Other alkaline halide melts

Some studies of acid-base equilibria in molten alkaline halides have been performed by Rybkin et al. The empirical acidity scales have been constructed on the base of potentiometric studies in molten KCf and Csl. The practical significance of the mentioned works consists in the proposition to use some buffer solutions - SO/ /SjOg, , P0/ /P207 as reference standards for the indicator electrode calibration and determinations of pO in molten alkaline halides. 

Rybkin and Banik obtained the empirical acidity scale in molten Nal at 700 C. The basicities of COs and OH were close which is incorrect since 0.01 mole/kg solutions were used for the scale constructing while equations 

We have performed a study of aeidie properties of boron (HI) and vanadium (V) oxo-compounds in molten Nal at 700 C (see Table 10.4.1). A eomparative study of strength of Lux bases OH , C03 , S04 was determined by potentiometrie titration using sodium pyrophosphate as acid. Two moles of the first base may be neutralized by 1 mole of P407 , while two other moles react with the acid in ratio 1 1. On the base of e.m.f. drop magnitude at the equivalence point, the bases have been arranged in sequence 0H C03 S04 of basicity decrease. The equilibrium constants were not estimated.  

There is no correlation between substances entered into melt and those really existing in it. E.g., potassium nitrate and nitrite are decomposed to K2O at temperatures considerably lower flian flie temperature of the experiment (700 C). Ditto referred to K2S2O7 and Na2S20g as acidic phosphates. In the latter work it has been found that pyrophosphate acidity is larger than that for metaphosphate. However, titration of phosphorus (V) oxo-compounds in melts runs according to the scheme [10.4.39], The acidity decreases. 

The absence of correlation between pH in aqueous solutions and pO in melts for the same substance is often noted, this conclusion is made mainly from data for oxocompounds of pv following reason. The stages of phosphoric acid neutralisation 

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Neodymium, along with lanthanum, cerium and praseodymium, has low melting points and high boiling points. The fluorides of these and other rare earth metals are placed under highly purified helium or argon atmosphere in a platinum, tantalum or tungsten crucible in a furnace. They are heated under this inert atmosphere or under vacuum at 1000 to 1500°C with an alkali or alkaline earth metal. The halides are reduced to their metals ... 

Some of the halides of the alkaline earth metals have a similar identity problem. Calcium chloride and magnesium chloride have melting points almost as high as that of sodium chloride. Those compounds are clearly held together by ionic bonds. Beryllium chloride, on the other hand, melts at about half the temperature of table salt. And it boils at 520°C compared to salt s 1,465°C. The differences in properties are due to the partially covalent bond formed between beryllium and chlorine. 

The presence of oxides and the formation of oxofluoro-complexes in molten electrolytes may be sometimes unwanted, but in many cases they are the fundamental features of the system. For instance, the formation of oxide complexes in alkali-alkaline earth chloride melts may be mentioned. The formation of oxofluoride complexes in molten cryolite-alumina melts, used as electrolytes for aluminum production, is typical as well. On the other hand, the presence of oxofluoride complexes in electrolytes used for niobium production was initially regarded as unwanted. Recently, however, it has been proven that their presence in niobium electrolytes plays an important role in the niobium electrodeposition. In the following, some technologically important examples of systems containing halides and oxides will be described. 

Pyridine is a tertiary amine its aqueous solution shows an alkaline reaction and precipitates the hydroxides of metals, some of which are soluble in an excess of the amine. Salts of pyridine like those of other amines form characteristic double salts with metallic halides. The ferrocyanide of pyridine and the addition-product of pyridine and mercuric chloride are difficultly soluble in water these compounds are used in the purification of the base. Pyridine is a very stable compound it can be heated with nitric acid or chromic acid without undergoing change but at 330° it is converted by a mixture of nitric acid and fuming sulphuric acid into nitropyridine, a colorless compound that melts at 41° and boils at 216°. At a high temperature pyridine is converted into a sulphonic acid by sulphuric acid. Chlorine and bromine form addition-products, e.g., C5H5N.CI2, at the ordinary temperature when these are heated to above 200°, substitution-products are formed. The hydroxyl derivative of pyridine is made by fusing the sulphonic acid with sodium hydroxide it resembles phenol in chemical properties. The three possible carboxyl derivatives of pyridine are known. The a-acid is called picolinic acid, the jS-acid nicotinic acid (664), and the 7-acid isonicotinic acid. 

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