Detection of the End Point Indicators— They Are Chelating Agents

Masking can be achieved by precipitation, complex formation, oxidation-reduction, and kinetically. A combination of these techniques may be employed. For example, Cu can be masked by reduction to Cu(I) with ascorbic acid and by complexation with I . Lead can be precipitated with sulfate when bismuth is to be titrated. Most masking is accomplished by selectively forming a stable, soluble complex. Hydroxide ion complexes aluminum ion [Al(OH)4 or AlOa ] so calcium can be titrated. Fluoride masks Sn(IV) in the titration of Sn(II). Ammonia complexes copper so it cannot be titrated with EDTA using murexide indicator. Metals can be titrated in the presence of Cr(III) because its EDTA chelate, although very stable, forms only slowly. 

We can measure the pM potentiometrically if a suitable electrode is available, for example, an ion-selective electrode (see Chapter 13), but it is simpler if an indicator can be used. Indicators used for chelometric titrations are themselves chelating agents. They are usually dyes of the o,o -dihydroxy azo type. 

Eriochrome Black T is a typical indicator. It contains three ionizable protons, so we will represent it by Hsin. This indicator can be used for the titration of Mg + with EDTA. A small amount of indicator is added to the sample solution, and it forms a red complex with pait. pf the Mg the color of the uncomplexed indicator is blue. As soon as all the free Mg. is titrated, the EDTA displaces the iudicator from the magnesium, causing a change in the color from red to blue  

This will occur over a pMg range, and the change will be sharper if the indicator is kept as dilute as possible and will stiU give a good color. 

Of course, the metal-indicator complex must be less stable than the metal-EDTA complex, or else the EDTA will not displace it from the metal. On the other hand, it must not be too weak, or the EDTA will start replacing it at the beginning of the titration, and a diffuse end point will result. In general, the metal-indicator complex should be 10 to 100 times less stable than the metal-titrant complex. 

---

DETECTION OF THE END POINT INDICATORS—THEY ARE CHELATING AGENTS... 

---