Feb., 1961
SITRIC -4cr1)EQUILIBRIA IS ~T.ITER-SULFURIC hcru
199
NITRIC ACID EQUILIBRIA IN FTA4TER-SULFURIC hCID' BY Y . C. DENO,HENRYJ. PETERSON AND EDWARD SACHER College of Chemistry and Physics, the Pennsylvania State University, University Park, Penna. Received February 11, 1960
ilbsorption spectra (220-400 mp) were measured for solutions of "03 in 0-99q', aqueous sulfuric acid. On the basis of nitration kinetics. it had been conclcded previously that concentrntions of S O 3 - and HSO, were,equal in 44% H2SO4 and that the shift of equilibrium from HSO, to X02+ was complete in 90yoHLSOI. The spectroscopic data confirm both conclusions. A species more strongly absorbing than HN03 or XOS+appears in 85-99?: H2S04. I t is tentatively identified as the mixed anhydride. 02NOS02H It is calculated that its relative concentration maximizes a t 90% H P S O where ~ it accounts for about 9'y0 of the added HSOs.
On the basis of rates of nitration of aromatic compounds, it was concluded that the maximum in the rates of nitnition a t 90co H2S04was due to the dominant conversion of HXOa to the active nitrating agent, SO?+.2 I t had also been concluded on the basis of the rates of nitration of anisole that concentrations of HNO, and KO3- were equal in about -IC(& H,S0,.3 The simple expedient of checking these conclusions by spectroscopic measurements has not been reported and this work was designed to accomplish this objective. I n water, HSO, is completely dissociated to XOa-. Of a number of studies reported on the absorption of S O , - in ~ v a t e r , ~the - ~ values of Amax 303 mp and E 6.0 reported by v. Halhan4 were exactly ieproduced in this mwk and are close to the most recently reported ~ a l u e s . X ~ report of a Amax at 385 mp for KOg-8 was not substantiated. The behavior of H K 0 3 in 100% HzSO4has been studied extensively by Raman spectra and cryoscopic measurements. The Raman spectra of HN03in several strong acids has a single characteristic emission at, 1400 cm.-1.9,10 The simplicity of the spectrum as well as other arguments have identified this species as K02+.9This identification is entirely in accord with the freezing point depression of HXO, in 100% H2S04,which mas four times that of a non-electrolyte.ll The presence of the nitrogen as a cation as shown by transference experiments further. supports X02+.12 Experimental The absorption spectra of solutions of nitric acid in sulfuric acid were examined from 220-400 mp and from 0-99% H2S04. A Beckman DU spectrophotometer was used. (1) This research was supported in part by a grant from the Petroleum Research Fund administered by the American Chemical Society. Grateful acknowledgment is hereby made t o the donors of thib fund. This research also was supported in part by a grant from the National Science Foiindation. Grateful acknowledgment is hereby made of this scpport. 12) F. H. JVestheinher and 31. Khnrasch, J . Am. Chem. Sac., 68, 1871 (194G); R . .J. 'Gillespie and D. G. Norton, J . Chem. Soc., 971 (1953). The latter reference summarizes much of the earlier literature. (3) K.n e n o and R . Stein, J . Am. Chem. Soc., 7 8 , 578 (1956). 14) €I. v. Halban, Trani. Faraday SOC.,21, 020 (1925). (5) R . N. Jones and G. D. Thorn, Can. J. Research, 27B,580 (1949). (6) R . A . 3 I x t o n and R . W. Riding, Proc. Roy. Soc. (London),8113, 717 (1926). (7) E. Siegler-Soru, Arch. phys. Biol., 5, 237 (1927). 18) K. Schaefer, 2.anorg. Chem., 97, 285 (1916); R. Schaefer and I€. Niggerman, ibid., 9f1, 77 (1916). (9) C . K. Ingold, I). J. Millen and H. G. Poole, J . Chem. Soc., 2576 (1950): D. J. RIillen, ibid., 2589 (1950). (10) J. Chedin, Ann. chim., 8 , 302 (1937). (11) R. J. Gillespie. J. Graham, E. D. Hughes, C . K. Ingold and E. R . Peeling, .I. Chem Soc., 2504 (1950). (12) G. M. Bennett, J. C. Brand and G. Williams, {bid.,809 (1946).
