Gravimetric Methods of Analysis Version 2012 Updated on 030212 Copyright © All rights reserved Dong-Sun Lee, Prof., Ph.D. Chemistry, Seoul Women’s University Chapter 8 Gravimetric Methods of Analysis

Gravimetry Gravimetric methods are quantitative methods that are based on determining the mass of pure compound to which the analyte is chemically related. Precipitation gravimetry: the analyte is separated from a solution of the sample as a precipitate and is converted to a compound of known composition that can be weighed. Volatilization gravimetry: the analyte is separated from other constituents of a sample by conversion to a gas of known chemical composition. The weight of this gas then serves as a measure of the analyte concentration. Electrogravimetry: the analyte is separated by deposition on an electrode by electrical current. The mass of this product then provides a measure of the analyte concentration. Atomic mass spectrometry: use a mass spectrometer to separate the gaseous ions formed from the elements making up a sample of matter. The concentration of the resulting ions is then determined by measuring the electrical current produced when they fall on the surface of an ion detector.

Procedure for gravimetry based on precipitation 1) Preparation of sample solution 2) Precipitation : Gravimetric precipitating agent should react specifically or at least selectively with the analyte. rR + aA = RrAa (solid) 3) Digestion 4) Filtration and washing 5) Drying, ignition and weighing 6) Computation of results

Ex. Precipitation method for determining calcium in natural water (AOAC) 2NH3 + H2(COO)2  2NH4+ + (COO)22– Ca2+ + (COO)22–  Ca(COO)2 (s) Ca(COO)2 (s)  CaO (s) + CO (g) + CO2 (g) 

[Cl – ] = ? = 0.3048 M Ex. Examples of gravimetry Ag+ + Cl– = AgCl 1 mol 1 mol 1 mol = 143.321g x x mol = 0.4368g x = 3.048 ×10–3mol [Cl– ] = 3.048 ×10–3 mol / 10.00ml = 0.3048 M Excess AgNO3 AgCl 0.4368 g 10.00 ml Cl– T.W. Richards : Nobel Prize-winning research

Properties of Precipitates and Precipitating Reagents Requirements of the ideal product of a gravimetry : 1) very insoluble 2) easily filterable : large crystal particle 3) very pure 4) known and constant composition

1) Solubility of precipitate T   solubility  Vant Hoff equation log Ks = –(H / 2.3RT) + J Polarity of solvent   solubility  H<0 log Ks H>0 1/T Hexane CCl4 Benzene Toluene CHCl3 Pyridine HAC Acetone EtOH MtOH CH3CN Water Polar Non-polar

2) Filterability : large particle size  Precipitation mechanism 1) Induction period (few min) : A precipitant(counterion) is added 2) Nucleation : form small aggregates : Nucleus has 4 molecules 3) Particle growth to form larger crystal 4) Adsorption 5) Electrostatic attraction to coalesce : electric double layer

Colloids and dialysis Colloids are particles with diameters in the range 1-100 nm. They are larger than molecules, but too small to precipitate. They remain in solution indefinitely, suspended by the Brownian motion (random movement) of solvent molecules. Colloidal particles show no tendency to settle from solution and are not easily filtered. The particles of crystalline suspension with the dimensions on the order of tenths of a millimeter or greater tend to settle spontaneously and are easily filtered. Large molecules remain trapped inside a dialysis bag (semipermeable membrane that has pores with diameters of 1-5nm), whereas small molecules diffuse through the membrane in both direction. Dialysis is used to treat patients suffering from kidney failure.

Increase particle size Techniques to decrease supersaturation(promote particle growth)  T   S   Q   degree of relative supersaturation  The precipitant is added slowly with vigorous mixing. Low concentrations of analyte and precipitant Low pH (acidic) Relative supersaturation = (Q–S) / S von Weimarn equation where Q = actual concentration of solute = supersaturation S = concentration at equilibrium Rate Crystal growth Nucleation (Q– S) / S Increase particle size

Coagulation of colloids Heating  particles’ kinetic energy Increase the concentration of electrolyte(HNO3) Silver chloride lattice Primary adsorption layer Counter ion layer Electric double layer

The growth of sodium acetate crystals from a supersaturated solution. A colloidal silver chloride particle suspended in a solution of silver nitrate.

Effect of AgNO3 and electrolyte concentration on the thickness of double layer surrounding a colloidal AgCl particle in a solution containing excess AgNO3.

