Equilibrium partial pressures of strong acids over concentrated saline solutions—II. HCl

AImospkric

Environmenr Vol. 22, No. I, pp. Il7-129.

1988.

ooo4-698l/fl8

Printed in Great Britain.

s3.00+0.00

Pergamon Journals Ltd.

EQUILIBRIUM PARTIAL PRESSURES OF STRONG ACIDS OVER CONCENTRATED SALINE SOLUTIONS-II. HCl S. L.CLEGG*~ and P. BRIMBLECOMBE School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, U.K. (First received 19 February 1987 and received for publication 8 July 1987) Abstract-Equilibrium partial pressures of HCl have been measured at 25°C over concentrated solutions containing the ions H+, Cl-, Mg* +, NH:, Al3+ SOi- ClO; , CH,SO; , and those of the alkali metals. Measurements agree closely with values predicted using a Henry’s Law constant of 2.04 x lo6 mol* kg-ratm-’ for the reaction HCI,,, = H&,, + Cl&,, and the Pitzer activity coefficient model. The following parameter values have been estimated for the model using the partial pressure and other thermodynamic data: 0,, at,, - 0.0120; I)“, at,, c1, - 0.012; OH,NH,r - 0.01; $n, NH,,cl, - 0.009; I)“, NH,, ar, -0.0104. Key word index: Hydrochloric acid, partial pressure, Henry’s Law, saline solutions, solubility, Pitzer, activity coefficient, aerosols, space shuttle, aluminium, alkali metals.

1. INTRODUCTION

coefficient model to calculations of HCl solubility. Important ions in the marine aerosol include H+, NH:, CH$O; (Saltzman et al., 1983) and some of the major ions of seawater: Na+, Cl-, K+, Mg’+ and SO:-. The interactions of these ions with H+ and Clare investigated here. The determination of the effects of individual ions is emphasized rather than studies of complex mixtures. These investigations have been extended to systems containing A1C13which are important in the mining industry and in local pollution caused by the space shuttle.

of strong acids such as H2S04, HNOs (MSA) with tropospheric aerosols is important for both geochemical cycling and an understanding of atmospheric pollution, These acids are highly soluble and will accrete on dry particles and partition strongly into aqueous phases. At relative humidities sufficient for the aerosol to remain aqueous, this process will result in concentrated acidified brine droplets, perhaps in combination with an insoluble solid phase. Hydrogen chloride is the principal volatile component in acidified marine seasalt aerosols (Cicerone, 1981; Clegg and Brimblecombe, 1985a). The seasalt aerosol is estimated to be the major natural source of HCl at about 20 Tg (HCI) a- ‘, based on field studies suggesting a displacement of at least 3 ‘x of total aerosol chloride (Chesselet et al., 1972) and a seasalt flux of 5 x lo3 Tga-’ (Blanchard, 1985). The purpose of the experiments described below was to gather new data on HCl partial pressures over saline media and verify the theoretical approach to strong acid solubility described in Clegg and Brimblecombe (1986). In a previous paper (Clegg and Brimblecombe, 1988) we presented new measurements of HN03 partial pressures above acidified salt mixtures. This work describes similar but more extensive measurements of HCI partial pressures over acidified salt mixtures containing the chloride anion. The results are used to show the applicability of the Pitzer activity

The interaction and CH3S03H

2.THEORY As was the case for HN03, the equilibrium of HCl between aqueous and gas phases is represented by:

HCl,,, = H :a,.,+ Cl,,,. The thermodynamic Henry’s (mo12kg-2atm-‘) is given by: K H=

* Present address: The Marine Biological Association of the United Kingdom, The Laboratory, Citadel Hill, Plymouth PLl 2PB, U.K. t To whom correspondence should be addressed.

mH + . t&l-

Law constant

. y;,-,/pHCl

(1) Ku (2)

where prefix m represents molality, ync, is the mean activity coefficient of H’ and Cl- ions in solution and pHC1 the equilibrium partial pressure of HCl. Existing partial pressure data and tabulated mean activity coefficients have been used to derive a Henry’s Law constant for HCl of 2.04 x lo6 molZkg~2atm-’ at 25°C (Clegg and Brimblecombe, 1986). In addition the present apparatus has been used to determine a value of 1.94 x lo6 mo12kg-2atm’ (Clegg and Brimblecombe, 1986). In the highly non-ideal solutions studied here ync, differs significantly from unity, and the Pitzer activity coefficient model (e.g. Pitzer, 1979; Harvie and Weare, 1980; Millero, 1982) has been used

117

S. L. CLEGGand P.

118

to estimate its value. A short description of the model is given in Clegg and Brimblecombe (1988) together with a list of parameter values. Briefly the model uses three or four parameters to describe osmotic and activity coefficients of single electrolytes. In multicomponent solutions further parameters Bij account for interactions between ions of like sign in solution mixtures, and tiijk for interactions between two ions of one sign and one ion of opposite sign. These two parameters are particularly

important

3.

in very concentrated

solutions.

EXPERIMENTAL

A large number of aqueous electrolyte mixtures varying in both composition and concentration are of interest to atmospheric chemists. The present experiments were constrained to considering just 24 different aqueous systems each containing a particular group of ions. Most experiments were conducted at a constant stoichiometric ionic strength of 5.0 mol kg-’ in order to maximize deviations from ideal behaviour in solution, while not exceeding the solubility of any of the salts present. Very high H+ concentrations (0.5-5.0 mol kg-‘) were maintained in the test solutions. These provide the most severe test of the model by maximizing association with SO:and the effect of Bi, and tiijl parameters involving H +. Partial pressures were measured with the apparatus described in Clegg and Brimblecombe (1985b)and Clegg (1986). Measurement errors have both random and systematic components, the latter due to a transfer of test solution as an aerosol within the apparatus. From experiments using non-volatile acids as test solutions the value of the systematic error was estimated to be 0.7 x lo-’ to 2.8 x lo-’ atm (mol kg ’ H ‘) ‘, depending on the experimental conditions. In the experiments with HNO, (Clegg and Brimblecombe, 1988) the measured partial pressures were corrected for this error. The same errors occurred in the present experiments, and two treatments were used. In Systems 1-12 the H+ concentration was 1.5 mol kg-’ and the systematic error (aerosol transfer) generally a rather small fraction of the measured partial pressure. Therefore no attempt was made to correct measured values for this error. The random error associated with each measurement was estimated as pre-

BRIMBLECOMBE

viously described by Clegg aqd Brimblecombe (1988). The upper error bar assigned to each data point was equal to the random error. The lower error bar assigned to each data point was determined from the larger of either (i) random error or (ii) aerosol transfer error. In Systems 1324 the test solutions contained different concentrations of H+, often greater than 1.5 mol kg-‘. The error due to aerosol transfer was both variable and greater than before, beingequal to 2.8 x lo-’ atm (molkg-’ H+)-‘. The measured partial pressure was corrected by this amount. An upper error bar was then assigned, equal to the greater of (i) the uncorrected measured partial pressure or (ii) the corrected partial pressure plus random variation. The lower error bar was assigned equal to the corrected partial pressure less the random variation. It must be emphasised that this procedure for estimating and correcting errors ensures that there is no experimental point for which the uncorrected partial pressure lies outside the error bars of the corrected value. This is true of all the partial pressure measurements in this work. The treatment of errors is described in more detail by Clegg (1986). 4. RESULTS

