NOTES

Limnol. Oceanogr., 33(2), 1988, 269-274 0 1988, by the American Society of Limnology and Oceanography, Inc.

The dissociation of hydrogen sulfide in seawater’

Abstract-The pK,* for the dissociation of H,S has been measured in artificial seawater as a func- tion of salinity (S = 5-40) and temperature (Y- 25°C). Results have been fitted to an equation of the form

pK,* = pK, + AS” + BS

where pK, is the value at infinite dilution given by

pK, = -98.080 + 5,765.4/T + 15.0455 In T A = -0.1570 B = 0.0135

(V = 0.033 in pK*, mol kg-’ seawater). The results at S = 35 show a 0.08 shift in pK* from the results in the literature, as was also found in 0.7 m NaCl solutions. The results are in good agree- ment with the apparent pK,’ results when ad- justed to the same pH scale. Using values for the apparent activity coefficient of the proton, we combined our results with those results to derive the consensus values of

A = -0.1498 B = 0.0119

(u = 0.028 in pK,*).

In our studies (Hershey et al. 1988) of the pK,* for the dissociation of H,S in NaCl solutions, we found that our measured re- sults at 0.7 m were 0.08 lower than the re- sults of Almgren et al. (1976) at 5” and 25°C. The published results for the pK,* in sea- water adjusted to the same pH scale also show that the results of Almgren et al. (1976) are 0.08 higher than the results of Gold- haber and Kaplan (1975) and Savenko ( 1977). We were unable to account for these differences in a simple manner, so we de- cided to remeasure the pK,* of H2S in sea- water as a function of salinity and temper- ature.

Values of pK,* for the ionization of H,S in seawater media were determined from

l This work was supported by the Office of Naval Research (NO001 4-87-G-O 116) and the Oceanograph- ic Section (86-00284) of the National Science Foun- dation.

potentiometric titrations of dilute NaHS so- lutions (0.005 m) with 1.0 M HCl. Titra- tions were made in a 250-cm3, water-jack- eted closed cell used in previous studies (Thurmond and Miller0 19 82). HCl was de- livered by an automatic buret (Metrohm E535). The titration system is fully auto- matic and controlled with a computer (Ap- ple II). The electromotive force of the glass (Corning) and double-junction reference electrode (Orion) was measured with a pH meter (Metrohm). The outer filling solution of the reference electrode was 3 M KCl. The Ag, AgCl reference electrode used in our earlier work (Thurmond and Miller0 1982; Miller0 and Thurmond 1983; Hershey et al. 1986) could not be used due to interference by HS-.

The emf of the electrode system is related to the total concentration of the proton [H’] T bY

E = E* - (RT/F)ln[H+], (1)

where E* is an apparent standard potential in the ionic media (and includes liquid junc- tion effects). Values of E* and the pK1* = -log K,*,

K,” = [H+],- [HS-],/[H,S], (2)

were determined from the titrations with a nonlinear, least-squares program (Arena et al. 1979). Values of K,* for the ionization of water were obtained from the equations of Miller0 (1979, 198 1) which are based on the experimental work of Dyrssen and Hansson (197 3), Culberson and Pytkowicz ( 197 3) and Dickson and Riley ( 19 79).

Seawater solutions were made by weight with reagent grade NaCl, Na,SO,, MgCl,, and CaCl, according to the composition of artificial seawater from Khoo et al. (1977). Concentrations of the MgCl, and CaCl, stock solutions were determined from density measurements. Solutions of NaHS were

269

270 ivotcs

Table 1. The pK,* for the dissociation of H,S in seawater.