The electrical double layer of a colloid consists of a layer of charge adsorbed on the surface of the particle (the primary adsorption layer) and a layer of opposite charge (the counter-ion layer) in the solution surrounding the particle. Increasing the electrolyte concentration has the effect of decreasing the volume of the counter-ion layer, thereby increasing the chance for coagulation.

Treatment after precipitation Peptization: a process by which a coagulated colloid returns to its dispersed state. Self purification : Internal ripening Ostwald ripening Digestion : following ppt, for a period of standing with heating the precipitate in contact with its mother liquor accelerate the ripening of crystalline particles. Washing Reprecipitation ppt Mother liquor

Impurities of ppt : coprecipitation 1) surface adsorption 2) mixed-crystal formation: inclusions: impurity ions that randomly occupy crystal lattice 3) occlusions : pockets of impurities that are literally trapped inside the growing crystal 4) mechanical entrapment A B C D Types of coprecipitation: A: surface adsorption B: inclusion-isomorphic carrying C: inclusion-mechanical entrapment in crystals D: occlusion-mechanical entrapment in colloidal aggregate.

A coagulated colloid. This figure suggests that a coagulated colloid continues to expose a large surface area to the solution from which it was formed. Increase in surface area per unit mass with decrease in particle size.

Controlled precipitation Control the solubility of a precipitate through chemical means such as pH or complexing ion control  enhance particle size Ex. ppt of calcium oxalate from hot acidic solution Ca(COO)2 = Ca2+ + (COO)22– (COO)22– + H+ = H(COO)2–

Ex. ppt of ferric formate by using urea and formic acid Homogeneous ppt Control the supersaturation by using slow generating of precipitant by means of chemical reaction  enhance particle size Ex. ppt of ferric formate by using urea and formic acid NH2CONH2 + 3H2O  CO2 + 2NH4+ + 2OH– OH– + HCOOH  HCOO– + H2O 3HCOO– + Fe3+  Fe(HCOO)3. nH2O Aluminum hydroxide formed by the direct addition of ammonia (left), and the homogeneous production of hydroxide (right).

Masking and masking agent

Drying and Ignition of precipitates Precipitate form Ignition (strong heating) is used to change the chemical form of some precipitate. Weighing form Thermogravimetric curve for calcium salicylate.

Schematic thermobalance Effect of temperature on precipitate mass.

Calculations of gravimetry Ex. Relating mass of product to mass of reactant piperazine + 2 CH3COOH  piperazine diacetate FW 84.121 FW 60.053 FW 204.227 = 1 mol x mol ppt 0.7121 g = x mol y g  x = 0.003487 mol y = 0.2933 g Weight percent of piperazine in the commercial material (0.3126 g) = (0.2933 / 0.3126)× 100 = 93.83 %

Confirmation of a molecular formula is satisfactory when the Combustion analysis Combustion methods are useful for the analysis of carbon, hydrogen, nitrogen, sulfur, and halogen. Catalyst Pt gauze CuO PbO2 MnO2 O2 Sample in Pt boat Phosphorous pentoxide Ascarite Absorb water Absorb CO2 Confirmation of a molecular formula is satisfactory when the calculated and experimentally determined percentages agree within 0.3%.

Gravimetric combustion analysis for carbon and hydrogen. Apparatus for determining the sodium hydrogen carbonate content of antacid tablets by a gravimetric volatilization procedure. NaHCO3 (aq) + H2SO4 (aq)  CO2 (g) + H2O (l) + NaHSO4 (aq) 2NaOH + CO2  Na2CO3 + H2O

Ex. Determination of masses of C, H by combustion analysis Organic compound 5.714mg combust CO2 14.414 mg H2O 2.529mg CO2  C 44.010g = 1mol 12.011g 14.414 mg = x mol y mg x = 0.3275 m mol y=3.934mg  C % = (3.934/5.714) ×100 = 68.84 % H2O  2 H 18.0152g =1mol 2×1.0079g 2.529mg = x mol y mg x= 0.1404 mmol 0. 2830 mg  H% = (0.2830 / 5.714)×100 = 4.952 %

Summary Gravimetry Precipitation, prcipitant, precipitate Precipitation process, Solubility of ppt, Filterability, Nucleation, Crystal growth Relative supersaturation, particle size Controlled ppt, Homogeneous ppt Coagulation of colloids Electric double layer Digestion, Coprecipitation, Occlusion Masking, ppt form, weighing form Combustion analysis