Gravimetry Part 3 PDF

Practical Steps 1)Precipitation 2)Digestion 3)Filtration 4)Washing 5)Drying or ignition 6)Weighing Weighing the sample Forming a precipitate Filtering the solution Weighing the dry precipitate  2)Digestion Digestion means standing the ppt for sometime in presence of the mother liquor t...

Practical Steps 1)Precipitation 2)Digestion 3)Filtration 4)Washing 5)Drying or ignition 6)Weighing Weighing the sample Forming a precipitate Filtering the solution Weighing the dry precipitate  2)Digestion Digestion means standing the ppt for sometime in presence of the mother liquor to allow coagulation. The individual particles agglomerate to share a common counter ion layer allowing coagulation and cement together. Also during digestion, small nuclei dissolves and larger ones grow at the expense of smaller nuclei which dissolve. Digestion also improves the internal perfection of the crystals (ripening) where occluded impurities are expelled out by the host ions. 3) Filtration Filtration is done to separate the ppt from the mother liquor Crystalline ppt easy and rapid filtration Colloidal ppt low filtration rate and can pass through filter paper into the filtrate leading to loss of the ppt 4)Washing It serves to remove surface adsorbed contaminants.  Properties of washing liquid: 1- Should not dissolve the ppt or hydrolyse it 2- Should not peptize the ppt ( Peptization is the reverse of coagulation for colloidal ppts only) 3- Should not cause further precipitation of impurities Examples - The washing liquid for AgCl ppt in Volhard’s method is acidulated water with HNO3 - The washing liquid for calcium oxalate ppt is 5% aqueous ammonia  Role of electrolytes during coagulation of colloidal ppt At beginning of After complete precipitation precipitation There are solvent molecules present between the two layers (2ry layer is loosely bound)  Role of electrolytes during coagulation of colloidal ppt At beginning of After complete precipitation precipitation Stability of colloidal ppt is due to the repulsive forces between particles by the charged double layer surrounding each particle preventing close approach of particles  Role of electrolytes during coagulation The presence of electrolytes causes the counter ions to be forced into closer contact with the primary layer, so the counter layer neutralizes the primary layer giving neutral coagulated particles. conc. of counter ions↑ → shrinkage of the counter-ion layer→ particle can then approach one another more closely and agglomerate  Colloidal ppts tend to peptize if washed with pure water because water will dilute and remove electrolytes. Peptization is the reverse of coagulation (washing with water containing Electrolyte) Coagulation Colloidal solution Coagulated ppt Peptization (washing with pure water) To prevent peptization, add electrolyte to the washing liquid (eg: volatile HNO3 or NH4OH) 5) Drying and ignition Drying: heating the ppt at temperature 250°C usually accompanied with change in chemical structure of ppt Ex: CaC2O4.H2O Pyrolysis curve It is a graph recording the wt. of ppt. (on Y axis) versus temperature (on X axis)