Precipitation Method. Filtration. Steps in Gravimetric Analysis (precipitation) gravi metric (weighing - measure)

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1 Quantitative Analysis Classical Gravimetric mass of analyte volumetric (or titrimetric) volume of solution containing sufficient reagent to react completely with analyte Instrumental Electroanalytical properties resulting from Ox./Red. behavior of analyte Spectroscopic measures electromagnetic radiation absorbed or emitted by analyte Chromatographic separates a mixture into its components 1 gravi metric (weighing - measure) Chemical analysis based on the determination of weight of a substance of known composition (the final product) that is chemically related to the analyte. 2 Precipitation: Dissolved sample converted to sparingly soluble precipitate Ag + + Cl - AgCl(s) Combustion: A sample is burned in excess oxygen and products are measured measure C, H, N, S, and halogens in organic compounds Volatilization: A sample is volatilized at suitable temperature; the volatile product is collected and weighted NaHCO 3 (aq)+h 2 SO 4 (aq)co 2 (g)+h 2 O(l)+NaHSO 4 (aq) CO 2 (g)+2naoh(s)na 2 CO 3 (s)+h 2 O(l) Precipitation Method (1) The desired substance: completely precipitated. "common ion" effect can be utilized: Ag + + Cl - AgCl(s) excess of Cl - which is added (2) The weighed form: known composition. (3) The product: "pure", easily filtered. 3 4 Steps in Gravimetric Analysis (precipitation) 1) Dry and weigh sample 2) Dissolve sample 3) Add precipitating reagent in excess 4) Coagulate precipitate usually by heating & wait for some time (Aging) 5) Filtration-separate precipitate from mother liquor 6) Wash precipitate 7) Dry and weigh to constant weight ( mg) Filtration Mother liquor avoid colloidal suspension, ideally, produce crystals 5 6 1

2 Particle Size & Filterability - Precipitates Colloids (d = 10-6 to 10-4 mm) -invisible to naked eye -don t settle out of solution -difficult or impossible to filter Particles (d = 0.10 mm or greater) -spontaneously settle out of solution -readily filtered and washed free of impurities -more desirable (typically of higher purity than Mechanisms of Precipitation Two competing processes: (1) Nucleation When a small number of ions, atoms, molecules initially unite. (2) Particle growth The 3-D growth of a particle nucleus into a larger crystal colloids) 7 8 Relative supersaturation (RSS) RSS = (Q-S)/Q Where Q = concentration of solute; S = equilibrium solubility of solute RSS can be used estimate/control the type of precipitate that is formed: large: nucleation, small particles (colloids) small: particle growth, crystalline solid likely 9 10 The goal is to form crystalline precipitates so RSS must be minimized. This can be done by: Increasing S Decreasing Q Recall: RSS = (Q-S)/Q = 1-S/Q Q = [solute] S = solute s Equil. Sol. 11 Techniques to promote crystal growth (1) Raising the temperature (increase S). (2) Adding precipitant slowly with vigorous mixing (decrease Q). (3) Keeping the volume of solution large (decrease Q). 12 2

3 !"#$ % & ' ' & ' ( ' ( 13 (1) Increasing the electrolyte concentration Colloidal Particle of AgCl Boundary of ionic atmosphere Decreasing the vol. of the counter-ion layer Increasing the chance for coagulation 14 (2) Using a volatile electrolyte Avoid peptization Ex. AgCl, wash with HCl. Drying precipitate at 110 C will remove HCl. This displace the less volatile, excess counter ion. (3) Digestion and aging Digestion: Heating the solution for about an hour after precipirate formation. This helps to remove weakly bound water Aging: Storing the solution, unheated, overnight. This allows trapped contaminates time to work their way out. Both can result in a denser precipitate that is easier to filter Representative Gravimetric Analyses

4 Advantages/Disadvantages ) *+" ) ),%-,%'. ) / 00 -, "# )!-1" Calculation Design of experiment Content Calculation Evaluation of the results % of analyte, % A Calculation %A = weight of analyte x 100 weight of sample weight of ppt directly obtained %A How Do We Get %A from ppt? % A = weight of ppt x gravimetric factor (G.F.) x 100 weight of sample G.F. = Gravimetric Factor a (FW of analyte) b (FW of precipitate) G.F. = a (FW of analyte) b (FW of precipitate) G.F. = # gms of analyte per 1 gm ppt Analyte ppt G.F. CaO CaCO 3 FeS BaSO 4 UO 2 (NO 3 ) 2. 6H 2 O U 3 O 8 Cr 2 O 3 Ag 2 CrO

5 Gravimetric Factor Thermogravimetry Analyte ppt G.F. CaO CaCO 3 CaO/CaCO 3 FeS BaSO 4 FeS/BaSO 4 UO 2 (NO 3 ) 2 U 3 O 8 3UO 2 (NO 3 ) 2 /U 3 O 8 Cr 2 O 3 Ag 2 CrO 4 Cr 2 O 3 /2Ag 2 CrO Exercise Consider a g sample containing 75% potassium sulfate (FW ) and 25% MSO 4. The sample is dissolved and the sulfate is precipitated as BaSO 4 (FW ). If the BaSO 4 ppt weighs g, what is the atomic weight of M 2+ in MSO 4? ANS: Mg 2+ Answer The hard part is setting up the correct equation 0.75* * = x Rearranging and solving: = ; x = 24.12( x Mg 2+ )