2How to Perform a Successful Gravimetric Analysis What steps are needed?Sampled dried, triplicate portions weighedPreparation of the solutionPrecipitationDigestionFiltrationWashingDrying or ignitingWeighingCalculation
3Gravimetric 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
5Desirable properties of analytical precipitates Readily filtered and purifiedLow solubility, preventing losses during filtration and washingStable final form (unreactive)Known composition after drying or ignition
7Filterability of Precipitates Colloidal suspensions10-7 to 10-4 cm diameterNormally remain suspendedVery difficult to filterCrystalline suspensions> tenths of mm diameterNormally settle out spontaneouslyReadily filterable
8R.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.
9Important 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)
10Important 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
11Important Factors for Gravimetric Analysis DigestionPrecipitation heated for hour(s) in contact with solution which it was formed
26Diverse 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)
30Calculating 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.
31Calculating 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.
32Calculating 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.