Titrations with Oxidizing Agents

Types of titrations for which potendometric methods of determining endpoints are particularly useful including dtrarions of halide mixtures, of various metals, of alkaloids, in non-aqneous solvents, and various titrations with oxidizing agents, such as permanganate, dichromate, iodate, and cenc sulfate. 

In alkaline solutions when suitable reducing agents are titrated with oxidizing agents such as H2O2 or a hypohalite in the presence of luminol and lucigenin, at the endpoint the redox potential of the solution suddenly moves towards a more positive value and the CL reaction occurs. In acidic solution (pH<3.5), siloxene has been used as a CL redox indicator. An outline of the use of these indicators in redox titrations is presented in Table 4B. 

Fig. 4.1 Curves for the potentiometric titration of Redn with oxidizing agent Ox2. Before 100% titration, curve I is for...

Although mercury seldom is used as an indicator electrode in redox titrations (because it is so readily oxidized), it is used extensively for potentiometric titrations with complexing agents such as ethylenediaminetetraacetic acid... 

In analytical chemistry, a redox titration is based on an oxidation-reduction reaction between analyte and titrant. Common analytical oxidants include iodine (I2), permanganate (MnOJ), cerium(IV), and dichromate (Cr207 ). Titrations with reducing agents such as Fe " (ferrous ion) and Sn " (stannous ion) are less common because solutions of most reducing agents need protection from air to prevent reaction with O2. 

Andrews deration An important titration for the estimation of reducing agents. The reducing agent is dissolved In concentrated hydrochloric acid and titrated with potassium iodale(V) solution. A drop of carbon tetrachloride is added to the solution and the end point is indicated by the disappearance of the iodine colour from this layer. The reducing agent is oxidized and the iodate reduced to ICl, i.e. a 4-eiectron change. 

Probably the most extensively applied masking agent is cyanide ion. In alkaline solution, cyanide forms strong cyano complexes with the following ions and masks their action toward EDTA Ag, Cd, Co(ll), Cu(ll), Fe(ll), Hg(ll), Ni, Pd(ll), Pt(ll), Tl(lll), and Zn. The alkaline earths, Mn(ll), Pb, and the rare earths are virtually unaffected hence, these latter ions may be titrated with EDTA with the former ions masked by cyanide. Iron(lll) is also masked by cyanide. However, as the hexacy-anoferrate(lll) ion oxidizes many indicators, ascorbic acid is added to form hexacyanoferrate(ll) ion. Moreover, since the addition of cyanide to an acidic solution results in the formation of deadly... 

Iodide ion, a moderately effective reducing agent, is used extensively for the deterrnination of oxidants. In such appHcations, the iodine Hberated by reaction between the analyte and the unmeasured excess of potassium iodide is ordinarily titrated with a standard solution of sodium thiosulfate. The reaction is as foHows ... 

The quantitative conversion of thiosulfate to tetrathionate is unique with iodine. Other oxidant agents tend to carry the oxidation further to sulfate ion or to a mixture of tetrathionate and sulfate ions. Thiosulfate titration of iodine is best performed in neutral or slightly acidic solutions. If strongly acidic solutions must be titrated, air oxidation of the excess of iodide must be prevented by blanketing the solution with an inert gas, such as carbon dioxide or... 

Chlorate Analysis. Chlorate ion concentration is determined by reaction with a reducing agent. Ferrous sulfate is preferred for quaHty control (111), but other reagents, such as arsenious acid, stannous chloride, and potassium iodide, have also been used (112). When ferrous sulfate is used, a measured excess of the reagent is added to a strong hydrochloric acid solution of the chlorate for reduction, after which the excess ferrous sulfate is titrated with an oxidant, usually potassium permanganate or potassium dichromate. 

With the exception of iron(II) and uranium(IV), the reduced solutions are extremely unstable and readily re-oxidise upon exposure to air. They are best stabilised in a five-fold excess of a solution of 150g of ammonium iron(III) sulphate and 150 mL of concentrated sulphuric acid per litre [approximately 0.3M with respect to iron] contained in the filter flask. The iron(II) formed is then titrated with a standard solution of a suitable oxidising agent. Titanium and chromium are completely oxidised and produce an equivalent amount of iron(II) sulphate molybdenum is re-oxidised to the Mo(V) (red) stage, which is fairly stable in air, and complete oxidation is effected by the permanganate, but the net result is the same, viz. Mo(III)- Mo(VI) vanadium is re-oxidised to the V(IV), condition, which is stable in air, and the final oxidation is completed by slow titration with potassium permanganate solution or with cerium(IV) sulphate solution. 

A common laboratory technique for determining the concentration of a solute is titration (Fig. L.2). Titrations are usually either acid-base titrations, in which an acid reacts with a base, or redox titrations, in which the reaction is between a reducing agent and an oxidizing agent. Titrations are widely used to monitor water purity and blood composition and for quality control in the food industry. 

Conductometric titration rests on the marked changes that occur near the titration endpoint in the relation between conductivity and the amount of titrant added (an extreme or inflection point). It is used in particular for the titration of acids with base (and vice versa) in colored and turbid solutions or solutions containing reducing and oxidizing agents (i.e., in those cases where the usual color change of acid-base indicators cannot be seen). 

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