Standard Solutions of Oxidation and Reduction Reagents

Macro quantities of selenium can be determined gravimetrically after reduction to the elemental form by various reagents such as tin (II) chloride, potassium iodide, or ascorbic acid (I). Ooba described a technique whereby the element is precipitated from perchloric acid solution with hydrazine (2). Selenium may be titrated with standard solutions of sodium thiosulfate, iodide, and ferrous, chromous, or trivalent titanium salts after oxidation to Se(VI) (I). Photometric and fluorometric methods based on formation of the piaselenol with diaminobenzidine or 2,3-diaminonaphthalene has been used for the determination of selenium (I, 3,4,5). Interfering elements such as As, Co, Cr, Cu, Fe, Hg, and Ni, are masked with EDTA or other chelating agents. 

In titrimetric analysis (often termed volumetric analysis in certain books), the substance to be determined is allowed to react with an appropriate reagent added as a standard solution, and the volume of solution needed for complete reaction is determined. The common types of reaction which are used in titrimetry are (a) neutralisation (acid-base) reactions (b) complex-forming reactions (c) precipitation reactions (d) oxidation-reduction reactions. 

Titrations are widely used in analytical chemistry to determine acids, bases, oxidants, reductants, metal ions, proteins, and many other species. Titrations are based on a reaction between the analyte and a standard reagent known as the titrant. The reaction is of known and reproducible stoichiometry. The volume, or the mass, of the titrant needed to react essentially completely with the analyte is determined and used to obtain the quantity of analyte. A volume-based titration is shown in this figure, in which the standard solution is added from a buret, and the reaction occurs in the Erlenmeyer flask. In some titrations, known as coulometric titrations, the quantity of charge needed to completely consume the analyte is obtained. In any titration, the point of chemical equivalence, experimentally called the end point, is signaled by an indicator color change or a change in an instrumental response. 

To detemriine the concentration of a particular solute in a solution, chemists often carry out a titration, which involves combining a sample of flie solution with a reagent solution of known concentration, called a standard solution. Titrations can be conducted using acid-base, precipitation, or oxidation-reduction reactions. Suppose we have an HQ solution of unknown concentration and an NaOH solution we know to be 0.100 M. To determine die concentration of die HQ solu-don, we take a specific volume of diat solution, say 20.00 mL. We dien slowly add die standard NaOH solution to it until die neutralization reaction between die HQ and NaOH is complete. The point at which stoichiometrically equivalent quantities are brought together is known as die equivalence point of the titration. 

The standard reduction potential of Cr " (Table 2) shows that this ion is a strong reducing agent, and Cr(II) compounds have been used as reagents in analytical chemistry procedures (26). The reduction potential also explains why Cr(II) compounds are unstable in aqueous solutions. In the presence of air, the oxidation to Cr(III) occurs by reaction with oxygen. However, Cr(II) also reacts with water in deoxygenated solutions, depending on acidity and the anion present, to produce H2 and Cr(III) (27,28). 

The standard dissolution procedure for titanium oxides is the ammonium sulfate-sulfuric acid mixture developed by Rahm [24]. The most commonly used method in industry is based on the use of metallic aluminum as the reductant, and ferric ammonium sulfate as the titrant. The use of ferric ion as the reagent is preferred, since relatively few species will interfere with its reaction with reduced titanium solutions. 

---