Chapter 4 Gravimetric Analysis PDF

Fundamenta ls of Analytical Chemistry Gravimetric Analysis  Gravimetric Analysis: Definition – Quantitative methods that are based on determining the mass of a pure compound to which the analyte is chemically related.  Gravimetric Analysis Steps Commonly Observed i...

Fundamenta ls of Analytical Chemistry Gravimetric Analysis  Gravimetric Analysis: Definition – Quantitative methods that are based on determining the mass of a pure compound to which the analyte is chemically related.  Gravimetric Analysis Steps Commonly Observed in the Gravimetric Analysis: 1. Preparation of a solution with the use of an acknowledged weight of the sample analyte. 2. Separation of the preferred ion/element/radical in pure forms by using diverse separation methods 3. After the ion has been separated, the amount of the natural insoluble compound is formed. 4. Calculating the value of the component of interest, primarily based on the weight of the compound observed.  Types of Gravimetric Analysis A. Precipitation Gravimetry B. Volatilization Gravimetry C. Electrogravimetry D. Thermogravimetry E. Gravimetric Titrimetry F. Atomic mass spectrometry  Types of Gravimetric Analysis A. Precipitation Gravimetry  Determining analyte concentration by measuring the mass of a solid precipitate formed during a chemical reaction.  Example: Measuring chloride ions in water by precipitating silver chloride (AgCl) and weighing the resulting solid.  Types of Gravimetric Analysis B. Volatilization Gravimetry  Analyzing volatile substances by measuring mass changes when they are converted to gases upon heating.  Example: Determining sulfur content in a sample by heating it to convert sulfur to sulfur dioxide (SO2) gas and measuring mass loss.  Types of Gravimetric Analysis C. Electrogravimetry  Quantifying analyte concentration by depositing or dissolving it on an electrode using an electric current.  Example: Measuring copper ion concentration by depositing copper metal on an electrode and measuring the mass gain.  Types of Gravimetric Analysis D. Thermogravimetry Studying a sample's composition and thermal properties by monitoring mass changes during controlled heating. Example: Analyzing polymer decomposition by heating it while measuring mass changes.  Types of Gravimetric Analysis E. Gravimetric titrimetry Determining analyte concentration through the mass of a precipitate formed during a titration. Example: Measuring sulfate ion concentration by titrating with barium chloride to form barium sulfate precipitate  Types of Gravimetric Analysis F. Atomic mass spectrometry Identifying elements and isotopes by measuring mass-to-charge ratios of ions. Example: Analyzing carbon isotopes to date archaeological materials using radiocarbon dating. Common Procedure Involved in Gravimetric Analysis 1. Preparation of the sample 5. Washing 2. Precipitation 6. Drying of Igniting 3. Digestion 7. Weighing 4. Filtration 8. Calculation  Advantages/Disadvantages of Gravimetric Analysis Advantages: Experimentally simple and elegant Accurate Precise (0.1 - 0.3%) Disadvantages: Macroscopic technique-requires at least 10 mg pp to collect and weigh properly Time-consuming  Precipitation Gravimetry – analyte is converted to a sparingly soluble precipitate – precipitate is filtered, washed free of impurities, converted to a product of known composition (by suitable heat treatment), and weighed Example: Determination of calcium in water  Precipitation Gravimetry Properties of Precipitates and Precipitating Reagents Gravimetric precipitating agents should: – react specifically or at least selectively with the analyte SPECIFIC REAGENTS ‒ react only with a single chemical species ‒ rare ‒ E.g. Dimethylglyoxime for Nickel test in alkaline solutions SELECTIVE REAGENTS ‒ react with a limited number of species ‒ Common ‒ E.g. AgNO3 to test halogens (Cl, Br, I) Precipitation Gravimetry Properties of Precipitates and Precipitating Reagents Gravimetric precipitating agents should react with analyte to give a product that is: 1. easily filtered and washed free of contaminants; 2. of sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing; 3. unreactive with constituents of the atmosphere; 4. of known chemical composition after it is dried or, if necessary, ignited Precipitation Gravimetry Particle Size and Filterability of Precipitates ‒ Precipitates with large particles are generally desirable Factors That Determine the Particle Size of Precipitates  precipitate solubility  Temperature  reactant concentrations  rate at which reactants are mixed  Precipitation Gravimetry Particle Size and Filterability of Precipitates This equation is known as the Von Weimarn equation in recognition of the scientist who proposed it in 1925. In this equation, Q is the concentration of the solute at any instant, and S is its equilibrium solubility. If Q < S then solution is unsaturated If Q = S then solution is saturated If Q > S then solution is supersaturated  Precipitation Gravimetry Particle Size and Filterability of Precipitates Mechanism of Precipitate Formation Effect of relative supersaturation on particle size can be explained in two