Marine Chemistry, 24 (1988) 203-213 203 Elsevier Science Publishers B.V., $~msterdam - - Printed in The Netherlands

Linear Free Energy Techniques for Estimation of Metal Sulfide Complexation Constants

SCOTT ELLIOTT

Department of Chemistry, University of California, Irvine, CA 92717 (U.S.A.)

(Received March 23, 1987; revision accepted November 11, 1987)

ABSTRACT

Elliott, S., 1988. Linear free energy techniques for estimation of metal sulfide complexation con- stants. Mar. Chem., 24:203-213

Unknown sulfide complex formation constants have been estimated through linear free energy comparisons of known values with sulfide solubility products, and with reactions of the model ligand dithizone. Results of the two techniques are in agreement for complexes of the + 2 metals with two sulfide ligands. Data are currently insufficient to establish a slope for free energy lines relating single ligand ÷ 2 complexes. Alternative methods for deducing single ligand formation include interpolation across oxidation states and correlation of bisulfide formation constants with the analogous hydroxide values. By linear free estimates, several trace metals speciate predomi- nantly as sulfide complexes over the bisulfide concentration ranges calculated from carbonyl sul- fide hydrolysis in the mixed layer. Recent preliminary measurements of total dissolved sulfide confirm this result for Cu, and perhaps also Hg.

INTRODUCTION

Measurements of metal sulfide complex formation constants are scarce (Martell and Smith, 1976), and in several areas of chemical oceanography demand has recently stimulated the development of linear free energy (LFE) techniques for the estimation of missing values. Elliott and coworkers identi- fied carbonyl sulfide hydrolysis as a quantifiable source of the hydrogen sul- fides in mixed-layer seawater (Elliott, 1984; Elliott et al., 1985a,b, 1987), and in order to evaluate the potential for seawater metal sulfide chemistry, com- pared sulfide complexation equilibria with solubility products (Elliott, 1984, Elliott et al., 1986). Dyrssen (1985) introduced dithizone as a model ligand in a study of metal speciation in anoxic waters, and has now applied the technique to an equilibrium analysis of preliminary total sulfide measurements made by Cutter and coworkers in the open Atlantic {Cutter and Oatts, 1987; Krahforst and Cutter, 1987; Dyrssen, 1988). In the present work, these sulfide free energy

0304-4203/88/$03.50 © 1988 Elsevier Science Publishers B.V.

204

comparisons are cross-checked and some general limitations of the method are explored.

LFE comparisons are most useful when data are sufficient for correlation as opposed to interpolation. At the trace sulfide levels now calculated for and measured in oxic seawater, mass action suggests that complexes involving sin- gle sulfide ligands may be dominant. There is currently only one data point available for such species in the + 2 oxidation state. Dyrssen (1988) circum- vents this problem by applying free energy slopes from larger sulfide complexes both within and across valences. Here, evidence is presented to suggest that slope may not be independent of the number of sulfide tigands and two alter- native strategies are offered. First, it is noted that some single sulfide constants can be obtained by free energy interpolation between the + 2 and + 1 states. A linear correlation between bisulfide formation constants and the analogous hydroxide values may also provide significant limits. In the final section, the metal-sulfide chemistry of oxic seawater is discussed in the light of LFE results.

METAL-SULFIDE COMPLEXATION

The generalized reaction

Mm+ +sS2-+hH+-~MS~Hr~ -2s+h (Ksh) (1)

encompasses many of the sulfide complexes which have been studied in the laboratory. Because the bisulfide ion SH - is the major unbound form of the hydrogen sulfides at pH 8, it is often convenient for oceanographic purposes to focus on the subset s = h, and the bisulfide formation processes

M m+ + n S H - ~ M ( S H ) ~ ' .... (K sH ) (2)

where n = s = h. The species s # h are then treated as acid-base conjugates. The non-parenthetical entries in the first four columns of Table i give a fairly com- prehensive collection of the bisulfide formation constants K sH (eq. (2) ) which can be applied at oceanographic temperatures. Nriagu (1971) measured for- mation for Pb (SH)~- at 90 ° C, and it may be reasonable to extrapolate to 25 ° C via the Gibbs-Helmhol tz relation, but this is not essential for our purposes here. We will focus on the + 2 oxidation state because its sulfide data are the most complete. Note, however, that even among the divalent metals, n = 2 con- stants are available only for Cd and Hg, and n = 1 only for Cd. LFE correlation is therefore impossible for the lower molecular weight complexes likely to dom- inate at trace sulfide levels, and even interpolation is excluded for n = 1.

