Nitric acid, as solvent

The argument for the S 2 process, when the transition from acetic acid as solvent to nitric acid as solvent is considered, is less direct, for because of the experimental need to use less reactive compounds, zeroth-order nitration has not been observed in nitric acid. It can be estimated, however, that a substance such as nitrobenzene would react about 10 faster in first-order nitration in nitric acid than in a solution of nitric acid (7 mol 1 ) in acetic acid. Such a large increase is understandable in terms of the S z mechanism, but not otherwise. 

The heat of combustion by the rotating-bomb method has been measured by Good et al. (66). The bomb contained nitric acid as solvent for the solid oxidation products of Pb, and the final solution was a uniform solution of Pb(N03)2 in excess dilute nitric acid. For the idealized reaction... 

Considering first pure nitric acid as the solvent, if the concentrations of nitronium ion in the absence and presence of a stoichiometric concentration x of dinitrogen tetroxide are yo and y respectively, these will also represent the concentrations of water in the two solutions, and the concentrations of nitrate ion will be y and x- y respectively. The equilibrium law, assuming that the variation of activity coefficients is negligible, then requires that ... 

An improved solvent extraction process, PUREX, utilizes an organic mixture of tributyl phosphate solvent dissolved in a hydrocarbon diluent, typically dodecane. This was used at Savannah River, Georgia, ca 1955 and Hanford, Washington, ca 1956. Waste volumes were reduced by using recoverable nitric acid as the salting agent. A hybrid REDOX/PUREX process was developed in Idaho Falls, Idaho, ca 1956 to reprocess high bum-up, fuUy enriched (97% u) uranium fuel from naval reactors. Other separations processes have been developed. The desirable features are compared in Table 1. 

The reaction conditions necessary to obtain a good yield of the title compound (a difficult isomer), and to avoid hazards during the nitration of resorcinol, are critical and strict adherence to those specified is essential. The necessary 80% white fuming nitric acid must be completely free from oxides of nitrogen and nitrous acid, and procedures for this are detailed. Then the temperature dining addition of the diacetate must be kept between -10 and 0°C by regulating the rate of addition. The alternative use of 80% sulfuric acid as solvent for the 80% nitric acid (5 equiv.) is preferred as more reliable, but both methods have led to violent exothermic decomposition, accompanied by fume-off, after an induction period. In any event, the explosive 2,4,6-trinitroresorcinol ( styphnic acid ) is produced as a by-product. 

The possibility of using nitric acid as an oxidizing agent in a leaching operation is often prohibitive because many reagents used in solvent extraction are not stable in an oxidizing solution. By the AX method, however, a nitric acid (nitrate) leach solution can be converted to a weak sulfuric acid (sulfate) solution, well suited for a solvent extraction treatment. 

Roberts and his associates (1954) re-examined the nitration of the halobenzenes using fuming nitric acid as the reagent in both acetic anhydride and nitromethane. These workers detected a significant solvent effect on the relative rate. Iodobenzene was nitrated by nitric acid in acetic anhydride at a rate 0.13 times that of benzene, in nitromethane this reagent provided a rate ratio of 0.22. 

The clearest evidence for microscopic diffusion control in nitration comes from the kinetic studies of Coombes et al. (1968), with low concentrations of nitric acid in 68.3% sulphuric acid as solvent. In this medium, the concentration of nitronium ions is proportional to the concentration of molecular nitric acid as required by (24) and, since the concentration of nitronium ions is very small, the concentration of molecular nitric acid is effectively equal to the stoicheiometric concentration of nitric acid. At a given acidity, the reactions have the kinetic form (25). Nitric acid is written out in full in this equation to show that the rate coefficient is calculated with reference to the stoicheiometric concentration of the acid. This convention assists the comparison of reaction rates over a wide range of acidity. 

Oxides, hydroxides, free metals, and simple alloys If a solid substance is found to contain no anions, it may be an oxide, or hydroxide, or a metal or a mixture of metals, or an alloy. Metals and alloys have certain characteristic physical properties many metals evolve hydrogen on treatment with dilute acids. As a rule, nitric acid must be employed as solvent, and it will then be necessary to remove the excess nitric acid (as already described above) before proceeding to the Group analysis. 

Nitric acid combines with ammonium nitrate to form acid nitrates. The trinitrate, NH4N03,2HN03, forms prismatic needles, m.p. 29° to 30° C., and the dinitrate, NH4N08,HN03, six-sided laminae, m.p. about 12° C.14 Fig. 12 gives Groschuff s solubility-curves of the nitrate and its acid salts in nitric acid, as well as those of the potassium salt and its acid salts in the same solvent, and those of the trinitrates in water. 

Conditioiijng of feed. Before solvent extraction, the concentrations of nitric acid and uranyl nitrate are brought to the desired values by addition of water and/or nitric acid, as required. Preferred concentrations are HNO3, 2 to 2.5 M UOjO Os) , 1.2 to 1.4 Af. 

Initial Work Nitric Acid as Nitrating Agent. Because of the generally large role of nitric acid in nitration, its specific action during nitration in media such as methylene chloride was important to establish initially. The acid exists virtually entirely in its molecular form in such solvents, and past accounts of nitrations in such media included complicated kinetics with orders in N(V) up to values of 5 or 6 (II). 

G. Van Weert, T. van Sandwijk and P. Hogeweg, "Solvent Extraction of Ferric Iron fi om Zinc Sulphate Solutions with D2EHPA-Investigation of Nitric Acid as Stripping Agent", EPD Congress 1998. B. Mishra, Ed., The Minerals, Metals and Materials Society, Warrendale, PA, U.S.A., 1998,245-266. 

P. Hogeweg, "The Application of Nitric Acid as a Stripping Agent in the Solvent Extraction of Ferric Iron with D2EHPA from Zinc Sulphate Solutions", PhD Thesis. Delft University, The Netherlands, 1996. 

Potassium nitrate is made synthetically in Israel by Israel Mining Industries (IMI) by low temperature reaction of potash with nitric acid with solvent extraction of the coproduct hydrochloric acid 116]. The HCI is re-extracted from the solvent with water and then distilled as about a 20% HCI solution. The solvent is recycled to the first extraction stage (Figure 15.9). 

Attempts have also been made to solvent-extract thorium directly from the sulphate liquor obtained by the acid breakdown of monazite. Tributyl phosphate can be used as the solvent provided a large concentration of nitric acid is added to the liquor before extraction. In order to make the process economic, a high proportion of the nitric acid must then be recovered by distillation of the raffinate liquor. Processes are also being developed which are based upon the use of higher alkyl phosphate or amine solvents to extract from sulphate solutions without the addition of nitric acid, as in the case of uranium. For example bis(l-isobutyl 1-3-5 dimethyl-hexyl) amine, di-2-ethylhexyl hydrogen phosphate and Primene JM-T have been used. 

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