Lanthanum, precipitation titration

The indicator electrode for a precipitation titration is often the metal from which the reacting cation is derived. Membrane electrodes that are sensitive to one of the ions involved in the titration process may be used. For example, fluoride-sensitive membrane electrode is used in the determination of the fluoride content of toothpastes. Lanthanum(III) solution is used as a precipitant. 

The extremely low solubility of lead phosphate in water (about 6 x 10 15m) again suggests potentiometric analysis. Selig57,59 determined micro amounts of phosphate by precipitation with lead perchlorate in aqueous medium. The sample was buffered at pH 8.25-8.75 and a lead-selective electrode was used to establish the end-point. The detection limit is about 10 pg of phosphorus. Anions which form insoluble lead salts, such as molybdate, tungstate or chromate, interfere with the procedure. Similar direct potentiometric titrations of phosphate by precipitation as insoluble salts of lanthanum(III), copper(II) or cadmium(II) are suggested, the corresponding ion-selective electrodes being used to detect the end-point. 

The precipitated LafOHjj was filtered, redissolved in HCl, and titrated with EDTA to obtain total lanthanum content. Combining this result with the first titration Eq. (5), the La/Ni ratio, y, was found to be 2.00 0.01, in agreement with the commerical analysis. 

In addition, further errors can be made in titration procedures which are purely chemical or physico-chemical in origin. Supersaturation effects can sometimes be observed. This arises when for short periods of time the solution contains more ions than it should on the basis of the solubility product of the corresponding precipitate. i>e-cipitate formation, and thus the electrode EMF, is a time-dependent process (such as the titration of traces of fluoride with a lanthanum solution. Fig. 50). For this reason caution is required when working with automatic titration devices. 

Phosphate, for example, can be determined in this way by precipitation with excess lanthanum, and then back-titrating the remaining lanthanum with fluoride ion as usual. 

---