Quantitative analysis, or the accurate determination of quantities of a substance present, can be divided into two very general categories – wet test methods and instrumental methods. Wet test methods can be subdivided into categories, including volumetric analysis and gravimetric analysis. The former attains results through the use of volume measurements, as in titration. Gravimetric analysis uses the properties of mass and molecular weight, and usually requires the conversion of a soluble substance into an insoluble one. Which of these two methods to use is determined by the analyst, who takes into consideration a number of factors, including ease and economy of procedure, as well as accuracy of results.
To illustrate: an analytical chemist desires to know how much barium chloride (BaCl2) is present in 25 ml. (5 ml. = 1 tsp.) of an aqueous solution. Although it is possible to employ volumetric methods for the determination, gravimetric analysis is simpler in this instance. Sulfuric acid (H2SO4) is added to the unknown to convert all the chloride into sulfate, according to the reaction BaCl2 + H2SO4 → BaSO4↓ + 2 HCl. Note that the sulfate precipitates or settles out, as the down arrow indicates. Gravimetric analysis provides excellent accuracy in this instance because barium sulfate is completely water insoluble.
Given a 150-ml. beaker containing 15 ml. of barium chloride solution, an analytical chemist estimates it to contain 4 to 7 g. (453.6 grams = 1 lb.) BaCl2 dissolved in pure water. While stirring the solution with a glass stirring-rod, he slowly pours in enough 10% reagent grade sulfuric acid to convert the chloride into sulfate. He pulls the rod out – rinses with deionized water the traces of sulfate remaining on the rod back into the beaker – and prepares for vacuum filtration. Swirling and pouring the contents of the beaker onto the filter paper, and adds the rinsings from the sides of the beaker, as well. Next, the precipitate remaining on the filter paper is rinsed with small portions of deionized water.
Before the filtration process, the unused filter paper was rinsed with deionized water, carefully dried, cooled and weighed accurately, giving 2.0174 g. Now the same filter paper with the BaSO4 precipitate collected onto it is placed into the oven and allowed to dry completely, being careful not to lose any of the sulfate product. Removed and cooled, the paper weighs 8.6112 g. Subtracting the weight of the unused paper from this number, gives 6.5938 g BaSO4. The molecular weights of barium sulfate and barium chloride are 233.3896 and 208.2330, respectively, so the amount of BaCl2 that was in the original solution was 6.5938 × (208.2330 ÷ 233.3896) = 5.3881 g – which is well within the range the analytical chemist had anticipated.
Gravimetric analysis affords certain advantages. The volume measurements of volumetric analysis are subject to temperature fluctuation, whereas mass does not vary with temperature. Losses are generally more likely to occur if volumetric methods are employed. In addition, a precipitate may be limited to one chemical substance that is known – in this instance, barium sulfate – while it can be harder to be certain of substance identity in a volumetric analysis.