Page 122 - Pharmaceutical Analytical Chemistry III

B) Post precipitation

• Pption of impurity occur after analyte
ppt has formed

• It is due to a difference in rate of
crystallisation

• It is increased by digestion
• Ex: Impurity of Mg2+ in

determination of Ca2+ as oxalate

• So it is necessary to filter the analyte
ppt as soon as its pption is complete

• Elimination: separation or
complexation before pption