The results of the experiments for each aqueous system are presented both graphically and in table form in Appendix 1 where the test solution compositions are given together along with calculated water activities. The graphs show partial pressures, with error bars, and also partial pressures calculated using the Pitzer model with two values of K,: 1.94 x lo6 mol* kg-* atm-’ determined by using the present apparatus; and 2.04 x lo6 mol* kg-‘atm-’ determined from the data of Fritz and Fuget (1956). 4.1. System 1: H+Na+Cl-ClO; 2: H+Na+Cl-CH$O; 3: H+Na+Cl-SOiThese three systems, Fig. 1, were regarded as the simplest analogues of real acidified marine aerosols, being essentially solutions of NaCl acidified by the strong acids HC104, H*SO, and CH3S03H. Per-

~~:i~~~;

3.0

mCI_-

5,o

$0

mCI-_-

50

,

I

3,o

mCI_-

5.0

Fig. 1. (a) Partial pressure measurements for System 1, at ionic strength (I) equal to 5 mol kg-‘. Measured (corrected) partial pressures are marked by open circles, with error bars. Boxes show partial pressures estimated using the Pi&r model and the following and 2.04 x 10e6 molZ kge2 atm- ‘. values of K,: 1.94 x 10m6molzkg-Zatm-’ (b) Results for System 2, ! = 5mol kg-‘. (c) Results for System 3. Stoichiometric I = 5 mol kg-‘.

Equilibrium partial pressures of strong acids over concentrated saline solutions

chloric acid is unrealistic in atmospheric terms, but was used as an example of an ‘ideal’ strong monobasic acid, which is very strongly dissociated and has no significant vapour pressure over aqueous solutions. The results may be interpreted as follows. In System 1 the solution containing 5.0 mol kg-’ Cl- has a composition of 1.5 mol kg- ’ H+, 3.5 mol kg- ’ Na * and 5.0 mol kg- ’ Cl-. Moving along the ‘mCl_’ axis towards lower con~ntrations corresponds to repiacing the chloride, on an ion-for-ion basis, by the ClO; anion. The same is true for System 2 involving the CH,SO; ion. In the case of System 3, and all others containing doubly or triply charged ions, the concentration of a third ion must also change to maintain both charge balance and a constant stoichiometri~ ionic strength. For example, in System 3 the concentration of the Na+ ion also varies. Total H+ concentration is the same in each solution. The results for System 1 show a straight line relationship between partial pressure and Cl- concentration. This corresponds to a constant value of yn,-t for all the solutions, and is in agreement with early EMF

40

39

20 10 _mK*

O.&

0.8

4.2. System 4: 5: 6: 7:

1.2

mMg-

mNH, .

I.

3.5 38

_

2.0 mNa*

l,o

-

0.0

data at lower ionic strengths (Murdoch and Barton, 1933). The predicted partial pressures using the Pitzer model agree well with the experimental values, despite the parameters Bct,cto. and $Ct,C1O,,H being unknown. The results suggest that they have negligible values. Results for System 2 containing CH,SO; (MS-) show that this moderately strong acid anion interacts sufficiently with H+ to reduce partial pressures relative to the linear relationship shown by System 1. The calculated partial pressures are in reasonable agreement with measured values, despite the fact that only single electrolyte interaction parameters are known. The higher terms @CI,MB +CI,MS,H and I//H.N~,Ms may have significant values. The complete lack ofactivity or osmotic coefficient data for mixtures confining the CH,SO; ion makes it very difficult to assign values to these parameters. However, the relative contributions of each to yHC, are such that the most important parameters are ect,Ms and $ft,Ms.u. System 3 shows that mixtures containing both H+ and SOi- show large deviations from linearity. The model calculations suggest that the concentration of free Hf ion is reduced by about 50 y0 for the mixture containing the most SO:-, effectively increasing the solubility of HCl. The fact that the true ionic strength of the mixture is less than 5.0 mol kg- ’ also contributes to this increase. It is clear that atmospheric modelling using this system as an analogue of acidified seawater without allowing for the formation of HSO, would result in a serious overestimate of the HCl partial pressure. Model calculated values of the partial pressure are in good agreement with the measurements. This is to be expected since this system is fully parameterized.

OP

3P 20 IQ 0.0 _ mNt(

0,O

119

ID

0.5

_

mkfg”

0.0

Fig. 2. (a) Partial pressure m~surements for System 4. f = 5 mol kg- ‘. The curve is calcufated using the Pitzer model with the #n,Nn, and +n,NH,,Ct parameters estimated in this study, and Kn equal to 1.94 x lo6 mo12kg-* atm-r. (b) Results for System 5, I = 5 mol kg-‘. (c) Results for System 6, I = 5 mol kg-‘. (d) Results for System 7, I = 5 mol kn- *. The curve is calculated using the Pitzer model with the an, mtI and +n, t-~n~it parameters estimated in this study, and K, equal to 1.94 x 106molz kg-2atm-‘. For explanation of symbols, see caption to Fig. la.

H* Na+ NH: ClH’Na+K+ClH’Na+Mg’+ClH+NH,+Mg*+Cl-

These systems, Fig. 2, were studied in order to observe the effects of positive ions on the partial pressure of HCl. Systems 4 and 5 have constant H+ and Cl- concentrations and show very clearly that the presence of NH: and Kf decreases the partial presure of HCl relative to a solution containing only Na’. The effect is large: a reduction of partial pressure of about 50 y0 when completely replacing Na + by K + or NH:. The predicted partial pressures for System 5, for which all interaction parameters are known, are in good agreement with measured values. However, for System 4 the partial pressures of HCl are overestimated by the model. Mixtures ~n~injng HN03 show a similar effect (Clegg and and NHf Brimblecombe, 1988), demonstrating the need for additional parameters involving the NH: ion. The following interaction parameters for the System 4 affect YHcI:~H,N~ eH,NH,~ "I/H,Na,Cb $H.NH,,Cir J/N*,NH,,C,.Of

unknown.

these parameters,

O~Y

~~,NH~,=,

is

However, sensitivity testing revealed that

120

S. L. CLEGG~~~

adopting reasonable values of this parameter had almost no effect on thevalue of yHC,and so data used to derive the other parameters were re-examined. The values of 0~,~~and $H,N,a appear reliable as they are firmly based on activity coefficient data at high ionic strengths (Hawkins, 1932) and have been used successfully in modelling studies of saturated solutions (Harvie and Weare, 1980; Harvie et al., 1984). The parameters 8,,,,* and tjH,NH,,CIwere therefore recalculated and found to take values -0.01 and -0.009, respectively. The derivation of these parameters and corresponding ones involving Br- are given in Appendix 2. The curve in Fig. 2a shows improved agreement between the partial pressures calculated using these values and the measurements. System 6 shows that the Mg’ + ion also causes a moderate depression of partial pressure relative to the Na+ ion. This is well predicted by the Pitzer model, since all the parameters are known. System 7 contains both NH: and Mg* + and the

P. BRIMBLECOMBE deviation between measured and predicted partial pressures increases with increasing NH: concentration in a similar way to those of System 4. Use of the doublet and triplet NH: parameters estimated in Appendix 2 results in only a slight improvement of the fit, which is shown by the curve in Fig. 2d. 4.3. System

8: H’ Mg* + Cl- SO:-9: H+Na+Mg2+ClVSOzIO: H+NH$Cl-SO:11: H+NH:Mg’+Cl-SO:12: H’Na+NH:Cl-SO:-

These systems, Fig. 3, are more complex than those listed so far, containing both SO:- and one or two positive ions in addition to H’. All the graphs are strongly curved, due to the formation of HSO;. Calculations suggest that a maximum of 30-50x of the total Hi concentration is present as HSO;. Interaction parameters for these systems are generally known, except for those involving the NHf ion.

