Unit IV Gravimetric Analysis !1. Gravimetric Analysis Gravimetric methods – are quantitative methods in which the mass of the analyte or some compound that is chemically related to the analyte is determined. PRINCIPLE : Gravimetric analysis is the process of isolating and weighing an element or a definite compound of the element in as pure a form as possible. The element or compound of the element is separated from a weighed portion of the substance being examined. A large portion of the determinations in gravimetric analysis is concerned with the transformation of the element or radical to be determined into a pure stable compound which can be readily converted into a form suitable for weighing. The weight of the element or radical may then be readily calculated from a knowledge of the formula of the compound and the relative atomic masses of the constituent elements. The separation of the element or of the compound containing it may be effected in number of ways, the most important of which are 1. Precipitation Methods 2. Volatilisation or evolution methods 3. Electro analytical methods - Electro Gravimetry 1. Precipitation Methods The most important is the precipitation method in gravimetric analysis, which is explained in detail below. A weighed sample of the substance to be analysed is brought into solution by a suitable method, and the element to be determined is then precipitated as an insoluble compound. The precipitate is filtered off, washed throughly, ignited (or dried) and weighed accurately. The content of the element is calculated from the weight of the precipitate and its formula and expressed as a percentage of the sample taken. The precipitate may be collected on a filter paper or in a Gooch crucible. 2. Volatilisation or evolution methods In addition to precipitation, the other methods are also used. This Volatilisation or Evolution method depends essentially upon the removal of volatile constituents. This may be effected in several ways. (i) By simple ignition in air or in a current of an indifferent gas. (ii) By treatment with some chemical reagent whereby the desired constituent is rendered volatile and (iii) By treatment with a chemical reagent whereby the desired constituent is rendered non-volatile. The volatilised substance may be absorbed in a weighed quantity of a suitable medium when the estimation is a direct one, or the weight of the residue remaining after volatilisation of a component is determined, and the proportion of the constituent calculated from the loss in weight. 3. Electro analytical methods - Electro Gravimetry In electro-gravimetric analysis, the element to be determined is deposited electrolytically upon a suitable electrode. Filtration is not required, and provided the experimental conditions are carefully controlled, the co-deposition of two metals can often be avoided. In each of these analytical methods, the amount of a given component is found from the results of weighing . These methods can therefore be regarded as different varieties of gravimetric analysis. Unit IV Gravimetric Analysis !2. Steps Involved in Gravimetric Analysis 1. Preparation of the solution 2. Precipitation 3. Digestion 4. Filtration 5. Washing of the Precipitate 6. Drying or igniting 7. Weighing 8. Calculation 1. PREPARATION OF THE SOLUTION This may involve several steps including adjustment of the pH of the solution in order for the precipitate to occur quantitatively and get a precipitate of desired properties, removing interferences, adjusting the volume of the sample to suit the amount of precipitating agent to be added. 2. PRECIPITATION or NUCLEATION Gravimetric precipitation are usually made in beakers. Except during the actual precipitation, the beaker is covered with the clock-glass. A thin stirring rod, rounded at each end, is also required. NUCLEATION In practice, supersaturation influences one of the mechanisms of precipitation, namely nucleation and crystal growth. Nucleation is the first stage of precipitation and consists in the formation of stable microcrystals that can grow spontaneously, i.e. crystallization nuclei. Further precipitation leads to competition between new and existing nuclei (particle growth). If nucleation predominates, then a crystal will form that is easy to filter. Low supersaturation encourages the growth of existing microcrystals rather than the formation of new nuclei. Higher temperatures also favour the formation of crystalline precipitates, because they increase the solubility of the precipitate (S), and other factors that encourage crystal growth are diluting the solution (Q), and adding the precipitation agent slowly while shaking energetically. PRECIPITATION The solution of the precipitating reagents is usually added slowly from a teat pipette or burette. with efficient stirring, to a suitably diluted solution of the sample. Efficient stirring. is necessary to avoid the possible high local concentrations of precipitating reagent which would tend to give contamination of precipitate due to co-precipitation. The reagent is introduced down the side of the beaker to avoid splashing. Precipitation is usually made from hot dilute solutions, a procedure which tends to give an easily Unit IV Gravimetric Analysis !3. filterable precipitate and reduces the possibility of co-precipitation. The formation of coarse particles is favoured by slow addition of the precipitant. vigorous stirring of the solution during precipitation and by carrying out the precipitation from hot solutions. Only a moderate excess of precipitating reagent is required. When the precipitate has settled somewhat. a few more drops of precipitant should be added to test for the completeness of precipitation; if further precipitation occurs. then the process of addition of precipitant, stirring and allowing the precipitation settle and testing again must be carried out until there is no doubt about completeness of precipitation. The stirring rod should not come into contact with the sides or bottom of the beaker during stirring in order to avoid scratching particles of glass from the surfaces. Also, a scratched surface is difficult to wash clean. Reagents should be examined for the clarity before use and filtered if necessary. Before filtration, precipitates may need to be digested by allowing them to stand overnight, or by heating the precipitate and its supernatant liquid nearly to boiling point for some time. Digestion in this manner increases the degree of coarseness of the precipitate: A further check for completeness of precipitation should be carried out when the supernatant liquid becomes clear during the period of digestion. No further precipitate should be produced when a few drops of the must be added as described above. If the precipitate is much more soluble in hot water than in cold, the solution is allowed to cool to room temperature, before filtration. Otherwise, solutions are filtered hot to speed up the filtration. (i) CO-PRECIPITATION When a precipitate separates from a solution, it is not always perfectly pure: it may contain varying amounts of impurities dependent upon the nature of the precipitate and the conditions of precipitation. The contamination of the precipitate by substances which are normally soluble in the mother liquor is termed co- precipitation. Hence the term co-precipitation is a phenomenon in which the soluble compounds are removed from solution during precipitate formation. It is important to understand that the solution is not saturated with the co- precipitated species. Moreover, contamination of a precipitate by a second substance whose solubility product has been exceed does not constitute co-precipitation. Types of Co-precipitation There are four types of co-precipitation: (i) Surface adsorption (ii) Mixed crystal formation (iii) Occlusion (iv) Mechanical entrapment. Surface adsorption and mixed crystal formation are equilibrium processes., where as occlusion and mechanical entrapment arise from the kinetics of crystal growth. (i) Surface adsorption: Adsorption is a common source of co-precipitation that is likely to cause significant contamination of precipitates with large specific surface areas i.e., coagulated colloids. Note the extensive internal surface area exposed to solvent Although adsorption does occur in crystalline solids, its effect on purity are usually undetectable because of the relatively small, Unit IV Gravimetric Analysis !4. specific surface area of these solids. Coagulation of a colloid does not significantly decrease the amount of adsorption because the coagulated solid still contains large internal surface areas that remain exposed to the solvent which can be seen in the figure. The co-precipitated contaminant on the coagulated colloid consists of the lattice ion originally adsorbed on the surface before coagulation and 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. For example The coagulated silver chloride formed in the gravimetric determination of chloride ion is contaminated with primarily adsorbed silver ions along with nitrate or other anions in the counter-ion layer. As a consequence, silver nitrate, a normally soluble compound, is co-precipitated with the silver chloride. (ii) Mixed - Crystal formation Mixed-crystal formation is a type of co-precipitation in which a contaminant ion replaces an ion in the lattice of crystal. In mixed-crystal formation,