COMPARISON WITH K~p

One method for estimating unknown bisulfide formation constants involves comparison of free energies in eq. (2) with values for the reaction to solid sulfide (Elliott, 1984; Elliott et al., 1986)

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TABLE1

Formation constants for bisulfide complexes (eq. (2)), with reciprocal solubility products (eq. (4)) and dithizone extraction constants (eq. (5) ). Values in parentheses are estimated with slope 0.96 interpolated from an n = 2 dithizone comparison, and intercept established by the measured Cd 2 + constant. All dithizone and Cd equilibria taken from Dyrssen (1988). Others are from Mar- tell and Smith (1976) or Barnes and Czamanske (1969) (BC). The data apply to a temperature of 25°C and an ionic strength of 1.0 in most cases, otherwise 0.0 or 3.0. No activity coefficient adjustments have been attempted

Metal Log K sH Ka2 Log Ke. ion Log

n = 1 2 3 4

Cu 2+ (14.2) (21.6) (23.8) 23.7(BC) 22.3 10 Pd 2+ (45.4) (52.8) (55.0) 42.5 Zn 2+ 14.2 (BC) 10.6 2.0 Cd 2 + 6.4 13.8 16.0 18.4 12.0 1.9 Hg 2 + (30.3) 37.7 40.3 (BC) 37.2 26.8 Ag + 13.3 17.2 7.6 T1 + 2.3

M 2+ + S2--,MSs (K~p I ) (3)

In order to facilitate an analogy drawn with solid oxides in a subsequent sec- tion, we add the second acid dissociation for H2S (Ka2), and rewrite as

M2+ + S H - - , M S ~ + H + (Ka2/Ksp) (4)

Values for log KaJK, p are also listed in Table 1, and logarithms for K sH are plotted against them in Fig. 1. Unknown formation constants can be interpo- lated for n = 2-4, with the relationships for 2 and 3 suggesting a slope of ~ 1. The formula for a line connecting the Hg and Cd n = 2 points is

] I I I

40 *3

::: 30 / C

4 I S J

0 or 20 4// / 0 __I 3 "3 /

I0

0 i i i i I o 20 50 40

L o g IO (Ka2 /Ksp)

Fig. 1. Free energy comparison of bisulfide complex formation constants measured for the divalent metals (eq. (2)) with reciprocal solubility products (eq. (4)). Integers in the field are n values (eq. (2) ). The solid line connects the n = 2 points, and the broken line extends their slope to n = 1.

206

log ~._~H =0.95 log (K,~2/Ksp) + 2.4

Cu will be highlighted in the remainder of the paper because it is the most, abundant of the strong sulfide interactors. The solubility product comparison gives log K sH for Cu of 23.6.

COMPARISON WITH DITHIZONE

In a recent investigation of speciation in deep Baltic anoxic/oxic transition zones, Dyrssen (1985) chose dithizone, an analytical reagent popular in the 1960s for determination of trace metals by extraction (Irving, 1977 ), as a model ligand for sulfide complexation. Equilibrium constants for the dithizone reaction

M 2+ + 2HDz(CC14 )--~ MDz2 (CC14) + 2 H + (Kex) (5)

where Kex is the extraction constant, are also given in Table I (Dyrssen, 1988). For n = 2 in this model, interpolation between Cd and Hg gives log K sH = 0.96 log Kex+ 12.0, and a Cu value of 21.6. On the one hand, reasonable agreement between the LFE techniques on Cu K ss lends credibility to both estimates. On the other, a two order of magnitude spread serves as a reminder that LFE estimates carry inherent uncertainties. The n=2 formation constants also compare favorably for Zn.

CONSTANT SLOPE ASSUMPTION

Dyrssen (1985) restricted LFE estimates to the n = 2 complexes, apparently ignoring larger species in anticipation of mass action effects and without men- tion of the single sulfides (n = 1 ). Elliott (1984)and coworkers (Eltiott et al., 1986, 1987 ) pointed out that n = 1 could dominate at the trace levels expected in the open sea and Dyrssen (1988) circumvented the lack of n = 1 data for the + 2 metals by extending the n-- 2 dithizone slope to the sole CdSH + data point. An equivalent procedure is shown for the solubility product comparison as the broken line in Fig. 1. The assumption is certainly valid as a means of generating preliminary estimates. In some related systems, however, it can be shown that the slopes of analogous correlation lines are not independent of n. In Table 2, for example, are listed equilibrium constants for the hydroxide analogs of eq. (2)

