Lecture - Gravimetric Analysis 1. Precipitation Methods dissolved analyte converted to sparingly soluble precipitate. a. readily filtered b. low solubility c. converted to product of known composition (heat) Ex. Excess of oxalic acid (H C O - ) added carefully to measured volume of Ca +. (1) In basic sol n: Ca + ( aq) + C O ( aq) CaC O ( s) () CaC O (s) is collected in a filtering crucible then dried (3) CaC O ignited to produce calcium oxide: CaC O CaO CO ( s ) (s) + (g) + () CaO(s) cooled, weighed CO ( g) (5) Original concentration of Ca + computed 1
. Volatilization Methods a. Analyte is volatilized at suitable temperature b. Volatile product is collected and weighed 3. Precipitates Particle ize & Filterability a. Colloids (d = 10-7 to 10 - cm) -invisible to naked eye -not easily filtered, don t settle out of solution b. Particles (0.10 mm or greater) -spontaneously settle out of solution -readily filtered and washed free of impurities -more desirable *ize of particles influenced by relative supersaturation of the solutions in which is formed: Relative upersaturation = Q Where Q = concentration of solute, = solute s equilibrium constant
-Precipitate solubility -Temperature -Reactant concentration -Rate of reactant mixing If Q is large = small particles (colloids) If Q is small = crystalline solid likely c. Crystalline Formation (1) Raising temperature (increases ) () Using dilute solutions (minimizes Q) (3) low addition of precipitating agent w/stirring (minimizes Q). Mechanism of Precipitate Formation a. Nucleation formation of a stable solid due to # of atoms, ions or molecules join together, e.g. formation on surface of suspended contaminants (dust particles). b. Particle Growth growth on existing nuclei 3
Q high rate of nucleation increases Q low particle growth dominates, excluding nucleation *Nucleation dominates results in a large # of very fine particles *Particle growth dominates small # of larger particles 5. Gravimetric Calculations % Analyte= Wanalyte x100 Wsample CaCl + AgNO AgCl s ) 3 ( + Ca(NO 3 ) FW CaCl 1mol CaCl wt CaCl wt AgCl x ( x = FW AgCl mol AgCl Gravimetric Factor (F) )
F Relates mass of product to mass of analyte, stoichiometry F = a(fw of b(fw of substance A) substance B) where a and b are the coefficients of A and B, respectively Ex. Calculate the % Phosphorus in a 0.3516 g detergent sample. Final yield is 0.161 g Mg P O Mass Analyte %Analyte = x 100 Mass ample a. Mass P = 0.161g Mg P O x Mass product Factor 1mol Mg P.57 g Mg P O O mol P x 1mol Mg P O Gravimetric 30.97g P x 1mol P = 0.061 g P 0.061g P % P = = 0.3516g sample or 17.10 % 5
F = a b x FW analyte FW sample 1 30.97g.57g F = x = 0. 7833 (0.161g Mg P O )(0.7833) %P= 7 x100 = 17.10% 0.3516g Ex: A 10.00 ml solution containing Cl - was treated with excess AgNO 3 to precipitate 0.368 g of AgCl. What was the molarity of Cl- in the unknown? Formula mass of AgCl = 13.31. A precipitate weighing 0.368 g contains: 0.368g AgCl 13.31g AgCl / mol AgCl = 3.08 x 10-3 mol AgCl *Note: 1 mol of AgCl contains 1 mol of Cl - 3.08 x 10-3 mol AgCl [Cl-] = = 0.308 M 0.0100L * IN CLA PROBLEM TO FOLLOW! 6
Ex1:Phosphate is precipitated from its solution with ammonium molybdate, as (NH ) 3 [PMo 1 O 0 xh 0]. ince the precipitate does not have a constant composition with regard to water content, it is dissolved in ammonia and the molybdate is precipitated with Pb(NO 3 ), as PbMoO. a) What is the value of the gravimetric factor for the calculation of %P? b) If the final precipitate weighs 0.100 g, what is the weight of P in the initial sample? 7
Ex: A 0.05 g sample consisting of only BaCl and KCl required 0.5 ml of 0.100 M AgNO 3 solution for the quantitative precipitation of chloride. Calculate the %Ba and %K in the sample. Ex3: A 0.99 g sample of a hydrate of CuO xh O, is heated to a constant weight of 0.318 g (total loss of water). Calculate the value of x. 8
Ex: In the gravimetric determination of sulfate in a 0.81 g sample of pure Na O, a BaO precipitate weighing 0.60 g was obtained. The weight of the precipitate was smaller than the theoretical one, since some BaO was converted to Ba during the heating process. a) Calculate the per cent of Ba in the precipitate (Hint: best to solve algebraically). b) The per cent error of the analysis (Hint: compare calculated with weight stated in problem). 9