14

r----Y lb1 61

1 10

Fig. 3. (a) Partial pressure measurements for System 8. Stoichiometric I = 5 mol kg- ‘, (b) Results for System 9. Stoichiometric 1 = 5 molkg- I. (c) Results for System 10. Stoichiometric I = 5 mol kg- ‘. Curve was calculated using the Pitzer model with the values of @HNH+ and $H,NH,,Ci estimated in this study, and K, equal to 1.94 x lo6 mo12k~-2atm-‘. (d) Partial pressure measurements for System 11. Stoichiometric I = 5 mol kg-‘. Curve was calculated using the Pitzer model with the values of eH,~p, and $H,NH,,a estimated in this study, and K, equal to 1.94 x lo6 mo12kg- atm-‘. (e) Results for System 12. Stoichiometric I = 5 mol kg-‘. For explanation of symbols, see caption to Fig. la.

Equilibrium partial pressures of strong acids over concentrated saline solutions

samples of MSA were found to be contaminated with small amounts of a volatile acidic compound. The results for System 1 showed that the ClO; ion did not significantly alter the properties of Cl- and H+ in the solution (apart from an ionic strength effect). Therefore HC104 is a suitable ‘substitute’ acid for the atmospherically important acids HN03 and MSA. Since the anions of both acids have stronger interactions with other ions in solution than ClO; it is expected their presence will result in lower partial pressures of HCl. The measured partial pressures for these systems therefore represent an upper limit below which mixtures containing weaker monobasic acids will fall. In all four systems the agreement between calculated and measured partial pressures is excellent, even for System 13, where a maximum ionic strength of almost 7.0 mol kg-’ is reached. Results also show that the percentage of free H+ ion, for a given H+ concentration, decreases with increasing ionic strength of the saline mixture. Agreement is achieved without values for any of the Bij and $i,t parameters involving ClO; , suggesting that they are likely to be close to zero.

Agreement between predicted and measured values is good except in the case of System 10, where the NH: concentration is particularly high. Agreement is improved both for this system and System 11 when the parameters estimated from the previous group of systems are used. Although System 12 also contains NH:, agreement is good. This is probably because NH: is present in greatest concentration in mixtures which give the lowest partial pressures and errors swamp any differences between measured and calculated values. The prediction of thermodynamic properties of systems containing NH:, SOi- and H+ ions could be improved by fitting unknown model parameters such as /Y”‘, /I”’ and C@for the NH:-HSO; interaction from the data of Tang and Munkelwitz (1977). However, because of the reasonable agreement found between measured and predicted partial pressures in this study, the fitting has not been attempted. 4.4. System 13: I 14: I 15: I 16: I

= = = =

5.0 4.0 3.0 2.0

seawater seawater seawater seawater

+ + + +

121

HClO, HCIOL HC104 HClO,

Results are shown in Fig. 4. The ‘seawater’ used in Systems 13 : 21 is a major ion recipe consisting of Na+, Mg ’ +> Cl - SO:-, with concentrations in the ratio 1: 0.1319 : 11145:0.05934 (Millero, 1982). The test solutions were prepared so that the molal concentrations of the seawater ions were constant for all the solutions in each system, and only the acid concentration varied. These systems are realistic models of the sea salt aerosol, acidified by a strong monobasic acid. Perchloric acid was chosen on both practical and theoretical grounds. Firstly, HN03 could not be used because of its volatility. Secondly, many commercial

4.5. System

17: I = 2.0 seawater + MSA

Here a seawater mixture of ionic strength 2.0 mol kg- ’ was acidified with pure MSA, Fig. 5. The samples of MSA used for these experiments were purified by flushing with large volumes of N2 gas. The results show firstly that the measured partial pressures are much lower than for the same system with HC104, as expected. Secondly, the predicted partial pressures are too high, by up to 25 %. The principal reason for this is the lack of /I(“, fi”’ and C@’parameters for the Mg2 +- CHJSO; interaction. These cannot be evalu-

301

‘,

’ 7

0,o

mH’-

2.0

4.0

Fig. 4. (a) Partial pressure measurements for System 13. Stoichiometric ionic strength varies from 5.3 to 7.2 mol kg- I. (b) Results for System 14. Stoichiometric ionic strength varies from 4.5 to 6.5 mol kg-‘. (c) Results for System 15. Stoichiometric ionic strength varies between 3.5 and 6.0 mol kg-‘. (d) Results for System 16. Stoichiometric ionic strength varies between 3.0 and 6.0 mol kg-‘. For explanation of symbols, see caption to Fig. la.

122

S. L. CLEGGand P.

Fig. 5. Partial pressure measurements for System 17. Stoichiometric

BRIMBLECOMBE

90

mH*.

2.0

ionic strength varies between 3.0 and 6.0 mol kg-‘. See caption to Fig. la for explanation of symbols.

ated because of the lack of thermodynamic data for the salt Mg(CH$O&.

4.6. System 18: I 19: I 20: I 21: I

= = = =

5.0 4.0 3.0 2.0

seawater seawater seawater seawater

+ + + +

H2S04 H2S04 HzS04 H*SO,

These systems, Fig. 6, are the most important and realistic of those examined in this study. The Pitzer model is fully parameterized for the above solutions. Agreement between measured and predicted partial pressures is generally close, a good result considering that in System 18 for example, a stoichiometric ionic strength > 10 mol kg- ’ is reached. The graphs have similar form to those involving HC104, but partial pressures, for a given H + concentration, are only about one quarter of those obtained with the monobasic acid. A comparison with the seawater/MSA system still shows large difference--the partial pressure obtained with the seawater/H$O, mixture is about one-third of that obtained with MSA at the same ionic strength. This may have important implications for the relative efficiency with which different acids displace HCl from the marine aerosol, although H+ ion concentrations are unrealistically high. This is discussed below.

Fig. 6. (a) Partial pressure measurements for System 18. Stoichiometric ionic strength varies between 5.5 and 10.5 mol kg-‘. (b) Results for System 19. Stoichiometric ionic strength varies between 5.1 and 9.5 mol kg-‘. (c) Partial pressure measurements for System 20. Stoichiometric ionic strength varies between 5.1 and 10.5 mol kg-‘. (d) Partial pressure measurements for System 21. Stoichiometric ionic strength varies between 4.0 and 9.2 mol kg-‘. See caption to Fig. la for explanation of symbols.

