Downloaded by KTH ROYAL INST OF TECHNOLOGY on September 8, 2015 | http://pubs.acs.org Publication Date: June 1, 1985 | doi: 10.1021/j100259a032
2864
J. Phys. Chem. 1985, 89, 2864-2869
formation. For the ethylene-water complex, the complex shifts of the water and ethylene fundamentals were used together with the assumption of a linear relation between complex shift and energy of formation to estimate an energy of formation of -2 kcal/mol.'O The calculated global energy minimum of the water-benzene interaction energy given in ref 1 is -3 kcal/mol. When the zero-point vibration energyI5 of the external vibrations of the complex are added to this value, the result is expected to be close to the experimental estimate. The calculated potential energy surface' was found to be rather insensitive to the precise orientation of the water molecule around the energy minimum. For instance, it was not possible to decide whether the complex has a single or a bifurcated hydrogen bond.I6 This observation may provide an explanation for the tine structure of the complex adsorption bands. While most water complexes studied in matrices have OHstretching bands with a single maximum, it is not uncommon to find bands with two maxima, but in these cases the shape of the band is identical for HOH--, HOD-., and DOD-. The v l (HOH-be) and vl(DOD.be) bands are similar in shape but v l (HOD-be) is entirely different (Figure 1). A matrix perturbation should produce similar structures in all three bands and is therefore not a likely source of the structure. Another possible cause of (15) See L. A. Curtiss, D.J. Fruip, and M . Blander, J. Chem. Phys., 71, 2703 (1979). for an estimate of the zero-point vibration energy of the water dimer. (16) Personal communication from the authors of ref 1.
the fine structure is the presence of a relative motion of the two complex components. Normally the fine structure resulting from, for instance, hindered rotation depends strongly on the matrix temperature, since the populations of the motional states vary with temperature and different relaxation processes become faster at higher temperatures. We lowered the temperature from 17 to 12 K in one experiment and observed some changes in the relative intensities of the components of v ~ ( H O H - C ~ H (Figure ~) 2). Unfortunately, the cryostat used did not allow us to lower the temperature further. It should be noted that if the motional states which are accessible at 20 K are within a few wavenumbers in energy, the changes in the populations in the 10-20 K interval may be obscured by changes in shape and overlap of the different components of the band. The dissimilarity in the structure of vl(HOD.be) and q(HOH-be) may be understood if H,O (or D20) is able to switch rapidly between the two possible hydrogen bonds. This would be consistent with the flatness of the calculated potential energy surface. In conclusion we suggest that water is hydrogen bonded to the flat surface of the benzene ring in such a way that two or more states of relative motion are accessible in the 11-17 K interval. The water molecule is not locked into a fixed position relative to a specific benzene carbon but moves over the ring in such a way that the interaction has effective C, symmetry and the two hydrogens of H 2 0 (or D 2 0 ) are equivalent. Registry NO. C6H6, 71-43-2; C6D6, 1076-43-3; CbHsD, 1120-89-4; H,O, 7732-18-5; DZO, 7789-20-0; HDO, 14940-63-7.
Equilibria of Nitric Acid in Sulfuric and Perchloric Acid at 25 O C by Raman and UV Spectroscopy M. Sampoli, A. De Santis, N. C. Marziano,* F. Pinna, and A. Zingales Facoltci di Chimica Industriale, Universitii, Dorso Duro 21 37, 301 23 Venezia, Italy (Received: June 1 1 , 1984; In Final Form: December 12, 1984)
Solutions of nitric acid in aqueous sulfuric (1-98 wt %) and perchloric acid (1-70 wt %) are studied by Raman and UV spatroscopy, and the concentrations of HN03,NO3-, and NOz+species are determined vs. medium composition. The variations of [HNO,] / [NO3-] and [NO,+]/ [HNO,] are analyzed to evaluate the corresponding dissociation constants. Different assumptions about the behavior of the activity coefficients of the species involved in the equilibria are tested. The M,activity coefficient function is found able to describe the protonation and the protonation-dehydration equilibria of nitric acid.
