LLE and glucose. These promote the rate over the blank at every
pH, but the difference in case of LLE is only significant at pH=8. In the
case of glucose, the overall effect is very weak, and there is little difference
in the rate at pH=4 and pH=6, but it is considerably reduced at
pH=8, although the overall effect in comparison to the blank remains
promotive. Welch and Vandevivere (1994) also note that the effect of
sugars on feldspar dissolution kinetics is only very slight, too. A passivating
effect, as is often observed with polyfunctional ligands capable of simultaneously
blocking several metal centres (c.f. our experiments with
DFOB in the following section) does not come into play for steric reasons:
the vicinal hydroxyl groups are not far enough apart to be able
to span more than one coordination site.