4.7. System 22: H + Al3 + Cl - (I = 7) 23: H+A13+Cl- (I = 9) These systems, Fig. 7, are of predominantly theoretical interest as examples of highly unsymmetrical mixtures-containing both univalent and trivalent ions-which are expected to behave in a particularly non-ideal way. However, HCl/AIC13 mixtures are also important in the mining industry (Brown et al., 1979; Gokcen, 1980) for mineral processing. They may also be relevant to the atmosphere. The space shuttle exhaust consists largely of a mixture of A1203, HCl and steam (Cofer et al., 1985). Adsorption of HCI by the A1,03 particles produces a surface which behaves, in many ways, like AlCI, (Cofer, 1978; Cofer et al.,

mH+___

0.0

2.0

mH+-.-

4.0

Fig. 7. (a) Partial pressure measurements for System 22, I = 7 mol kg-‘. Curve [a] was calculated using the Davies activity coefficient equation, and [b] was calculated assuming yHClequal to 1.0. (b) Results for System 23, I = 9 mol kg- ‘. Curve [a] was calculated using the Davies activity coefficient equation, and [b] was calculated assuming yHClequal to 1.0. See caption to Fig. la for explanation of symbols.

Equilibrium

partial

pressures

of strong

1984). The production of cloud condensation nuclei and the partitioning of HCl between particle surfaces, liquid water and the vapour phase has implications for local pollution around the shuttle launch site. This has been the subject of a number of studies (Hindernan et al., 1980, 1982; Radke et a/., 1982). Atmospheric measurements suggest shuttle fallout aerosols continuously degas HCl (Cofer et al., 1985). Partial pressure measurements were made at two very high ionic strengths 7.0 and 9.0 mol kg- ’ as a test of the Pitzer model. The system is fully parameterized from EMF measurements made up to ionic strengths of 5.0 mol kg-’ (Pitzer, 1975). These measurements are essentially a test of the EBijfunctions in the Pitzer equations. It can be seen that for both systems the agreement with predicted partial pressures is excellent. It is possible to demonstrate the non-ideality of these systems and the inability of very simple theories to accurately predict activity coefficients. Curves are shown of predicted partial pressures, (a) using the Davies (1938) extended Debye-Huckel equation to predict yHc,, and (b) assuming yHC,equal to 1.0. Both these predictions are grossly in error, Figs 7a, b. 4.8. H+, (Li+, Na+, K+, Rb+, Cs’) ClResults are given in Fig. 8. In this group of measurements the test solutions consisted of 1.5 mol kg-’ H+, 5.0 mol kg-’ Cl- and 3.5 mol kg-’ of each of the alkali metal ions in succession. The measurements were made in order to demonstrate the effects of different cations on the HCI partial pressure and also to look for relationships within the periodic group. The results are plotted as partial pressure against the hydration energy of the alkali metal ion on the assumption that the strength of hydration might

0.0

20

30

LO

9

hydration energy / IO’J mo?

Fig. 8. Partial pressure measurements for alkali metal chloride/HCI mixtures at I = 5.0 mol kg-‘. The concentrations of the ions present are: H+, lSmolkg_‘;‘ Cl-, 5.0 mol kg-‘; alkali metal ion, 3.5 mol kg- I. See caption to Fig. la for explanation of symbols.

acids over concentrated

saline solutions

123

correlate with a ‘salting out’ effect on the HCl, and hence its partial pressure. This proves to be the case. The graph shows very clearly the large changes of partial pressure caused by the different ions and also the ordering according to each metal ion’s position in the periodic group. Note also the large error for the HCI/RbCI mixture for which eH,a,, and +“.R~.cI parameters are not available. While it is reasonable to assume that plotting the partial pressures against hydration energy would result in a smooth curve, the apparently linear relationship was a surprise. However, calculations of yHC, for HCl/alkali metal chloride mixtures for which all interaction parameters are known showed that this was fortuitous. The values of yHC,for HCl/alkali metal chloride mixtures of the same relative composition always lie on smooth curves, but slope and curvature were found to vary considerably. These relationships are further discussed below.

5. DKXXJSSION

Partial pressures are in general well predicted, as was the case for similar aqueous systems containing HNO, (Clegg and Brimblecombe, 1988). Here the largest errors occur for systems containing the CH,SO; ion. The lack of activity or osmotic coefficient data for salts such as Mg(CH,SO,), means that important strong interactions cannot be incorporated into the model. Furthermore, no eij or $ijt parameters are available. The results for systems containing the ions NH:, K+ and Mg’+ show that these ions reduce the partial pressure of HCl by up to 50%. They are important constituents of the marine aerosol, thus the common approximation of treating seasalt as an NaCl solution (e.g. Clegg and Brimblecombe, 1985a) could result in positive errors in calculated partial pressures. The presence of SOi- results in a substantial lowering of partial pressure relative to solutions containing only non-associating anions. This appears to imply that in the atmosphere, acidification of the seasalt aerosol by H2S04 might result in less displacement of HCl for a given aqueous concentration of H+ than acidification by a strong monobasic acid. However, these results were obtained using unrealistically high concentrations of acid, where very large amounts of sulphate are being added to the seawater. Accordingly, the Pitzer model was used to calculate partial pressures of HCl over seawater mixtures, at ionic strength 4.0 mol kg-‘, acidified by very small amounts of both H,SO, and HCIO,. The partial pressures of HCl for concentrations of added H,S04 ranging from 1 x 10m5 to 0.1 mol kg-’ were always close to double those arising from the same molal concentrations of HC104. This suggests that in real aerosols the degassing of Cl _ is simply proportional to the concentration of added H + and is unaffected by the form in which it is added. However, this may only be true in the early stages of acidification, since an aged

124

S. L.

CLEGGand P.

aerosol acidified by HzS04 may build up a substantial concentration of added SO:-, and retain more H+ as HSO;. By contrast, acidification by HN03 could result in equilibrium being reached between gas and aqueous phase HN03, which may reduce the loss of HCl. Only in the case of acidification by MSA and HISO, should Cl- degassing continue throughout all stages of acidification. Dynamic factors such as these are clearly important but are beyond the scope of this study. The relationship between partial pressure and hydration energy shown by the alkali metals in the last group of measurements suggests that the unknown interaction parameters Oab n and tin ahc, for the system HCl/RbCl can be estimated from ‘the properties of the other HCl/alkali metal chloride mixtures. To do this the Pitzer equations were used to generate values of ~uc, for the mixtures HCl/LiCl, HCl/NaCl, HCl/KCl, HCl/CsCl over a range of concentrations, and values of yrra in HCl/RbCl mixtures were then estimated graphically by plotting against hydration energy. The activity coefficient was also calculated with 0n,a,,, $n.a4c,set to zero. The quantity 6 In ~nc,/mRb+ was then plotted against 0.5 (mH+ + mCl_) in the usual way (Pitzer and Kim, 1974). The set of estimated activity coefficients was self-consistent and yielded e u,ab equal to -0.020 and $u,ab.c, equal to -0.012. Nesbitt (1984) has investigated an analogous set of relationships for the mean activity coefficients of alkali metal and alkaline earth chlorides in NaCl media. The author used an equation derived by Robinson and Stokes (1959) for the mean activity coefficient of one electrolyte present at trace levels in a solution of a second. The activity coefficient was expressed in terms of the hydration numbers of the electrolytes. It was shown that for concentrated media the equation was dominated by a term in the hydration number of the salt of interest-thus mean activity coefficient and hydration number were closely related. Nesbitt used the relationship to determine Harned’s Rule coefficients for the system RbCl/NaCl/H,O.

