Alkalimetry

ALKALIMETRY, one of the methods of volumetric analysis, aimed at determining the quantitative content of alkali in a substance being tested. The methodology of alkalimetry is closely connected with the methodology of acidimetry, which has the objective of determining the quantitative content of free acids or hydrogen ions. The typical reaction underlying these methods is the reaction of mutual neutralization of acids and bases: R'.H⁺ + M^-.OH' = MR + H₂O (acid + base = salt + water). Both methods consist of testing a specific volume of the analyzed dissolved substance by neutralization drop by drop from a buret with an exact titrated solution. By the change in the color of the indicator at the moment following the achieved neutralization, it is possible to establish the point of neutral reaction, after which, knowing the number of added cubic centimeters of the solution and its titer, one can stoichiometrically calculate (see Stoichiometry) the percentage of alkali (respectively acid) in the test solution. The starting substance for establishing the titer must be such a chemical compound that can be easily obtained in a pure form, does not change in air, withstands weathering, and has a constant chemical formula. Usually, anhydrous soda (well-recrystallized, free of impurities, and dry), freshly recrystallized oxalic acid from water, or pure succinic acid are used. The latter, despite its high cost, has the advantage that it crystallizes without water of crystallization and therefore does not weather. Benzoic acid also differs by the same advantages. Oxalic acid, C₂O₄H₂ · 2H₂O; molecular weight 126.05; 1 cubic centimeter of normal solution contains 0.063025 g of C₂O₄H₂. Succinic acid, C₂H₄(COOH)₂; molecular weight 118.05; 1 cubic centimeter of normal solution contains 0.059025 g of C₄H₆O₄. Soda (anhydrous), Na₂CO₃; molecular weight 106.00; 1 cubic centimeter of normal solution contains 0.053 g of Na₂CO₃. As indicators are used: in the titration of free alkalis—phenolphthalein, in the titration of carbonate alkalis—methyl orange; it is also possible to titrate carbonate alkalis with phenolphthalein, but in this case it is necessary to take into account that CO₂ decolorizes phenolphthalein, and therefore it is necessary to immediately add an excess of titrated HCl and, after heating the acidic solution to remove CO₂, back-titrate the excess hydrochloric acid with titrated caustic alkali until a red color appears.