Visual Detection of the End Point

The above two methods of indication do not depend on the half-reaction potentials, although the completeness of the titration reaction and hence the sharpness of the end point do. Examples of these first two methods of visual indication are few, and most types of redox titrations are detected using redox indicators. These are highly colored dyes that are weak reducing or oxidizing agents that can be oxidized or reduced the colors of the oxidized and reduced forms are different. The oxidation state of the indicator and hence its color will depend on the potential at a given point in the titration. A half-reaction and Nemst equation can be written for the indicator  

E% must be near the equivalence point potential. A potential change of 120 raV is needed for a color change for n = 1 (of the indicator half-reaction) and 60 mV for n = 2. 

Indicator Reduced Form Oxidized Form Solution 0(V) 

Diphenylaminesulfonic acid Colorless Purple Diluted acid 0.84 

E% is near the equivalence point potential of the titration, where there is a rapid change in potential in excess of 0.12 V, then the color change occurs at the equivalence point. Again, this is analogous to the requirement that the pKa value of an acid-base indicator be near the pH of the equivalence point. 

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Discussion. Iodine (or tri-iodide ion Ij" = I2 +1-) is readily generated with 100 per cent efficiency by the oxidation of iodide ion at a platinum anode, and can be used for the coulometric titration of antimony (III). The optimum pH is between 7.5 and 8.5, and a complexing agent (e.g. tartrate ion) must be present to prevent hydrolysis and precipitation of the antimony. In solutions more alkaline than pH of about 8.5, disproportionation of iodine to iodide and iodate(I) (hypoiodite) occurs. The reversible character of the iodine-iodide complex renders equivalence point detection easy by both potentiometric and amperometric techniques for macro titrations, the usual visual detection of the end point with starch is possible. 

Figure 19.10—Karl Fischer method for determination of water. The conventional burette titration with visual detection of the end point leads to imprecise results. Thus, a cell containing two small platinum electrodes is used. As long as no iodide is present in the solution, the current between the electrodes is weak. When excess iodide is present in the solution at the instant the equivalence point is reached, a significant current is registered.

The end-point of titrations with cerium(IV) solutions can be detected visually (without or with use of a redox indicator) or potentiometrically. Whereas the intense purple color of a permanganate solution allows an easy visual detection of the end point, the yellow-orange color of cerium(IV) solutions is often not intense enough to act as an indicator. Only in a limited number of cases, for instance when oxalic acid or hydrogen peroxide is the analyte, can the titration be made without a redox indicator, provided that the concentrations of the analyte are not too low and that an appropriate blank correction is made. It is easier to detect the end point in hot solutions than in cold solutions, because of an intensification of the yellow color of the cerium(IV) ion with a rise in temperature. A large blank correction is required... 

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