LEAD OXIDE 
COBALT HYDROXIDE 
MAGNESIUM HYDROXIDE 
STRONTIUM SULFATE 
IRON HYDROXIDE 
CALCIUM CARBONATE 
MANGANESE OXIDE 
CALCIUM PHOSPHATE 
i ■ 1 ■ 1 J-J-- » 
6 7 8 9 10 pH 
Figure 4.6. Saturation pH for various metal solids in Aquil. 
These various solid formation processes are dependent on kinetic factors 
which are controlled by the particular temperature and pH regime of the 
medium. These, in turn, depend on the conditions and duration of autoclaving, 
as well as on the size of the containers and the mixing conditions. Precipitates 
are rarely seen with filter sterilization. Figure 4-6 shows the onset of saturation 
for various solids as pH is increased in the medium Aquil, normally designed to 
avoid precipitates. None of the four solids that are saturated at pH = 8 in Aquil 
are actually seen to precipitate, even after autoclaving if the volumes are kept 
smaller than 100 ml. If larger volumes are autoclaved, immediate bubbling with 
carbon dioxide prevents precipitation. Avoidance of calcium carbonate 
precipitate is very important for maintaining iron and manganese in solution, as 
the presence of CaCO^(s) will catalyze the formation of hydrous oxides of 
manganese and iron (43). For other trace metals the formation of these 
precipitates creates difficulties mostly through adsorption processes (see next 
section). 
For toxicity studies, trace metals are sometimes introduced in algal cultures 
in excess of the chelating agent concentration. Precipitates are then often 
expected to lorm mostly oxides, hydroxides and carbonates, depending on the 
metal. For example, a hydroxide (Cu(OH) 2 ), an oxide (CuO, tenorite) when a 
carbonate (Cu 2 C 03 ( 0 H) 2 , malachite) become quickly saturated in Aquil when 
copper exceeds the EDTA concentration. Although the hydroxide is not the 
50 
