The conductivity of water having parts per billion concentrations of oxygen
, hydrogen, and bicarbonate was measured while the water was irradiated by
a low-pressure mercury vapor lamp, which was turned on and off periodically
. A cell normally used for measurement of dissolved oxidizable carbon was m
odified for use in these measurements. When the lamp is turned on, the cond
uctivity increases (sometimes decreases) with a time constant of about 50 m
s; when the lamp is turned off, the conductivity changes in the opposite di
rection with a time constant of about 275 ms, but does not return tb its va
lue before the lamp is turned on. The lamp step (difference between conduct
ivity with lamp on and conductivity with lamp off) depends on the intensity
of radiation and on the concentrations of oxygen, hydrogen, and bicarbonat
e. It is negative when [O-2] is less than approximate to 10(-10) M and posi
tive for higher [O-2]. increasing to a maximum at [O-2] approximate to 10(-
7) M. The presence of dissolved H-2 increases the lamp step. The lamp step
increases in magnitude when the lamp intensity increases, without being pro
portional to intensity. Experiments were performed that show that the react
ions responsible for the changes in conductivity occur in bulk solution and
not at the cell electrodes. A theoretical model to explain the changes in
conductivity was developed. It assumes that the absorption of a photon of u
ltraviolet radiation converts one molecule of water to a hydrogen and a hyd
roxyl radical (H . and . OH), and that these react with H+, OH-, and other
dissolved species. Some thirty bimolecular reactions are considered, with r
ate constants taken from the literature. The differential equations giving
the changes in the concentrations of twelve species are solved numerically.
The rate of generation of H . and . OH is varied with time to represent th
e turning on and off of the ultraviolet lamp. From the species concentratio
ns, the conductivity is calculated as a function of time, yielding calculat
ed lamp steps in general agreement with our experimental results. The speci
es responsible for the lamp steps can then be identified, and the important
reactions elucidated. The conductivity is always dominated by the contribu
tion of H+. It is shown that a substantial negative lamp step, found for ve
ry low oxygen concentrations, cannot occur in completely pure water. Dissol
ved carbon that has been oxidized to bicarbonate must be present. Hydroxyl
radicals produced by irradiation react with HCO3- to give the carbonate rad
ical anion, C .O-3(-). Because the pK of the parent acid HC .O-3 is substan
tially larger than that of H2CO3, formation of C .O-3(-) leads to a decreas
e in [H+] and hence a decrease in conductivity. If dissolved oxygen is pres
ent, it may be converted by H . to perhydroxyl radical H .O-2, which dissoc
iates to H+ and superoxide anion .O-2(-). raising the conductivity. Further
more, superoxide can reduce HC .O-3 back to HCO3-, countering the conductiv
ity-lowering effect of bicarbonate. Because superoxide is destroyed mainly
by reaction with perhydroxyl radical, and the concentration of perhydroxyl
is much smaller than that of superoxide, superoxide is a long-lived species
. Thus the conductivity after the lamp is turned on and then off is larger
than the conductivity before the sequence. If hydrogen is present in additi
on to oxygen, it reacts with . OH to generate .H, which leads to the format
ion of more H .O-2.
In addition, the reaction of . OH with H .O-2, which would convert the latt
er back to O-2, is prevented. For both reasons, hydrogen makes the conducti
vity step larger, as observed. The concentration of superoxide is limited b
ecause high [O-2(-)] leads to high [H .O-2], so the reaction of O-2(-) With
H .O-2, which destroys O-2(-), becomes important. The experimental observa
tion that the conductivity step goes through a maximum as a function of O-2
concentration is not explained by our model, but is believed to be associa
ted with absorption of ultraviolet radiation by superoxide, H2O2, or other
species formed from O-2.