Introduction to Analytical Chemistry Chapter 4 Gravimetric Methods of Analysis

Gravimetric Methods of Analysis 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. 4-2

Gravimetric Methods of Analysis 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. 4-3

Gravimetric Methods of Analysis 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. 4-4

Figure 4-1 Figure 4-1 Apparatus for determining the sodium hydrogen carbonate content of antacid tablets by a gravimetric volatilization procedure. 4-5

4A Properties Of Precipitates And Precipitating Reagents Ideally, a gravimetric precipitating agent should react specifically, or if not, then selectively with the analyte. 4-6

4A Properties Of Precipitates And Precipitating Reagents The ideal precipitating reagent would react with the analyte to give a product that is 1. Readily filtered and washed free of contaminants 2. Of sufficiently low solubility so that no significant loss of the solid occurs during filtration and washing 3. Unreactive with constituents of the atmosphere 4. Of known composition after it is dried or, if necessary, ignited 4-7

4A-1 Particle Size and Filterability of Precipitates Precipitates made up of large particles are generally desirable in gravimetric work because large particles are easy to filter and wash free of impurities. A colloid is a solid made up of particles having diameters that are less than 10-4 cm. 4-8

4A-1 Particle Size and Filterability of Precipitates What Factors Determine Particle Size? Colloidal particles show no tendency to settle from solution, nor are they easily filtered. The particles, with dimensions on the order of tenths of a millimeter or greater, temporary dispersed in the liquid phase is called a crystalline suspension. The particles of a crystalline suspension tend to settle spontaneously and are readily filtered. 4-9

4A-1 Particle Size and Filterability of Precipitates What Factors Determine Particle Size? The particle size of a precipitate is influenced by such experimental variables as precipitate solubility, temperature, reactant concentrations, and the rate at which reactants are mixed. 4-10

4A-1 Particle Size and Filterability of Precipitates What Factors Determine Particle Size? The particle size is related to relative supersaturation, where Q is the concentration of the solute at any instant and S is its equilibrium solubility. (4-1) 4-11

4A-1 Particle Size and Filterability of Precipitates What Factors Determine Particle Size? When (Q - S)/S is large, the precipitate tends to be colloidal; when (Q - S)/S is small, a crystalline solid is more likely. 4-12

4A-1 Particle Size and Filterability of Precipitates How Do Precipitates Form? Assume that precipitates form in two ways, namely by nucleation and by particle growth. The particle size of a freshly formed precipitate is determined by which way is faster. 4-13

4A-1 Particle Size and Filterability of Precipitates How Do Precipitates Form? Nucleation is a process in which a minimum number of atoms, ions, or molecules join together to produce a stable solid. Further precipitation then involves a competition between additional nucleation and growth on existing nuclei (particle growth). 4-14

4A-1 Particle Size and Filterability of Precipitates How Do Precipitates Form? The rate of nucleation is believed to increase enormously with increasing relative supersaturation. In contrast, the rate of particle growth is only moderately enhanced by high relative supersaturations. 4-15

4A-1 Particle Size and Filterability of Precipitates Controlling Particle Size Experimental variables lead to crystalline precipitates include elevated temperatures to increase the solubility of the precipitate, dilute solutions, and slow addition of the precipitating agent with good stirring. Larger particles can also be obtained by pH control, provided the solubility of the precipitate depends on pH. 4-16

4A-2 Colloidal Precipitates Coagulate, or agglomerate, the individual particles of most colloids to give a filterable, amorphous mass that will settle out of solution. 4-17

4A-2 Colloidal Precipitates Coagulation of Colloids Coagulation can be hastened by heating, stirring, and adding an electrolyte to the medium. The charge on a colloidal particle formed in a gravimetric analysis is determined by the charge of the lattice ion that is in excess when the precipitation is complete. 4-18

4A-2 Colloidal Precipitates Coagulation of Colloids Adsorption is a process in which a substance (gas, liquid, or solid) is held on the surface of a solid. In contrast, absorption involves retention of a substance within the pores of a solid. The charge on a colloidal particle formed in a gravimetric analysis is determined by the charge of the lattice ion that is in excess when the precipitation is complete. 4-19

Figure 4-2 Figure 4-2 A colloidal silver chloride particle suspended in a solution of silver nitrate. 4-20

Figure 4-3 Figure 4-3 Effect of AgNO3 and electrolyte concentration on the thickness of the double layer surrounding a colloidal AgCl particle in a solution containing excess of AgNO3 . 4-21

Figure 4-4 Figure 4-4 The electric 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 chances for coagulation. 4-22

4A-2 Colloidal Precipitates Coagulation of Colloids Coagulation of a colloidal suspension can often be brought about by a short period of heating, particularly if accompanied by stirring. An even more effective way to coagulate a colloid is to increase the electrolyte concentration of the solution. 4-23

4A-2 Colloidal Precipitates Peptization of Colloids Peptization is a process by which a coagulated colloid returns to its dispersed state. 4-24

4A-2 Colloidal Precipitates Treatment of Colloidal Precipitates Colloids are best precipitated from hot, stirred solutions containing sufficient electrolyte to ensure coagulation. 4-25

