1. Gravimetric Analysis l A Gravimetric analysis is based upon the measurement of the weight of a substance that has a KNOWN composition AND IS chemically related to the analyte.

2. l Accurate and precise. l Possible sources of errors can be checked. l It is an ABSOLUTE method. l Relatively inexpensive Gravimetric Analysis

3. l Precipitation methods. l Volatilisation methods. l Electrogravimetry. l Thermogravimetry. Gravimetric Analysis

4. l In precipitation methods, the species to be determined is precipitated by a reagent that yields a sparingly soluble product that has a known composition or can be converted to such a substance.

5. l Precipitation methods. v Analyte (or chemically related species) isolated as a sparingly soluble precipitate of known composition. v Analyte (or chemically related species) isolated as a sparingly soluble precipitate that can be converted by heat to species of known composition. Gravimetric Analysis

6. l Determination of silver. v A solution of Ag + is treated with an excess of NaCl or KCl solution, the precipitate is filtered off, washed well with water to remove soluble salts, dried at 130 - 150°C and weighed as AgCl.

7. Gravimetric Analysis l Frequently the constituent being estimated is weighed in a form other than that it was precipitated in. l Mg 2+ : precipitated as Mg(NH 4 )PO 4.6H 2 0 but is weighed as magnesium pyrophosphate Mg 2 P 2 O 7 after ignition.

8. Gravimetric Analysis l Conditions: 1. Must be a stoichiometric reaction. 2. A stable product; no oxidation, dehydration or gelatinous precipitates. 3. Must avoid side reactions which result in coprecipitates.

9. l Accuracy v Solubility Products. v Solubility. v Particle size. v Coprecipitates. v Drying and ignition. Gravimetric Analysis

10. l Solubility Products v Even the most insoluble products have at least a certain solubility. It is therefore more correct to call these compounds sparingly soluble substances, eg: AgCl Gravimetric Analysis

11. l Equilibrium between AgCl precipitate and the saturated solution. Gravimetric Analysis AgCl(s) Ag + (aq) + Cl - (aq)

12. l The corresponding thermodynamic equilibrium constant K T is given by: Gravimetric Analysis [Ag + ][Cl - ] [AgCl] K T =

13. l AgCl is in a solid phase therefore [AgCl] = 1 [AgCl] = 1 Gravimetric Analysis K T SP = [Ag + ][Cl - ]

14. l Solubility. v Common ion effect v Ionic strength v Fractional precipitation v Complex ions v Temperature v Solvent Gravimetric Analysis

15. l Particle size. v Colloidal suspension (10 -6 - 10 -4 mm diameter) to crystalline precipitate. v Depends on nucleation and particle growth. Gravimetric Analysis

16. l Coprecipitates. v Removal during precipitation of compounds which are otherwise soluble. v Sources: n Surface adsorption n Mixed crystal formation n Occlusion n Mechanical entrapment Gravimetric Analysis

17. Drying and ignition. Drying and ignition. v Removes solvents and volatiles v Decomposition to known form Gravimetric Analysis

18. l Inorganic: v H 2 S, AgNO 3, HCl, BaCl 2 l Organic v 2,4-DNP Gravimetric Analysis

19. l 8-Hydroxyquinoline Gravimetric Analysis Mg 2+ + 2 N OH + 2H + N O Mg O N l Selectivity through pH control

20. l 8-Hydroxyquinoline Examples Gravimetric Analysis

21. l Dimethylglyoxine Gravimetric Analysis l Weakly alkaline conditions l Nickel salt bright red Ni 2+ + 2 CH 3 CCCH 3 NN HOOH + CH 3 CCCH 3 NN Ni OO OONN CH 3 CCCH 3 H H 2H +

22. Summary l Principles v Solution reaction between analytes and reagents to give sparingly soluble products. v Drying or ignition of precipitates. v Weighing l Apparatus v Flasks, beakers, pipettes, crucibles and filter papers. v Oven or furnace and a dessicator. v Analytical quality balance.

23. Summary l Applications v Extensive numbers of inorganic ions are determined with excellent precision and accuracy. v Routine assays of metallurgical samples. v Relative precision 0.1 to 1%. v Good accuracy

24. Summary l Disadvantages v Careful and time consuming. v Scrupulously clean glassware. v Very accurate weighing. v Coprecipitation.