Introduction Equilibria of nitric acid in aqueous sulfuric acid have been extensively studied by different spectroscopic techniques.'-7 Raman,'-, UV,fs IR,6 and NMR7 spectroscopy, for instance, have been used as suitable means to identify molecular and ionic species present in the solutions. It is well-known that the solute dissociation gives rise to different ionic species which are described (1) J. Chbdin, C. R. Acad. Sci., 200, 1397 (1935); Ann. Chim., 8, 243-315 (1937); MEm.Seru. Chim. h a r , 31, 113 (1944); J. Chain, S . Ftntant, M6m. Seru. Chim. E r a , 40, 292 (1955). (2) C. K. Ingold, D.J. Millen, and H. G. Poole, J . Chem. SOC.,2576 (1 950). (3) N. C. Marziano, P. G. Traverso, A. De Santis, and M. Sampoli, J . Chem. SOC.,Chem. Commun., 873 (1978). (4) N. C. Deno, M. J. Peterson, and E. Secher, J . Phys. Chem., 65, 199 (1961). ( 5 ) N. S. Bayliss and D. W. Watts, Austr. J. Chem., 16, 943 (1963). (6) R. A. Marcus and J. M. Frescoe, J . Chem. Phys., 27, 564 (1957). (7) F. Seel, V. Hartman, and W. Gombler, Z . Naturforsch. B, 278, 325 (1972); D. S. Ross, K. F. Kulmann, and R. Malhotra, J . Am. Chem. SOC., 105, 4299 (1983).
0022-3654/85/2089-2864$01.50/0
by equilibria (1) and (2) in going from diluted to concentrated sulfuric acid.'-9 Direct evidence for the nitronium ion in the
+ NO3s NOz+ + H,O
HNO, s H" HN03
+ H"
(2)
concentrated acidity range (equilibrium 2) is of importance in nitration studies of aromatic compounds by sulfuric acid-nitric acid mixtures, since NO2+is the reactive species of the Moreover, the estimation of its concentration is particularly relevant and indeed necessary before the mechanism and kinetics can be interpreted. As for the evaluation of the concentrations of the solute species, few studies are a ~ a i l a b l e . l - ~Our $ ~ .purpose ~ is to determine these concentrations as a function of medium composition by Raman and UV spectroscopy as well as possible. ~~
(8) P B D De la Mare and J H Ridd, 'Aromatic Substitution, Nitration and Halogenation", Butterworths, London, 1959 (9) K Schofield, "Aromatic Nitration", Cambridge University Press, London, 1980
0 1985 American Chemical Society
Equilibria of Nitric Acid in Sulfuric and Perchloric Acid
Downloaded by KTH ROYAL INST OF TECHNOLOGY on September 8, 2015 | http://pubs.acs.org Publication Date: June 1, 1985 | doi: 10.1021/j100259a032
These two techniques possess certain advantages as well as certain limitations. The use of both should provide a more reliable picture than either technique alone. The present work extends to the whole acidity range of sulfuric acid (1-98 wt %) preliminary measurements, carried out at high acidity., Further additional measurements of the dissociation of nitric acid in aqueous perchloric acid (1-70 wt %) are reported. This allows meaningful comparisons to be made between the behaviors of the solute in different acid solutions. To arrive at a simple description of the protonation (reaction 1) and protonation-dehydration processes of nitric acid (reaction 2), a procedure analogous to that applied to the usual indicators in concentrated acids is used. This procedure implies some assumptions about the behavior of activity coefficients. The relevant assumptions suggested in the literature"I4 are tested in the present work. Moreover, the problem of the meaning of the pK, values is discussed.
