This may be attributed to the dierence
in the anity of organic matter for the two metals. The
stability constants (log) for the complexes of Cu(II) with
citric acid (H4L), for example, are 6.1 (CuHL) and 18
(CuL), and those for Pb(II) are 5.2 (PbHL) and 12.3 (PbL).
Although both the Cu(II) and Pb(II) complexes have high
stability, the stability constants for the complexes of Cu(II)
are greater than those for Pb(II).