Abstract
The author's recent studies on how ionic interactions affect the rates of oxidation of Fe (II) and Cu (I) with O
2 in natural waters are reviewed. The oxidation of these metals has been measured as a function of pH, temperature, ionic strength and ionic composition. The oxidation of Fe (II) was found to be second order with respect to H
+ or OH
− over a wide range of temperature, ionic strength and ionic composition. These results indicate that Fe(OH)
2
0 is the reactive Fe(II) species. At a constant pH and ionic strength, various anions were found to change the rates of oxidation of Fe(II). The rate constants were in the order HCO
3
− > Br
− > ClO
4
− > NO
3
− > Cl
− > SO
4
2− > B(OH)
4
−. This order was attributed to the relative strength of the interactions of these anions with Fe
2+. The strong interactions of Fe
2+ with SO
4
2− and B(OH)
4
− were used to estimate the stability constants of log
β
FeSO4=1.8±0.1 and log
β
FeB(OH)
4
= 3.2±0.1 at 25°C. The addition of Mg
2+ at a constant pH was found to decrease the rate of oxidation of Fe(II). This was attributed to the decrease in Fe(OH)
2
0 as a result of the formation of MgOH
+.
The oxidation of Cu(I) was strongly dependent on the Cl
− concentration. At low ionic strengths Cu
+ and CuCl
0 are the reactive species, whereas at 6 m the CuCl
2
− is also reactive. The effect of Mg
2+ and HCO
3
− on the rate was determined as a function of chloride concentration (1–6 m). The addition of Mg
2+ causes the rate to decrease and the addition of HCO
3
− causes the rate to increase. The possible causes of these effects are discussed.
The rates of oxidation of Cu(I) have been measured in mixtures of NaX+NaClO
4, where X=Cl
− Br
− and I
−. The rate constants at a given halide concentration are in the expected order,
κ
Cl >
κ
Br >
κ
I. The rate constants for CuCl
0 and CuBr
0 were the same within experimental error. By assuming that the rate of oxidation of CuI
0 is the same as these species, a value of log
β
1
∗ = 5.7 ± 0.7
has been calculated for the stability constant of CuI
0. This value agrees with the expected trend in the stability constants CuCl
0 < CuBr
0 < CuI
0.