M2+ + n O H - - ~ M ( O H ) ~ -n (K °n ) (6)

and fbr the reaction to solid oxide

M2+ +OH--~MO,~ + H + (Kr, o ) (7)

Plotted together (Fig. 2), the hydroxide and oxide data behave in a manner

207

TABLE 2

Formation constants for hydroxide complexes (eq. (6); Martell and Smith, 1976) and reaction to solid oxide (eq. (7); Latimer, 1952 ). The n -- 1-4 data apply to a temperature of 25 o C and an ionic strength of 1.0 in most cases, otherwise 0.0 or 3.0

Metal Log K °H Log

ion Kmo n = 1 2 3 4

Mn 2 + 3.0 7.7 - 3.8 Co 2+ 3.9 8.5 9.7 10.2 - 1.0 Ni 2+ 3.8 8 11 1.9 Cu 2+ 6.3 12.8 14.5 15.6 6.1 Pd 2 + 13.0 25.8 16.4 Zn 2+ 5.0 8.3 13.7 18.0 2.4 Cd 2+ 4.1 7.7 10.3 12.0 - 1.8 Hg 2+ 10.1 21.1 20.9 11.6 Sn 2+ 10.4 13.0 Pb 2+ 6.3 10.9 13.7 1.3

very similar to the sulfides for n > 2, but describe a distinctly different corre- lation for n = 1. Least squares best fits to the n-- 2 and n = 1 points are shown, with slopes of 1.0 _+ 0.1 and 0.5 + 0.05. The same pat tern is evident for corre- lations of chloride versus hydroxide formation. In view of these results it ap- pears that sulfide equilibria generated in the constant slope assumption should be treated with caution. With dithizone as model ligand, and CdSH + alone determining the intercept, log K sH ----0.96 log Kex + 4.6. It may be noted from Table 1 for purposes of later comparison, log K s n - - 14.2 and 30.3 for Cu and Hg by this reasoning.

25

2O Oc

~ J5 0

r i i I ] J . 2

4 ,// 4 , 2 . .3 / . ~

3.,'~ I ,. ",

• 4 . 2 . ~

. , 2 : ' " ~ '

. ] I I I -5 o 5 IO 15

Logto K~o

Fig. 2. Linear free energy correlation of hydroxide complex formation (eq. (6)) with reaction to solid oxide (eq. (7)) for the + 2 metals. Integers in the field are n values (eq. (6)) . The solid and broken lines are least squares best fits to the n = 2 and n = 1 data.

208

ALTERNATIVE n = 1 ESTIMATES

Figure 2 indicates that it might be fruitful to explore n = 1 sulfide LFE re- lationships with larger basis sets. For example, data are sufficient for a cross- valence interpolation between Cd and Ag, giving log K sn = 1.21 log Kex + 4.1 against dithizone. Extrapolation is required to reach Cu, but the value K sH = 16.2 lends some support to the constant slope evaluation. Elliott (1984) and coworkers (Elliott et al., 1986) proposed correlation of the analogous hy- droxide and bisulfide formation constants (eqs. (2) and (6)) as a route to K sH. Results for actual bisulfide formation measurements are shown in Fig. 3, and seem to describe a single correlation line independent of n. Regression gives the formula log K sH = 1.9 log Kn °H - 2.7, with a correlation coefficient of 0.98. Several workers (e.g.D. Dyrssen, personal communication, 1987) have since noted that the relationship does not include some of the strongly hydro- lyzed metals, an example being Zn. The dithizone technique, however, can be used to validate the correlation for those species generally classified as 'soft' and for Cu, which is the strongest sulfide interactor within the transition se- ries. Following the example of Dyrssen (1988), the constant slope assumption gives log KSH=0.96 log Kex+14.2 for n = 3 with the intercept set by C d ( S H ) ~ . Similar results are obtained on interpolation from Cd or Zn to Hg. The available dithizone n-- 1-3 points for Cu and for the 'soft' metals Hg and Pd are listed in Table 1 and also included in Fig. 3. The n = 2 and 3 data are colinear with formation constant measurements, but n = 1 may form an alter-

5 0

z c 4 0

o 30

o -J 2 0

I0

! !

a, • Meosured o Dithizone ° 2 o / / /

,

I0 20

Loglo K°n H

Fig. 3. Free energy correlation of bisulfide and hydroxide formation constants (eqs. (2) and (6) ) for complexes of like n. Integers in the field are n values. Solid circles represent the bisulfide complexation measurements, and the line is a regression through them. For open circles, K sH has been estimated assuming a slope independent of n in the di*~hizone technique (v~ms given in parentheses in Table 1 ). The open triangle represents zinc.

