vital and highly sensitive portion of are present in cells in very minute
the cell, but no one knew what this quantitiesand yet are absolutelyes-
might be. The researches of Green sential as catalystsfor metabolicac-
(2) and his coworkers at Columbia tivity.
Universityhave now provideda satis- These ideas do not, however,fur-
factorysolution. Accordingto Green, nish a completeanswerto the problem
the death of the organismresultsfrom of chlorinedisinfection,for,when the

1111
100H£*>J.
= = = =
50yJp= I' ^
^
N
ppm.CI
., _0."05
20 '~
'' ' ' A'

¡ !115=111
0.05 ppm.CI

¡'ïËSpli 3) '| ' 0.07 ppm.CI

L 'OQ7 ppm CI~X^ ZZZ^I

0.5 1' == ~~ ' =
' I I Vi
„zEBEEpz
014 ppm' ' '
0.2
CI

J TTMI 1 KI
' -0.14 ppm.
V I I
'
'

0 5 10 15 20 25 0 20 40 60 80 100 120
Time-min
Contact Square of ContactTime- (min.)2

Fig. 1. Lengthof Survivalof Esch. coli Fig. 2. Lengthof Survivalof Esch. coli

a chemicalreactionof HOC1 with an enzymes are removed from the cell,

enzymesystemin the cell that is es- they are equally susceptibleto attack
sentialforthe metabolicfunctioning of by many oxidizing agents, such as
the organism. The enzyme probably H2O2 and KMnO4, whereas chlorine
attacked is triosephosphatedehydro- is markedlysuperiorin its attackon in-
genase, which is found in practically tact cells. The differencecan be ex-
all cells and is essentialforthe utiliza- plained on the basis that, for the in-
tion of glucose. Attack on other en- tact cell, not only the ability of the
zymesis not excluded,but triosephos- reagentto react withthe enzymeis of
phate dehydrogenasewas foundto be concern,but also its abilityto gain ac-
especiallysensitiveto oxidizing chlo- cess to the enzyme by penetrating
rine. The reactionwas presumedto through the cell wall. Laboratory

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October1948 CHLORINE BEHAVIOR 1057

studies indicate that it is this latter standing of the disinfectionprocess.

factor- the rate of diffusionof the Up to the
present,the only thorough
active agent throughthe cell wall- studies of these factorsas they relate
which largely determinesthe rate of to the action of chlorinesolutionson
disinfectionand the relativeefficiency bacteria have been made by Butter-
of various disinfecting materials. fieldet al (3) and by Wattieand But-
From this pointof view, it is not pri- terfield(4, 5). Even theiradmirable
marilythe strongoxidizingpower of results,which formthe basis for.the
HOC1 that makes it a superiordisin- followingdiscussion,are not entirely
fectant,though that is essential,but satisfactory fortheoreticalcalculations,
rather its small molecular size and however,and it is hoped that the ex-
electricalneutrality.These allow it to tremedifficulty of the experimentsthat
pass readilythroughthecell membrane. mustbe conductedwill notpreventfur-
Confirmation ofthishypothesis mustbe ther studies of this type with theo-
sought in more extensive fundamental reticalobjectivesin mind.

>j 1' '^

10 1 ■
*1 I 1THJ1II
I I i i i mi i ■ ■ , ■..TI

-5S 5Z.pHlO.7---
0.5 S- O- n-0.8-- 5- ------■" uzzi::::

E-
f^-fht"
pH 9.8 V
> 5
»«
i,pH9.5
V ii-1.3
I 0.2 S n«13 ^s- 2 ^ V |
¿ 0.1 N^- X:s.
= i::::¡ -
|
5 ==
EEjEjEi::::
J0,=^:::::S-^¡;.
0.05 ~^-pH8.5-::::
|
1 | I ■■ ^y*™ ^'^
""
OpH7
-^-|
5 PH7 .5* ö¡-Sii?
0.02 n«1.3 V N
0.2 1- ! -S^-
I
o.oi 1- 1 I I 11111 1- I I I I111
12 5 10 20 50 100 o.iI 1- I M M111 1- I 111III
ContactTimefor99% Kill- min. 10 20 50 100 200 500
ContactTime for50% Kill-min.
Fig. 3. Free Chlorine Requirements
Fig. 4. Chloramine Requirements

