Equivalence Point Detection

Before continuing with other examples, it is important to consider how the equivalence point in an acid-base titration is found and what relationship this has with titration curves. As we have said, the inflection point at the center of these curves occurs at the equivalence point, the point at which all of the substance titrated has been exactly consumed by the titrant. The exact position for this in the case 

FIGURE 5.4 The family of titration curves for various bases titrated with a strong acid. 

FIGURE 5.5 Some acid-base indicators and their color change ranges. R = red, Y = yellow, B = blue, P = pink, C = colorless, and A = amber. 

FIGURE 5.6 Titration curve of 0.10 N H2S04 titrated with 0.10 AT NaOH. 

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Ethylenediaminetetra-acetic acid, largely as the disodium salt of EDTA, is a very important reagent for complex formation titrations and has become one of the most important reagents used in titrimetric analysis. Equivalence point detection by the use of metal-ion indicators has greatly enhanced its value in titrimetry. 

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. 

Amperometric titrations have an even wider range of application than polarography. Although the titrant may be added from a burette, in many applications it is electrically generated in a coulometric cell (p. 261). Such an arrangement lends itself to complete automation and is particularly valuable for the titration of very small quantities. For examples of coulometric titrations with amperometric equivalence point detection see Table 6.5. 

Working and counter-electrodes source of constant current and timer, or a potentiostat and integrator equivalence point detection system. 

Element or compound determined Titrant generated Means of equivalence point detection Example of application... 

ELEMENT OR COMPOUND DETERMINED TITRANT GENERATED MEANS OF EQUIVALENCE POINT DETECTION EXAMPLE OP APPLICATION... 

The coulometric generation of titrants is widely applicable to redox, pre cipitation, acid-base and complexing reactions. Of particular value is the determination of many organic compounds with >romine and of mercaptans and halides with the silver ion. Amperometric equivalence point detection is the most common. An attractive feature of the technique is that the need to store standard and possibly unstable reagent solutions is obviated. In fact many applications involve the use of electrdgenerated reagents such as halogens and chromium(II) which are difficult or impossible to store. The technique is especially useful for the determination of veiy small amounts. 

It turns out that amitriptyline and propranolol hydrochlorides can be titrated in a water/ethanol mixture with a 0.1 mol/L sodium hydroxide solution with a pH-metric equivalence point detection. Levomepromazine hydrochloride can be titrated in the same conditions but with propanol instead of ethanol as cosolvent. Propanol is preferred to ethanol because of the very high lipophilicity of the levomepromazine base. 

The usefulness of the fundamental reaction of direct and indirect lodometries has its roots, at least in part, in the fact that it is accompanied by the disappearance or appearance of the yellow-brown color in the solution due to tri-iodide ions. Tri-iodide ions are their proper indicator. Hence, its equivalence point detection is particularly easy. In some difficult cases, starch may be used. The partitioning of iodine into an organic phase at the equivalence point may also be used. Finally, some internal indicators of intermediary standard potential values such as variamine blue (E° 0.60 V) may also be used. The difficult cases are those in which the tri-iodide coloration is masked by that of the solution. 

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