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Index of refraction of sulfuric acid in the IR region

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					II-1.2.4

INDEX OF REFRACTION OF SULFURIC ACID IN THE IR REGION

Ozone layer observation was conducted with the solar occulation sensor ILAS (Improved Limb Atmospheric Spectrometer) on board the ADEOS (Advanced Earth Observing Satellite). ADEOS was launched on August 17, 1996 and stopped its operation on June 30, 1997. ILAS has provided optical properties of the stratospheric aerosol from visible and infrared extinction measurements. To interpret these optical aerosol data, information on the index of refraction of aqueous sulfuric acid is needed as a function of both composition and temperature in the infrared region. At the start of the LAMOCS project very few data on the index of refraction of sulfuric acid were available [Remsberg et al., 1974, Palmer and Williams, 1975; Pinkley and Williams, 1976]. We therefore decided to include this basic physical quantity in our study. To test our experimental set-up we used water at 298 K as reference [Querry et al., 1969; Irvine and Pollack, 1968; Hale and Querry, 1973; Gertie and Zhida, 1996]. The comparison, given in Figure 1, indicates that our index of refraction is accurate within ±0.01. For experimental details, see Appendix A-x.
1.75 1.70 1.65 1.60 1.55
Querry et al J. of Optical Soc. of Am., 39, 1299-1108, 1969 Irvine and Pollack Icarus, 8, 324-360, 1968 Hale and Querry Applied Optics, 12, 555-563, 1973 Data from this work

Index of Refraction

1.50 1.45 1.40 1.35 1.30 1.25 1.20 1.15 1.10 1.05 1.00 10000

8000

5000

4000

3000
-1

2000

1000

Wavenumber /cm

Figure 1.

Comparison of values for index of refraction of H2O at 298 K in the region 6000400 cm-1.

Figure 2 shows the index of refraction for 50 wt% H2SO4 from Palmer and Williams (1975) at 298 K, compared to our data for 48.5 wt% H2SO4 at the same temperature in the region 6700400 cm-1.

1.9 1.8 1.7
Palmer and Williams Applied Optics, Vol 14, 1, 1975 Our data

Refractive index

1.6 1.5 1.4 1.3 1.2 1.1 6000 5000 4000 3000
-1

2000

1000

Wavenumber /cm

Figure 2:

Comparison of index of refraction for 48.5 wt% H2SO4 for our data and 50 wt% H2SO4 from Palmer and Williams, (1975) at 298 K in the region 6700-400 cm-1.

We have measured the index of refraction in the region 6700-400 cm-1 for liquid sulfuric acids of 40 to 80 wt% and at temperatures from 300 K to as low as possible. Figure 3 illustrates the variation in index of refraction with acid concentration at constant temperature, while Figure 4 shows the variation with temperature of a 48.5 wt% solution.
2.2 2.1 2.0 1.9 1.8

77 wt% 48 wt% 40 wt%

Index of refraction

1.7 1.6 1.5 1.4 1.3 1.2 1.1 1.0 5000

4500

4000

3500

3000

2500

2000
-1

1500

1000

500

Wavenumber /cm

Figure 3: The variation in index of refraction with acid concentration at constant temperature, T=298 K.

1.9

1.8

1.7

T= 298 K T= 273 K T= 234 K T= 213 K

Index of Refraction

1.6

1.5

1.4

1.3

1.2

1.1 2200 2000 1800 1600 1400 1200 1000 800

600

400

Wavenumber /cm

-1

Figure 4:

Variation in index of refraction with temperature for 48 wt% sulfuric acid in the region 6700-400 cm-1.

As can be seen from Figures 3 and 4 the index of refraction varies significantly with both composition and temperature - a variation which is not consistent with the simple concept of molar refractivity. The variation is obviously connected with the ionic speciation in sulfuric acid. However, we have so far not been able to derive a consistent set of molar refractivities for H2SO4(sa), H2O4(sa)-, and SO4(sa)-. We conclude that the method of approximating the refractivity of liquids like sulfuric acid by a linear superposition of molar refractivities of the pure components, and subsequently relating the calculated refractivity to the index of refraction of the solution through the Lorentz-Lorenz relation, is at best naive.

II-1.2.4 REFERENCES Bertie, J. E., and Z. Lan, Infrared intensities of liquids XX: Intensity of the OH stretching band of liquid water revisited, and the best current values of the optical constants of H2O(l) at 25 ºC between 15,000 and 1 cm-1, Appl. Spectrosc., 50, 1047-1057, 1996. Hale, G. M., and M. R. Querry, Optical constants of water in the 200-nm to 200-m wavelength region, Appl. Opt., 12, 555-563, 1973. Irvine, W. M., and J. B. Pollack, Infrared optical properties of water and ice spheres, Icarus, 8, 324-360, 1968. Palmer, K. F., and D. Williams, Optical constants of sulfuric acid; application to the clouds of Venus?, Appl. Opt., 14, 208-219, 1975. Palmer, K. F., and D. Williams, Optical properties of water in the near infrared, J. Opt. Soc. Am., 64, 1107-1110, 1974. Pinkley, L. W., and D. Williams, The optical constants of sulfuric acid at 250 K, J. Opt. Soc. Am., 66, 122-124, 1976. Querry, M. R., B. Curnutte, and D. Williams, Refractive index of water in the infrared, J. Opt. Soc. Am., 59, 1299-1108, 1960. Remsberg, E. E., D. Lavery, and B. Crawford, Jr., Optical constants for sulfuric and nitric acids, J. Chem. Eng. Data, 19, 263-265, 1974. Robertson, C. W., B. Curnutte, and D. Williams, The infrared spectrum of water, Mol. Phys., 26, 183-191, 1973.


				
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