Gravimetric Methods of Analysis PDF

Republic of the Philippines President Ramon Magsaysay State University (Formerly Ramon Magsaysay Technological University) Iba, Zambales, Philippines Tel/ Fax No: (047) 811- 168...

Republic of the Philippines President Ramon Magsaysay State University (Formerly Ramon Magsaysay Technological University) Iba, Zambales, Philippines Tel/ Fax No: (047) 811- 1683 COLLEGE OF ARTS AND SCIENCES CHEM BIO II: ANALYTICAL METHODS FOR BIOLOGY Chapter 5 GRAVIMETRIC METHODS OF ANALYSIS LEARNING OUTCOMES At the end of the lesson, the student must be able to: Describe the different gravimetric methods of analysis Perform calculations involving gravimetric methods OVERVIEW OF GRAVIMETRIC METHODS Gravimetric analysis is one of the most accurate and precise methods of macro- quantitative analysis. In this process, the analyte is selectively converted to an insoluble form. The separated precipitate is dried or ignited, possibly to another form, and is accurately weighed. From the weight of the precipitate and a knowledge of its chemical composition, we can calculate the weight of the analyte in the desired form. Gravimetric analysis is an analytical technique used for the quantitative determination of an analyte based on the mass of a solid. The element to be identified is precipitated from a solution using this method of analysis with the addition of a suitable precipitating agent. The precipitate should either have a known composition or, through heating, should be changed into another compound with a known composition. For example, to determine the sulphate ions (SO4 2- ) contained in ammonium sulphate (NH4)2SO4 solution, the solution is treated with barium chloride (BaCl2) first. When all sulphate ions have precipitated as barium sulphate (BaSO4), the precipitate of barium sulphate is filtered, washed, dried, ignited, and finally weighed. By knowing the weight of the precipitate, BaSO4, the amount of sulphate ion present in the given volume of ammonium sulfate can be determined using a suitable stoichiometric relationship. TYPES OF GRAVIMETRIC METHODS Gravimetric methods: The quantitative methods that are based on determining the mass of a pure compound to which the analyte is chemically related. 1. 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. Precipitation gravimetric analysis separates ions from a solution by using the precipitation process (the reaction that creates an insoluble solid product from the reaction of two soluble solid products). The chemical responsible for precipitate formation in the precipitation reaction is referred to as the precipitating agent.  For instance, the white precipitate of silver chloride is produced when aqueous silver nitrate and sodium chloride solutions react. Sodium chloride is utilized as a precipitating agent in this process. AgNO3 + NaCl → AgCl + NaNO3 2. Volatilization gravimetry: The analyte is separated from other constituents of a sample by converting it to a gas of known chemical composition that can be weighed. The analytical technique of gravimetric volatilization separates the masses using thermal or chemical energy to determine their masses. In this method solid reactant molecules are converted into gaseous molecules using thermal or chemical energy. Different volatile gases (that can be easily evaporative), like carbon dioxide, chlorine, etc., can be separated with the help of volatilization gravimetry. For example, the aqueous solution of sulfuric acid helps to separate carbon dioxide gas (a volatile gas) molecules from sodium bicarbonate. 2NaHCO3 (aq) +H2SO4 (aq) → 2CO2 (g) +2H2O (l) +Na2SO4 (aq) 3. Electrogravimetry: The analyte is separated by deposition on an electrode by an electrical current. Electro gravimetric method is employed to separate the ions of a substance, often a metal. In this method, the analyte solution is electrolyzed. As a result of the electrolytic reduction, the analyte is deposited in the cathode. PRECIPITATION GRAVIMETRY In precipitation gravimetry, the analyte is converted to a sparingly soluble precipitate by adding a precipitating reagent, or precipitant, to a solution containing the analyte. This precipitate is then filtered, washed free of impurities, converted to a product of known composition by suitable heat treatment, and weighed. Example: precipitation method for determining calcium in water - an excess of oxalic acid, H2C2O4, is added to an aqueous solution of the sample. Ammonia is then added, which neutralizes the acid and causes essentially all of the calcium in the sample to precipitate as calcium oxalate. 2 NH3 + H2C2O4 → 2NH4+ + C2O42- Ca2+(aq) + C2O42- (aq) → CaC2O4 (s) - The CaC2O4 precipitate is filtered using a weighed filtering crucible, then dried and ignited. This process converts the precipitate entirely to calcium oxide. Δ CaC2O4 (s) → CaO (s) + CO (g) + CO2( g) - After cooling, the crucible and precipitate are weighed, and the mass of calcium oxide is determined by subtracting the known mass of the crucible. 1|P age PROPERTIES OF PRECIPITATES AND PRECIPITATING REAGENTS Precipitant or precipitating agent refers to the chemical that is used to cause precipitation. A gravimetric precipitating agent should ideally react specifically or selectively with the analyte. The ideal precipitating reagent would react with the analyte to produce a precipitate in addition to specificity and selectivity. 1. Inorganic precipitating agent- although they have less selectivity inorganic precipitants like S2-, PO4 3-, and CO3 2- are used for gravimetric analysis. 