The absorption cells were t,hermostated at 25 f 1". The absorption from 320-400 mp was negligible. A brief summary of the data from 220-310 mp is recorded in Table I. The detailed data are presented elsewhere.l3 The solutions were 10-2 to 10-4 molar in HSOa and were prepared by adding through a micropipet 0.01 ml. of a freshly prepared one molar stork solution of "03 to the appropriate % &SO,. Urea was added to the "03 stock solution in an amount which ultimately resulted in a to 10-6 molar solution of urea in the solutions spectroscopically studied. With this expedient, the reported extinction coefficients for N03- in water4-5 were reproduced easily, and the data were more reproducible than when purification of the "0s alone was used. The urea presumably destroys lower oxidation states of nitrogen which continuously form The stock by the light-catalyzed decomposition of " 0 3 . solution of HN03 must be freshly prepared. In the region 80-99y0 H2S04,identical results were obtained with or without added urea. Presumably the strong acid converts "02 and/or NO2 to NO plus "02. TABLEI EXTINCTION COEFFICIENTS (E) FOR SOLUTIOXS OF HXO, O-99Y0 AQUEOUS SULFURIC ACID %
HzSO~
220
Extinction coefficients a t mp listed250 270 290
IN
310
O.Ob 33 X IO2 9.37 1.85 5.34 6.06 20.0" 33 X lo2 8.75 3.08 6.45 5.81 41.6 20 X lo2 6.77 5.90 6.80 3.8 69.0d 2.7 X lo2 8.76 7.30 4.45 0.7 80.3 2.7 X IO2 8.05 8.90 4.10 .7 83.9 .7 2.7 X lo2 9.53 9.35 4.16 89.4 7.1 X lo2 54 16.7 4.02 .7 94.5 3.1 x 102 33 1.8 .4 7.12 99.4 1.3 x 102 1.9 0.4 .2 12 4 The extinction coefficient is the ratio of optical density to concentration in moles/l. In computing the numbers in this Table, the t.otal stoichiometric concentration of HNO3 was used without regard for the form in which the "01 existed. A,, 303 mp, E 6.90. A,, 301 mp, e 7.19. A,, 265 mp, E 9.05. For picrate ion in water solution, = 8.38, 11.6, 13.2, and 13.1 a t 320, 340, 350 and 360 mp, respectively. These values were assumed to obtain in 0-30% H2S04. For picric acid, lo-% exhibited a small increase from 4070% H2S04a t 320-360 mp. The increase was linear with % ' H&04 and E had the following respective values a t 40.5 and 69.0 % H2S04: 3.38 and 3.59 (320 mp), 4.00 and 4.17 (340 mp), 3.53 and 3.71 (350 mp), and 2.51 and 2.67 (360 mp). The values of E for picric acid a t 5-307, H2S04were obtained by extrapolation of these linear plots. The absorption spectra of solutions of picric acid in &69% HS04 were measured and are published in detail elsewhere.'* Values of the ratio of concentrations of picrate anion to picric acid were the same whether calculated from data at 340, 350 or 360 mp and nearly the same for data a t 320 mp. These ratios are recorded in Table 11. (13) Ph. D. Thesis of Edward Sacher, Pennsylvania State Univ., 1960. (14) Ph. D. Thesis of Henry J. Peterson, Pennsylvania State Univ., 1960.
