I
A. A. KRAWETZ' and
T. F.
YOUNG
George Herbert Jones Chemical Laboratory, University of Chicago, Chicago, 111.
Purification and Stabilization of Nitric Acid decomposition of nitric acid during storage not only decreases oxidizing power but also builds u p pressure in the containers. Photochemical decomposition is less important but may be significant when acid is stored in glass bottles or carboys. Except for extensive dilution with water, these problems have not been solved by use of additives to inhibit the decomposition rate (4). Consideration of thermal decomposition as a n equilibrium has led to a satisfactory solution. Addition of 3 to 5% of NO2 and about the same percentage of water to 100% nitric acid represses the pressure of oxygen sufficiently. For many purposes this ternary mixture is essentially as good as the pure acid, but in some applications nitrogen dioxide-free nitric acid is desirable, if not essential. T h e gaseous products of the decomposition of nitric acid could also be removed by passing a stream of inert gas through the liquid. This flushing procedure is inefficient and leads to some loss of acid strength. T h e process described is suggested as more useful. Oxvgen was passed through a n electric discharge and the resulting gas containing about 5% ozone bubbled through nitric acid solutions a t a rate regulated to produce vigorous but not violent agitation. Pl'itric acid (707,) which had turned a deep yellow after being stored in a loosely stoppered glass bottle for more than a year was exposed for 5 minutes to the ozone-containing gas stream. T h e yellow color disappeared, leaving the acid water-white. Undecomposed 70Yo nitric acid was contaminated by addition of a small amount of a n oxidizable organic impurity. T h e impurity was immediately digested by the acid with little immediate darkening, but after standing 24 hours, the acid had become conspicuously yellow. Ozonization for not more than 5 minutes restored the original appearance, which then remained unchanged for several months. Several samples of fuming acid (907c) decolorized by ozone. were stored several months without evidence of further decomposition. Optical transmittance was measured for ozonized nitric acid of concentrations u p to 857, acid. Blue light of wave length 4600 A., which is strongly absorbed by even small amounts of nitrogen dioxide impurity. was used. A IO-cm. absorption cell ensured sensitivity sufficient to reveal trace impurities. After THERMAL
' Present address, Wright-Patterson Air Force Base, Ohio.
several months no sample had a transmittance significantly less than unity. Some acid fumes were flushed from the most concentrated acid samples by the stream of gas; no appreciable changes in concentration were detected by titration of the acid with standard base before and after ozonization. R a m a n spectra of ozonized nitric acid samples show no bands other than those attributed to nitric acid or to nitrate ion ( 3 ) . T h e ozone treatment does not, therefore, appear to have contaminated the acid by formation of oxidation products other than nitric acid itself. T h e reaction is simply 2x02 0 3 HzO 4 2HN03 0 2 . If other products had been produced in significant concentrations, they would have been accompanied by their characteristic R a m a n bands. Untreated nitric acid (50 to 90Y0) samples used for R a m a n spectral studies decomposed appreciably (as indicated by the appearance of a yellow color) during the measurements (1 hour). This photochemical decomposition is induced by the high intensity mercury arc light used to excite R a m a n radiation. Traces of nitrogen dioxide in the sample are excited by the blue light of the mercury arc and thereby sensitize the solution to decomposition. Samples exposed to the exciting radiation a t elevated temperatures decomposed still more rapidly. At the higher temperature increased thermal decomposition apparently produces additional nitrogen dioxide which, becoming photochemically excited, sensitizes further decomposition. Samples of ozonized nitric acid were exposed for 1 hour to high-intensity mercury arc radiation a t 25" and 50" C. T h e extent of decomposition was tested
by measurement of optical transmittance before and after exposure. Ozonized nitric acid a t 25' C. does not decompose significantly under these conditions. Decreases in optical transmittance were noted, however, for solutions more concentrated than 60% acid when tested a t 50" C. The smaller photochemical decomposition in the ozonized nitric acid illustrates the degree to which ozone scavenges nitrogen dioxide from solution. Ozonization is particularly useful for removing decomposition products of aqueous nitric acid, Nitrogen dioxide is oxidized in szdu to restore the decomposed acid and give a purified acid containing only oxygen and some ozone ( 7 , 5) as contaminants. The remaining ozone augments the stability of the acid by scavenging it of decomposition products (thermal or photochemical) as they are formed. 'The nitric acid should be stable as long as the ozone remains. Small amounts of nitrogen dioxide affect photochemical decomposition. The ozonized acid apparently maintains its stability to thermal decomposition to a certain degree even after all ozone must have disappeared from solution ( 5 ) . This observation is consistent ivith that of hfellor ( 2 ) ,that there is a n induction period for the thermal decomposition of nitric acid, implying that small concentrations of decomposition products m a s play a n autocatalytic role. The residual stability of the ozonized acid mal- result from removal of all appreciable amounts of decomposition products by ozone. T h e possibility of enhancing the thermal stability of pure or nearly pure nitric acid by ozonization has not been studied in this work. Future experiments may indicate that effects of ozonization on stability are less pronounced for the anhydrous acid
Ozonized Nitric Acid Solutions Do Not Decompose Significantly on Exposure to Mercury Arc Light Optical Transmittance Nitric Arid, a t 4600 A. Wt. % Before After Test at 25' C.
literature Cited (1) Kilpatrick, M. L., Herrick, C. C.: Kilpatrick, M., J . .4m. Cherii. SOG.7 8 , 1784 (1956). (2) Mellor, J. W., "Comprehensive Treatise on Inorganic and Theoretical Chemistry," vol. 8, p. 554, Lonqmans, Green, Kcw \-ark, London, 1928. (3) Redlich, O., Nielsen, I,. E., J . . 4 ~ . Chem. SOG. 6 5 , 654 (1943). (4) Robertson, G. D., Jr., Mason. D. hf.! Corcoran, W. H., J . f'hjs. Chern. 5 9 , 683 (1955). (5) Taube, H., Trms. F u ~ - Q Soc. ~ Q J53, ) 6.56 (1957).
+
75.8 79.4 83.4 87.2 91.2
54.0 55.8 59.3 65.7 67.8
+ +
1.01 1 .oo 1 .oo 1.00 0.96 Test at 50° C. 1.01 1.01 1.01 1.00 1.01
1.01 1.01 1.00 1.00 0.97 1.01 1.01 1.01 0.99 0.97
RECEIVED for review June 19, 1957 .ACCEPTED .\ugust 25, 1958 Work supported in part by the Office of Naval Research. VOL. 51, NO. 2
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FEBRUARY 1959
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