Gravimetric Analysis 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. Gravimetric Analysis Accurate and precise. Possible sources of errors can be checked. It is an ABSOLUTE method. Relatively inexpensive Gravimetric Analysis Precipitation methods. Volatilisation methods. Electrogravimetry. Thermogravimetry. Gravimetric Analysis 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. Gravimetric Analysis Precipitation methods. Analyte (or chemically related species) isolated as a sparingly soluble precipitate of known composition. Analyte (or chemically related species) isolated as a sparingly soluble precipitate that can be converted by heat to species of known composition. Gravimetric Analysis Determination of silver. 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. A Gravimetric Analysis Frequently the constituent being estimated is weighed in a form other than that it was precipitated in. Mg2+: precipitated as Mg(NH4)PO4.6H20 but is weighed as magnesium pyrophosphate Mg2P2O7 after ignition. Gravimetric Analysis 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. Gravimetric Analysis Accuracy Solubility Products. Solubility. Particle size. Coprecipitates. Drying and ignition. Gravimetric Analysis Solubility Products 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 Equilibrium between AgCl precipitate and the saturated solution. AgCl(s) Ag+(aq) + Cl-(aq) Gravimetric Analysis The corresponding thermodynamic equilibrium constant KT is given by: +][Cl-] [Ag KT = [AgCl] Gravimetric Analysis AgCl is in a solid phase therefore [AgCl] = 1 KTSP = [Ag+][Cl-] Gravimetric Analysis Solubility. Common ion effect Ionic strength Fractional precipitation Complex ions Temperature Solvent Gravimetric Analysis Particle size. suspension (10-6 - 10-4 mm diameter) to crystalline precipitate. Colloidal Depends on nucleation and particle growth. Gravimetric Analysis Coprecipitates. Removal during precipitation of compounds which are otherwise soluble. Sources: Surface adsorption Mixed crystal formation Occlusion Mechanical entrapment Gravimetric Analysis Drying and ignition. Removes solvents and volatiles Decomposition to known form Gravimetric Analysis Inorganic: H2S, AgNO3, Organic 2,4-DNP HCl, BaCl2 Gravimetric Analysis 8-Hydroxyquinoline O Mg2+ + 2 N Mg N N OH Selectivity O through pH control + 2H+ Gravimetric Analysis 8-Hydroxyquinoline Examples Metal pH pH Initial Ppt. Complete Metal pH pH Initial Ppt. Complete Ppt Ppt Aluminium 2.9 4.7 – 9.8 Manganese 4.3 5.9 – 9.5 Bismuth 3.7 5.2 – 9.4 Molybdenum 2.0 3.6 – 7.3 Cadmium 4.5 5.5 – 13.2 Nickel 3.5 4.6 – 10.0 Calcium 6.8 9.2 – 12.7 Thorium 3.9 4.4 – 8.8 Cobalt 3.6 4.9 – 11.6 Titanium 3.6 4.8 – 8.6 Copper 3.0 >3.3 Tungsten 3.5 5.0 – 5.7 Iron(III) 2.5 4.1 – 11.2 Uranium 3.7 4.9 – 9.3 Lead 4.8 8.4 – 12.3 Vanadium 1.4 2.7 – 6.1 Magnesium 7.0 >8.7 Zinc 3.3 >4.4 Gravimetric Analysis Dimethylglyoxine CH3 Ni2+ + 2 CH3 C C CH3 N N HO OH C C O N N O H H + 2H+ Ni O N N O CH3 C C Weakly alkaline conditions CH3 Nickel salt bright red CH3 Summary Principles Solution reaction between analytes and reagents to give sparingly soluble products. Drying or ignition of precipitates. Weighing Apparatus Flasks, beakers, pipettes, crucibles and filter papers. Oven or furnace and a dessicator. Analytical quality balance. Summary Applications Extensive numbers of inorganic ions are determined with excellent precision and accuracy. Routine assays of metallurgical samples. Relative precision 0.1 to 1%. Good accuracy Summary Disadvantages Careful and time consuming. Scrupulously clean glassware. Very accurate weighing. Coprecipitation.