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6.1 Gravimetry Methods of Analysis Objectives: by Th Title: GRAVIMETRIC METHODS OF ANALYSIS Unit Title: Same as above Sub-Units: 6.1 Gravimetry Methods of Analysis Objectives: By the end of the lesson students should be able to: i) Define gravimetric ii) Mention four types of gravimetric methods of analysis iii) Differentiate between inclusion, occlusion, surface adsorption, and isomorphous replacement. 6.0 Main Content 6.1 Gravimetry encompasses all techniques in which we measure mass or change in mass. Measuring mass is the most fundamental of all analytical measurements, and gravimetry is the oldest analytical technique. This suggests that there are at least two ways to use mass as analytical signal. An analyte mass can be measure directly by placing it on a balance and recording its mass. In anoother way, an analyte can be removed and used as a change in mass as the analytical signal. For instance, moisture content of the food sample may be determined by heating the same at certain temperature and the mass difference before and after heating is recorded and expressed at percentage moisture content. In other words, an analyte can be determined gravimetrically by directly determining its mass, or the mass of the compound containing the analyte. Alternatively, an analyte can be determined indirectly by measuring a change in mass due to its loss, or mass of a compound formed as a result of reaction involving the analyte. Gravimetric analysis can thus be defined as a quantitative method of analysis in which insoluble products after reaction between a reagent and specie is weighed and from the weight the mass of the analyte is determined. Note: The most fundamental and frequent measurement made in an analysis laboratory is that of weight (or mass). But there is fundamental difference between the two terms. Mass is the quantity or amount of a substance being measured. This quantity is the same no matter where the measurement is made – on the surface of the earth. Weight is a measure of the earth’s gravitational force exerted on a quantity of matter. Weight can measure the quantity of a substance by measuring the earth’s gravitational effect on it. Weighing devices are calibrated in grams, which is defined as the basic unit of mass in the metric system. 6.2 Types 6.2.1 Precipitation Gravimetry: Here, the species to be determined is precipitated by a reagent that yields a sparingly soluble product that either has a known composition or can be converted to 3- such a substance. The indirect determination of PO3 by precipitating Hg2Cl2 is a representative example, as is the direct determination of Cl- by precipitating AgCl. 6.2.2 Electrogravimetry: The analyte is deposited as a solid film on one electrode in an 2+ electrochemical cell. The oxidation of Pb and its deposition as PbO2 on a Pt anode is an example of electrogravimetry. Reduction may also be used in electrogravimetry. The electrodeposition of Cu on a Pt cathode, for e.g., provides direct analysis for Cu2+. In other words, electrogravimetry is a gravimetric method in which the signal is the mass of an electrodeposit on the cathode or anode in an electrochemical cell. 6.2.3 Volatilization Gravimetry: Thermal or chemical energy is used to remove a volatile spp. i.e. the analyte or its decomposition products are volatilized at an appropriate temperature. In determination of moisture content of food, thermal energy vaporises the water. The amount of carbon in organic compound may be determined by using the chemical energy of combustion to convert carbon to carbon dioxide. 6.2.4 Particulate Gravimetry: The analyte is determined following its removal from the sample matrix by filtration or extraction. E.g. the determination of suspended solids. 6.3 Precipitation Gravimetry This is based on the formation of an insoluble compound following the addition of a precipitating reagent, or precipitant (a reagent that causes the precipitation of a soluble spp), to a solution of the analyte. In other words, the analyte is precipitated from the sample solution in a form which is practically insoluble such that no appreciable loss occur when the precipitate is separated by filtration and washing. The following are the attributes or properties of precipitation gravimetry: the precipitate must be of low solubility, high purity, and of known composition. Further, the precipitate must be in a form that is easy to separate from the reaction mixture; unreactive with the constituents of the atmosphere. 