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[效力级别] [学科分类] 化学（综合）
[关键词] niobates;chromium niobates;electrometry. INTRODUCTION An outstanding character of niobates is their strong tendency to form condensed ions in solutions1;2 and from these polyniobates crystallize;depending upon the H+ concentration which plays an important role on the aggregation process. It has been reported3 that sodium ortho-niobate;Na3NbO4;dissolves in water giving a strongly alkaline solution due to its hydrolysis to form Nb6O198- which upon decreasing the pH may successively change to HNb6O197- and (Nb6O186-)n . The value of about 3 for n;has been suggested but Conard and Land4 concluded that the value of n should be about 6. A large number of niobates with conflicting reports have been described5 in which the ratio of alkali metal to acid (M2O/Nb2O5) is given variously as 5/1;4/1;3/1;2/1;16/5;3/2;4/3;4/5;1/1;6/7;3/4;2/3;1/2;1/3;and 1/4. There is no doubt;however;that the study of niobates is complicated by the fact that many of the hydrated salts do not exist in anhydrous state;nor do they form congruently saturated solutions;while in both anhydrous and hydrated systems;metastability and slowness to attain equilibrium contribute to the variability of composition. Some of these compounds have ferroelectric and piezoelectric properties and have been found to be attractive alternatives to quartz as frequency filters in communications devices. A number of nonstoichiometric bronzes are also known6 which like tungsten bronzes;are characterized by high electrical conductivity and characteristic colors. There has also been a rapid increase in recent years in the use of niobium in steel industry. Small amounts of niobium markedly increase the yield strength of mild steel plates and prevent weld decay and intergranular corrosion in stainless steel7. In addition to its continued use in the production of chrome and stainless steel;it is now being used as a component for nonferrous superalloys;generally nickel-based8;9;which have been developed for the manufacture of jet engines;turbines;and missiles. It has also been found that certain niobium alloys;especially with tin and zirconium;become superconducting at easily available temperatures;and retain their superconductivity in magnetic fields10;11. Some work has appeared on the electrodeposition of niobium alloys using niobate solutions in baths12;13. The literature surveyed clearly reflects the usefulness of niobium alloys in imparting specific properties;but almost no electrolytic bath has been developed until now which can deposit an alloy of good quality of some commercial value. Adequate control of the baths and complicated chemistry of niobates seem to be the main sources of the problem. Niobium compounds are also employed as catalyst in many reactions (dehydrogenation;oxidation-reduction;etc) of industrial importance. Recently;interest in dehydrogenation processes has increased due to high demand for production of oxygenated compounds in reformulated gasoline and C9-C14 mono-olefines to obtain biodegradable detergents14;15 . The catalysts Pt/Nb2O5 and Pt-Sn/Nb2O5 have been reported to be excellent and highly selective catalysts in dehydrogenation reactions16-18. Structure of the metal oxide catalysts may have a profound effect on catalytic processes19;20. The study of chemistry of niobate polyanions may help in unfolding some structural aspects of the oxides. Due to their high charge and large size niobate polyanions exhibit many of the properties of infinite lattices21. For such reasons these polyanions can be used to examine the catalytic properties of the metal oxides by methods which are inapplicable to solids;such as absorption spectroscopy and polarography. Further these anions can be used as convenient models for testing theories of bonding and metal-metal interactions in oxide lattices21. In an earlier publication22 the author has shown that the solution containing the species NbO43- on treatment with dilute acid decomposes into Nb2O5.aq via formation of various intermediate anions depending upon the hydrogen ion concentration of the medium. In view of the interesting results obtained and the importance of niobium poly-compounds;it was considered worthwhile to investigate precisely the composition of chromium niobates obtained by the interaction of chromium chloride on various niobate species at different pH levels by employing electrometric techniques which have provided more conclusive evidences on the composition of these and related compounds22-25. There is;however;no reference available in the literature on the study of chromium niobates as a function of pH. EXPERIMENTAL [时效性]