Experimental Section Nitric acid vacuum distilled from concentrated sulfuric acid was used and stored at -50 O C . Nitric acid distilled once or twice does not lead to different results. Sulfuric and perchloric acid solutions were prepared by diluting pure concentrated reagents suitable for optical investigations. The percentage compositions of solutions was determined by potentiometric titrations against standard solutions of sodium hydroxide. Two different normal solutions of HC1 were used for the standardization of normal solutions of NaOH. An Amel instrument equipped with a digital buret was used for titrations. Each percentage value of acid solutions was the average of several measurements and the estimated error was about 1%. Acid solution of appropriate concentration was weighed in a Raman or UV cell and a small volume (0.1-0.01 mL) of pure nitric acid (or a solution of it) was added in by means of a precision micrometric syringe. Nitric acid solutions were prepared by weighing both reagent and solvent. The concentration of the reagent was calculated by measuring the solution density. The nitric acid concentrations in the sample were 10-2-10-1 M for Raman measurements and 10-3-10-2 M for UV measurements. Test measurements showed that the addition of urea (10-5-10-4 M) does not improve the stability and/or accuracy over the whole range of sulfuric acid (1-98%).4 On the contrary, in the range 80-98% the UV measurements were complicated by this addition. All samples are thermostatted at 25 f 1 "C for UV measurements and at 23 f 2 OC for Raman. The UV integrated absorbance of samples was measured at selected wavelengths by a Perkin-Elmer 55 UV spectrophotometer. The Raman spectra were recorded by a fairly conventional apparatus. The incoming beam of an argon-ion laser operating at 488 nm, vertically polarized, was focused gently on the sample and kept at constant low power (300 f 1 mW) to avoid local turbulences. The scattered light at 90' (VV + VH scattering geometry) was analyzed by a Coderg T800 monochromator, detected by a lownoise cooled photomultiplier tube and processed by a single-photon counting system. Different resolving powers, from 2 to 5 cm-I, have been employed in the different cases. The integrated areas were corrected for background scattering by manual and/or polynomial interpolation. The indeterminate nature of the background line was the major source of error that has been estimated to be less than 5% in most of the investigated region. For lack of a more detailed theory of the medium polarization around the species under study, the integrated intensities have been corrected for the usual local field factor, (Le., ((n2 2)/3)4 where n is the refractive index calculated from standard literature
+
(10) C. H. Rochester, "Acidity Functions", Academic Press, London, 1970. (1 1) L. P. Hammett, "Physical Organic Chemistry", 2nd ed,McGraw-Hill, New York, 1970. (12) M. Liler, "Reaction Mechanism in Sulphuric Acid", Academic Press, London, 1971. (13) N. C. Marziano, G. M. Cimino, and R. Passerini, J . Chem. SOC., Perkin Trans. 2, 1915 (1973); N. C. Marziano, P. G . Traverso, and R. Passerini, J . Chem. SOC.,Perkin Trans. 2, 306 (1977). (14) N. C. Marziano, P. G. Traverso, A. Tomasin, and R. Passerini, J . Chem. SOC.,Perkin Trans. 2,309 (1977); N. C. Marziano, A. Tomasin, and P. G. Traverso, J . Chem. SOC.,Perkin Trans. 2, 1070 (1981).
