Gravimetric Analysis By: Dr. O. Rajabi (Pharm.D.- Ph.D.) Associate Professor of Chemistry Department of Medicinal Chemistry Mashad University of Medical Sciences

gravi – metric (weighing - measure) Definition: a precipitation or volatilization method based on the determination of weight of a substance of known composition that is chemically related to the analyte analyte - chemical element or compound of interest

Reaction: aA + rR -----> AaRr ppt where: a is # of moles of analyte A r is # of moles of reagent R AaRr is a pure, insoluble precipitate which we can dry and weigh or ignite to convert to something we can weigh ppt=precipitate

1914 Nobel Prize to T.W.Richards (Harvard University) for the atomic weights of Ag, Cl, and N Richards and his group determined atomic weights of 55 of the 92 known elements using gravimetry

T.W.Richards: “Every substance must be assumed to be impure, every reaction must be assumed to be incomplete, every method of measurement must be assumed to contain some constant error, until proof to the contrary can be obtained.”

7 Steps in Gravimetric Analysis Dry and weigh sample Dissolve sample Add precipitating reagent in excess Coagulate precipitate usually by heating Filtration-separate ppt from mother liquor Wash precipitate (peptization) Dry and weigh to constant weight

Precipitation: Dissolve sample Add ppt’ing reagent Filter Dry Weigh

Suction Filtration Filter flask Buchner funnel Filter paper Glass frit Filter adapter Heavy-walled rubber tubing Water aspirator

Mother liquor

Identify insoluble form Two considerations: Minimize errors due to limited precipitate solubility Minimize errors due to precipitation process Finite solubility of precipitate ideally, Ksp = 0 (i.e., completely insoluble) Some come close: ~10-38 for Fe(OH)3 ~10-50 for Ag2S For AgCl, Ksp = 1.78 x 10-10

For a 0.1000 g AgCl precipitate in 200 ml H2O: For example: what would be the % error introduced in gravimetric analysis by the solubility of AgCl? For a 0.1000 g AgCl precipitate in 200 ml H2O: Note: Error is independent of mass of precipitate,  relative error will decrease as precipitate mass increases (i.e., 0.038% error for 1.000 g AgCl)

Precipitation process ideally, we’d like a precipitate that forms quickly. This implies: Large, pure crystals Low solubility Easily filtered Easily washed How does precipitation occur? As Ksp is exceeded, solution becomes “supersaturated” At some point nucleation begins At the same time, crystal growth begins

Two points to remember: Crystal growth is independent of degree of supersaturation Nucleation increases with degree of supersaturation Minimization of supersaturation will produce the largest particles Two particle size classes Colloids very small difficult to handle experimentally Crystals “large” (~ 10-1 mm) easily and rapidly filtered high purity

What affects degree of supersaturation? Ksp Temperature solubility  as T  Reagent addition speed slower addition gives precipitation a chance to begin at lower supersaturation levels Solution concentration low reagent concentration equals low supersaturation But, even with the above precautions, we will often obtain colloid instead of a crystal!

Keys to successful colloid precipitation: Add precipitant slowly and in slight excess Digest precipitate (Heat, stir, sit) What about crystalline precipitate? Similar to colloids: Dilute solution Slow precipitant addition Elevated temperature Heat unstirred Contaminants can escape from crystal lattice Increase crystal bridges

Particle Size / Filterability produce particles large enough to be 'caught‘ ideally, produce crystals avoid colloidal suspension particle size = 1 - 100 nm

Precipitate Formation crystallization nucleation: particles join to produce aggregates crystal growth aggregate grows and 'fall out' of solution   We want a few big chunks of precipitate! supersaturation: more solute than should be present in solution relative supersaturation: a measure of supersaturation, (Q-S)/S Q = actual solute concentration S = equilibrium solute concentration

Controlling Precipitation Increase S Increase temperature Decrease Q Dilute solution Well mixed (stirring)

What Do We Get Out of Gravimetry? % of analyte, % A %A = weight of analyte x 100 weight of sample

How Do We Get %A? G.F. = a FW[analyte] b FW[precipitate] % A = weight of ppt x gravimetric factor (G.F.) x 100 weight of sample G.F. = a FW[analyte] b FW[precipitate] G.F. = # gms of analyte per 1 gm ppt

Gravimetric Factor X apples + Y sugar = Z apple pies What is this relationship in chemistry?

The Gravimetric Factor G.F. = a FW[analyte] b FW[precipitate] Analyte ppt G.F. CaO CaCO3 FeS BaSO4 UO2(NO3)2.6H2O U3O8 Cr2O3 Ag2CrO4

Analyte. ppt. G. F. CaO. CaCO3. CaO/CaCO3 FeS. BaSO4 Analyte ppt G.F. CaO CaCO3 CaO/CaCO3 FeS BaSO4 FeS/BaSO4 UO2(NO3)2 U3O8 3UO2(NO3)2/U3O8 Cr2O3 Ag2CrO4 Cr2O3/2Ag2CrO4

Problem Consider a 1.0000 g sample containing 75% potassium sulfate (FW 174.25) and 25% MSO4. The sample is dissolved and the sulfate is precipated as BaSO4 (FW 233.39). If the BaSO4 ppt weighs 1.4900, what is the atomic weight of M2+ in MSO4? ANS: Mg2+

Answer The hard part is setting up the correct equation (good stoichiometry skills are essential here!): Rearranging and solving:

Problem A mixture of mercurous chloride (FW 472.09) and mercurous bromide (FW 560.99) weighs 2.00 g. The mixture is quantitatively reduced to mercury metal (At wt 200.59) which weighs 1.50 g. Calculate the % mercurous chloride and mercurous bromide in the original mixture. ANS: 0.5182 g

Answer Again, important to set up correct equation: Rearranging and solving:

Homogeneous Precipitation (NH2)CO + 3 H2O + heat  HCOOH + OH- + CO2 + 2 NH4+

High Electrolyte Concentration to Aid Precipitation Excess charge on colloid creates ionic atmosphere around particle

Composition by Gravimetric Analysis Ni2+ (aq) + H2DMG  Ni(DMG)2 + 2 H+ A 0.8234 g org sample produced 0.1397 g of bis(dimethylglyoximate) nickel (II) (FW = 288.91 g/mol). Find the nickel content. Explain how to create a large, filterable precipitate.

Combustion Analysis Find the empirical formula for a 13.72 mg organic sample that produced 6.97 mg of water and 28.44 mg of carbon dioxide

Gravimetric Overview Simple Cheap Specific Timely (1/2 day) Accurate Glassware Reagents ovens, etc. Balances Specific Timely (1/2 day) Accurate Precise (0.1-0.3 %) Sensitive