Analytical Chemistry: Gravimetric Methods

o ANALYTICAL CHEMISTRY QUANTITATIVE ANALYSIS: GRAVIMETRIC METHOD o o INTRODUCTION GRAVIMETRIC METHOD • • • Gravimetric methods of analysis are based on the measurement of mass. These analyses are based on mass measurements with an analytical balance, an instrument that yields highly accurate and precise data. In fact, if you perform a gravimetric chloride determination in the laboratory, you may take some of the most accurate and precise measurements of your life. There are two major types of gravimetric methods: precipitation gravimetry and volatilization gravimetry. In precipitation gravimetry, 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. Another form of gravimetric analysis is volatilization gravimetry. In this technique, 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. PARTICLE SIZE AND FILTERABILITY OF PRECIPITATES • • STEPS OF A GRAVIMETRIC ANALYSIS The steps required in gravimetric analysis, after the sample has been dissolved are summarized below: 1. Preparation of the solution 2. Precipitation 3. Digestion 4. Filtration 5. Washing 6. Drying or igniting 7. Weighing 8. Calculation • PRECIPITATION GRAVIMETRY • This method of analysis involves isolation of an ion in solution by a precipitation reaction, filtering, washing the precipitate free of contaminants, conversion of the precipitate to a product of known composition, and finally weighing the precipitate and determining its mass by difference. From the mass and known composition of the precipitate, the amount of the original ion can be determined. PROPERTIES OF PRECIPITATES AND PRECIPITATING REAGENTS • • Ideally, a gravimetric precipitating agent should react specifically or at least selectively with the analyte. Specific reagents, which are rare, react only with a single chemical species. Selective reagents, which are more common, react with a limited number of species. In addition to specificity and selectivity, the ideal precipitating reagent would react with the analyte to give a product that is o Readily filtered and washed free of contaminants Precipitates consisting of large particles are generally desirable for gravimetric work because these particles are easy to filter and wash free of impurities. In addition, precipitates of this type are usually purer than are precipitates made up of fine particles. FACTORS THAT DETERMINE THE PARTICLE SIZE OF PRECIPITATES LESSON 1 - GRAVIMETRIC ANALYSIS BY PRECIPITATION • Of sufficiently low solubility that no significant loss of the analyte occurs during filtration and washing Unreactive with constituents of the atmosphere Of known chemical composition after it is dried or, if necessary, ignited. The particle size of solids formed by precipitation varies enormously. At one extreme are colloidal suspensions, whose tiny particles are invisible to the naked eye (10-7 to 10-4 cm in diameter). Colloidal particles show no tendency to settle from solution and are difficult to filter. At the other extreme are particles with dimensions on the order of tenths of a millimeter or greater. The temporary dispersion of such particles in the liquid phase is called a crystalline suspension. The particles of a crystalline suspension tend to settle spontaneously and are easily filtered. The particle size of a precipitate is influenced by o Precipitate solubility o Temperature o Reactant concentrations o The rate at which reactants are mixed. The net effect of these variables can be accounted for, at least qualitatively, by assuming that the particle size is related to a single property of the system called relative supersaturation. RSS = (Q-S) / S Q: The concentration of the solute at any instant S: Its equilibrium solubility. Generally, precipitation reactions are slow so that, even when a precipitating reagent is added drop by drop to a solution of an analyte, some supersaturation is likely. Experimental evidence indicates that the particle size of a precipitate varies inversely with the average relative supersaturation during the time when the reagent is being introduced. Thus, when (Q-S)/S is large, the precipitate tends to be colloidal, and when (Q-S)/S is small, a crystalline solid is more likely. MECHANISM OF PRECIPITATE FORMATION • • • The effect of relative supersaturation on particle size can be explained if we assume that precipitates form in two ways: by nucleation and by particle growth. The particle size of a freshly formed precipitate is determined by the mechanism that predominates. In nucleation, a few ions, atoms, or molecules (perhaps as few as four or five) come together to form a stable solid. Further precipitation then is governed by the competition between additional nucleation and growth of existing nuclei (particle growth). If nucleation predominates, a precipitate containing a large number of small particles results, and if growth predominates, a smaller number of larger particles is produced. At high relative supersaturations, the rate of nucleation tends to predominate. High relative supersaturation produces colloidal suspensions. ANACHEM • 1 • At low relative supersaturations, the rate of particle growth tends to predominate. Low relative supersaturation produces crystalline suspensions. • EXPERIMENTAL CONTROL OF PARTICLE SIZE • Experimental variables that minimize supersaturation and thus produce crystalline precipitates include o Elevated temperatures (increase S) o Dilute solutions (minimize Q) o Slow addition of the precipitating agent (minimize Q) o Good stirring (minimize Q) LESSON 2 - PURITY OF PRECIPITATES • PRECIPITATES • • Crystalline Precipitates o Crystalline precipitates are generally more easily filtered and purified than coagulated colloids. In addition, the size of individual crystalline particles, and thus their filterability, can be controlled to a degree. Colloidal Precipitates o Individual colloidal particles are so small that they are not retained by ordinary filters. Fortunately, however, we can coagulate, or agglomerate, the individual particles of most colloids to give a filterable, amorphous mass that will settle out of solution. WAYS TO PURIFY PRECIPITATES OR MINIMIZE IMPURITIES • COAGULATION OF COLLOIDS • • • • Coagulation: colloids come together to form a filterable solid Coagulation can be hastened by heating, by stirring, and by adding an electrolyte to the medium. Colloidal suspensions are stable because all of the particles of the colloid are either positively or negatively charged and thus repel one another. The charge results from cations or anions that are bound to the surface of the particles. The process by which ions are retained on the surface of a solid is known as adsorption. o Adsorption is a process in which a substance (gas, liquid, or solid) is held on the surface of a solid. o Absorption is the retention of a substance within the pores of a solid. Attached directly to the solid surface is the primary adsorption layer, which consists mainly of adsorbed barium ions. Surrounding the charged particle is a layer of solution, called the counter-ion layer, which contains sufficient excess of negative ions to just balance the charge on the surface of the particle. The primarily adsorbed barium ions and the negative counter-ion layer constitute an electric double layer that imparts stability to the colloidal suspension. o As colloidal particles approach one another, this double layer exerts an electrostatic repulsive force that prevents particles from colliding and adhering. o Coagulation of a colloidal suspension can often be brought about by a short period of heating, particularly if accompanied by stirring. o Heating decreases the number of adsorbed ions and the thickness of the double layer. The particles may also gain enough kinetic energy at a higher temperature to overcome the barrier to a close approach imposed by the double layer. An even more effective way to coagulate a colloid is to increase the electrolyte concentration of the solution. If we add a suitable ionic compound to a colloidal suspension, the concentration of counter-ions increases in the vicinity of each particle. As a result, the volume of solution that contains sufficient counter-ions to balance the charge of the primary adsorption layer decreases. The net effect of adding an electrolyte is thus a shrinkage of the counter-ion layer. The particles can then approach one another more closely and agglomerate. Peptization of Colloids o Peptization is a process by which a coagulated colloid returns to its dispersed state. o Washing the precipitate with pure water causes peptization. The problem is usually solved by washing the precipitate with a solution containing an electrolyte that volatilizes when the precipitate is dried or ignited. Colloids are best precipitated from hot, stirred solutions containing sufficient electrolyte to ensure coagulation. • Digestion o Digestion is 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. During this process, weakly bound water appears to be lost from the precipitate. The result is a denser mass that is easier to filter. o Digestion of crystalline precipitates (without stirring) for some time after formation frequently yields a purer, more filterable product. The improvement in filterability undoubtedly results from the dissolution and recrystallization that occur continuously and at an enhanced rate at elevated temperatures. Recrystallization apparently results in bridging between adjacent particles, a process that yields larger and more easily filtered crystalline aggregates. This view is supported by the observation that little improvement in filtering characteristics occurs if the mixture is stirred during digestion. Washing o Washing a coagulated colloid with a solution containing a volatile electrolyte may also be helpful because any nonvolatile electrolyte added earlier to cause coagulation is displaced by the volatile species. Washing generally does not remove much of the primarily adsorbed ions because the attraction between these ions and the surface of the solid is too strong. Exchange occurs, however, between existing counter ions and ions in the wash liquid. o For example, in the determination of silver by precipitation with chloride ion, the primarily adsorbed species is chloride. Washing with an acidic solution converts the counter-ion layer largely to hydrogen ions so that both chloride and hydrogen ions are retained by the solid. Volatile HCl is then given off when the precipitate is dried. o Regardless of the method of treatment, a coagulated colloid is always contaminated to some degree, even after extensive washing. The error introduced into the ANACHEM • 2 • analysis from this source can be as low as 1 to 2 ppt, as in the coprecipitation of heavy metal hydroxides on the hydroxides of trivalent iron or aluminum can result in errors as large as several percent, which is generally tolerable. Reprecipitation o A drastic but effective way to minimize the effects of adsorption is reprecipitation or double precipitation. Here, the filtered solid is redissolved and reprecipitated. The first precipitate ordinarily carries down only a