The Journal of Physical Chemistry, Vol. 89, No. 13. 1985 2865 TABLE I: Normalized Area (AN, 76) of H N 0 3 Band for the Protonation Equilibrium of Nitric Acid in Aqueous HC104 by Raman Spectroscopy
AN,7% %HClO4 HNOp 4.97 9.40 9.84 14.79 15.40 19.58 20.32 24.11 25.26
%HClO4
AN,% HNO,
28.72 29.95 33.96 34.90 38.81 40.16 43.22 44.46 48.20
7.27 7.53 11.67 13.02 19.82 21.85 31.29 35.05 50.55
0.81 0.94 0.80 1.61 1.72 2.64 2.94 4.50 4.95
AN, %
%HClO4 HNOp 49.80 52.77 54.57 57.89 59.30 62.71 64.39 65-70
57.29 69.05 77.06 87.30 90.60 95.55 97.54 100
TABLE 11: Normalized Area (AN,%) of HNOBBand for the Protonation Equilibrium of Nitric Acid in Aqueous H#04 by Raman SwctroscoDv ~
AN, 76
% H2S04 HN03
19.62 23.45 24.37 28.42 29.49 32.96 33.65 34.05 37.97 38.88 39.24 40.04 40.68
too 80
AN, %
% H2S04 HN03
3.53 4.59 5.10 6.80 8.27 10.26 10.86 11.43 15.54 17.37 18.92 19.67 19.66
41.20 42.60 43.40 43.85 44.60 45.39 45.91 47.36 48.04 48.67 49.47 50.26 50.72 5 1.90
21.91 24.43 26.05 27.29 29.61 30.98 33.32 36.80 40.04 41.52 43.30 47.75 50.13 53.55
~~
~~
AN,
% H2S0,
HNO,
52.80 53.40 57.11 57.74 58.64 61.98 62.94 63.80 66.97 67.68 68.68 71.77 78-82
56.21 58.99 71.10 72.26 74.74 82.86 84.94 86.48 91.78 93.04 93.80 96.56 100
-
Po 4 4 0
-
20
0.0
I
I
10
20
I
30 WI
Y
I
I
40
SO
nao,
I
1
80
70
Figure 1. Normalized area of HNO, band vs. wt % of aqueous HC104 by Raman spectroscopy.
data).I5 This correction is of some importance in the case of sulfuric acid solutions.
Results Equilibrium between HNO, and NO3-. Equilibrium 1 of nitric acid has been studied in perchloric and sulfuric acid in the range 0-70 wt % and 0-80 wt %, respectively. The Raman spectra of the solutions exhibit a strong band at 1300 cm-I, characteristic of undissociated nitric acid: sufficiently resolved from the solvent bands in the overall acidity range. The concentration of the undissociated species was derived from the area of the band. The nitric acid concentration was kept between 0.05 and 0.5 mol/L. Tests made at different medium compositions have shown good linearity between the area and nitric acid concentration (from 0.01 ( 1 5) Landolt Bornstein, 'Physikalisch-Chemische Tabellen",11, Springer-Verlag, Berlin, 1923, p 988. 'Handbook of Chemistry and Physics", CRC Press, Boca Raton, FL, 1980, E-387.
2866
The Journal of Physical Chemistry, Vol. 89, No. 13. 1985
Sampoli et al.
TABLE III: Normalized Molar Extinction Coefficients (a) for the Protonation Equilibrium of HN03 in Aqueous H#O, by UV Spectroscopy t
Downloaded by KTH ROYAL INST OF TECHNOLOGY on September 8, 2015 | http://pubs.acs.org Publication Date: June 1, 1985 | doi: 10.1021/j100259a032
% H2S04
9.96 9.97 10.11 10.17 14.72 14.81 15.16 15.17 15.17 17.18 17.19 19.44 19.54 24.12 24.23 24.62 24.64 24.64 24.64 29.20 29.29 29.44 29.47 29.47 29.47 33.62 33.65 33.91 33.92 34.56 34.58 35.09 37.02 37.04 38.92 39.14 39.46 39.46 40.86 41.18 41.47 41.50 43.59 43.78 44.12 44.13
4.49 3.77 5.67 4.89 10.03 8.23 9.97 1 1.79 1 1.54 11.89 15.72 14.27
270nm 0.25 0.97 0.82 0.50 4.31 7.66 4.23 4.22 3.04 ' 7.83 5.27 8.15 7.17 12.73 11.98 13.75 15.38 15.70 16.68 20.32 19.53
17.95 19.28 18.95 25.01 23.72
21.96 23.93 22.56 29.83 29.17
23.41 26.48 26.19 29.1 1 33.17 3 1.97 36.80 37.02 38.13 39.76 41.01 44.86 45.69
29.05 30.79 30.9 1 35.48 39.36 37.35 42.44 42.82 44.22 45.69 5 1.70 50.62 50.83