209

native correlation curve. The measurement line provides rough lower limits on K sH for Cu and Hg of 10 and 15, respectively.

ACIDITY OF BISULFIDE COMPLEXES

Where pKa values are known for the M(SH)n ~-n species, they lie near or well below the oceanographic pH, so that conjugate bases will be dominant sulfides in the sea. A well established set of constants are pKal and pK, e for Hg(SH)2, at 6.2 and 8.3. Dyrssen (1988) has recently realized that a pH-in- dependent minimum solubility of ~ 10 -7 M observed for Cd (Ste. Marie et al., 1964) is probably best explained by the neutral single sulfide complex CdS °, product of the general formation reaction

M2+ + S2---,MS ° (8)

Taken together with the solubility product log K~ = - 25 (Dyrssen, 1988) and the Table 1 K sH, this intrinsic solubility yields a pK~ of ~ 2 for CdSH + dis- sociation. Acidities for other + 2 metal complexes must currently be estimated by assuming equality with the mercury values in the n = 2 case, or Cd for n = 1 (Dyrssen, 1985, 1988). It should be stressed, however, that orders of magnitude of uncertainty may be involved in the approximation.

APPLICATION TO SEAWATER

Elliott (1984) and coworkers (Elliott et al., 1985a,b, 1986, 1987) identified carbonyl sulfide hydrolysis as a quantifiable source of the hydrogen sulfides in mixed layer seawater, and coupled hydrolysis with rate constants for direct oxidation by molecular oxygen (Millero et al., 1987) to conclude that non- thermodynamic bisulfide ion levels could reasonably be expected to lie within the pico- to nanomolar range. They also pointed out that picomolar sulfide is sufficient for complete conversion of Cu, Hg, and perhaps other sulfide-inter- active metals to sulfide complexes in equilibrium models. In this section, the role of LFE estimates in such calculations is illustrated.

Oceanic metals simultaneously complex sulfide and better known ligands (L) in reactions analogous to ( 1 ) or (2) to form, schematically, MSsHhLz with equilibrium constants which we will call K,m. If s = 0, the complex is a previous standard speciation, and if l= 0, it is a pure sulfide. Mixed species with both s and I non-zero are also likely. In equilibrium models of seawater chemistry, the total dissolved inorganic concentration M t is conserved for any one metal.

M t = M m + ( l + ~ Ksh~(se-)8(H+ )h(L)Z)=Mm+ (I + E) \ s,h,l

(9)

2 1 0

)< ID

. _ q ~ . I

~ o t0 -z

;.,c t0 -4.

. . . . ~ CuS °

C u S H S ~ X x X x \ \ \ \

I " ~ 1 - I 0 -15

Logfo (SH-)

Fig. 4. Fraction of total dissolved Cu speciating as the CuSHS- and CuS ° complexes in an equi- librium model of seawater complexation chemistry, as a function of bisulfide ion concentration (M).

where unsubscripted ~ refers to summation over all possible s, h, and 1. The fraction harbored in sulfide complexes is then ( ~ ) / ( 1 + Z ). The sulfide con-

s ¢ 0

centration at which this fraction becomes one half, obtained by solving ( ~ ) = (1 + ~ ), gives a feeling for the lowest levels of importance in specia-

s ¢ 0 s = 0

tion. We will call this the half speciation point, and calculate it in terms of the bisulfide ion.

The carbonate CuC03 is traditionally assigned as the dominant inorganic Cu complex in open seawater, with a value ( ~ ~ 20) (Dyrssen, 1988). Taking

s = 0

a central figure tog K sn = 1022.5 from the various LFE methods, and the Hg pK,1 andpKaz, log (1+ ~ ) = l . 3 , a n d l o g ( ~ ) =24.8+21og ( S H - ). The n = 2

s = 0 s ¢ 0

half speciation point is ( S H - ) = 1-2 pM. The fraction of total Cu present as CuSHS- is plotted as a function of bisulfide in Fig. 4. For the related n = 1 calculation, we adopt the rough range log K sn =10-15, and pKa---2. Log ( ~ ) = 16.2 +log ( S H - ) to 21.2+1og ( S H - ) , giving a half speciation range of

s ¢ 0

femtomolar or less. The fraction of total Cu speciating as CuS ° is again shown in Fig. 4. At equilibrium in this system, Cu speciates as sulfide complex to well below pM. Evidence is accumulating that organic ligands exist in seawater which bind Cu with log (1+ ~ ) up to ~ 4 (e.g. Buckley and van den Berg,

s = 0

1986). If the sulfides equilibrate with such Cu organic complexes, the half spe- ciation bisulfide concentrations would rise slightly to < 40 pM, and ~< pM.

