REACTIONS BETWEEN Pb2+ AND (I-+I7) IN ACIDIC SILICA GELS1

In this work it was found that reactions between Pb2+ and (I-+I3-) in acidic silica gels are limited to the formation of Pbl2 from Pb2++2 I- and from Pb2++2I J, the latter occur-ring with the coincident formation of I2 crystals. There was no evidence of pseudomorphic conversion of I2 to Pbl2, even by the presence of large excesses of Pb2+, in contrast to reactions in acidic gels between excess Hg2+, Ag+, or T l + and I2, in which instances these metal cations convert I2 pseudomorphically to the respective iodides. When a deficiency of Pb2+ existed, there was no zone separation of Pbl2 and I2, as occurs with Hgl2, Agl, or Til and I2 in metal-ion deficient systems, and the distributions of Pbl2 and I2 crystals were random. Reactions between Pb2+ and I ~ in acidic and slightly basic silica gels have been described by Holmes (1917, 1934). In sufficiently acidic gels, Holmes found that the normal iodide, Pbl2, formed, whereas in only slightly acidic and in basic gels, a basic iodide resulted, presumably Pb(OH)I or PbI2-PbO. Holmes (1917, 1934) 'Manuscript received August 15, 1964