Fig. 4. Macroscopic dissolution behavior of aged Ni precipitates on silica showing the relative amount of Ni remaining on the surface following extraction with (a) 1 mM EDTA at pH 4.0, (b) 1 mM EDTA at pH 7.5, (c) 3 mM oxalate at pH 4.0, (d) 3 mM acetylacetone at pH 6.0, (e) HNO₃ at pH 4.0, and (f) HNO₃ at pH 6.0 plotted against the total number of replenishments. The stability of the Ni precipitates increases with aging time