The same conclusions apply to Hg, but uncertainties in the LFE methods leave it unclear whether the single or double sulfide complexes are more im- portant. Log ( 1 + ~ ) = 14.2 for the complexes HgCl~- c (Martell a n d Smith,

~=0 1976), and for n=2, log ( Z ) = 4 0 . 0 + 2 log ( S H - ) , so that at sulfide half spe-

s ~ 0

ciation ( S H - ) -0 .1 pM. For the rough log K sn range 15-30, values are log

211

(SH - ) = - 7 to - 22. Formation constants lying nearer the CdSH + point in Fig. 1 are better constrained. It is interesting to note that half speciation for Cd is also in the picomolar range. The currently accepted predominant species are CdCl~ -c with log (1+ • )=1.6 (Martell and Smith, 1976), and log

s = 0

( ~ ) = 12.6+ log (SH- ) . This indicates that transition metals with similar sO=0

binding properties, such as Zn, for example, are candidates for sulfide specia- tion as well.

Cutter and Oatts (1987) and Krahforst and Cutter (1987) have recently reported measurements of 0.1-1.0 nM of total dissolved sulfide in open Atlan- tic mixed layer seawater. Because Cu is a strong sulfide interactor, and its concentrations are near the Cutter group totals, it is an element likely to exert a controlling influence on sulfide speciation. Dyrssen (1988) has inserted the Cutter levels into a full-scale equilibrium complexation model with total dis- solved Cu concentrations set at 1.2 nM, an average for the western Atlantic (Bruland, 1983). With copper present in excess, total sulfide is t i trated and converted quantitatively to CuS °. We have simulated the more elaborate model by focusing on the likely controlling reaction, the CuS ° formation (eq. (8)), and within this system, we vary total sulfide. The results are illustrated in Fig. 5. At the measured total levels, free sulfide is maintained below femtomolar. Equilibrium dictates significant sulfide speciation for Cu, and perhaps also Hg.

Current understanding of the fraction of Cu kinetically accessible to inor- ganic complexation does not rule out an excess sulfide situation. For example, oceanic Cu organic complexes seem to have conditional stability constants of 1012 or greater (Buckley and van den Berg, 1986; J. Moffett, personal com- munications, 1987) and may be quite non-labile (D. Dyrssen, personal com- munication, 1987). Furthermore, Moffett and Zika (1987a,b) have shown that a substantial portion of dissolved Cu is present as photochemically-supported Cu (I), and so is unavailable to CuS ° formation. It is not clear at present whether total dissolved Cu measurements distinguish the two oxidation states. Excess

o u

0 ~ -20

I I

cuZ++sZ--..CuS o

I

I I 0.5 I.O

Total Dissolved Sulfide, nM

Fig. 5. Bisulfide ion concentration (M) in equil ibrium with I n M of to ta l dissolved Cu for reaction (8) as a function of total sulfide. The solid curve corresponds to a rough lower l imit for n = 1 binding (log K sH = 10), the broken curve to an upper l imit (log K sH = 15).

212

sulfide produces the right-hand third of the titration curve in Fig. 5, with free, non-metal bound concentrations sufficient for sulfide conversion of metals less interactive than Cu and Hg.

Models of the remote marine troposphere are currently interpreting H2S vapor measurements made over the open ocean as evidence for a sea-to-air H2S flux (Toon et al., 1987). This would require total sulfide in excess of Cu, and bisulfide ion concentrations of several hundred pM (Elliott et al., 1987). Cooper and Saltzman (1987) have shown, however, that an OCS interference has been operative in previous gas phase H2S measurements, casting doubt on all the available data. The direction of H2S flux at the sea-air interface remains to be established.

CONCLUSIONS

Several lines of evidence now indicate that the hydrogen sulfides are viable ligands in mixed layer seawater, and possess a rich metal chemistry there. Metal- sulfide binding constants are scarce and can be estimated from a variety of linear free energy perspectives, but LFE techniques contain inherent uncer- tainties, sometimes many orders of magnitude in breadth. Measurements of formation for some of the single sulfide complexes MSH ÷ would eliminate much of this problem. A logical initial system would be Hg because a knowledge of its K sH would permit preliminary estimates of n = 1 constants for interme- diate sulfide binders through interpolation with Cd.