265 nm 0.81 3.17 6.64 2.64 2.59
52.38 49.00 53.40 54.55
59.11 54.96 59.23 61.08
t
305 nm 97.84 96.32 97.92 97.73 96.22 96.79 95.15 93.52
310nm 96.55 96.55 96.53 96.55 93.48 94.37 93.51
95.33 92.01 93.77
93.10
74.30 78.01 76.96 73.84
90.69 89.89 85.20 85.02 84.14 83.04 84.90 83.97 77.07 76.18 76.80 77.13 78.35 75.18 73.16 66.96 67.81 65.98 64.00
73.88 69.57 67.26 65.39 64.73 62.97 62.41 62.10 60.12 58.12 60.89 54.10 49.64 5 1.06 50.23
67.95 59.39 58.03 53.45 53.25 51.77 52.77 5 1.94 50.64 48.42 51.15 44.49 40.07 42.33 40.75
90.10 90.26 88.98 89.20 89.17 88.34 84.93 84.25 83.95 83.43 83.72 82.96
to -0.6 mol/L of HN03). The results obtained in perchloric and sulfuric acid by Raman spectroscopy are reported in Tables I and 11, respectively. The corresponding titration curves are shown in Figures 1 and 2. The NO3- band at 1052 cm-I was not analyzed owing to the difficulties in resolving it from overlapping bands of the solvent. The UV spectra of nitric acid in water-sulfuric acid mixtures show a large spectral shift according to previous result^.^*^ This behavior is related to a solvent effect yielding a poor definition of the isosbestic point for NO3-and HN03bands, whose maxima occur at 303 and 265 nm, respectively. The extinction coefficients were then measured at four selected wavelengths (265, 270, 305, and 310 nm) and the normalized values are given in Table 111. In Figure 2 the titration curves a t 3 0 5 and 270 nm are shown. The observed behavior points out considerable uncertainties in the UV data. Indeed owing to the solvent effect, different percentages of sulfuric acid at half protonation could be estimated by UV measurements at each selected wavelength. Therefore, in our opinion, Raman results are more reliable. In 0-80 wt % H2S04 the titration curves obtained by using KN035or LiNO, or HN03as starting compound are practically the same. It must be noted that the UV normalized extintion coefficient is independent of the starting compound at each selected wavelength. Equilibrium between HNO, and NO2+.According to previous studies's2 the Raman spectra of nitric acid in concentrated sulfuric
-
7% HzSO4 44.66 45.64 45.76 46.43 46.46 47.10 48.39 48.57 49.32 49.33 49.34 50.52 50.81 51.16 5 1.53 5 1.55 51.55 53.06 53.29 54.00 54.00 54.01 54.01 54.7 1 58.15 58.53 58.70 58.73 58.74 58.74 63.38 63.41 63.67 63.71 63.71 63.75 67.95 68.69 68.88 68.92 68.92 68.93 70.55 72.87 73-80
265 nm 54.53 57.81 59.07 62.23 60.42 61.30 65.69 67.00 70.10 65.93 68.12 72.58 72.47 73.97 76.53
270nm 61.25 64.56 66.77 68.79 67.80 68.59 72.07 73.35 76.66
74.85 79.25 78.68 85.87 81.36 8 1.78 80.22 82.88 90.08
8 1.37 84.73 84.03 89.97 85.96 85.66 84.52 87.88 93.81
94.57 91.94 90.36
98.08 95.14 94.17
95.84
98.20
98.88 96.61
97.75
96.64 98.47
97.78 99.15
74.71 78.54 78.87 78.50 82.58
99.98 97.07 98.75
98.30
98.47 100
100
305 nm 49.44 45.19 46.67 43.43 43.64 42.05 37.31 38.39 37.37 35.03 36.06 32.51 32.04 31.48 30.93 29.44 31.81 26.51 24.95
310 nm 40.94 37.65 38.16 33.57 34.61 32.28 30.29 3 1.79 28.47 27.05 27.80 27.85 24.80 23.87 22.81 23.46 24.31 19.68 19.10
24.58 26.27 23.31 23.21 17.24 13.26 17.20 14.85 15.19 13.56 9.22 5.99 6.52 9.49 6.52 6.59 4.02 1.53 2.29 3.43 0.48 3.21 3.10
17.74 18.07 16.10 15.35 12.23 10.13 11.98 10.50 9.67 8.61 5.04 4.70 5.06
0.00
0.00
4.42 4.86 2.37 0.51 0.20 3.54 2.06 3.57
900.