6. CONCLUSIONS The measured and predicted partial pressures agree well for most systems in this study, as was the case for previous work involving HN03. The results for systems containing AlCl, show the importance of nonideal effects and the inability of very simple models to estimate them. The Pitzer model may be used to predict the activity coefficients of the major ions in atmospheric aqueous systems where solution behaviour is highly non-ideal, subject to the concentration limitations discussed in a previous paper (Clegg and Brimblecombe, 1988). Even in the cases where the parameters oij and tiijr are unavailable the calculated partial pressures are in reasonable agreement with those measured. Using an empirical relationship with hydration

BRIMBLECOMBE

energy and a re-assessment of existing data, new values for the following parameters are recommended: 0”. ab, -0.0120; ti”,Rb,c,, -0.012; ~H,NH., -0.01; $H,NH,,CI, - 0.0°9; I(/n. NnI,or, -0.0104. Parameters were not evaluated for interactions involving the CH,SO; ion. Acknowledgements-This grant GT4/82/APPS/20.

work

was supported

by NERC

REFERENCES Blanchard D. C. (1985) The oceanic production

of atmospheric seasalt. J. geophys. Rex 90, 961-963. Brown R. R., Dam G. E., Mrazek R. V. and Gokcen N. A. (1979) Solubility and activity of aluminium chloride in aqueous hydrochloric acid solutions. Report of investigations, U.S. Bureau of Mines; 8379. Chesselet R., Morelli J. and Buat-Menard P. (1972) Some aspects of the geochemistry of marine aerosols. In The Changing Chemistry of the Oceans (edited by Dyrrsen D. and Jagner D.). Wiley, London. Cicerone R. J. (1981) Halogens in the atmosphere. Rev. Geophys. Space Phys. 19, 123-139. Clegg S. L. (1986) The atmospheric chemistry of extremely concentrated solutions. Ph.D. thesis, University of East Anglia. Clegg S. L. and Brimblecombe P. (1985a) Potential degassing of HCl from acidified sodium chloride droplets. Atmospheric Environment 19, 465470. Clegg S. L. and Brimblecombe P. (1985b) The Henry’s Law constant of methanesulphonic acid and its implications for atmospheric chemistry. Envir. Tech. Left. 6, 269-278. Clegg S. L. and Brimblecombe P. (1986) The dissociation constant and Henry’s Law constant of HCI in aqueous solution. Atmospheric Environment 20, 2483-2485. Clegg S. L. and Brimblecombe P. (1988) Equilibrium partial pressures of strong acids over concentrated aqueous solutions-I. HNO,. Atmospheric Environment 22,91-100. Cofer W. R. (1978) Enhanced hydrophilicity of chlorided aluminium oxide particulates. NASA Technical Paper 1312. Cofer W. R., Bendura R. J., Sebacher, D. I., Pellett G. L., Gregory G. L. and Maddrea G. L. (1985) Airborne measurements of space shuttle exhaust constituents. AIAA J. 23, 283-287. Cofer W. R., Pellett G. L., Sebacher D. 1. and Wakelyn N. T. (1984) Surface chloride formation on space shuttle exhaust alumina. J. geophys. Res. 89, 25352540. Davies C. W. J. (1938) The extent of dissociation of salts in water-VIH. An equation for the mean ionic activity coefficient of an electrolyte in water, and a revision of the dissociation constants of some sulphates. J. Chem. Sot. 2093-2098. Downes C. J. (1975) Thermodynamics of mixed electrolyte solutions, comparison of HCl-NH4CI-H,O and HCl-KCl-H20. J. Chem. Sot. Faraday Trans. 1,425-434. Fritz J. J. and Fuaet C. R. f1956) Vaoour uressure of aaueous hydrogen chlo%de solutions( CkkT’Chem. Engng Data Ser. I, 10-12. Gokcen N. A. (1980) Partial pressures of gaseous HCI and H,O over aqueous solutions of HCI, AlCI,. and FeCI,. Report of investigations, U.S. Bureau of Mines; 8456. ” Harvie C. E., Molter N. and Weare J. H. f 1984) The Drediction of mineral solubilities in natural ‘waters: the Na-KMg-Ca-H-Cl-S04-OH-HCOa-C03-C02-Hz0 system to high ionic strengths at 25°C. Geochim. Cosmochim. Acta 48, 723-751. Harvie C. E. and Weare J. H. (1980) The prediction of mineral solubilities in natural waters: the Na-K-Mg-Ca-Cl-

Equilibrium

partial

pressures

of strong

acids over concentrated

SO,-Hz0 systems from zero to high concentration at 25°C. Geochim. Cosmochim. Acta 44, 981-991. Hawkins J. E. (1932) The activity coefficients of hydrochloric acid in uni-univalent solutions at constant total molality. J. Am. Chem. Sot. 54, 4481-4487. Hindeman E. E., Garvey D. M., Langer G., Odencrantz F. K. and Gregory G. L. (1980) Laboratory investigations of cloud condensation nuclei from combustion of space shuttle propellant. J. appl. Met. 19, 175-184. Hindeman E. E., Radke L. F. and Eltgroth M. W. (1982) Measurements of cloud nuclei in the effluents from launches of liquid and solid fuelled rockets. J. appl. Met. 21, 1323-1331. Miller0 F. J. (1982) Use of models to determine ionic interactions in natural waters. Thalassia Jugoslavica 18, 253-291. Murdoch P. G. and Barton R. G. (1933) The activity coefficients of hydrochloric acid in aqueous solutions containing either sodium dithionate or perchloric acid. J. Am. Chem. SIX. 55,4074. Nesbitt H. W. (1984) Activity coefficients of ions in alkali and alkaline earth chloride dominated waters including seawater. Chem. Geol. 43, 127-142. Pitzer K. S. (1975) Thermodynamics of electrolytes-V. Effects of higher order electrostatic terms. J. So/n Chem. 4, 249-265. Pitzer K. S. (1979) Theory: ion interaction approach. In Activity Coefficients in Electrolyte Solutions (edited by Pytkowicz R. M.), Vol. I. pp. 209-265. CRC Press, Boca Raton, Florida. Pitzer K. S. and Kim J. (1974) Thermodynamics of electrolytes-IV. Activity and osmotic coefficients of mixed electrolytes. J. Am. Chem. Sot. 96, 5701-5707. Radke L. F., Hobbs P. V. and Hegg D. A. (1982) Aerosols and trace gases in the efluents produced by the launch of large liquidand solid-fueled rockets. J. appl. Met. 21, 1332-1345. Robinson R. A., Roy R. N. and Bates R. G. (1974) The system H,O-HCI-NH,Cl at 25°C: a study of Harned’s rule. J. Soln Chem. 3, 837-846. Robinson R. A. and Stokes R. H. (1959) Hectrolyte Solutions. Butterworths, London. Roy R. N. and Swensson E. E. (1975) Thermodynamic properties of strong electrolytes: the HBr-NH,Br-H,O system at 25°C. J. So/n Chem. 4, 431-440. Saltzman E. S., Savoie D. L., Zika R. G. and Prosper0 J. M. (1983) Methane sulphonic acid in the marine atmosphere. J. geophys. Rex 88, 10,897-10,902. Tang 1. N. and Munkelwitz H. R. (1977) Aerosol growth studies--III. Ammonium bisulphate aerosols in a moist atmosphere. J. Aerosol Sci. 8, 32 l-330.