Temp (“C) Sallnltl (S? pK,* (mol kg ’ SW)

5 5.0

10.0 20.0

35.0

15

25

40.0 1.0 5.2 5.5 9.6

19.5

33.0 34.2 38.9 39.2

5.0 10.0

20.2

30.0

35.0

40.0

7.012 7.014 6.982 6.905 6.949 6.897 6.872 6.834 6.969 6.784 6.793 6.756 6.740 6.688 6.654 6.657 6.675 6.627 6.676 6.648 6.648 6.596 6.589 6.534 6.536 6.549 6.554 6.556 6.508

made by dissolving weighed crystals of NazS. 9H,O in degassed, ion-exchanged water. HS concentrations were determined by iodometric titrations. Stock solutions of NaHS were kept under nitrogen to prevent oxidation. Diluted seawater solutions were prepared by weight with ion-exchanged water.

Temperature of the titration cell was con- trolled to +O.O2”C with a Forma bath. Tem- peratures were set with a quartz crystal ther- mometer calibrated with a Pt resistance thermometer and G-2 Mueller bridge.

Values of pK, * for the dissociation of H,S obtained in this study at various salinities and temperatures are given in Table 1. The effect of salinity is shown in Fig. 1. As with other acids (Miller0 1979, 198 l), the salin- ity dependence can be accounted for with an equation of the form

pK,* = pK, + AS” + BS (3) where pK, is the dissociation constant at

631 01234567

G Fig. 1. Values of pK,* for the dissociation of H,S

in seawater as a function of salinity.

infinite dilution, and *4 and B are temper- ature-dependent parameters.

The effect of temperature on the pK,* for HS is shown in Fig. 2. The heats of ioni- zation, AH* = R7”-(d In K*IdT), are nearly independent of salinity. Values of AH* = 6.6kO.3, 6.4kO.2, and 6.2kO.2 kcal mol l were obtained at S = 0, 10, and 40. The value at S = 35 (AH* = 6.2-t0.2 kcal mol-l) can be compared to AH* = 7.4kO.1, 6.O-tO. 1, and 6.1 +O. 1 kcal mol-’ obtained by Almgren et al. (1976) Goldhaber and Kaplan ( 19 7 5) and Savenko ( 19 7 7) over the same temperature range. The agreement is quite reasonable. The large differences be- tween the various studies and the work of Almgren et al. cannot be easily explained. Since they made their measurements only at 5” and 25”C, part of the difference could be due to the limited temperature range studied. The effect of temperature on the pK, in pure water from 0” to 300°C can be obtained from (Hershey et al. 1988)

pK, = -98.080 + 5,765.4/T + 15.0455 In T (4)

(a = 0.05). The higher temperature data are derived from thermodynamic data (Barber0 et al. 1982) and the low temperature data (O”-45°C) are based on new measurements (Hershey et al. 1988). The differentiation of this equation with respect to temperature yields a AH, for the dissociation that is tem- perature-dependent (AH, = 7.2 and 5.9 kcal mol-l at 5” and 25°C).

Since the thermodynamic values are re-

Notes 271

631 3.3 34 35 3.6

( VT)103 Fig. 2. Values of pK,* for the dissociation of H,S

in seawater as a function of temperature (T”K).

liably known, we have determined the tem- perature coefficients of Eq. 3 by subtracting the value of pK, at a given temperature cal- culated from Eq. 4. The least-squares fit of these differences (pK,* - pK,) gives A = -0.1570 and B = 0.0135 (g = 0.033 in PC).

The measurements of Savenko (1977) and Goldhaber and Kaplan (1975) were made with the National Bureau of Standards (NBS) pH scale (Bates 1973) where

K,’ = a,‘[HS-],/[H,S],. (5) The measurements made by Almgren et al. (1976) and for our study were made on the total proton scale (Hansson 1973):

K1* = [H+],.[HS-],/[H,S],. (6)

The two pH scales are related by (Dickson 1984)

4I’ = LIw+17- (7) wheref, is the apparent activity coefficient of the proton. This value includes effects of liquid junctions, the definition of the NBS scale, and the activity coefficient of the pro- ton (Dickson 1984). Since different refer- ence electrodes give different values of&, it is not possible to determine its value from first principles.