-v
SO.
-
{ S O .
0
e
'O
#
e
10
10
20
30
40
50
e0
70
80
w i x n2so,
Figure 2. Percentage of HN03 species in HzS04 by Raman and UV spectroscopy: normalized areas (AN)of the HN03band (0) by Raman; normalized values of absorbance at 270 nm (*) and at 305 mn (A) by
uv.
acid exhibit a relatively narrow band at 1400 cm-I attributed to N02+.2Therefore above 80 wt % H2SO4 both the areas of the
Equilibria of Nitric Acid in Sulfuric and Perchloric Acid TABLE I V Normalized Areas (AN,%) of H N 0 3 and NO2+ Bands for the Protonation-Dehydration Equilibrium of H N 0 3 in Aqueous H S O , by Raman Spectroscopy
The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 2861 TABLE V Normalized Molar Extinction Coefficients (e) for the Equilibrium of H N 0 3 in Aqueous H # 0 4 by UV Spectroscopyo 1
Downloaded by KTH ROYAL INST OF TECHNOLOGY on September 8, 2015 | http://pubs.acs.org Publication Date: June 1, 1985 | doi: 10.1021/j100259a032
7% H2S04 88.71 88.89 89.01 89.02 89.12 89.23 89.24 89.41 89.65 89.73 89.78 90.43 90.45 90.55 90.61 90.72 90.80 90.87 91.55 91.55 9 1.63 9 1.67 91.83 9 1.94 91.95 92.83 93.07 93.39 94-97
IO0 IO0
8 1.45 81.79 82.61 83.14 83.52 83.84 84.27 84.30 84.93 85.18 85.25 85.27 85.80 86.36 86.71 86.72 87.31 87.31 87.45 87.52 87.63 87.65 87.75 87.83 87.85 87.90 88.34 88.35 88.68 88.69
98.95 98.03 97.75 97.49 95.71 96.18 93.27 90.48 88.12 85.02 79.02 69.68 71.57 60.96 60.87 60.36 55.61 55.60 57.15 52.51 49.58 48.27 47.35 40.49 40.63 31.12 32.63
1.55 1.85 2.68 3.31 4.54 4.61 8.74 9.64 11.38 11.76 15.18 21.40 26.65 25.52 36.57 35.31 38.65 40.74 42.31 44.84 47.95 51.12 50.30 50.26 58.92 58.84 66.15 66.93
31.91 28.19 28.52 25.92 24.16 22.95
16.80 1 1.42
d
68.88 70.50 72.87 71.23 73.64 75.69 74.86 79.14 8 1.93 82.20 84.31 90.57 89.68 90.18 90.21 90.38 92.46 92.28 94.84 95.02 95.37 95.77 96.51 96.99 96.52 98.60 98.91 99.13 100
/--
/'
82.57 82.62 82.70 83.07 83.72 83.97 84.05 84.26 84.35 84.66 84.73 84.94 85.19 85.38 85.47 85.70 85.74 86.02 86.03 86.07 86.67 86.68 86.76 86.78 87.15 87.63 87.63 87.92 87.93 88.07 88.53 88.85 88.90 88.95 88.98 89.51 89.77 91.26 91.74
260 nm 1.01 0.96 0.91 1.57 2.59
11.77 10.31 11.52 13.02 17.30
250 nm 0.58 0.85 0.69 2.05 1.59 2.00 1.90 4.06 4.05 4.47 5.31 7.76 9.60 10.82 8.16 10.25 12.09 15.60
17.48 23.07
15.30 23.21
23.54 25.49 33.92 41.78 42.84 48.91 49.38 50.88 59.68 66.68 69.38 70.71 69.31 79.05 83.51
21.12 20.88 33.65 40.08 42.02 48.17 46.18 57.53 66.54 70.45 72.66 70.21 80.88 84.29
94.03
87.81
3.09 4.48 5.62 6.66 8.33
245 nm 0.56 0.79 0.57 1.80 1.46 2.15 1.86 3.71 3.66 4.30 5.04 7.49 8.32 10.67 7.84 9.77 12.18 16.15 15.60 15.66 24.12 23.44 22.34 22.04 34.27 41.63 43.73 49.04 47.39 60.20 67.48 73.98 75.90 72.65 83.80 88.06 91.57 9 1.84
