2 How to Perform a Successful Gravimetric Analysis What steps are needed?Sampled dried, triplicate portions weighedPreparation of the solutionPrecipitationDigestionFiltrationWashingDrying or ignitingWeighingCalculation
3 Gravimetric AnalysisGravimetric Analysis – one of the most accurate and precise methods of macro-quantitative analysis.Analyte selectively converted to an insoluble form.Measurement of mass of materialCorrelate with chemical compositionWhy?SimpleOften required for high precision
5 Desirable properties of analytical precipitates Readily filtered and purifiedLow solubility, preventing losses during filtration and washingStable final form (unreactive)Known composition after drying or ignition
7 Filterability of Precipitates Colloidal suspensions10-7 to 10-4 cm diameterNormally remain suspendedVery difficult to filterCrystalline suspensions> tenths of mm diameterNormally settle out spontaneouslyReadily filterable
8 R.S. = (Q-S)/S Precipitate formation affected by: Solubility of PrecipitateTemperatureConcentration of reagensRate of mixing-RELATIVE SUPERSATURATION(R.S.)R.S. = (Q-S)/SS = Equilibrium Solubility of PrecipitateQ = Instantaneous ConcentrationLarger Q leads to colloidal precipitates.
9 Important Factors for Gravimetric Analysis NucleationIndividual ions/atoms/molecules coalesce to form “nuclei”Particle GrowthCondensation of ions/atoms/molecules with existing “nuclei” forming larger particles which settle outColloidal SuspensionColloidal particles remain suspended due to:small sizeadsorbed ions giving a net + or – charge(Brownian motion)
10 Important Factors for Gravimetric Analysis Coagulation, agglomerationSuspended colloidal particles coalesce to form larger filterable particles by:Heating, stirring and adding inert electrolytePeptizationRe-dissolution of coagulated colloids by :washing and removing inert electrolyte
11 Important Factors for Gravimetric Analysis DigestionPrecipitation heated for hour(s) in contact with solution which it was formed
26 Diverse Ion Effect on Solubility: Presence of diverse ions will increase the solubility of precipitates due to shielding of dissociated ion species.KSPo and Activity CoefficientsAgCl(s)(AgCl)(aq) Ag+ + Cl-Thermodynamic solubility product KSPoKSPo = aAg+ . aCl- = [Ag+]ƒAg+. [Cl-]ƒCl-KSPo = KSP ƒAg+. ƒCl-KSP = KSPo/(ƒAg+. ƒCl)
30 Calculating Results from Gravimetric Data The calcium in a ml sample of a natural water was determined by precipitating the cation as CaC2O4. The precipitate was filtered, washed, and ignited in a crucible with an empty mass of g. The mass of the crucible plus CaO (fwt g/mol) was g. Calculate the concentration of Ca (fwt g/mol) in the water in units of grams per 100 mL.
31 Calculating Results from Gravimetric Data An iron ore was analyzed by dissolving a g sample in concentrated HCl. The resulting solution was diluted with water, and the iron(III) was precipitated as the hydrous oxide Fe2O3.xH2O by addition of NH3. After filtration and washing, the residue was ignited at high temperature to give g pure Fe2O3 (fwt g/mol). Calculate (a) the % Fe (fwt g/mol) and (b) % Fe3O4 (fwt g/mol) in the sample.
32 Calculating Results from Gravimetric Data A g sample containing only NaCl (fwt g/mol) and BaCl2 (fwt g/mol) yielded g of dried AgCl (fwt g/mol). Calculate the percent of each halogen compound in the sample.