Phosphate in Coral

Some procedures call for washing away the mother liquor, redissolving the precipitate, and reprecipitating the product. In the second precipitation, the concentration of impurities is lower than in the first precipitation, and the degree of coprecipitation therefore tends to be lower (Table 7-4). 

Occasionally, a trace component that is too dilute to be measured is intentionally concentrated by coprecipitation with a major component of the solution. The procedure is called gathering, and the precipitate used to collect the trace component is said to be a gathering agent. When the precipitate is dissolved in a small volume of solvent, the concentration of the trace component is high enough for accurate analysis. 

Some impurities can be treated with a masking agent, which prevents them from reacting with the precipitant. In the gravimetric analysis of Be , Mg , Ca , or Ba with the reagent A -p-chlorophenylcinnamohydroxamic acid (designated RH), impurities such as Ag, Mn, Zn, Cd, Hg, Fe , and are kept in solution by excess KCN. 

Impurities might collect on the product while it is standing in the mother liquor. This process is called postprecipitation and usually entails a supersaturated impurity that does not readily crystallize. An example is the crystallization of magnesium oxalate (MgC204) on calcium oxalate (CaC204). 

Washing precipitate on a filter removes droplets of liquid containing excess solute. Some precipitates can be washed with water, but many require electrolyte to maintain coherence. For these precipitates, the ions in solution are required to 

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