fraction of the contaminant present in the original solvent. Thus, the solution containing the redissolved precipitate has a significantly lower contaminant concentration than the original, and even less adsorption occurs during the second precipitation. Reprecipitation adds substantially to the time required for analysis but is often necessary for such precipitates as the hydroxides of iron (III) and aluminum, which have extraordinary tendencies to adsorb the hydroxides of heavy-metal cations, such as zinc, cadmium, and manganese. • PRECIPITATION FROM HOMOGENEOUS SOLUTION • • Homogeneous precipitation is a process in which a precipitate is formed by a slow generation of a precipitating reagent homogeneously throughout a solution. Solids formed by homogeneous precipitation are generally purer and more easily filtered than precipitates generated by the direct addition of a reagent to the analyte solution. DRYING AND IGNITION OF PRECIPITATES • After filtration, a gravimetric precipitate is heated until its mass becomes constant. Heating removes the solvent and any volatile species carried down with the precipitate. 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. • LESSON 3 ERRORS OF PRECIPITATION ANALYSIS COPRECIPITATION • Coprecipitation is a process in which normally soluble compounds are carried out of solution by a precipitate. When otherwise soluble compounds are removed from solution during precipitate formation, we refer to the process as coprecipitation. Contamination of a precipitate by a second substance whose solubility product has been exceeded is not coprecipitation. • FOUR TYPES OF COPRECIPITATION • Surface Adsorption o Adsorption is a common source of coprecipitation and is likely to cause significant contamination of precipitates with large specific surface areas, that is, coagulated colloids. In adsorption, a normally soluble compound is carried out of solution on the surface of a coagulated colloid. This compound consists of the primarily adsorbed ion and an ion of opposite charge from the counter-ion layer. o Although adsorption does occur in crystalline solids, its effects on purity are usually undetectable because of the relatively small specific surface area of these solids. Adsorption is often the major source of contamination in coagulated colloids but of no significance in crystalline precipitates. Mixed-Crystal Formation o In mixed-crystal formation, one of the ions in the crystal lattice of a solid is replaced by an ion of another element. For this exchange to occur, it is necessary that the two ions have the same charge and that their sizes differ by no more than about 5%. Furthermore, the two salts must belong to the same crystal class. o For example, barium sulfate formed by adding barium chloride to a solution containing sulfate, lead, and acetate ions is found to be severely contaminated by lead sulfate. This contamination occurs even though acetate ions normally prevent precipitation of lead sulfate by complexing the lead. In this case, lead ions replace some of the barium ions in the barium sulfate crystals. Other examples of coprecipitation by mixedcrystal formation include MgKPO4 in MgNH4PO4, SrSO4 in BaSO4, and MnS in CdS. o 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. Mixed-crystal formation is a particularly troublesome type of coprecipitation because little can be done about it when certain combinations of ions are present in a sample matrix. This problem occurs with both colloidal suspensions and crystalline precipitates. When mixed-crystal formation occurs, the interfering ion may have to be separated before the final precipitation step. Alternatively, a different precipitating reagent that does not give mixed crystals with the ions in question may be used. Occlusion o Occlusion is a type of coprecipitation in which a compound is trapped within a pocket formed during rapid crystal growth. 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. Because supersaturation and thus growth rate decrease as precipitation progresses, the amount of occluded material is greatest in that part of a crystal that forms first. Mechanical Entrapment o Mechanical entrapment occurs when crystals lie close together during growth. Several crystals grow together and in so doing trap a portion of the solution in a tiny pocket. o 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 often reduces the effects of these types of coprecipitation. Undoubtedly, the rapid dissolving and reprecipitation that occur at the elevated temperature of digestion open up the pockets and allow the impurities to escape into the solution. o Mixed-crystal formation may occur in both colloidal and crystalline precipitates, but occlusion and mechanical entrapment are confined to crystalline precipitates. • ANACHEM • 3 COPRECIPITATION ERRORS • REDUCING AGENTS Coprecipitated impurities may cause either negative or positive errors in analysis. If the contaminant is not a compound of the ion being determined, a positive error will always result. Therefore, a positive error is observed whenever colloidal silver chloride adsorbs silver nitrate during a chloride analysis. In contrast, when the contaminant does contain the ion being determined, either positive or negative errors may occur. For example, in the determination of barium by precipitation as barium sulfate, occlusion of other barium salts occurs. If the occluded contaminant is barium nitrate, a positive error is observed because this compound has a larger molar mass than the barium sulfate that would have formed had no