4A-3 Crystalline Precipitates Improving Particle Size and Filterability The particle size of crystalline solids can often be improved significantly by minimizing Q, maximizing S, or both. Digestion of crystalline precipitates (without stirring) for some time after formation frequently yields a purer, more filterable product. 4-26

4A-4 Coprecipitation There are four types of coprecipitation: surface adsorption, mixed-crystal formation, occlusion, and mechanical entrapment. Coprecipitation is a process in which normally soluble compounds are carried out of solution by a precipitate. 4-27

4A-4 Coprecipitation Surface Adsorption Adsorption is a common source of coprecipitation that is likely to cause significant contamination of precipitates with large specific surface areas, coagulated colloids . 4-28

4A-4 Coprecipitation Surface Adsorption Coagulation of a colloid does not significantly decrease the amount of adsorption. The net effect of surface adsorption is therefore the carrying down of an otherwise soluble compound as a surface contaminant. 4-29

4A-4 Coprecipitation Surface Adsorption Minimizing Adsorbed Impurities on Colloids. The purity of many coagulated colloids is improved by digestion. Regardless of the method of treatment, a coagulated colloid is always contaminated to some degree, even after extensive washing. 4-30

4A-4 Coprecipitation Surface Adsorption Reprecipitation. A drastic but effective way to minimize the effects of adsorption is reprecipitation, or double precipitation. 4-31

4A-4 Coprecipitation Mixed-Crystal Formation In mixed-crystal formation, one of the ions in the crystal lattice of a solid is replaced by an ion of another element. The extent of mixed-crystal contamination is governed by the law of mass action and increases as the ratio of contaminant to analyte concentration increases. 4-32

4A-4 Coprecipitation Occlusion and Mechanical Entrapment When a crystal is growing rapidly during precipitate formation, foreign ions in the counter-ion layer may become trapped, or occluded, within the growing crystal. Occlusion is a type of coprecipitation in which a compound is trapped within a pocket formed during rapid crystal growth. 4-33

4A-4 Coprecipitation Occlusion and Mechanical Entrapment Mixed-crystal formation may occur in both colloidal and crystalline precipitates, whereas occlusion and mechanical entrapment are confined to crystalline precipitates. Mechanical entrapment occurs when crystals lie close together during growth. 4-34

4A-4 Coprecipitation Occlusion and Mechanical Entrapment Both occlusion and mechanical entrapment are at a minimum when the rate of precipitate formation is low, that is, under conditions of low supersaturation. In addition, digestion is often remarkably helpful in reducing these types of coprecipitation. 4-35

4A-4 Coprecipitation Coprecipitation Errors Coprecipitation can cause either negative or positive errors. 4-36

4A-5 Precipitation from Homogeneous Solution Homogeneous precipitation is a process in which a precipitate is formed by slow generation of a precipitating reagent homogeneously throughout a solution. In general, homogeneously formed precipitates, both colloidal and crystalline, are better suited for analysis than a solid formed by direct addition of a precipitating reagent. 4-37

4B Drying And Ignition Of Precipitates 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. This new compound is often called the weighing form. 4-38

4B Drying And Ignition Of Precipitates Recording thermal decomposition curves isoften called thermogravimetry or thermalgravimetric analysis, and the mass vs. temperature curves are called thermograms. 4-39

Figure 4-7 Figure 4-7 Effect of temperature on precipitate mass. 4-40

4C Calculating Results From Gravimetric Data The results of a gravimetric analysis are generally computed from the mass of sample and the mass of a product of known composition. 4-41

Example 4.2 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) the % Fe3O4 (231.54 g/mol) in the sample. 4-42

Example 4.2 For both parts of this problem, we need to calculate the number of moles of Fe2O3 . Thus, 4-43

Example 4.2 (a) The number of moles of Fe is twice the number of moles of Fe2O3 , and 4-44

Example 4.2 (b) As shown by the following balanced equation, 3 mol of Fe2O3 are chemically equivalent to 2 mol of Fe3O4 . That is, 4-45

4D Applications Of Gravimetric Methods 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. 4-46

4D-1 Inorganic Precipitating Agents These reagents typically form slightly soluble salts or hydrous oxides with the analyte. Most inorganic reagents are not very selective. 4-47

4D-3 Organic Precipitating Agents Numerous organic reagents have been developed for the gravimetric determination of inorganic species. One forms slightly soluble nonionic products called coordination compounds; the other forms products in which the bonding between the inorganic species and the reagent is largely ionic. 4-48

4D-3 Organic Precipitating Agents Chelates are cyclical metal-organic compounds in which the metal is a part of one or more five- or six-membered rings. 8-Hydroxyquinoline and dimethylglyoxime are two widely used chelating reagent. Sodium tetraphenyl borate is a near-specific precipitation agent for potassium and ammonium. 4-49

4D-3 Organic Precipitating Agents 8-Hydroxyquinoline 4-50

4D-3 Organic Precipitating Agents Dimethylglyoxime 4-51

4D-3 Organic Precipitating Agents Sodium Tetraphenylborate Sodium tetraphenylborate, (C6H5) 4B–Na+, is an important example of an organic precipitating reagent that forms salt-like precipitates. 4-52

4D-5 Volatilization Methods 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. 4-53

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