studiesof the rate of kill of organisms Time of contact. The effectof con-
by various disinfecting agents and of tact time on the
variationsin the rate with changing has killingof organisms
generallybeen expressedin terms
conditions. of Chick's Law, whichmay be written
DisinjectionEfficiency in the form:
The principalfactorsthat affectthe
efficiencyof destructionof a particular log-§-=-kt
ivo
(10)
species of organismsare: timeof con-
tact, concentrationof organisms,con- Here N/No is the fractionof the orig-
centrationof disinfectant,
temperature, inal numberof organismsremainingat
and nature of the disinfectant. Dy- timet, and k is a proportionalitycon-
namic studies of all these elements stant. A plot of log AT/Aro against t
are essential for a thoroughunder- forvarioustimesof contactshouldgive

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1058 G. M. FAIRET AL. Jour.AWWA
a straightline. Figure 1, prepared of concentrationof bacteriaon the rate
fromthe data of Butterfieldet al. (3) of disinfection. Butterfield(3) re-
for the destructionof Esch. coli by fersto one series of experimentswith
chlorineat pH 8.5 and 2°-5°C, shows an initial concentrationof organisms
that Chick's Law does not hold for tenfoldsmallerthanthatnormallyem-
disinfection
bychlorine. Similarcurves ployed and states that no significant
are obtained at other pH values, at differencein the percentagedestroyed
othertemperatures, forotherspeciesof at varioustimeswas observed. This is
bacteria* and for chloraminesas well the resultone would intuitivelyexpect,
as freechlorine. but it should be checked more ex-
Linear relationsare createdfor the tensively.
resultsin Fig. 1 if log N/No is plotted Concentration of disinfectant .
againstt2ratherthant. This is shown Changes in disinfectionefficiency
with
in Fig. 2. A relationof this character
can be explainedin two ways: ( 1) as i|
-
| | | | | |
a combination of slow diffusionthrough
0.5 - =
p^--^
M^
TABLE 4 2_
Temperature
Dependence Action
ofDisinfecting 02 -/y
ofAqueousChlorine
and Chloramines
0.1 (-
pH E Oio
cal.
s 0.05 zm-Lzm - izz
7.0 8,200 1.65
Aqueous 8.5 6,400 1.42 1 0.02 J-
Chlorine 9.8 12,000 2.13
10.7 2.50 °
15,000 0.01 J-
i y
u/
7.0 12,000 2.08 0.005 - ■=- ■
Chloramines 8.5 14,000 2.28
9.5 20,000 3.35
0.002

the cell wall and a rate of killingde- o.ooiI 1 1 1 1 1 1

pendentupon the concentration of dis- 5 6 7 8 9 10 11 12
pH
infectingmaterialinside the cell; (2) Fig. 5. Relative DisinfectingEfficiency
of
on the assumptionthatthereare three HOC1 and OC1"
to fouractive centersin the organism
and thatthe organismis not dead until concentrationof the disinfectantcan
all of thesecentershave been destroyed be expressed mathematicallyby the
(6). Further studies are needed to equation:
clarifythese points. Cnt= k (11)
Concentration of organisms. Virtu-
is known about the effect In this equation,C representsthe con-
ally nothing
* There are some exceptionsto the gen- centrationof disinfectant, t represents
eral failureto obey Chick's Law. For ex- the time requiredfor a constant per-
ample,destruction by centage of the organisms to be de-
of Shigella dysenteriae
chloraminesbelow pH 8 follows the law stroyedand mis a constantwhichmay
quiteclosely. Explorationof thereasonsfor be called the concentration
this mightyield information of considerable exponent.
value. High values of n indicatethatthe dis-

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October1948 CHLORINE BEHAVIOR 1059