2. Organic precipitating agent- In comparison to inorganic precipitants, organic precipitants like dimethglyoxime and 8-hydroxyquinoline are more selective. They form a less soluble precipitate with the analyte. The drawback of organic precipitants is that they frequently produce an unknown-formula precipitate with the analyte, which is then burnt to produce metal oxide. A gravimetric precipitating agent should react specifically or at least selectively with the analyte and give precipitates that are: 1. Enough particle size for retaining on filter paper 2. High purity (free of contaminants) 3. Low solubility that no significant loss of the analyte occurs during filtration and washing 4. Unreactive with air (stable) 5. Known stoichiometric composition after it is dried or, if necessary, ignited. PARTICLE SIZE AND FILTERABILITY OF PRECIPITATES 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. Name Diameter Characteristics Ion ~10-8 cm (A°) Dissolved Colloid 10-7~10-4 cm (nm-µm) Suspended Crystalline >10-4cm (µm) Settled from solution (filterable) Precipitates consisting of large particles are generally desirable for gravimetric work because these particles are easy to filter and wash free of impurities. Large particles are desired since they are filtered much easier. In order to obtain these large precipitate particles, we must understand the factors that affect the particle size and use them to our benefit. The particle size of a precipitate is influenced by: 1. precipitate solubility, 2. temperature, 3. reactant concentrations, and 4. the rate at which reactants are mixed 2|P age  To account for the effect of these variables on particle size, chemist have theorized that particle size is related to a single property called the supersaturation ratio, or relative supersaturation, given by: RSS = Q-S S where Q is the concentration of the solute at any instant, and S is its equilibrium solubility. Q – S is a measure of the degree of saturation. This ratio is also called the Von Weimarn ratio. Accordingly, the rate of nucleation increases exponentially with the supersaturation ratio, while the rate of growth increases linearly. 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. RSS Process Result Large Nucleation dominate Smaller particles (colloidal) Small Particle growth dominate Larger Particle (Crystalline) MECHANISM OF PRECIPITATE FORMATION Precipitates form in two ways: by nucleation and by particle growth 1. Nucleation: The initial formation process in which a minimum number of atoms, ions, or molecules join together to give a stable solid. Often, these nuclei form on the surface of suspended solid contaminants, such as dust particles Nucleation – molecules in solution come together randomly and form small aggregates 2. Particle growth: The subsequent growth after nucleation. Particle growth – addition of molecules to a nucleus to form a crystal If nucleation is faster than particle growth: - a large number of small aggregates occur giving colloidal suspensions If particle growth is faster than nucleation: - only a few, large particles form giving pure crystals COLLOIDAL PARTICLES Many precipitates do not give a favorable von Weimarn ratio, especially very insoluble ones. Hence, it is impossible to yield a crystalline precipitate (small number of large particles), and the precipitate is first colloidal (large number of small particles). Colloidal particles are very small (1 to 1000 nm) and have a very large surface-to-mass ratio, which promotes surface adsorption. Adsorption is a process in which a substance (gas, liquid, or solid) is held on the surface of a solid. a. Coagulation of Colloids 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. 3|P age When a precipitate’s particles are electrically neutral, they tend to coagulate into larger particles that are easier to filter. For instance, coagulating a precipitate of AgCl by adsorption: Figure 1: Adsorption process The adsorption creates a primary layer that is strongly adsorbed and is an integral part of the crystal. It will attract ions of the opposite charge in a counter-ion layer or secondary layer so the particle will have an overall neutral charge. There will be solvent molecules interspersed between the layers. Normally, the counter-ion layer completely neutralizes the primary layer and is close to it, so the particles will collect together to form larger-sized particles; that is, they will coagulate. However, if the secondary layer is loosely bound, the primary surface charge will tend to repel like particles, maintaining a colloidal state. Coagulation can be hastened by heating, by stirring, and by adding an electrolyte to the medium. To understand the effectiveness of these measures, we need to look into why colloidal suspensions are stable and do not coagulate spontaneously. Adsorption: A process in which a substance (gas, liquid, or solid) is held on the surface of a solid. Absorption: A process in which a substance within the pores of a solid. PEPTIZATION OF COLLOIDS The term “coprecipitated” is generally meant to include only the contamination of a precipitate by normally soluble substance. Coprecipitated impurities, especially those on the surface, can be removed by washing the precipitate after filtering. The precipitate will be wet with another mother liquor, which is removed by washing. Many precipitates cannot be washed with pure water, because peptization occurs. Peptization is the process by which a coagulated colloid reverts to its original dispersed state. - When a coagulated colloid is washed, some of the electrolyte responsible for its coagulation is leached from the internal liquid in contact with the solid particles. - Removal of this electrolyte has the effect of increasing the volume of the counter-ion layer. The repulsive forces responsible for the original colloidal state are then reestablished, and particles detach themselves from the coagulated mass. The washings become cloudy as the freshly dispersed particles pass through the filter. 