I 



IRON HYDROXIDE 

 CALCIUM CARBONATE 



I 



MANGANESE OXIDE 



I 



CALCIUM PHOSPHATE 



I I 



LEAD OXIDE 

 COBALT HYDROXIDE 



MAGNESIUM HYDROXIDE 

 STRONTIUM SULFATE 



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 soHds 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 CaC03(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 form mostly oxides, hydroxides and carbonates, depending on the 

 metal. For example, a hydroxide (Cu(0H)2), an oxide (CuO, tenorite) and a 

 carbonate (Cu2C03(OH)2, malachite) become quickly saturated in Aquil when 

 copper exceeds the EDTA concentration. Although the hydroxide is not the 



50 