infectant decreasesrapidlyin effective- Nature of disinfectant. Figure 3

ness as it is diluted; with low values shows thatthe disinfection of
efficiency
of n, the timeof contactbecomesmore chlorine decreases markedlywith an
important thanthedosage. increasein pH. This can be accounted
Values of n are determinedby plot- for by postulatingthat HOC1 is a far
tinglog C againstlog t and measuring strongerdisinfectant than OC1~, since
theslope of thelines. A seriesof such the percentageof the free chlorinein
plots,using the times requiredfor 99 the solutionin the formof HOCl de-
per cent of the bacteria to be killed, creases very rapidly with increasing
is shown in Fig. 3 for free chlorine pH, as shown previously. By assum-
solutions. A similarseries forchlora- ing an additive disinfectingefficiency
mines appears in Fig. 4, except that for HOCl and OC1", one can calculate
the times are for 50 per cent kills of a theoreticalcurveforthetotalamount
the bacteria,sufficient data for 99 per of freechlorinerequiredto produce a
centkillsnotbeingavailable. No theo- given percentageof kill in a specified
reticalsignificancecan be attachedto timeat variouspH values. The equa-
the values of n obtained; the equation tion employedis :
itselfis withouttheoreticalfoundation
and is justan expressionfor the cor-
relationof data.
Temperature of disinfection. If *-+-3%- a«
l+£-
eitherthe rate of diffusionthroughthe (H+)
cell wall or the rateof reactionwithan in whichR is the requiredtotal chlo-
enzymedeterminesthe rate of disin- rine,A is the concentrationof HOCl
fection,its variationwith temperature alone requiredto produce the desired
is bestexpressedby theequation:
percentageof kill and B is the ratio of
the efficiency of OC1~ ion to that of
log^ECrwo v
(i2); HOCl. In 5, the observed con-
*t* 4.575rir2 Fig.
centrationsof aqueous chlorine re-
The symbolsT2 and 7' stand for two
quiredto kill99 per centofEsch. coli in
temperatures(in degrees Kelvin) be- 30 minutesat 2°-5°C. have been
tweenwhichthe rates are being com- ted plot-
against the pH and fittedwith a
pared; txand 12 are the timesrequired theoreticalcurve whose
forequal percentagesof kill at a fixed equation,cor-
responding to Eq. 14, is :
concentrationof disinfectant;and E,
called the activationenergy,is a con- 2.2 X10-8
stant characteristicof the reaction. (H+)
When T2 - 7' = 10, the ratio tjt2 is ^ = 0.005-
0Q12x22xl0_8
frequently called Q10. For studies in +
the vicinityof 20°C., Q10 is approxi- (H+)
matelyrelatedto E by the formula: The value of B used, 0.012, indicates
thatthe OC1" ion possesses about 1/80
(13) of the disinfecting efficiencyof HOCl
IogÖw=39|ö-0 underthe statedconditions.
Values of E and Q10 for the destruc- Since Fig. 5 showsthatat pH 7 and
tionof Esch. coli by chlorineand chlor- 8.5 essentiallythewholedisinfecting ac-
amines, computedfrom the work of tion is due to HOCl, whereas at pH
Butterfieldet al. (3) , appearinTable 4. 10.7 it is chieflydue to OC1",the char-

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1060 G. M. FAIRET AL. Jour.AWWA