A comparison of our results for pK,* with the earlier measurements of Almgren et al. (1976) is shown in Table 2. At 5°C our sea- water results are O.lO+O.O 1 lower than theirs; at 25°C our results are 0.07 kO.02 lower. Measurements made in NaCl solu- tions between 0.4 and 0.7 m also show a similar difference (Table 2). We have no logical explanation for this offset. Since the

Table 2. Comparison of our pK,* results with the values of Almgren et al. 1976.

5 5 7.043 7.112 10 6.964 7.046 20 6.894 6.990 30 6.871 6.974 35 6.870 6.979 40 6.873 6.982

25 2.5

10 20 30 35 40

5 m (NaCl)

0.4 0.7

25 0.4 6.655 6.734 0.7 6.675 6.729

6.766 6.805 6.697 6.739 6.619 6.698 6.548 6.644 6.526 6.617 6.524 6.60 1 6.527 6.596

7.015 7.138 6.989 7.078

0.069 0.082 0.096 0.103 0.109 0.109

Avg 0.095 kO.0 1

0.039 0.042 0.079 0.096 0.09 1 0.077 0.069

Avg 0.07iO.02

0.123 0.089

Avg O.llkO.01

0.079 0.054

Avg 0.07 kO.0 1

‘b Equations 3 and 4 wth A = ~ 0.1570 and B = 0.0135

272 Notes

0 05 0

:..;:‘i. 0 5 IO 15 20 25 30

“C 005

0

0 -0.05

0 IO 20 30 40 50 S

Fig. 3. Residuals of errors for the fit of pK,’ with the data of Goldhaber and Kaplan 1975 (0) and Sa- venko 1977 (0).

differences show up as an offset in both sea- water and NaCl, they could arise from a systematic error in the electrode system or the method of calculating pK1*.

A comparison of the apparent constants for the pK,’ of H,S was made with the fitted equation of Miller0 (1986~~) based on the work of Goldhaber and Kaplan (1975).

These comparisons demonstrate that the pK, results of the two studies agree to kO.02 in pK with a maximum error of 0.05. The combined data for pK,’ have been fitted to Eq. 3 where A = 0.0057 - 19.9WTand B = 0.0028 (c = 0.019 in pK,‘). Examination of the residuals between the measured and cal- culated results shown in Fig. 3 demonstrates that the two studies are in good agreement and that the electrode systems gave similar values offH. The earlier fit (Miller0 1986~1) of the data of Goldhaber and Kaplan (1975) is in agreement with the combined fit to within the experimental error (0.02 in pK,‘). The lower standard error for the fit of pK,’ (compared to our pK,* data) is probably related to the experimental techniques of Goldhaber and Kaplan. They determined pK,’ with a spectroscopic method that re- sponds to changes in HS- and is not strongly affected by various polysulfide species that may influence the pH titration techniques we used.

A comparison of our pK,* values with the apparent pK,’ calculated from Eq. 3 and 4 is shown in Table 3. The differences in pK,* - pK,’ = ApK can be related to the apparent activity coefficient of the proton

fH= 10 T (APK). (8)

The values offH shown in Table 3 are com- pared to measurements made by Mehrbach et al. (1973), Culberson and Pytkowicz

Table 3. Comparison of the pK,* and pK,’ values of H,S in seawater and estimates off”.