Table

mNa+ 3.50 3.27 3.03 2.80 2.57 2.33

3A. Results

mHT

mHt?

mS0:

5.00 4.30 3.60 2.90 2.20 1.50

1.50 1.50 1.50 1.50 1.50 1.50

1.50 1.30 1.11 0.93 0.88 0.64

0.000 0.233 0.467 0.700 0.933 1.167

Refer to caption

APPENDIX

Table 1A. Results

125

1

for System 1 (H+ Na+ Cl- CIO;), strength 5.0 mol kg-’

mH+

mNa+

mC1-

mCl04

1.50 1.50 1.50 1.50 1.50 1.50

3.50 3.50 3.50 3.50 3.50 3.50

5.00 4.30 3.60 2.90 2.20 1.50

0.00 0.70 1.40 2.10 2.80 3.50

ionic

YHCI

aw

pHC1

1.830 1.824 1.818 1.812 1.806 1.800

0.783 0.786 0.788 0.791 0.794 0.796

12.80 10.80 9.76 7.55 5.16 4.07

Units: all concentrations (m)mol kg-‘, all partial pressures 10 m6 atm. The values of yHC, and water activity (a,) given for each solution were calculated using the Pitzer model, all unknown interaction parameters set to zero. For solutions containing SO: _ the calculated free H + concentration (HF+) is also given (the balance being present as HSO,). Superscript ‘T’ indicates a total or stoichiometric concentration. The ionic strength (I), taking into account HSO; formation, is listed for solutions containing SO:-.

Table 2A. Results

for System 2 (H+ Na* CI- CH,SO;), = 5.0 mol kg- 1

mH+

mCI-

mMS-

1:50 1.50 1.50 1.50 1.50 1.50

5.00 4.30 3.60 2.90 2.20 1.50

0.00 0.70 1.40 2.10 2.80 3.50

Refer to caption headings.

for System 3 (H’ Na+ Cl- SO:-), = 5.0molkg-’

mCI_

saline solutions

to Table 1A for explanation

YHCI

aw

3.50 3.50 3.50 3.50 3.50 3.50

1.830 1.796 1.764 1.731 1.700 1.669

0.783 0.785 0.787 0.790 0.792 0.794

to Table 1A for explanation

stoichiometric

YHCI

1.830 1.514 1.240 1.006 0.806 0.639

mNa +

0.783 0.810 0.835 0.859 0.881 0.902 of column

I

pHCl

5.00 4.60 4.22 3.86 3.55 3.28

12.80 7.78 4.41 2.46 1.37 0.58

headings.

I

pHCl 12.80 10.80 7.99 5.76 3.89 2.72 of column

I

126 Table

S.L.C~~~crand 4A. Results

for System 4 (H+ Na+ NHf = 5.0 mol kg-r

P. BRIMBLECOMBE

Cl-),

I

Table

5A. Results

for System 5 = 5.0 mol kg-’

mHf

mNa+

mNH:

mCl-

YHCI

aw

pHC1

mH+

‘mNa+

mK+

1.50 1.50 1.50 1.50 1.50 1.50

3.50 2.80 2.10 1.40 0.70 0.00

0.00 0.70 1.40 2.10 2.80 3.50

5.00 5.00 5.00 5.00 5.00 5.00

1.830 1.720 1.616 1.519 1,428 1.342

0.783 0.790 0.797 0.803 0.811 0.818

12.80 10.40 9.4.1 8.08 6.92 6.25

1.50 1.50 1.50 1.50 1.50 1.50

3.50 2.80 2.10 1.40 0.70 0.00

0.00 0.70 1.40 2.10 2.80 3.50

Refer to caption headings.

to Table

Table

for

6A. Results

1A for explanation

System

of column

mMgZ +

mCl-

YHCl

aw

pHC1

mH+

mNHa

1.50 1.50 1.50 1.50 1.50 1.50

3.50 2.80 2.10 1.40 0.70 0.00

0.000 0.233 0.467 0.700 0.933 1.167

5.00 4.77 4.53 4.30 4.07 3.83

1.830 1.784 1.742 1.703 1.668 1.636

0.783 0.794 0.804 0.815 0.827 0.839

12.80 11.40 10.80 9.64 8.39 7.78

1.50 1.50 1.50 1.50 1.50 1.50

3.50 2.80 2.10 1.40 0.70 0.00

of’ column

c1- SO:-),

Mg’+

mCI-

mHT

mHd

mS0:

1.167 1.109 1.050 0.992 0.934 0.875

3.83 3.37 2.90 2.43 1.97 1.50

1.50 1.50 1.50 1.50 1.50 1.50

1.50 1.38 1.26 1.14 1.03 0.93

0.175 0.350 0.525 0.700 0.875

Refer to caption

Table 9A. Results mNa + 3.50 2.80 2.10 1.40 0.70 0.00

12.80 10.30 8.89 7.66 7.11 6.14

0.000

to Table 1A for explanation

1A for explanation

a,

pHCl

0.000 0.233 0.467 0.700 0.933 1.167

5.00 4.77 4.53 4.30 4.07 3.83

1.342 1.398 1.457 1.515 1.574 1.636

0.812 0.816 0.821 0.826 0.832 0.839

6.25 6.51 6.92 7.26 7.80 7.78

stoichiometric

1A for explanation

I = 5.0 molkg-

YHCl

a,

I

pHC1

1.636 1.442 1.266 1.107 0.964 0.835

0.839 0.854 0.870 0.885 0.899 0.913

5.00 4.75 4.51 4.28 4.07 3.86

7.78 5.84 3.70 2.45 1.54 0.81

of column

for System 9 (H+Na+Mg’+Cl-SO:-),

’

headings.

stoichiometric

I = 5.0 mol kg-’

mCl_

mHT

mHF+

mS0:

YHCI

aw

1

pHC1

0.000 0.175 0.350 0.525 0.700 0.875

5.00 4.30 3.60 2.90 2.20 1.50

1.50 1.50 1.50 1.50 1.50 1.50

1.50 1.35 1.22 1.10 1.01 0.93

0.000 0.175 0.350 0.525 0.700 0.875

1.830 1.560 1.330 1.136 0.972 0.835

0.783 0.810 0.837 0.863 0.888 0.913

5.00 4.70 4.43 4.20 4.01 3.86

12.80 8.30 5.13 3.11 1.82 0.81

to Table 1A for explanation

Table 10A. Results for System 10 (H+ NHaClmHT

mHG

mS0:

1so

1.50 1.30 1.11 0.93 0.76 0.6 1

0.000 0.233 0.467 0.700 0.933 1.167

Refer to caption

mNHf 3.50 3.27 3.03 2.80 2.57 2.33

of column

headings.

SO:-),

stoichiometric

I = 5.0 mol kg-’

mC1 _

YHCI

a,

I

pHC1

5.00 4.30 3.60 2.90 2.20 1.50

1.342 1.148 0.970 0.809 0.664 0.536

0.8 12 0.833 0.854 0.874 0.893 0.912

5.00 4.60 4.22 3.85 3.51 3.21

6.25 3.75 2.28 1.29 0.70 0.34

to Table 1A for explanation

of column

Mg* + Cl-),

YHCI

to Table

headings.

1

of column

mCl-

mMgZ +

Refer to caption

1.50 1.50 1.50 1.50 1.50

0.783 0.790 0.797 0.803 0.808 0.813

mMgZ+

Refer to caption headings.