acteristicconstantsobtained at these Shigella, Salmonella and Vibrio spe-

pH values may be identifiedwith the cies) ; (2) intestinalprotozoa,themost
disinfectingprocessforthecorrespond- importantrepresentativeof which is
ing species. Thus the values n = 1.4 Entameba histolytica;(3) worms,such
and E - 7,000 calories are connected as the blood flukes(Schistosoma spe-
with disinfectionby HOC1, while the cies) ; and (4) viruses- for example,
values n =0.8 and E = 15,000 cal. re- the virus of infectioushepatitis.
ferto the reactionof OC1". It is of in- Each of these groups of organisms
terestto note that the value of E = differsin its reactionto chlorine,and
*
7,000 forthe action of HOC1 is in the thereis considerablevariation,too, in
range of activationenergiesfor diffu- the behavior of the differentspecies
sion, whereas the value of E = withinthe individualgroups. There is
15,000 for the action of OC1~ is more some evidence,furthermore, that the
characteristicof a chemical reaction. comparativereactionofdifferent organ-
Perhaps, therefore,the rate-determin- isms to one form of chlorine is not
ing processes are differentfor these necessarilymaintainedrelativeto other
two substances. forms.
A plotsimilarto thatin Fig. 5 'might Much researchwill have to be done
be made for the chloramines,but the beforethe patternof the biologicalre-
data are so scanty,and reliablevalues actionsof significant organismsis fully
forthe chloramineequilibriaare so re- established. Of the four groups of
cent,that this has not yet been done. pathogenicorganisms,the entericbac-
However,themannerin whichthedis- teria are most easily destroyedby or-
infectingefficiency of chloraminesolu- dinarychlorination procedures,whereas
tionsdecreaseswithincreasingpH (see the cystsof E. histolyticaappear to be
Fig. 4) indicates qualitatively that the most resistant(7).
NH2C1, which is favoredat the high Conclusion
disinfect-
pH values, is a less efficient
ant thanNHC12. In addition,a tenta- A remarkablerecordof accomplish-
tive quantitativeevaluationof the rate menthas been establishedby chlorina-
of change with pH suggeststhat the tion duringthe 40 years since it was
entiredisinfectingaction in the solu- firstappliedto a publicwatersupplyin
tions studied may have been due to America. Pride in the reductionof
NHC12 and that NH2C1 may be so water-bornetyphoid fever and dys-
weak a disinfectantthat its influence entery,however,must not obscurethe
was not felt. need for intensifying researches into
the chemistryand biology of water
Effectson Other Organisms chlorination.The cost of waterdisin-
The biological reactionsof chlorine fection,to be sure,is negligiblein terms
have been illustratedin this paper for of the savings that it has effectedand
but a single organism,Esch. coli, a will continueto effectin the preven-
"type organism of water contamina- tion of sickness,sufferingand death.
tion" ratherthana water-bornepatho- The total annual expenditurefor chlo-
gen. The pathogenicorganismsthat rine as a water disinfectantand its
shouldbe consideredin connectionwith significance in thepreventionof water-
water chlorinationinclude: (l' the borne disease are sufficiently large,
entericvegetativebacteria (Eberthella, however,to warrantthe cost of all the

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October1948 CHLORINE BEHAVIOR 1061

investigations thatmustyetbe entered Mode of Action of Chlorine. Jour.

to the under- A.W.W.A., 38:1301 (1946).
upon acquire necessary 3. Butterfield, C. T. et al. Influenceof
standingof this- surelythe most im- pH and Temperature on the Survivalof
portant- water works chemical. No Coliforms and EntericPathogensWhen
industrycan long affordthe luxuryof Exposed to Free Chlorine. U.S. Pub.
Health Rpts.,58:1837 (1943).
ignorance,even thoughit is tempered 4. Wattie, E. & Butterfield, C. T. Rela-
by accomplishment; neithercan a pro- tive Resistanceof Escherichiacoli and
fessionthat,like the waterworkspro- Eberthella typhosa to Chlorine and
Chloramines.U.S. Pub. Health Rpts.,
fession,is so clearlydevotedto thepro- 59:1661 (1944).
tection and promotionof the public 5. . Influenceof pH and Tempera-
health. ture on the Survivalof Coliformsand
Enteric PathogensWhen Exposed to
Chloramine. U.S. Pub. Health Rpts.,
References 61:157 (1946).
6. Rahn, O. Injuryand Death of Bacteria
1. ShILOV, E. A. & SOLODUSHENKOV, S. M.
by Chemical Agents. Biodynamica,
The Velocityof Hydrolysisof Chlorine. p, 12 (1945).
ComptesRendus Acad. Sci. U.R.S.S., 7. Fair, G. M.; Morris, J. C; & Chang,
3:1:17 (1936). S. L. The Dynamicsof Water Chlorin-
2. Green, D. E. & Stumpf, P. K. The ation. J.N.E.W.W.A., 61:285 (1947).

Editor's Note
The AWW.A. committeewhich has prepared the chapteron "Chlorina-
tion and Other DisinjectionPractices" for the forthcoming Manual of Water
Quality and Treatmenthas set up the terms"free available chlorineresidual"
and "combinedavailable chlorineresidual" to replace the terms
"free chlorine"
and "chloramine" respectively. The committeeconsiders that
free available
chlorinemay exist as elementalchlorine,hypochlorousacid
(HOCl) or hypo-
chloriteion (OCl~) - or as hypochlorousacid and eitherof the othertwo
forms
-depending on the pH of the solution. The concept of combinedavailable
chlorineenvisages the presenceof ammonia or other nitrogenous
compounds
whichhave the capacityio combinewith chlorine{or hypochlorous
acid) and
thusmodifyits rate of bactericidalaction.
Based on theforegoingdistinction, the committeehas suggestedthatchlo-
rinationpracticesbe classifiedas freeresidual chlorinationand combinedresid-
ual chlorination.

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