N.C. DEXO,H. J . PETERSON ASD E. S.XHICR
200
T'ol. 65
HzS04and were independent of the wave length used in their computation. This is compclling evidence that the equilibrium was betwcen only two AS A species. Derivative in respect to % ' HrSOi of From 6542% HzS04the spectra between 220 (Log cNor-/ (Log cx03-/ -Hob CMN03)' CHN03) and 400 mp are virtually invariant. In this region % H2so4 0.76 0.068 0.068 the added HXO3 exists as molecular HN03. 30 .49 ,054 .073 The Equilibrium between HN03,NOz+ and 0234 .080 .31 ,046 NOS03H.-From 85% H2S04,where HX03 pre38 dominates, to 99% HzS04where NOz+predominates, .12 ,052 ,088 42 - .09 ,056 marked changes occur in the absorption spectra. ,098 46 Throughout the 220-270 mp region, the extinction ,106 - .32 ,057 50 ,110 - .54 .060 coefficients increase sharply from S5-9OyO H,SO, 54 ,110 - .81 ,072 and subsequently fall to low values at 99% HzS04. 58 It is evident that a spectroscopic species other than (Log CP-CHP) (Log C P - / C I I P ) HSO, or NOz+ has appeared. This species (I) is 4.8 -0.01 0 03 tentatively identified as the mixed anhydride, 9 8 - 356 ,070 0.078 02NOS03H,on the basis of the following arguments. 18 2 - .95 .070 .069 From 89-1007, HzS04,the Raman spectra data 29 6 -1 8 ,070 ,068 of ChedinlO are used to calculate the % " 0 3 conCalculated from data presented in ref. 13, an abbreviated verted to NO2+,and these estimates are summarized HOis the acidity portion of which appears in Table I. function introduced by L. P. Hammett, which has been in Table 111. *ifter correcting for the water profound to correlate the shift of B/BH equilibria with acidity duced by the reaction TABLE I1
VALUES
CP-/CHP ( H P = FUNCTION OF % HzSOl
O F CN03-/CHN03
AND
PICRIC
ACID)
0
+
for a wide variety of bases (F. A. Long and M. A . Paul, Chem. Revs., 57, 1, 935 (1957)). The last two columns demonstrate that Ho shows a fair correlation with the NO3-/
HNOI and P - / H P equilibria. Data on the P-/HP equilibria have been reported briefly (D. J. Shepherd quoted in a footnote by C. D . Coryell and R. C. Fix, J . Inorg. Nuclear Chem., l , 119 (1955)).
Discussion The Equilibrium between H N 0 3 and N03-.The spectrum of NO,- in water is characterized by a weak maximum a t 303 mp and a strong maximum below 220 mp. A convincing argument has been advanced identifying the 303 band as an n -t T* transition.ls The strong maximum below 220 is probably a T T* transition as judged by E 3280 a t 220 mp. From 0-2070 H2S04there are small changes in the spectra which can be related to the changing properties of the solvent system. In particular, the n -+ T* transition a t 303 shifts to 301 mp. This is related to the increased hydrogen bonding ability of the solvent. From 0-20% HzS04. H30+ is increasingly introduced and by virtue of its greater acidity it will more strongly hydrogen bond to the non-bonding electrons on the oxygen of the Nosion. The effect of this bonding to the n-electrons is to produce a blue shift of the n + R* band.l5,I6 From 2040% H2S04a large spectral shift occurs due to the shift of equilibrium from NOS- to HKO,. The identification of the spectrum in 60% HzS04 as due to molecular HN03 rests most simply on the fact that the spectrum is nearly identical with that reported for 100% "0,6, which is known to consist principally of molecular HXO,. The spectrum of HN03 exhibits a maximum a t 265 mp. This is the n +-T* band which has undergone a pronounced blue shift as the n-electrons in NO3- become more localized when the proton is added to form HN03.15,16 Values for the relative concentrations of NOSand "0, (Table 11) were calculated from 20-6001, ( 1 5 ) H. McConnell, J . Chem. Phys., 20, 700 (1952).