ways: Nucleation ‒ a process in which a minimum number of atoms, ions, or molecules join together to give a stable solid ‒ predominates at high relative supersaturation ‒ Produces large number of very fine particles Particle growth ‒ growth of existing nuclei ‒ Predominates at low relative supersaturations ‒ produces crystalline suspensions (DESIRED OUTCOME)  Precipitation Gravimetry Particle Size and Filterability of Precipitates Experimental Control of Particle Size Goal: minimize supersaturation to produce crystalline precipitates  Increase S (solubility of the precipitate): ‒ elevate temperatures ‒ control pH (depends on type of precipitate)  minimize Q (concentration of the solute): - Use dilute solutions ‒ slow addition of the precipitating agent with good stirring  Precipitation Gravimetry Colloidal Precipitates ‒ very small and hard to filter ‒ do not settle (Brownian motion) ‒ Can be coagulated or agglomerated to be filterable Coagulation of Colloids Coagulation ‒ a process of aggregation or accumulation of colloidal particles to settle down as a precipitate ‒ increased by heating, stirring, and adding an electrolyte  Precipitation Gravimetry Colloidal Precipitates Peptization of Colloids ‒ the process by which a coagulated colloid reverts to its original dispersed state Practical Treatment of Colloidal Precipitates ‒ colloids are best precipitated from hot, stirred solutions containing sufficient electrolyte to ensure coagulation  Digestion ‒ a process in which a precipitate is heated in the solution from which it was formed (the mother liquor) and allowed to stand in contact with the solution  Precipitation Gravimetry Crystalline Precipitates Methods of Improving Particle Size and Filterability – Can be done by digestion (without stirring) Coprecipitation – a process in which normally soluble compounds are carried out of solution by a precipitate Four Types of Coprecipitation 1. surface adsorption 2. mixed-crystal formation 3. Occlusion 4. mechanical entrapment  Precipitation Gravimetry Coprecipitation Surface Adsorption – common source of coprecipitation – major source of contamination in coagulated colloids Minimizing Adsorbed Impurities on Colloids – by digestion and washing with a solution of volatile electrolyte Reprecipitation – drastic but effective way to minimize the effects of adsorption – filtered solid is redissolved and reprecipitated – solution containing the redissolved precipitate has a significantly lower contaminant concentration than the original  Precipitation Gravimetry Coprecipitation Mixed-Crystal Formation – a type of coprecipitation in which a contaminant ion replaces an ion in the lattice of a crystal – troublesome type of coprecipitation because little can be done about it when certain combinations of ions are present in a sample matrix – occurs with both colloidal suspensions and crystalline precipitates Occlusion and Mechanical Entrapment – a compound is trapped within a pocket formed during rapid crystal growth – confined to crystalline precipitates  Applications of Gravimetric Methods Inorganic Precipitating Agents  Applications of Gravimetric Methods Inorganic Precipitating Agents Applications of Gravimetric Methods Reducing Agents Organic Precipitating Agents  Applications of Gravimetric Methods Organic Precipitating Agents 8-Hydroxyquinoline (Oxine) Magnesium complex with 8-hydroxyquinoline  Applications of Gravimetric Methods Organic Precipitating Agents Dimethylglyoxime  Applications of Gravimetric Methods Organic Precipitating Agents Sodium Tetraphenylborate  Applications of Gravimetric Methods Organic Functional Group Analysis Volatilization Gravimetry  Calculation of Results from Gravimetric Data Problem 1 The calcium in a 200.0-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 26.6002 g. The mass of the crucible plus CaO (56.077 g/mol) was 26.7134 g. Calculate the concentration of Ca (40.078 g/mol) in water in units of grams per 100 mL of the water.  Calculation of Results from Gravimetric Data Problem 1: Solution  Calculation of Results from Gravimetric Data Problem 2 An iron ore was analyzed by dissolving a 1.1324-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 the addition of NH3. After filtration and washing, the residue was ignited at a high temperature to give 0.5394 g of pure Fe2O3 (159.69 g/mol). Calculate a) the % Fe (55.847 g/mol); and b) (b) the % Fe3O4 (231.54 g/mol) in the sample.  Calculation of Results from Gravimetric Data Problem 2: Solution  Calculation of Results from Gravimetric Data Problem 2: Solution Cont…  Calculation of Results from Gravimetric Data Problem 2: Solution Cont…  Calculation of Results from Gravimetric Data Problem 3 A 0.2356-g sample containing only NaCl (58.44 g/mol) and BaCl2 (208.23 g/mol) yielded 0.4637 g of dried AgCl (143.32 g/mol). Calculate the percent of each halogen compound in the sample. Solution  Calculation of Results from Gravimetric Data Problem 3: Solution Cont…  Calculation of Results from Gravimetric Data Problem 3: Solution Cont…  Calculation of Results from Gravimetric Data Problem 3: Solution Cont…  Calculation of Results from Gravimetric Data Problem 3: Solution Cont…