Table 8A. Results for System 8 (H+ Mg”

1.830 1.711 1.602 1.501 1.408 1.321

for System 7 (H+ NH; = 5.0 mol kg-’

7A. Results

mNa+

1A for explanation

pHC1

Table

mH+

to Table

aw

to Table

= 5.0mol kg-’

Refer to caption headings.

5.00 5.00 5.00 5.00 5.00 5.00

YHCI

Refer to caption headings.

1

6 (H+ Na+ Mg”‘Cl-),

mW

(H + Na+ K+ Cl-),

of column

I

Equilibrium partial pressures of strong acids over concentrated saline solutions

127

Table 11A. Results for System 11 (H’ NH: Mg’+Cl- SO:-), stoichiometric I = 5.0 molkg-’ mHT

mH$

r&0:

1.50 1.50 1.50 1.50 1.50 1.50

1.50 1.35 1.22 1.10 1.00 0.93

o.WO 0.175 0.350 0.525 0.700 0.875

mNHa

mC1-

mMg’+

YHCl

3.50 2.80 2.10 1.40 0.70 0.00

5.00 4.30 3.60 2.90 2.20 1.50

0.000 0.175 0.350 0.525 0.700 0.875

1.342 1.232 1.123 1.019 0.922 0.835

I

pHCl

5.00 4.71 4.44 4.20 4.00 3.86

6.25 4.99 3.21 2.34 1.19 0.81

a,

0.812 0.831 0.851 0.871 0.892 0.913

Refer to caption to Table 1A for explanation of column headings

Table 12A. Results for System 12 (H+ Na+ NHf Cl- SO:-), stoichiometric I = 5.0 molkg-’ mHT

mH$

mSOf

1.50 1.50 1.50 1.50 1.50 1.50

1.50 1.30 1.11 0.93 0.76 0.61

0.000 0.233 0.467 0.700 0.933 1.167

mNHa

mNa +

mCl-

0.000 0.466 0.934 1.400 1.870 2.334

3.50 2.80 2.10 1.40 0.70 0.00

5.00 4.30 3.60 2.90 220 1.50

YHCl

1.830 1.455 1.150 0.902 0.760 0.536

a,

0.783 0.813 0.840 0.866 0.890 0.912

I

pHCl

5.00 4.60 4.22 3.86 3.52 3.21

12.80 6.67 3.32 1.82 0.70 0.34

Refer to caption to Table 1A for explanation of column headings.

Table 13A. Results for System 13 (I = 5.0 seawater + HCIO,) mHT

mH;

mS0:

2.20 1.a0 1.40 1.00 0.60 0.30

2.02 1.62 1.23 0.84 0.45 0.19

0.204 0.204 0.204 0.204 0.204 0.204

mC10; 2.20 1.80 1.40 1.00 0.60 0.30

pHCl

YHCI

aw

I

2.687 2.330 2.019 2.738 1.462 1.220

0.705 0.732 0.759 0.785 0.810 0.828

6.83 6.44 6.05 5.67 5.31 5.08

30.50 18.20 11.10 5.59 2.17 0.70

The seawater mixture contains the ions Na’, Mg’+, Cl- and SOi- in the mole ratios 1.0:0.1319 : 1.145:0.0593, respectively. Calculated values of the free H+ concentration and true ionic strength of the solutions are given. Refer to caption to Table 1A for explanation of column headings.

Table 14A. Results for System 14 (I = 4.0 seawater + HCIO,) mHT

mH;

mso:

2.50 2.10 1.70 1.30 0.90 0.50

2.36 1.96 1.56 1.17 0.77 0.39

0.163 0.163 0.163 0.163 0.163 0.163

mC10; 2.50 2.10 1.70 1.30 0.90 0.50

YHCl

2.454 2.134 1.858 1.618 1.400 1.184

a,

0.727 0.754 0.781 0.806 0.831 0.855

I

pHC1

6.2 1 5.82 5.42 5.03 4.65 4.28

23.00 15.90 9.08 4.97 2.49 0.91

The seawater mixture contains the ions Na’, Mg’+, Cl- and SOi- in the mole ratios 1.0:0.1319: 1.145: 0.0593, respectively. Calculated values of the free H’ concentration and true ionic strength of the solutions are given. Refer to caption to Table 1A for explanation of column headings.

128

S. L. CLEGG and P. BRIMBLECOMBE Table 15A. Results for System 60:

trtHT 2.89 2.40 1.90 1.40 0.91 0.42

3.00 2.50 2.00 1.50 1.00 0.50

mc10;

0.122 0.122 0.122 0.122 0.122 0.122

3.00 2.50 2.00 1.50 1.00 0.50

15 (I = 3.0 seawater

YHCI 2.409 2.020 1.706 1.445 1.223 1.017

The seawater mixture contains the ions Nat, mole ratios 1.0: 0.1319 : 1.145 : 0.0593, respectively. H+ concentration and true ionic strength of the caption to Table 1A for explanation of column

Table 16A. Results for System mHT

mHF+

mS0:

4.00 3.40 2.80 2.20 1.60 1.00

3.93 3.33 2.73 2.13 1.54 0.94

0.0815 0.0815 0.0815 0.0815 0.0815 0.0815

r&10; 4.00 3.40 2.80 2.20 1.60 1.00

a,

1

pHC1

0.734 0.769 0.802 0.833 0.863 0.890

5.79 5.29 4.80 4.30 3.81 3.34

22.00 11.90 7.33 3.49 1.55 0.63

Mg* +, Cl and SOi- in the Calculated values of the free solutions are given. Refer to headings.

16 (I = 2.0 seawater ilHCl

2.848 2.290 1.856 1.519 1.253 1.040

The seawater mixture contains the ions Na+, mole ratios 1.0: 0.1319 : 1.145 : 0.0593, respectively. H+ concentration and true ionic strength of the caption to Table 1A for explanation of column

Table

+ HCIOJ

+ HCIOJ

a,

1

pHC1

0.705 0.749 0.790 0.830 0.866 0.899

5.86 5.26 4.66 4.07 3.47 2.88

27.40 13.60 7.47 3.89 1.80 1.07

Mg’ +, Cl-

and SOi- in the Calculated values of the free solutions are given. Refer to headings.

Table 17A. Results for System

17 (I = 2.0 seawater+

MSA)

mHT

mHd

mS0:

mMS_

YHCI

0,

I

pHC1

4.00 3.40 2.80 2.20 1.60 1.00

3.93 3.33 2.74 2.14 1.54 0.94

0.0815 0.0815 0.0815 0.0815 0.0815 0.0815

4.00 3.40 2.80 2.20 1.60 1.00

2.249 1.935 1.664 1.429 1.225 1.042

0.740 0.774 0.808 0.841 0.872 0.902

5.87 5.27 4.67 4.07 3.47 2.88

10.90 7.83 4.41 3.04 1.18 0.57

The seawater mixture contains the ions Na’, mole ratios 1.0: 0.1319 : 1.145: 0.0593, respectively. H+ concentration and true ionic strength of the caption to Table 1A for explanation of column

Mg*+, Cl- and SOi- in the Calculated values of the free solutions are given. Refer to headings.

for

19A. Results

18A. Results

mHT

mHf

mS0:

3.650 2.990 2.000 1.000 0.666 0.333

1.84 1.51 1.00 0.49 0.32 0.15

2.027 1.699 1.202 0.703 0.537 0.371

System + H,SO,)

