An experimental procedure is described for the potentiometric titration of the acid K,Cr,O, with the base KOH in molten KNOs at 370". An oxygen electrode is used as indicator electrode. The titration curves obtained are characterized by a potential drop of cu 700 mV at the equivalent point. The curves are analysed theoretically. Above 400" the dichromate attacks the nitrate substrate, as also does the acid CrO, at 370". The titration of molten KOH with K,Cr,O, does not follow expectation. R&urn&--Description dune methode experimentale de titrage potentiometrique du bichromate de potassium par la potasse, en milieu de nitrate de potassium fondu a 370°C. Une electrode a oxygene sert d&&rode indicatrice. Les courbes presentent un saut de potentiel d'environ 700 mV au point d'equivalence. Elles sont interp&tes theoriquement. Au dessus de 4OOC, le bichromate reagirait sur le nitrate, comme d'ailleurs le ferait deja l'anhydride chromique a 370°C. Le titrage de KOH fondue par K2Crs0, n'est pas conforme a l'attente. Zusarnmenfassung-Es wird eine Methode zur potentiometrischen Titration von K,Cr,O, mit KOH in geschmolzenem KNO, bei 370°C beschrieben. Als Indikator-Elektrode dient eine Sauerstoff-Elektrode. Die Titrationskurven weisen einen Potentialsprung von etwa 700 mV beim Auquivalenzpunkt auf. Die Kurven werden theoretisch analysiert. Oberhalb 400" greift das Bichromat das Nitrat an, wie es bei 370" such die Siure CrO, tut. Die Titration von geschmolzenem KOH mit K&,0, entspricht nicht den Erwartungen.
THE rapidly increasing interest in the field of molten salts calls for suitable analytical methods. The present methods imply generally the solidification of a sample of the melt, its dissolution in a suitable reagent, and the analysis of the aqueous solution by known standard methods. Such procedures, though supplying accurate results, suffer from being time-consuming. Polarography has received some attention as a tool for analysing melts l. However, the polarographic waves are not always well developed. Also, when a solid (metal or oxide) is deposited on the indicator electrode, the area of the latter continuously changes in an unknown manner making the quantitative interpretation of the results rather difficult. The feasability of chronopotentiometry as an analytical tool for melts has recently been investigated2 and the method is thought to possess a high degree of accuracy. Jordon and co-workers3 described a thermometric method for the continuous titration in fused salts. Van Norman and Osteryoung4 determined acidic oxy-anions in nitrate and chloride melts by measuring the quantities of CO, which evolved when an excess of Na,CO, was added to the melt.
In the present series of papers it will be shown that classical potentiometric titrations, beside offering theoretical information on the melt under investigation, can be applied for its accurate analysis in situ.
Very little has been done on potentiometric titrations in molten salts. Lux5 observed that the addition of Na,O to sulphate melts containing the acid NaPO, resulted in a potential drop of ca 1 V at the molar ratio of 1: 1 of the two salts. Flengas and Rideal potentiometrically determined silver in a nitrate melt by adding weighed quantities of chloride, bromide, iodide, cyanide and chromate. Laitinen and * Manuscript received