6.4 Steps in Gravimetric Analysis 6.4.1 Preparation of Solution: Here, preliminary separation may be necessary to eliminate interfering materials. Also, the condition of the solution must be adjusted to maintain low solubility of the precipitate. Factors to be considered in preparation of solution include the following: (i) volume of the solution (ii) the concentration range of the test substance (iii) presence and concentration of other constituents (iv) temperature (v) pH. pH is essential because it influences both the solubility of the precipitate and the possibility of interference from other substance. For instance, hydroxide precipitates such as FeOH2 are more soluble at lower pH at which the concentration of OH- is small. Also, calcium oxalate is insoluble in basic medium but at low pH the oxalate ion combine with H+ ions to form weak acid. Al3+ions can be precipitated at pH 4, but the concentration of the anion form oxime is too low at this pH to precipitate magnesium ions. 6.4.2 Precipitation: This has to do with the addition of precipitating agent to a test solution to form a precipitate e.g. AgCl. The precipitation process involves heterogeneous equilibrium which is not instantaneous. The first step in precipitation process is super saturation i.e. the solution phase contains more of the dissolved salt that occurs at equilibrium. This is started by Nucleation. The higher the degree of super saturation, the greater the rate of nucleation. 6.4.3 Digestion: This is done to make larger precipitate and more pure crystals. The precipitate is allowed to stand in the presence of mother liquor i.e. the solution from which it was precipitated. In digestion, similar particles tend to dissolve and re-precipitate on the surfaces of the larger crystals. As precipitate forms, the ions are re-arranged in a fixed pattern for e.g. in AgCl, there will be alternating Ag+ and Cl- ions on the surface. Other steps are filtration, washing, drying or igniting, weighing and then calculation. 6.5 Impurities in Precipitates Precipitates tend to carry some impurities causing the precipitate to become contaminated. Any impurities must be removed before weighing. The greatest source of impurities results from chemical and physical interactions occurring at the precipitate’s surface. Precipitate particles grow in size because of electrostatic attraction between charged ions on the surface of the precipitate and oppositely charged ions in solution. Some common types of impurities are as follows: 6.5.1 Inclusion: This occurs when ions, whose size and charge are similar to a lattice ion are trapped within the crystal lattice by chemical adsorption, provided that the interferent precipitates with the same crystal structure. The probability of forming an inclusion is greatest when the interfering ion is present at substantially higher concentrations than the dissolved lattice. Inclusions are hard to remove the included material is chemically part of the crystal lattice. The only way to remove included material is through reprecipitation. 6.5.2 Occlusion: This is a second type of coprecipitated impurity, which occur when physically adsorbed interfering ions become trapped within the growing precipitate. i.e. when the material that is not part of the crystal structure is trapped within a crystal. Occlusions from in two ways: i) When physically adsorbed ions are surrounded by additional precipitate before they can be desorbed or displaced. Here, the precipitate weight is greater than expected. ii) When rapid precipitation traps a pocket of solution within the growing precipitate. Here, the precipitate mass increased because the trapped solution contains dissolved solids. The mass of precipitate is less than expected. Occlusions may be minimized by keeping the precipitate in equilibrium with its supernatant solution for a period of time. This process is called digestion (i.e. the process by which a precipitate is given time to form larger, purer particles) which may be carried out at room temperature or at a higher temperature. 6.5.3 Surface adsorption: The surface of the precipitate which may be chemically or physically adsorbed, constitute the third type of coprecipitated impurity. The surface adsorption is minimized by decreasing the precipitate’s available surface area. Surface adsorbates may also be removed by washing the precipitate. Occlusion, inclusion, and surface adsorption are called coprecipitates because they represent the soluble species that are brought into solid forms along with the desired precipitate. 6.5.4 Isomorphous replacement: Two compounds having the same type of formula and crystallize in similar geometric form are said to be isomorphous. When their lattice dimension are almost the same, one ion can replaces another in a crystal, resulting in a mixed crystal. This process is called isomorphous replacement.e.g. In precipitation of Mg2+, as magnesium ammonium + + phosphate, K has the same ionic size as NH4 and can replace it to form magnesium potassium phosphate. Whenever isomorphous replacement occurs, little or nothing can be done about it. It is rarely used in analytical analysis. 6.5.5 Post precipitation: In a situation where the precipitate is allowed to stand in contact with the mother liquor, a second substance will slowly form a precipitate with the precipitating agent. This is known as post precipitation. 6.6 Gravimetric Calculations Example 1: 0.3126 g sample of piperazine was dissolved in 25 mL of acetone, and 1 mL of acetic acid was added.
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