higher initial concentrations of manganese, with their higher concentrations of uncomplexed Mn(II), would have a greater tendency to form MnO2. However, the fraction of total manganese present as Mn02 on the electrode cannot be large. An experiment in which the solution was removed from one coulometric cell after 50% of the Mn(I1) had been oxidized and then the remaining Mn(I1) determined in a second cell yielded within 0.1 % of the calculated amount, based on no Mn02 formation. Assuming that an adsorbed product or electrodeposit does inhibit the oxidation of Mn(II), and that part of the deposit is lost at open circuit, the shape of the log i us. t curve at high concentrations of Mn(I1) can be explained as follows. Before the control potential is applied to the electrode, the coverage is such that a relatively rapid electrolysis is possible. When a potential of $1.10 V us. SCE is imposed on the electrode, oxidation of Mn(I1) commences along with the formation of the deposit, and the current decreases more rapidly than normal because product build-up is concurrently inhibiting the Mn(I1) reaction. Eventually the current reaches a value at which the rate of Mn(I1) consumption does not rapidly decrease the Mn(I1) concentration, and a steady state is approached. Finally, the concentration of Mn(I1) decreases to the region where the limiting current density is less influenced by a species on the electrode surface, and at this point the log i us. t curve becomes concave downward, finally becoming linear as in an uncomplicated electrolysis. Interruption of the electrolysis allows dissolution of the surface deposit, again permitting more rapid electrolysis and causing the current-time behavior of Figure 7. Just as the disproportionation Reaction 2 is probably involved in the current interruption, so also must it be a factor in the electrode surface reaction, if Mn(1V) is present. As mentioned earlier, the characteristics of the oxidation of Ce(II1) to Ce(1V) in Na4P20, solution at pH 2 are similar to those of the Mn(I1) oxidation, with an even more pronounced enhancement of the electrolysis current upon interruption and even more distinctive peaking of the voltammetric curves. The maximum rate of electrolysis of Ce(II1) is reached at +0.9 V us. SCE and is considerably slower at $1.10 V. Examination of the electrode by ion microprobe mass spectrometry after electrolysis at +1.10 V [as was done with Mn(II)], also revealed significant amounts of cerium on its surface. On the other hand, vanadium(1V) was oxidized at +1.10 V us. SCE in Na4P2O7 solution at pH 2, and the
electrode analyzed by mass spectrometry, but no vanadium was found. The oxidation of V(IV), also totally irreversible, exhibits none of the unusual voltammetric and controlledpotential electrolysis characteristics observed for Mn(I1) and Ce(II1). Although it was beyond the scope of the present work, a further investigation of the Mn(I1)-Mn(II1) system in pyrophosphate medium should include a study of the effect of electrode material, the use of fast potentiostatic measurements, and a more complete study of the reaction as a function of pH. The newer techniques such as the in situ optical examination methods and the use of ring-disk electrodes would probably also be useful. ACKNOWLEDGMENTS
Fred B. Stephens assisted in obtaining some of the voltammetric data. Ronald K. Stump performed the analyses with the ion microprobe mass spectrometer, and Raymond L. Ward carried out the electron spin resonance measurements. RECEIVED for review November 25, 1968. Accepted March 3, 1969. This work was performed under the auspices of the U. S. Atomic Energy Commission. Presented at the 157th Meeting, ACS, Minneapolis, April, 1969.
Correction Identification of Surface Functional Groups on Active Carbon by Infrared Internal Reflection Spectrophotometry In this article by James S. Mattson, Harry B. Mark, Jr., and Walter J. Weber, Jr. [ANAL.CHEM.,41,355 (1969)], the following errors appeared. In the title the word “Spectrophotometric” was substituted for “Spectrophotometry”. On page 356, column one, lines 15 and 18 under Experimental, the words “absorbs” and “absorbed” were substituted for “adsorbs” and “adsorbed”. On page 357, column one, line 16 under Results and Discussion, “H202”was printed for “H20”.
VOL. 41, NO. 6, MAY 1969
765
