Conductivity of irradiated pure water

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.