ACKNOWLEDGMENTS

The author thanks D. Dyrssen, J. Moffett, G. Cutter and C. Krahforst for helpful discussions and preprints. This work was supported by the U,S. De- par tment of Energy Grant No. AT03-76ER-70126.

REFERENCES

Barnes, H.L. and Czamanske, G.K., 1969. Solubilities and transport of ore minerals. In: H.L. Barnes (Editor), The Geochemistry of Hydrothermal Fluids. Academic Press, New York, pp. 334-378.

Bruland, K.W., 1983. Trace elements in seawater. In: J.P. Riley and R. Chester {Editors), Chem- ical Oceanography, Vol. 8. Academic Press, London, pp. 157-220.

Buckley, P.J.M. and van den Berg, C.M.G., 1986. Copper complexation profiles in the Atlantic Ocean. Mar. Chem., 19: 281-296.

Cooper, D.J. and Saltzman, E.S., 1987. Uptake of carbonyl sulfide by silver nitrate impregnated filters" implications for the measurements of low level atmospheric H2S. Geophys. Res: Lett., 14: 206-209.

Cutter, G.A. and Oatts, J.J., 1987, Determination of dissolved sulfide and sedimentary sulfur speciation using gas chromatography-photoionization detection, Anal. Chem., 59:717-721.

213

Dyrssen, D., 1985. Metal complex formation in sulphidic seawater. Mar. Chem., 15: 285-293. Dyrssen, D., 1988. Sulfide complexation in surface seawater. Mar. Chem., 24' 143-153. Elliott, S., 1984. The chemistry of some atmospheric gases in the ocean. Ph.D. Thesis, University

of California, Irvine. Elliott, S., Lu, E. and Rowland, F.S., 1985a. The oceanic sink for carbonyl sulfide (abstr.), paper

presented at the IAMAP/IAPSO Joint Assem., Honolulu, HI. Elliott, S., Lu, E. and Rowland, F.S., 1985b. Marine tropospheric H2S and OCS imply significant

levels of the sulfide anions in seawater (abstr.). EOS Trans. Am. Geophys. Union, 66(51): 1309.

Elliott, S., Lu, E. and Rowland, F.S., 1986. Speciation of several soft and transition metals as sulfide complexes in open surface seawater (abstr.). EOS Trans. Am. Geophys. Union, 67 (16): 295.

Elliott, S., Lu, E. and Rowland, F.S., 1987. Carbonyl sulfide hydrolysis as a source of hydrogen sulfide in open ocean seawater. Geophys. Res. Lett., 14: 131-134.

Irving, H.M.N.H., 1977. "Dithizone" Analytical Science Monogr., No. 5. The Chem. Soc., Lon- don, 106 pp.

Krahforst, C.F. and Cutter, G.A., 1987. Dissolved hydrogen sulfide in surface waters of the western Atlantic (abstr.). EOS Trans. Am. Geophys. Union, 68(16): 339.

Latimer, W.M., 1952. Oxidation Potentials. Prentice-HaU, Englewood Cliffs, NJ. Martell, A.E. and Smith, R.M., 1976. Critical Stability Constants, Vol. 4. Plenum, New York, 257

pp. Millero, F.J., Hubinger, S., Fernandez, M. and Garnett, S., 1987. Oxidation of H2S in seawater as

a function of temperature, pH and ionic strength. Environ. Sci. Technol., 21: 439-443. Moffett, J.W. and Zika, R.G., 1987a. Photochemistry of copper complexes in seawater. In: R. Zika

(Editor), Photochemistry of Environmental Aquatic Systems. Am. Chem. Soc. Moffett, J.W. and Zika, R.G., 1987b. Reaction kinetics of hydrogen peroxide with copper and iron

in seawater. Environ. Sci. Technol., 21: 804-810. Nriagu, J.O., 1971. Studies in the system PbS-NaCI-H2S-H20: stability of lead(II) thio com-

plexes at 90°C. Chem. Geol., 8: 299-310. Ste. Marie, J., Torma, A.E. and Gubeli, A.O., 1964. The stability of thiocomplexes and solubility

products of metal sulphides I. Cadmium sulphide. Can. J. Chem., 42: 662-668. Toon, O.B., Kasting, J.F., Turco, R.P. and Liu, M.S., 1987. The sulfur cycle in the marine atmos-

phere. J. Geophys. Res., 92 (D1): 943-963.