HSOI
+
=
Hz0
+ KOz+ + HSO4-
(1)
it is evident that over 90% of the added H K 0 3 is converted to NOz+ from 92-100y0 HzS04. From 92-9870 H2S04, the absorption between 220-310 mp must be due to the new spectroscopic species (I) plus a small contribution from KO:!+. In particular, HNO, cannot significantly contribute. One example will suffice to show this. At 94.5% H2S04,all but a few per cent. of the added HN03 is converted to NOzf (Table 111). Even if 570 remained as HN03,it would contribute only 0.5 to the total extinction coefficient at 250 mp whereas the observed value was 33 (Table I).
-+
(16) 111. Kasha, Dtsc. Faraday Soc., 9,14 (1950); G.J. Brealey and hl. Kashs. J . A m . Chrm. S o r . , 77, 4402 (1955).
TABLE I11
70HXOa CONVERTED TO KO2+ AS
CALCVLATED FROM IXTEXSITY OF THE 1400 C x - ' RAMAN LINE'
% HsSOi added 6
% HKOs converted t o NOa+
THE
Effectivee % HzSOa
100 100 98.5 97.5 100 96 0 95 100 93.6 92 5 70 91.3 90 60 89.2 0 Calculated from the data in ref. 10. The solutions were made up by adding 95 parts of the sulfuric acid listed in this column to 5 parts of 100% " 0 3 . The conversion t o NOn+ produces H20 which effectively reduces of "03 the concentration of H?S04 below that listed in the first column.
Since the concentration of NOz+ is sensibly constant from 92-99% HzS04 (Table 111),a constant amount can be subtracted from the observed E values to give the component of E due solely to the new spectroscopic species (I). These values appear as €1 in Table IV. It is attractive at this point to attempt to identify I by relating EI to expectations based on equilibrium constant expressions. To make such an attempt, we are forced to use concentrations in place of activities from lack of data on activity coefficients. The validity of such an assumption is difficult to justify because of the lack of a satis-
XITRICACIDEQUILIBRIA IN WATER-SULFURIC ACID
Feb., 1961
201
2 would require that CI/CHSO,- be constant and thus EI/CHSO,- also be constant. This m s found to be true (Table IV). Two other observations support, the identificaea % I-IzSO4 260 mp er b EI/CH804-C El/aHiOd tion of I as 02T\'OSO,H. First, according to eq. 2, 3 2 3.2 99 6.3 addition of H S O c should increase CI and this in6.3 3.1 12.0 98 !) -1 crease should be proportional to the added HS0411.0 2.8 4.5 96 14.0 in 92-99% HzS04,where CNO2+is sensibly constant. 18.7 3.1 3.1 94 21.8 This effect was qualitatively observed in 98y0 H226.9 3.2 2.2 92 30.0 SO4 where CHSO,- is 2.0 molar.23 Addition of 120 a Extinction coeficients calculated from the total stoichioand 240 g. of NaHS04 per liter increases CHSO,- to metric conc,entration of HXOZ, which was 0.0649 molar. about 3 and 4 molar. These additions should inCorrected for the absorption due to NOI+ by subtracting the E a t 100% HzSO,, estimated by extrapolation, from E in crease CI and thus EI by factors of 1.5 and 2. Exthe previous column. Other values of e 1 / & ~ 0 , - were 1.5 perimentally, it was observed that from 260-290 a t 270 mp, 0.68 a t 280 mp, and 0.34 a t 290 mfi. No trends mp, EI increased by factors of 1.9 and 3.4. This is were shown at any of these three wave lengths. The values of &so4- w r e estimated by the method of J. Brand (ref. regarded as satisfactory agreement in view of the 19) which treats the reaction of HzO HzSO, to form H 3 0 + large amounts of NaHS04 added. + HS04- as complete. From 90-100% HnSOa, values Secondly, from a qualitative comparison of the estimated on this basis and values of &SO; estimated from absorption spectra of "03, I and K2O6(Table I), Raman spectra (T. F. Young, L. F. Maranville and H. M. Smith in W. J. Hamer, "The Structure of Electrolytic it is evident that I must have larger