1. GRAVIMETRIC ANALYSIS BY Dr.JAGADEESH

2. INTRODUCTION Gravimetric Analysis: It is a method of chemical analysis done by weighing a precipitate ( element or compound) isolated in pure form from a substance.

3. Advantages Precise and Accurate Possible errors can be checked (e.g. Impurity & Completion of Precipitation) Absolute method ( Direct weighing) Need only inexpensive apparatus

4. Applications For analyzing the standards used for calibration of instruments ( ensuring the reliability of standards) Analysis requiring high accuracy

5. Precipitation (requirements) Precipitate must be insoluble ( Solubility product control) No loss should occur while collecting it (Filtration and Washing steps)

6. Solubility Product It is the product of the concentrations of the ions raised to the power equal to stoichiometric coefficients in the dissociation equation AgCl Ag + + Cl - [Ag+] [Cl-] K eq = [AgCl]

7. Solubility Product K eq = [Ag] [Cl] = K sp since [Ag Cl] is unity For the dissociation, M m N n m[M + ] + n[N - ] K sp = [M + ] m [N - ] n For precipitation, ionic product >solubility product

8. Types of Precipitates Curdy Precipitate ( e.g. AgCl) Gelatinous Precipitate (e.g. Al(OH) 3 ) Crystalline Precipitate (e.g. BaSO 4 )

9. Criteria for Precipitation Precipitation should be done from dilute solution. Reagents mixed slowly-for large crystals. Precipitation done in hot solutions – prevents sol formation Crystalline precipitate digested for long- avoids copreciptiation. Precipitated washed with suitable electrolyte If impurities still present adopt reprecipitation

10. Steps involved in Gravimetric Analysis. Preparation of the solution of the substance Precipitation Digestion Filtration Washing Drying and / igniting Weighing Calculation

11. Preparation of the solution Weighing out a definite quantity of the substance Adjusting pH, volume, concentration range, temperature etc for low solubility of the precipitate, good size, purity etc.

12. Precipitation Consider the factors below: Solubility Product Type of the precipitate Temperature Concentration Volume of the solution pH

13. Digestion The process of heating the precipitate along with the solution from which it is formed over a steam bath. To give good size and shape for easy filtration and washing

14. Filtration and Washing Porcelain filters, Sintered crucible, filter paper in a funnel etc can be used. Gelatinous precipitate can not be filtered in porcelain or sintered crucible due to clogging. Washing takes care of impurities Washings can be tested to ensure purity

15. Drying and Igniting Precipitate dried to remove water ( 110 -120 degree Celsius for 1-2 hrs) Ignition done to convert any other suitable form for weighing ( Iron hydroxide converted to Iron Oxide, Aluminium Hydroxide to Alumina)