uThe 100% limit value of L has been estimated by comparing Raman and UV titration curves.
J
i
,/ LO
30
wt Y
n,ro,
Figure 3. Percentage of the species NO2+for the protonation-dehydration equilibrium of HNO, in aqueous H2S04by Raman and UV spectroscopy: from normalized areas of the NO2+(0)and HN03 ( 0 )bands by Raman; from normalized values of absorbance at 260 nm (A)by UV.
bands at 1300 and 1400 cm-' have been measured. Their variations vs. solvent acid concentration are reported in Table IV and compared in Figure 3. The evaluated concentrations of the HNO,
and NOz+species are consistent within experimental uncertainties. New bands, ascribable to other hypothesized species: were not identified. A slow decrease of the NO2+ area in time has been observed above 90 wt % HzSO4. We have no firm explanation for that. However, the use of freshly mixed reagents provides reproducible data, which is the requirement for the present work. The UV measurement in the range 80-98 wt % H2S04 shows two main difficulties. The first is a net increase of the absorbance with time, so that an extrapolation back to the time of mixing of reagents has to be employed. The second is a blue shift of HN03absorption at 265 nm concomitant with its own decrease a t high a c i d i t i e ~ . ~This , ~ leads to a rather large uncertantainty on the tgH+ value. It was suggested by Deno4 that a new species, identified as 0 2 N O S 0 3 H , is formed above 90 wt %. However, the Raman and UV behavior in this range remains to be ascertained. Indeed kinetic measurements of nitric acid in sulfuric acid, using different concentrations of reagents, have been attempted without conclusive experimental evidence for the existence of a new species. The results of UV studies carried out at 245, 250, and 260 nm are given in Table V. The comparison between UV and Raman result is shown in Figure 3. It can be seen that the UV and Raman data are in fair agreement in spite of all the experimental complications. Discussion
The measurements previously reported allow us to determine with some confidence the concentration of ionic and molecular species of nitric acid vs. medium acidity. At the low stoichiometric concentrations under consideration the nitric acid can be con-
2868
The Journal of Physical Chemistry, Vol. 89, No. 13, 1985 n o in
Downloaded by KTH ROYAL INST OF TECHNOLOGY on September 8, 2015 | http://pubs.acs.org Publication Date: June 1, 1985 | doi: 10.1021/j100259a032
00
1
2
3
Sampoli et al.
ncio.
4
5
6
7
.*.
c
*.
t
I
I
1
0.0
1
2
I
3 no in
1
4 u2so.
I 5
1
1
6
7
/ / I
1
I
I
" 15
1
1
,
16
I
#
*.
I
I
I
b
8
17 in n 2 a 0 4
Figure 4. Study of the equilibria of HNOpin aqueous HCIO4 and H2S04: (a) log [HNO,]/[NO
