Dr. Suraj B. Ade� Associate Professor�Department of Chemistry &
Analytical chemistry� M.S.P. Mandal’s� Shri Shivaji College, Parbhani-431401 � Cell : +919890081354,
(B.Sc.- II Year) (Semester-III)
Subject:- Chemistry
Paper No.-VII (Physical + Inorganic Chemistry)
(CCC III)(Section-B) Inorganic Chemistry-Part- (II)
Unit-V-B-(Theory of gravimetric Analysis)
INTRODUCTION TO GRAVIMETRIC ANALYSIS
Gravimetric analysis describes a set of methods in analytical chemistry for the quantitative determination of an analyte based on mass of the solid. Gravimetric analysis is potentially one of the most accurate classes of analytical method available, as weight can be measured with greater accuracy than almost any other fundamental property.
Steps In Gravimetric Analysis
The steps required in gravimetric analysis, after the sample has been dissolved, can be summarized as follows: preparation of the solution , precipitation , digestion, filtration , washing , drying or igniting , weighing and finally calculation .
1. Preparation of the Solution: This may involve several steps including adjustment of the pH of the solution in order for the precipitate to occur quantitatively and get a precipitate of desired properties, removing interferences …etc.
2. Precipitation: Precipitation is an ionic reaction in which the positive ion of one substance in solution combines with the negative ion of another substance, also in a solution, to form a sparingly soluble substance.
3- Digestion of the Precipitate:
4- Washing and filtration of precipitate
5. DRYING AND IGNITION OF PRECIPITATES
The purpose of drying (heating at about 120-150 oC in an oven) or ignition in a muffle furnace at temperatures ranging from 600-1200 oC is to get a material with exactly known chemical structure so that the amount of analyte can be accurately determined
TYPES OF PRECIPITATE
Precipitates are classified into crystalline, curdy and gelatinous precipitates. Crystalline precipitates are relatively pure and consist of easily filterable particles. Curdy precipitates are agglomerates of colloidal particles and are of filterable size. However, they are more easily contaminated than crystalline precipitates and hence must be washed with an electrolyte solution. Gelatinous precipitates are flocculated colloids. The particle size is smaller than that of curdy precipitates and hence is difficult to filter. They must also be washed with an electrolyte solution to prevent peptization. Following were the example Crystalline Precipitate-BaSO4,CaC2O4
Curdy Precipitate- AgCl, AgBr, Gelatinous Precipitate-Al(OH)3,Fe(OH)3.
FACTORS AFFECTING PRECIPITATION
*Effect of temperature: The solubility product of a substance is constant only when its temperature is unaltered. Usually the solubility increases with the increase in temperature. When the precipitation is carried out at higher temperature, the precipitate formed is of high purity due to better crystal structure. Hence, wherever possible, precipitation which is carried out at higher temperature is most advantageous but then it should be cooled before filtration.
*Effect of pH: The solubility of the precipitate with the change in pH of the solution is inevitable. The effect depends on the type of precipitate. Generally, the precipitate of metal hydroxides and those of sparingly soluble salts of weak acids are precipitated only in alkaline or neutral pH ranges. Smaller the dissolution constant for the acid, higher is the pH required for practically complete precipitation of its salt . The selectivity of organic reagents can always be improved by the control of pH. 5.