ChemInform Abstract: Chemistry in Nitric Acid Solutions

Abstract

High field multinuclear n.m.r. spectroscopy can be applied with advantage to the study of inorganic solutes in 100% nitric acid and mixtures with dinitrogen tetraoxide or pentaoxide. The behaviour of phosphorus haloand oxo-compounds in high density acid, HDA (44 wt. % dinitrogen tetraoxide in pure nitric acid) reveals the hydrolytic character of this type of medium and in 100% nitric acid, indicates the latter to be more highly protonating than its Hammett acidity function would suggest. In the present lecture, attention is focussed on5olutions 9 vanadium and aluminium compounds, especially in relation to V and Al n.m.r. studies, and the results emphasise that metal chemistry occurring in nitric acid solutions can be much richer than might be inferred solely from a knowledge of isolable solid products. INTRODUCTION Pure (100%) nitric acid is in some respects a unique solvent system (ref. 1,2). The extensive and unusual self-dissociation ( 1) 2HNO3NO2 + NO3 + H20 . (1) (concentration of each dissociation product O.251mo11kg I) accounts for its exceptionally high electrical conductivity (K = 3.72 x 10 ohm cm at 25°C) and also for the presence of unprotonated but solvated water molecules which have a profound effect on chemistry in nitric acid solutions. Physical properties of 100% nitric acid and its solutions with dinitrogen tetraoxide (red fuming nitric acids, which are important oxidiser components of liquid rocket propeilants) and dinitrogen pentaoxide (nitric oleums) have been extensively studied (ref. 1,2) but little information was available hitherto on chemistry in these liquids. Our long-standing interest in these solvent media has been further stimulated by involvement with technological problems arising from the corrosion of metals by red fuming nitric acids when used as liquid rocket propellant oxidisers. In rocket engines, these liquids are in contact with stainless steels or aluminium alloys and contain a small amount (ca. 0.7 wt.%) of hydrogen fluoride, added to inhibit metal corrosion. Many other compounds have been tested as inhibitors, one of the most successful alternatives to hydrogen fluoride being phosphorus pentafluoride, and this has led to an interest in both metallic and non-metallic solutes in nitric acid and its dinitrogen tetraoxide and pentaoxide solutions. We are using high field multinuclear n.m.r. spectroscopy to characterise (sometimes novel) chemical species generated and stabilised in these solvent media and these species, in turn, help to elucidate the nature of the media themselves; both solvent and solute magnetic nuclei may be exploitable. NON-METAL COMPOUNDS AS SOLUTES The behaviour of phosphorus pentafluoride, difluorophosphoric acid and phosphoric oxide in a mixture of nitrogr tetraoxide (44 wt.%) and nitric acid (high density acid', HDA), studied by F and P n.m.r. spectroscopy, reveals the hydrolytic nature of this medium (ref. 3). The hydrolysis of all three solutes is, however, time—dependent and incomplete (except at low concentrations), equilibrium mixtures of fluorophosphate species (from phosphorus pentafluoride or difluorophosphoric acid) or phosphoric acids (from phosphoric oxide) being ultimately obtained. Preliminary results indicate that the reaction of phosphorus pentafluoride with 100% nitric acid is similar to that with HDA. The hydrolysis of phosphoric oxide in 100% nitric acid is more extensive and rapid than in HDA, although similar species are observed in both media. These hydrolytic reactions of phosphorus compounds suggest that solvation of the wat mocules 100% nitric acid or HDA limits their usual chemical reactivity (ref. 2). 0, F and P n.m.r. spectroscopy has been used (ref4) to investigate the protonating ability of 100% HNO3 towards POCl, POBr3, HPO2F2 and PO3F ; the results show the acid to be more strongly protonating than indicated by its