Sulfurous acid activity coefficients

The activity of any ion, a = 7m, where y is the activity coefficient and m is the molaHty (mol solute/kg solvent). Because it is not possible to measure individual ionic activities, a mean ionic activity coefficient, 7, is used to define the activities of all ions in a solution. The convention used in most of the Hterature to report the mean ionic activity coefficients for sulfuric acid is based on the assumption that the acid dissociates completely into hydrogen and sulfate ions. This assumption leads to the foUowing formula for the activity of sulfuric acid. 

Tables on the "reaction probalility or "uptake coefficient" have been summarized for various heterogeneous reactions in a recent review article [87], and by the IUPAC [88] and NASA-JPL [86] evaluation teams. For the purpose of this article, a rough comparison is made of the uptake rates for the reactions (1) to (5) on the different type surfaces. Three major type of surfaces have been considered a) NAT, or Type I PSC, b) Water ice, or Type II PSC and c) sulfuric acid aerosol, which is normally a liquid surface generally composed of 60-80 wt % H,S04 and 40-20 wt % H,0 also considered is the solid form SAT (sulfuric acid tetrahydrate) with a composition of 57.5 wt % H,S04. The importance of chlorine activation on sulfuric acid solutions has been demonstrated in a recent article [89]. Halogen activation on seasalt material will shortly be reviewed as part of the tropospheric processes.

The activity coefficients of sulfuric acid have been deterrnined independentiy by measuring three types of physical phenomena cell potentials, vapor pressure, and freeting point. A consistent set of activity coefficients has been reported from 0.1 to 8 at 25°C (14), from 0.1 to 4 and 5 to 55°C (18), and from 0.001 to 0.02 m at 25°C (19). These values are all based on cell potential measurements. The activity coefficients based on vapor pressure measurements (20) agree with those from potential measurements when they are corrected to the same reference activity coefficient. 

To calculate the open circuit voltage of the lead—acid battery, an accurate value for the standard cell potential, which is consistent with the activity coefficients of sulfuric acid, must also be known. The standard cell potential for the double sulfate reaction is 2.048 V at 25 °C. This value is calculated from the standard electrode potentials for the (Pt)H2 H2S04(yw) PbS04 Pb02(Pt) electrode 1.690 V (14), for the Pb(Hg) PbS04 H2S04(yw) H2(Pt) electrode 0.3526 V (19), and for the Pb Pb2+ Pb(Hg) 0.0057 V (21). 

The ESR spectrum of the pyridazine radical anion, generated by the action of sodium or potassium, has been reported, and oxidation of 6-hydroxypyridazin-3(2//)-one with cerium(IV) sulfate in sulfuric acid results in an intense ESR spectrum (79TL2821). The self-diffusion coefficient and activation energy, the half-wave potential (-2.16 eV) magnetic susceptibility and room temperature fluorescence in-solution (Amax = 23 800cm life time 2.6 X 10 s) are reported. 

Tables 3 and 4 contain values of the log water activity and log sulfuric acid activity in molarity units. These can be obtained at any temperature by using the polynomial coefficients supplied by Zeleznik,45 which are based on all of the preexisting thermodynamic data obtained for this medium. The numbers were converted to the molarity scale using the conversion formula given in Robinson and Stokes 46 Molarity-based water activities are given for HCIO4 in Tables 5 and 6. These are calculated from data obtained at 25°C by Pearce and Nelson,17...

Staples, B. R. "Activity and Osmotic Coefficients of Aqueous Sulfuric Acid" J. Phys. Chan. Ref. Data, in press. 

From the foregoing discussion we conclude that some sophisticated tools are now available by which the activity coefficient in hydrometal— lurgical systems can be addressed. What is lacking is the actual application of these tools by the industry. The next step in establishing the accuracy of the available approaches lies in providing a broader data base for complex multicomponent systems which can be used for parameter refinement. TTte lack of data is most serious in the weak electrolyte area, but even familiar systems such as those encountered in sulfuric acid leaching need attention. 

Sulfuric acid is a 2 1 electrolyte, and so (by using the data in Table 3.1) the ionic strengthlis three times the concentration, i.e.l = 0.03 moldm f Next, from the Debye-Huckel extended law equation (3.15), we can obtain the mean ionic activity coefficient y as follows ... 

The uptake coefficient on liquid sulfuric acid is a strong function on the water activity, in analogy to the hydrolysis of CIONO, and therefore depends upon the composition of the mixture [92]. It was suggested [93] that the CIONO, uptake due to reaction with HC1 is dependent on both bulk and surface concentrations of HC1 y varies by more than two orders of magnitude (0.3>y>10°), and depends strongly on the HC1 partial pressure... 