4|P age The problem is usually solved by washing the precipitate with a solution containing an electrolyte that volatilizes when the precipitate is dried or ignited. PRACTICAL TREATMENT FOR COLLOIDAL PARTICLES Colloids are best precipitated from hot, stirred solutions containing sufficient electrolyte to ensure coagulation. The filterability of a coagulated colloid often improves if it is allowed to stand for an hour or more in contact with the hot solution from which it was formed. During this process, which is known as digestion, weakly bound water appears to be lost from the precipitate. The result is a denser mass that is easier to filter. 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. Mother liquor is the solution from which a precipitate was formed. CRYSTALLINE PRECIPITATES Crystalline precipitates are generally more easily filtered and purified than are coagulated colloids. Particle size of crystalline solids can often be improved significantly by minimizing Q (by using dilute solutions, and adding the precipitating reagent slowly, with good mixing) or maximizing S (precipitating from hot solution or by adjusting the pH), or both. Digestion improves the purity and filterability of both colloidal and crystalline precipitates. The improvement in filterability undoubtedly results from the dissolution and recrystallization that occur continuously and at an enhanced rate at elevated temperatures. COPRECIPITATION 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. 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. 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. Four types of coprecipitation: 1. surface adsorption 2. mixed-crystal formation 3. occlusion 4. mechanical entrapment Surface adsorption and mixed-crystal formation are equilibrium processes, and occlusion and mechanical entrapment arise from the kinetics of crystal growth. a. Surface Adsorption The coprecipitated contaminant on the coagulated colloid consists of the lattice ion originally adsorbed on the surface before coagulation plus the counter-ion of opposite charge held in the film of solution immediately adjacent to the particle. The net effect of surface adsorption is, therefore, the carrying down of an otherwise soluble compound as a surface contaminant. 5|P age Minimizing Adsorbed Impurities on Colloids i. Digestion. - water is expelled from the solid to give a denser mass that has a smaller specific surface area for adsorption. ii. Washing a coagulated colloid with a solution containing a volatile electrolyte - any nonvolatile electrolyte added earlier to cause coagulation is displaced by the volatile species iii. Reprecipitation - the filtered solid is redissolved and reprecipitated o The first precipitate usually 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 an analysis. b. Mixed-Crystal Formation- A type of coprecipitation in which a contaminant ion replaces an ion in the lattice of a crystal. “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. ▪ 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. c. Occlusion and Mechanical Entrapment Occlusion- A type of co-precipitation in which a compound (foreign ions in the counter-ion layer ) is physically trapped within a precipitate during rapid precipitate formation Mechanical Entrapment- A type of co-precipitation in which coprecipitated physically trap a pocket of solution within a precipitate during rapid precipitate formation. ▪ 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 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. Under conditions of low supersaturation, both occlusion and mechanical entrapment are at a minimum. - digestion often reduces the effects of these types of coprecipitation - 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. 6|P age DRYING AND IGNITION OF PRECIPITATES ✓ A gravimetric precipitate is heated until its mass becomes constant. ✓ After the precipitate is filtered, it is heated until its mass becomes constant. The primary purpose of this is to drive off any solvent and any other volatile species that may be present. The temperature and time required to produce a suitable product varies from precipitate to precipitate. ✓ Some precipitates are also ignited. This process decomposes the solid, whose exact composition may not be known, and forms a new compound of known composition. This new compound is called the weighing form. VOLATILIZATION GRAVIMETRY A second approach to gravimetry is to thermally or chemically decompose the sample and measure the resulting change in its mass. Alternatively, we can trap and weigh a volatile decomposition product. Because the release of a volatile species is an essential part of these methods, we classify them collectively as volatilization gravimetric methods of analysis. Volatilization gravimetry usually requires that we know the products of the decomposition reaction. ▪ organic compounds typically decompose to form simple gases such as CO 2, H2O, and N2 ▪ inorganic compound products often depend on the decomposition temperature 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. 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. 7|P age https://www.youtube.com/watch?v=FxbpAAEzJro https://labmodules.soilweb.ca/gravimetric-soil-water-content/ 8|P age