Gravimetric Analysis

Gravimetric Analysis, that branch of quantitative chemical analysis in which the amount of the constituent part being determined, in contrast to volumetric and gaseous analysis, is established by means of weighing. Besides ordinary gravimetric analysis (macroanalysis), a special method called microanalysis is employed, in which the usual operations of gravimetric analysis are used, only being modified and refined in connection with the fact that all determinations in microanalysis are conducted with minimal amounts of substance. The essence of gravimetric analysis (both macro- and microanalysis) is that the constituent part of the substance being analyzed is completely isolated from it by various operations and then weighed. When weighing on analytical balances, it is necessary to observe a number of rules and precautions, of which the most important are the following: before beginning to weigh, it is necessary to allow the object being weighed to assume the temperature of the surrounding room, without placing hot, uncooled objects on the balance. The substance being weighed is always placed on the same pan of the balance, usually the left one, while the counterweights are placed on the right one. First, the "zero point" is determined (it often does not coincide with zero on the scale), for which the average of 3-5 (but not an even number) oscillations of the balance with empty pans is taken. Then the substance being weighed is placed on one pan (usually the left one), and counterweights on the other, starting with the larger ones. Placing the substance being weighed and all movements of counterweights on the balance pans must necessarily be done after preliminary arresting of the balance. The final stages of weighing, performed by moving the rider, are done with the door of the case in which the balance is placed closed. The constituent part of the substance being analyzed, with the exception of special cases of electrolytic determinations, is rarely isolated in a pure state; usually it is isolated and weighed in the form of some chemical compound of constant and precisely known composition; in this case, knowing the weight and quantitative composition of the compound obtained, it is not difficult to calculate the amount of the constituent part being contained in it. Let, for example, it be required to determine the percentage content of barium in barium chloride. An exact sample (for example, 0.5125 g) of barium chloride is weighed on analytical balances, dissolved in water, acidified with hydrochloric acid, heated, and then sulfuric acid is added gradually to this solution. In this process, all the barium present in the solution is converted into practically water-insoluble barium sulfate, and the latter is precipitated from the solution in the form of a white, fine-grained precipitate. The precipitate is allowed to settle, a little more sulfuric acid is added to ensure that enough has been taken and that no barium remains in the solution; then the liquid is decanted through a filter, the precipitate is transferred to it in the most careful manner without losses, and washed with hot water until the reactions for sulfuric acid disappear. The filter with the precipitate is dried, transferred to a pre-ignited and weighed crucible, the filter is burned, the crucible with the precipitate and the ash from the filter is moderately ignited, allowed to cool in a desiccator, and weighed. Ignition and weighing are repeated until constant weight is reached (see). Subtracting the weight of the crucible and the weight of the filter ash (previously known) from the found weight, the weight of barium sulfate is obtained; let it be equal to 0.4897 g. Knowing that in 233.44 g (the weight of a gram-molecule) of barium sulfate there is 137.37 g of pure barium, it is easy to calculate that in 0.4897 g of barium sulfate there is 0.2882 g of pure barium. Since this amount of barium corresponds to a sample of 0.5125 g of barium chloride, it can be calculated that the content of barium in barium chloride is 56.23%. Sometimes the form in which the constituent part being determined is isolated differs from the form in which it is weighed; this happens in cases where the form in which it is convenient to isolate a substance due to its difficult solubility does not at the same time possess the necessary stability and constancy of composition; for example, magnesium is usually isolated in the form of a very difficultly soluble double ammonium-phosphate-magnesium salt, and then by calcination is converted into magnesium pyrophosphate and weighed in this form. Sometimes the constituent part being determined is converted into a gaseous compound of definite composition, which is then absorbed by appropriate absorbents, and its weight amount is determined by the increase in weight of the latter; for example, in so-called elementary organic analysis, when determining carbon, it is converted by burning the substance being analyzed into carbon dioxide, and the latter is absorbed by a pre-weighed solution of caustic potash. The above example also shows that even in the case presented, which is generally the simplest case of analysis, a whole series of preliminary operations, following one another in a definite order, must be performed to obtain the final result, namely: weighing, dissolving, precipitating, filtering, washing, drying, burning, ignition, and again weighing. These operations are sometimes quite complex, requiring the most scrupulous observance of certain conditions developed by theory and practice, and in any case are sufficiently lengthy. Moreover, in the analysis of any new substance (of unknown composition), it is necessary to first carefully study the substance being analyzed, determine its qualitative composition, and develop a plan for isolating the substance to be determined, such a plan in which all processes would proceed quantitatively, i.e., completely, without the slightest loss of substance. It follows from this that gravimetric analysis requires a significant expenditure of time for its execution, and this explains the desire to replace it, in all possible cases, with more rapidly conducted methods of volumetric analysis. However, despite this drawback, gravimetric analysis, due to its accuracy and wide applicability