extinction illthough it is not evident Solutions," John TViley and Sons, New York, N. Y . ,11959, coefficients than "0,. Values of UHZO were taken from why the anhydride (I) should absorb more, the p. 51) are in agreement. ref. 18. same effect appears with nitric anhydride, n'2Oj. The extinction coefficients for N205 in CC14 at 270, factory theory relating activity coefficients to struc- 290 and 310 mp are 90, 30 and 12.j These may be in G9% H2S04 ture and media. However, such an assumption has compared with the values for "03 been used succeissfully to calculate the solubility of (Table I), which are 8.8, 4.5 and 0.7 at the same BaS04 in 94-10070 H2S04,17 the activity of H 2 0 in respective wave lengths. The most plausible alternative structure for I 81-98~oHzS04, values of the Ha acidity function from 83-99yo €€,SO4,14,19 and the approximate is H2N03+. This would equilibrate ivith KO2+acfreezing point depressions of a wide variety of in- cording to the equation organic and organic compounds in 97-100% XOz+ + H20 = HzNOa+ (3) ~ ~ ~ 0 ~ . 12 17 , 2 0 In 92-997, H2S04 where CNO%+is constant, eq. 3 The most glaying example of concentrations not that EI/CIH~Owould be constant if I were paralleling activities in concentrated sulfuric acids requires is the large increase in solubility of nitrobenzene H2K03+. The data in Table IT.' do not support an assumption. from 85-05% HzS04, but this behavior is conceiv- such I f the value of Kes ( K e s= C I / C N O ~ + C H S O ~ - ) for eq. ably the result of compound formation.22 Further 2 were known, the extinction coefficients and conexamples of concentrations not paralleling actiricentrations of HSOs, NO2+ and O&\'OSO3H could ties hare been observed in precise freezing point be computed from the spectroscopic data. The experiments in 08-1007, HzS04.2 1 The deviations results of a number of such using proreported do not appear large enough to affect the visional values for K showed calculations that values of K much following discusjion. 0.01 gave calculated values of CNO,+ larger below The argument will thus proceed using concentra- than those in Table 111. Values of K much above tions in place of activities. Equation 2 0.01 gave cI C N ~ greater ~ + than the HXO, added YO,+ + HSO4- = O?EOSOIH(I) which is impossible. A value of K = 0.01 is cer(2) expresses the equilibrium between X02+and the tainly not precise, but it can be used to estimate the mixed anhydride (I). From 92-99% H2S04, EI order of magnitude of the relative concentrations. would be proportional to the concentration of I. The results of such estimates are as follows. The Since CNO?+ is constant from 92-997, acid, equation HSO, existing as O2KOSO3H is about 10/0 in 85% H2S04,increases to a maximum of 9% in 907, acid, (17) L P. Ilammett and A J. Deyrup, J . A m . Chem. Soc., 55, 1900 and decreases to 5 and 1% in 95 and 99% IIzSO4. (1933). The % HN03 existing as XOz+follows the values in (18) N. Den0 and R . W.Taft, J r , zbzd., 7 6 , 2 4 4 (1954). Table I11 ultimately declining to ahout 10% in (19) J. C. E).Brand, J . Chem. Soc., 1002 (1950). 85% HzS04. The % H S 0 3 remaining as HN03 (20) M S. Newman, H. G. Kuivila and A. B. Garrett, J . A m . Chem. Soc., 6 7 , 7 0 4 (1945); 31.9.Newman and N. Deno. %bid.,7 3 , 3 6 4 4 , 3 6 5 1 decreases from 90% in 85% HzS04 t o zero in 9OY0 (1951); R. J. Gillespie and associates, J . Chem. Soc., 2473 (1950). acid. (21) 9. J. Bass, R. J. Gillespie and J. V. Oubridge, abad., 837 (1960). TABLE IV DERIOXSTRATION OF THE CONSTANCY OF E I / C ~ S ~ A ~N - D LACKOF CONSTANCY OF E I / C C ~ , ~
THE
,
,
+
+
(22) N. Deno and C . Perriaaolo, J . A m . Chem. S o c . , 79, 1345 (1967)
(23) Footnote c of Table IV.