The uptake coefficient on liquid sulfuric acid is due to QONO, hydrolysis and has been shown to depend strongly on the composition. It was indicated that y depends on the HjO activity of the mixture [93]. A detailed model for applying the laboratory uptake coefficient for this reaction to the small aerosol composition found in the stratosphere has been developed [43,96]. 

The uptake coefficients on various surfaces are listed [86]. On NAT-like substrates y is large, near 0.2. On water-ice substrates, y is even larger, on the order of 0.3. The uptake coefficient of HO on liquid sulfuric acid bulk solutions decreases with increasing activity of the mixture [41,82]. The solubility of HC1 in those mixtures is the controling factor. The uptake on sulfuric acid droplets was recently measured [102], The mass accommodation coefficient a was found to be inversely proportional to temperature and increases from 0.06 at 184 K to 1.0 at - 230 K. The uptake of HOC1 on water-ice in the... 

An experimental check on this assumption about activity coefficients is possible over a limited range of solvent acidity. If the composition of water-sulfuric acid mixtures is varied over the range in which all four species, Al5 A2, A1H +, and A2H+ are present in appreciable concentration, then, since OAlH + / aA2H+ 1S (W definition) constant, a constant ratio [A1][A2H+]/ [A2 [A1H + ] implies that the assumption of the ratio of y s being constant is correct in this range of solvents. Experimentally, for bases that are substituted anilines this test is fairly successful, a result that supports the validity of the method. The question of how similar two compounds must be to be sufficiently close in structure will be considered later. 

The discussion in the previous sections concerning solvated species indicates that a complete knowledge of the chemical reactions that take place in a system is not necessary in order to apply thermodynamics to that system, provided that the assumptions made are applied consistently. The application of thermodynamics to sulfuric acid in aqueous solution affords another illustration of this fact. We choose the reference state of sulfuric acid to be the infinitely dilute solution. However, because we know that sulfuric acid is dissociated in aqueous solution, we must express the chemical potential in terms of the dissociation products rather than the component (Sect. 8.15). Either we can assume that the only solute species present are hydrogen ion and sulfate ion (we choose to designate the acid species as hydrogen rather than hydronium ion), or we can take into account the weak character of the bisulfate ion and assume that the species are hydrogen ion, bisulfate ion, and sulfate ion. With the first assumption, the effect of the weakness of the bisulfate ion is contained in the mean activity coefficient of the sulfuric acid, whereas with the second assumption, the ionization constant of the bisulfate ion is involved indirectly. 

The mean activity coefficient of sulfuric acid is usually calculated in terms of Equation (11.91), where the weakness of the bisulfate ion is ignored. The relationship between the various activity coefficients when the incomplete ionization of the ion is included, and when it is not, is now readily obtained by the combination of the appropriate equations. Thus, when Equations (11.91) and (11.92) are equated,... 

Figure 15-2 (left) depicts several titration curves of Fe(II) with permanganate. Beyond the end point the experimental curves differ from the theoretical shape, which is nearly flat beyond the end point (5-equivalent reduction). The essential symmetry of the curves suggests that the potential is determined by the Mn(III)-Mn(II) couple beyond the end point. Evidence for this behavior can be seen in solutions containing sulfate or phosphate, which tend to stabilize Mn(III) (Section 17-1). That sulfuric and phosphoric acids have about the same effect before and after the end point is consistent with the similarity of the behavior of the Mn(III)-Mn(II) and the Fe(III)-Fe(II) systems with respect to changes in activity coefficients as well as with respect to hydrolysis and complex formation. 

Problem Assuming to remain constant, calculate the relative change in the mean ionic activity coefficient of 1 molal sulfuric acid solution from 0 to 25 C. 

The E.M.F. of a lead storage battery containing 2.75 molal sulfuric acid was found to be 2.005 volt at 25 C. The aqueous vapor pressure of the acid solution at this temperature is about 20.4 mm., while that of pure water is 23.8 mm. The mean ionic activity coefficient of the sulfuric acid is 0.136. Calculate the standard free energy change of the cell reaction at 25 C and check the values from tabulated free energy data. 

Stelson and Seinfeld (1981) have shown that solution concentrations of 8-26 M can be expected in wetted atmospheric aerosol. At such concentrations the solutions are strongly nonideal, and appropriate thermodynamic activity coefficients are necessary for thermodynamic calculations. Tang (1980), Stelson and Seinfeld (1982a-c), and Stelson et al. (1984) have developed activity coefficient expressions for aqueous systems of nitrate, sulfate, ammonium, nitric acid, and sulfuric acid at concentrations exceeding 1M. 

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