Volhard's Method

VOLHARD'S METHOD for the determination of chlorides, bromides, iodides, cyanides, and thiocyanates is based on their precipitation in the presence of HNO₃ with a titrated solution of AgNO₃, the excess of which is back-titrated with a solution of ammonium thiocyanate; a solution of iron-ammonium alum (a ferric iron salt) serves as the indicator. Silver precipitates in the form of silver thiocyanate, and once all of it has precipitated, an extra drop of ammonium thiocyanate produces a red coloration from the resulting ferric thiocyanate. Required solutions: 1) Titrated solution of AgNO₃: ⁿ/₁₀ or empirical, 1 cm³ of which corresponds to 0.01 g NaCl (29.0633 g AgNO₃ in 1 l of aqueous solution) or to a certain amount of Cl, Br, I, CN, CNS. The exact titer of the solution is established against an exact solution of NaCl as described below in the execution of the determination. 2) A cold-saturated solution of commercial iron-ammonium alum, free of chlorine. 3) Titrated solution of ammonium or potassium thiocyanate. 13-14 g of NH₄CNS is dissolved in 1 l of water and its titer is established against AgNO₃: 10 cm³ of the AgNO₃ solution is measured with a pipette into a beaker, 4 cm³ of 30% HNO₃ free of chlorine, 5 cm³ of the alum solution, and 80 cm³ of water are added. Then ammonium thiocyanate is added dropwise from a buret with constant stirring until the first persistent pink coloration appears. From the amount (a) of cubic centimeters of ammonium thiocyanate consumed, the number of cubic centimeters of silver nitrate corresponding to 1 cm³ of NH₄CNS is calculated: 1 cm³ NH₄CNS = k cm³ AgNO₃. 4) Chlorine-free 30% nitric acid (sp. gr. 1.2). Execution of the determination: 10 cm³ of the test solution (e.g., urine) is accurately measured with a pipette and poured into a 100 cm³ volumetric flask, 50 cm³ of distilled water, 4 cm³ of conc. HNO₃, and an accurately measured volume of AgNO₃ solution from a pipette are added in such a quantity that an excess of it remains after the precipitation of chlorine (not less than 10 cm³). Then, filling to the mark with water, it is mixed well and filtered through a dry Swedish filter into a dry 50 cm³ volumetric flask. The contents of the flask are poured into a beaker of 150-300 cm³ capacity, the flask is rinsed several times with water; 5 cm³ of the iron alum solution is added and titrated with the NH₄CNS solution until a persistent pink coloration appears. Calculation: the titration of 50 cm³ of the filtrate consumed x cm³ of NH₄CNS, which means the entire volume of liquid (100 cm³) corresponds to 2x cm³ of this solution. Consequently, the excess of the AgNO₃ solution taken for the precipitation of chlorides is = 2x · k cm³. The total amount of silver nitrate taken was y cm³. Hence, the amount of AgNO₃ consumed for the precipitation of chlorine is (y - 2x · k) cm³. If the titration was performed with an empirical AgNO₃ solution, each cubic centimeter of which corresponds to 0.01 g NaCl, then 10 cm³ of the test solution contain NaCl = 0.01 · (y - 2x · k) g. In the case of using an ⁿ/₁₀ solution of AgNO₃ — 5.845 · (y - 2x · k) mg, since 1 cm³ of it corresponds to 0.0058454 g NaCl. The calculation for Cl, Br, I, CN, CNS is conducted in a similar manner. Volhard's method can be used for the determination of chlorides, bromides, etc., in urine and other fluids and has the advantage over Mohr's method that it does not strictly require a neutral reaction of the medium. Mercury salts and oxidizing agents interfere with the titration. Proteins and albumoses must be preliminarily destroyed by incineration. Volhard's method gives accurate results, but at very low chloride contents, slightly high figures are obtained.