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• Gravimetric methods of analysis are based on the measurement of mass
• There are two major types of gravimetric methods: precipitation methods and volatilization methods
• In precipitation methods, the analyte is converted to a sparingly soluble precipitate. This precipitate is then filtered, washed free of impurities, and converted to a product of known composition by suitable heat treatment, and the product is weighed
• In volatilization methods, the analyte or its decomposition products are volatilized at a suitable temperature. The volatile product is then collected and weighed, or, alternatively, the mass of the product is determined indirectly from the loss in mass of the sample
• The ideal product of a gravimetric analysis should be pure, insoluble, and easily filterable, and should possess a known composition
• Few substances meet these requirements, but appropriate techniques can help optimize properties of gravimetric precipitates
• Precipitates consisting of large particles are generally desirable for gravimetric work because these particles are easy to filter and wash free of impurities
• Precipitates of large particles are usually purer than those made up of fine particles
• Colloidal suspensions (10^-7 to 10^-4 cm in diameter) do not settle from solution and are difficult to filter
• Crystalline suspensions (tenths of mm or greater) tend to settle spontaneously and are easily filtered
• The particle size of a precipitate is influenced by precipitate solubility, temperature, reactant concentrations, and the rate at which reactants are mixed
• The particle size is related to relative supersaturation (Von Weimarn equation)
• When (Q - S)/S is large, the precipitate tends to be colloidal
• When (Q - S)/S is small, a crystalline solid is more likely
• Precipitates form in two ways, namely by nucleation and particle growth
• Nucleation is a process in which a minimum number of atoms, ions, or molecules join to produce a stable solid. Further precipitation then involves a competition between additional nucleation and growth on existing nuclei (particle growth)
• Colloidal precipitates are so small that they tend to pass through the filter paper. The Brownian motion also prevents them from settling out of the solution under gravity
• In order to collect these colloidal precipitates, they need to be coagulated or agglomerated
• Adsorption is a process in which a substance is held on the surface of a solid. In contrast, absorption involves retention of a substance within the pores of a solid
• Improving Particle Size and Filterability of crystalline solids can often be achieved by minimizing Q, maximizing S, or both
• Coprecipitation is a process in which normally soluble compounds are carried out of solution by a precipitate
• There are four types of coprecipitation: Surface adsorption, Mixed-crystal formation, Occlusion, Mechanical entrapment
• Homogeneous precipitation is a process in which a precipitate is formed by slow generation of a precipitating reagent homogeneously throughout a solution
• After filtration, a gravimetric precipitate is heated until its mass becomes constant. Some precipitates are also ignited to decompose the solid and form a compound of known composition
• The results of the gravimetric analysis are generally computed from the mass of the sample and the mass of a product of known composition
• Calculation requires the correct identity of the products after ignition and stoichiometric relationships (balanced equation)
• Gravimetric methods have been developed for most inorganic anions and cations as well as for such neutral species. A variety of organic substances can also be readily determined gravimetrically
• Gravimetric methods do not require a calibration or standardization step because the results are calculated directly from the experimental data and molar masses
• The two most common gravimetric methods based on volatilization are those for water and carbon dioxide
• Carbonates are ordinarily decomposed by acids to give carbon dioxide, which is readily evolved from solution by heat
• Modern combustion analysis instruments can measure C, H, N, and S in a single operation
• A compound weighing 5.714 mg produced 14.414 mg of CO2 and 2.529 mg of H2O upon combustion. Find the wt% of C and H in the sample
• A 6.234-mg sample produced 12.123 mg CO2 and 2.529 mg of H2O. Find the wt% of C and H in the sample
• The calcium in a 200.0-mL sample of a natural water was determined by precipitating the cation as CaC2O4. Calculate the concentration of Ca in water in units of grams per 100 mL of the water
• A 0.2356-g sample containing only NaCl and BaCl2 yielded 0.4637 g of dried AgCl. Calculate the percent of each halogen compound in the sample