Physics
Scientific paper
Mar 1991
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1991gecoa..55..677z&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 55, Issue 3, pp.677-685
Physics
2
Scientific paper
The rate of oxidation of sulfite, S ( IV ), in seawater was measured as a function of pH (4.0- 8.5), temperature ( 15-45°C), and salinity (0-35). The observed rate constant, k , in seawater at a pH = 8.2 was found to be second order with respect to S ( IV ) and half order with respect to oxygen: - d [ S ( IV )]/ dt = k [ S ( IV )] 2 [ O 2 ] 0.5 . The resulting values of k ( M -1.5 min -1 ) have been fitted to a function of ionic strength, I , and temperature, T (K): log K = 19.54 - 5069.47/ T + 14.74 I 0.5 - 2.93 I - 2877.0 I 0.5 / T , and the standard error is 0.05 in log k . The energy of activation was found to be a function of salinity and has a value of 140 ± 6 kJ mol -1 at S = 35. The rates measured in 0.57 M NaCl were found to be higher than the rates in seawater. Measurements made in the major sea salts indicate that Ca 2+ , Mg 2+ , and SO 4 2- added to NaCl cause the decrease. Measurements made in artificial seawater (Na + , Mg 2+ , Ca 2+ , Cl - , and SO 4 2- ) were found to be in good agreement with the measurements in real seawater. The rate increased from pH 4 to a maximum at pH 6.5 and decreased at higher pH. The effect of pH on the rates was attributed to the rate-determining step involving the combination of HSO 3 - and SO 3 2- . This yields k = k '' HSO 3 - SO 3 2- where i is the molar fraction of species i . Values of k '' equal to 6.66 ± 0.06 and 6.17 ± 0.17 were found for NaCl and seawater, respectively. The larger range of k '' in seawater is due to it being a function of pH. The addition of Mn 2+ was found to increase the rate apparently due to the formation of MnSO 3 . Additions of Fe 3+ and Fe 2+ have a catalytic effect only before they hydrolyze to colloidal iron.
Millero Frank. J.
Zhang Jia-Zhong
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