coprecipitation occurred. If barium chloride is the contaminant, the error is negative because its molar mass is less than that of the sulfate salt. LESSON 4 - TYPICAL PRECIPITATION ANALYSIS ORGANIC PRECIPITATING AGENTS • APPLICATIONS OF GRAVIMETRIC METHODS • Gravimetric methods have been developed for most inorganic anions and cations, as well as for such neutral species as water, sulfur dioxide, carbon dioxide, and iodine. A variety of organic substances can also be determined gravimetrically. Examples include lactose in milk products, salicylates in drug preparations, phenolphthalein in laxatives, nicotine in pesticides, cholesterol in cereals, and benzaldehyde in almond extracts. Indeed, gravimetric methods are among the most widely applicable of all analytical procedures. • INORGANIC PRECIPITATING AGENTS • The table below lists common inorganic precipitating agents. These reagents typically form slightly soluble salts or hydrous oxides with the analyte. As you can see from the many entries for each reagent, few inorganic reagents are selective. • Numerous organic reagents have been developed for the gravimetric determination of inorganic species. Some of these reagents are significantly more selective in their reactions than are most of the inorganic reagents listed below. There are two types of organic reagents: one forms slightly soluble non-ionic products called coordination compounds, and the other forms of products in which the bonding between the inorganic species and the reagent is largely ionic. Organic reagents that yield sparingly soluble coordination compounds typically contain at least two functional groups. Each of these groups is capable of bonding with a cation by donating a pair of electrons. The functional groups are located in the molecule such that a five- or six-membered ring results from the reaction. Reagents that form compounds of this type are called chelating agents, and their products are called chelates. Metal chelates are relatively nonpolar and, as a consequence, have solubilities that are low in water but high in organic liquids. Usually, these compounds possess low densities and are often intensely colored. Because they are not wetted by water, coordination compounds are easily freed of moisture at low temperatures. ORGANIC FUNCTIONAL GROUP ANALYSIS • Several reagents react selectively with certain organic functional groups and thus can be used for the determination of most compounds containing these groups. A list of gravimetric functional group reagents is given below. Many of the reactions shown can also be ANACHEM • 4 used for volumetric and spectrophotometric determinations. VOLATILIZATION GRAVIMETRY • • • • • • The two most common gravimetric methods based on volatilization are those for determining water and carbon dioxide. Water is quantitatively distilled from many materials by heating. In direct determination, water vapor is collected on any of several solid desiccants, and its mass is determined from the mass gain of the desiccant. The indirect method in which the amount of water is determined by the loss of mass of the sample during heating is less satisfactory because it must be assumed that water is the only component that is volatilized. This assumption can present problems, however, if any component of the precipitate is volatile. Nevertheless, the indirect method is widely used to determine water in items of commerce. For example, a semiautomated instrument for the determination of moisture in cereal grains can be purchased. It consists of a platform balance on which a 10-g sample is heated with an infrared lamp. The percent moisture is read directly. An example of a gravimetric procedure involving the volatilization of carbon dioxide is the determination of the sodium hydrogen carbonate content of antacid tablets. A weighed sample of the finely ground tablets is treated with dilute sulfuric acid to convert the sodium hydrogen carbonate to carbon dioxide: NaHCO3(aq) 1 H2SO4(aq) S CO2(g) 1 H2O(l) 1 NaHSO4(aq) As shown in Figure 12-8, this reaction is carried out in a flask connected first to a tube containing CaSO4 that removes water vapor from the initial reaction stream to produce a stream of pure CO2 in nitrogen. These gases then pass through a weighed absorption tube containing the absorbent Ascarite II,8 which consists of sodium hydroxide absorbed on a nonfibrous silicate. This material retains carbon dioxide by the reaction 2NaOH 1 CO2 S Na2CO3 1 H2O • The absorption tube must also contain a desiccant such as CaSO4 to prevent loss of the water produced by this last reaction. Sulfides and sulfites can also be determined by volatilization. Hydrogen sulfide or sulfur dioxide evolved from the sample after treatment with acid is collected in a suitable absorbent. Finally, the classical method for the determination of carbon and hydrogen in organic compounds is a gravimetric volatilization procedure in which the combustion products (H2O and CO2) are collected selectively on weighed absorbents. The increase in mass serves as the analytical variable. QUALITATIVE APPLICATIONS • A precipitation reaction is a useful method for identifying inorganic and organic analytes. Because a qualitative analysis does not require quantitative measurements, the analytical signal is simply the observation that a precipitate has formed. Although qualitative applications of precipitation gravimetry have been largely replaced by spectroscopic methods of analysis, they continue to find application in spot testing for the presence of specific analytes. ANACHEM • 5