Kinetics and mechanism of oxidation of S(IV) by the ethylenebisbiguanide Ag(III) ion

The reaction, first-order both in Ag(H2L)3+ and S(IV), is not affected by dissolved oxygen and the rate is retarded by the hydrogen ion. The rate law in Eq. (i) is deduced from the mechanism given below where kobs is the pseudo first-order rate constant ([S(IV)] ≫ [Ag(H2L)]3+), and k1 and k2 are composite rate constants. equation(i)kobs=(k1+k2[H+]-1)[S(IV)]kobs=(k1+k2[H+]-1)[S(IV)]equation(ii)SO2(aq)⇌Ka1HSO3-+H+equation(iii)Ag(H2L)3++SO2(aq)⇌K1Ag(H2L)SO23+equation(iv)Ag(H2L)3++HSO3-⇌K2Ag(H2L)HSO32+equation(v)Ag(H2L)SO23+→k′HSO3+Ag(H2L)2++H+equation(vi)Ag(H2L)HSO32+→k″HSO3+Ag(H2L)2+equation(vii)Ag(H2L)2++HSO3+H2O→fastAg(H2L)++HSO4-+2H+ where k1=k′K1k1=k′K1 and k2=k″K1K2k2=k″K1K2 in Eq. (i). The inner-sphere mechanism is supported by similar activation parameters reported in other oxidations for similar reaction pathways.

Graphical abstractWeak intermediate are involved in the oxidation of SO2SO2 (aq) to SO42- by [ethylenebis(biguanide)]silver(II) in the acidic medium by successive one-electron transfer.Figure optionsDownload full-size imageDownload as PowerPoint slide