18

YHCI

aw

1.857 1.650 1.378 1.145 1.072 0.996

0.730 0.752 0.785 0.816 0.825 0.835

(I = 5.0

seawater

Table

I

pHC1

mHT

mH;

6.84 6.50 5.99 5.47 5.30 5.14

21.70 13.90 6.22 1.91 1.02 0.43

3.655 2.893 2.196 1.589 1.000 0,700

1.93 1.53 1.16 0.85 0.53 0.37

The seawater mixture contains the ions Na+, Mg’+, Cland SOi- in the mote ratios 1.0:0.1319: 1.145:0.0593, respectively. Calculated values of the free H + concentration and true ionic strength of the solutions are given. Refer to caption to Table 1A for explanation of column headings.

for

System + H&U

19

(I = 4.0

seawater

mSOf

YHCI

aw

I

pHC1

1.990 1.609 1.260 0.962 0.662 0.513

1.544 1.348 1.193 1.077 0.972 0.923

0.769 0.795 0.817 0.836 0.853 0.862

6.02 5.60 5.22 4.89 4.55 4.38

11.80 7.38 3.99 2.58 1.08 0.66

The seawater mixture contains the ions Na+, Mg2 ‘, Cl _ and SOi- in the mole ratios 1.0:0.1319: 1.145:0.0593, respectively. Calculated values of the free H + concentration and true ionic strength of the solutions are given. Refer to caption to Table 1A for explanation of column headings.

Equilibrium partial pressures of strong acids over concentrated saline solutions Table 20A. Results

for System + H2SW

20 (I = 3.0 seawater

Table 24A. Results for System 24, mixtures of HCI with the alkali metal chlorides

mHT

mH;

F&.0,:

YHCI

=w

1

pHCl

mM’

4.980 4.180 3.390 2.600 2.ooo 1.400

2.12 2.30 1.81 I.44 1.11 0.78

2.612 2.213 1.820 1.420 1.120 0.823

1.621 1.401 1.223 1.061 0.966 0.878

0.164 0.191 0.817 0.843 0.861 0.878

5.93 5.49 5.04 4.58 4.22 3.87

11.20 10.20 6.54 3.24 1.82 1.11

3.50 3.50 3.50 3.50 3.50

The seawater mixture contains the ions Na”, Mg’ ‘, Cland SO:- in the mole ratios 1.0:0.1319: 1.145:0.0593, respectively. Calculated values of the free H + concentration and true ionic strength of the solutions are given. Refer to caption to Table 1A for explanation of column headings. Table

21A. Results

for System + HJSW

mHT

mH;

mSO#$

4.812 4.046 3.270 2.505 1.928 1.351

2.14 2.33 1.89 1.46 1.13 0.79

2.488 2.104 1.718 1.334 1.045 0.157

I

pHCl

5.08 4.62 4.14 3.66 3.28 2.91

6.71 3.13 2.53 1.31 0.16 0.53

%

0.810 0.834 0.859 0.882 0.898 0.914

22A. Results

for System 22 (H+A13+Cl-), = 7.0 mol kg-’

mH+

mA13+

mCl_

YHCI

aw

2.00 1.60 1.20 0.80 0.60 0.40

0.833 0.900 0.967 1.030 1.OlO 1.100

4.50 4.30 4.10 3.90 3.80 3.70

2.160 2.020 1.900 1.780 1.720 1.670

0.793 0.810 0.827 0.844 0.852 0.861

I

pHC1 20.40 13.60 8.51 4.76 3.13 1.86

Refer to caption to Table IA for explanation of column headings. Table

mH”

1.20 1.00 0.80 0.60 0.40 0.20

23A. Results

mAP+

1.3ocl 1.333 1.367 1.400 1.430

1.410

Refer to caption headings.

for System 23 (H+A13+C1-), = 9.0 mol kg- ’

mCl_

5.10 5.00 4.90 4.80 4.70 4.60

mC1_

YHCl

aw

pHCl

1.50 1.50 1.50 1.50 1.50

5.00 5.00 5.00 5.00 5.00

2.395 1.830 1.321 1.440 0.967

0.743 0.183 0.813 0.808 0.964

20.80 12.80 6.14 5.14 3.43

Concentrations are as follows: mH’ = 1.5, mCl- = 5.0, mM’ = 3.5 mol kg-‘. Refer to caption to Table 1A for explanation of column headings.

APPENDIX 2

The seawater mixture contains the ions Na ‘, Mg2 +, Cl and SOi- in the mole ratios 1.0:0.1319: 1.145:0.0593, respectively. Calculated values of the free H + con~tration and true ionic strength of the solutions are given. Refer to caption to Table IA for explanation of column headings. Table

Li+ Na+ K’ Rb+ cs+

mH+

21 (I = 2.0 seawater

YHCl

1.307 1.140 0.997 0.881 0.805 0.740

129

YHCl

2.760 2.610 2.590 2.500 2.420 2.340

a,

0.762 0.771 0.780 0.790 0.800 0.809

to Table 1A for explanation

pHCl 22.20 11.60 13.10 8.80 5.00 2.32 of column

I

The parameters ~?H,NH,and $IH,NH+C~ have been derived from activity coefficient data for the two systems, H+/NHf/Cland H+/NH:/Br-, to maximum ionic strengths of only 3.0 mol kg-’ (Robinson er al., 1914; Roy and Swensson, 1975). Downes (1975) also reported activity coefficient measurements for the system H’/NHa/Cl. The data for both systems were interpreted in the light of Harned’s Rule, the two triplet (I,~H,NH~,~,$IH,NH,,B,)parameters set to zero and the ffH,~& parameter assigned the value - 0.019. These enabled the data to be fitted to within the limits of experimental error. Since higher interaction parameters have little effect on calculated activity coefficients over most of the ionic strength range of these experiments, it is not clear whether the chosen parameter values accurately represent the interactions at high concentrations. Comparison with the system H ‘/K ‘/Cl - is relevant as it can be seen that NH: and K+ have similar effects on YHC]in the solutions studied here. The parameter $~,?,a, derived from data to an ionic strength of 3.5 mol kg-’ (Pltzer and Kim, 1974), has a non-zero value and so the experimental data of Robinson et al. (1914) and Roy and Swensson (1975) were re-interpreted to see if the corresponding NH: parameters were significant. The original EMF data for both systems were first used to calculate YHC~and ?~a~. The values were then compared with those calculated by the Pitzer model assuming 0% NH,, +H,NH,.cI and I~H,NH+,B~equal to zero. The resulting plots of 6 In YHcl/mNH: and 6 In y~a,/mNHa against O.S(mH+ + mC]-) and OS(mH + + mBr_) yield intercept OH,NH, and Slope5 $H, NH,,cland $H!NH,,&, respectively. Values for these parameters were determmed by least squares fitting with the folfowing results: &,NH, equal to - 0.01, $H,NH.,c~equal to - 0.009 and $H, NH,,er equal to - 0.0104. The data are rather scattered so there is some uncertainty in these values. The fitting of the original data of Robinson et al. and Roy and Swensson is only slightly improved by adopting the above parameter values over those given by the authors, and the fit is not sensitive to small changes. However, at the high ionic strengths used in the present study, the effect of the parameters is pronounced. Their use improves the fit of predicted and measured partial pressures for systems containing the ammonium ion and they are therefore recommended for use in calculations for high ionic strength systems.