35

p&t fHS s Temp (“C) I 2 -A PK, This study 3 4 5

5 10 15 20 25 30 - 25 40

35 30 25 20 15 10 5

6.870 6.772 6.682 6.599 6.524 6.455 6.527 6.524 6.526 6.533 6.548 6.575 6.619 6.697

6.965 6.875 6.792 6.717 6.648 6.586 6.633 6.648 6.666 6.687 6.711 6.741 6.778 6.832

0.095 0.103 0.110 0.118 0.124 0.131 0.106 0.124 0.140 0.154 0.163 0.166 0.159 0.135

0.80 0.79 0.78 0.76 0.75 0.74 0.78 0.75 0.72 0.70 0.69 0.68 0.69 0.73

0.79 0.77 0.75 0.72 0.69 0.66 0.69 0.69 0.68 0.68 0.68 0.67 0.67 0.67

0.79 0.78 0.76 0.74 0.7 1 0.69 0.72 0.71 0.70 0.70 0.70 0.70 0.71 0.72

0.72 0.70 0.7 1 0.71 0.71 0.72 0.73 0.74

t l--Values of pK,* from Eq. 3 and 4 wth A = -0.1570 and B = 0.0135. Z-Values of pK,’ from Eq. 3 and 4 wth A = -0.0057 - 19,98/Tand B = 0.0028.

$3-Mehrbach et al. 1973; 4-Culberson and Pytkowcz 1973, 5-Mlllero 19866.

Notes 273

(1973), and Miller0 (1986b). At 25°C the values of fH are nearly independent of sa- linity. Our average results of& = 0.73 LO.04 can be compared to 0.68 -to.02 by Mehr- bath et al., 0.71 +O.Ol by Culberson and Pytkowicz, and 0.72+0.01 by Millero. The effect of temperature on the values of fH calculated in Table 3 are also in good agree- ment with the measured values of Mehr- bath et al. and Culberson and Pytkowicz.

The comparisons in Table 3 clearly dem- onstrate that our experimental pK,* meaurements are in good agreement with the results of Goldhaber and Kaplan (1975) and Savenko (1977) if adjusted to the same pH scale. To develop a consensus equation for the pK,* of H,S, we have used values of& from Mehrbach et al. (1973) Culber- son and Pytkowicz (1973), and Miller0 (19863) to convert the pK,’ data

pK,* = PK,’ + log.&,. (9)

To determine fH as a function of salinity and temperature (FK), we have fitted the experimentally derived values of Mehrbach et al., Culberson and Pytkowicz, and Mil- lero to the equation

fH = 0.739 AI 3.07 x 1o-2 s + 7.94 x 1o-5 s2 + 6.443 x 1O-5 T - 1.17 X lop4 TS (10)

(a = 0.006). It should be pointed out that the 25” and 35°C data of Mehrbach et al. have not been used because of large differ- ences (0.02-0.04) with the work of Culber- son and Pytkowicz and Millero. Equations 9 and 10 were used to adjust the results of Goldhaber and Kaplan (1975) and Savenko (1977) to the total proton pH scale. These pK,* results agree on the average with those of this study to kO.02 for Goldhaber and Kaplan and +0.03 for Savenko. These con- verted values of pK, * have been added to the measurements made in our study to de- rive a consensus equation.

The total data set consists of 72 values: 8 from Savenko ( 1977); 3 5 from Goldhaber and Kaplan (1975); 29 from this study. A least-squares fit of these data gives A = -0.1498 and B = 0.0 119 (a = 0.028 in pK,*). We feel these parameters should be

-000 4

0 5 IO 15 20 25 30

OC 0 00

A

004 - d 0 :. 2 .A A 00

A8 o A A APK; 0. .A . = &, AA

.

‘D l f3 I

. A r'O dI A. -004- A A*

0 A A

-0.00 - 4

0 IO 20 30 40

Fig. 4. Residuals of errors for the fit of pK,* with the adjusted data of Goldhaber and Kaplan 1975 (0) and Savenko 1977 (0) and the results of our study (a).

used to determine the pK,* of H2S in sea- water solutions from S = O-40 and T = O”- 35°C. The residuals shown in Fig. 4 dem- onstrate that the three studies are in good agreement when adjusted to the same pH scale.

Frank J. Millero Tinka Plese

Marino Fernandez

Rosenstiel School of Marine and Atmospheric Science

University of Miami 4600 Rickenbacker Causeway Miami, Florida 33 149

References

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Chem. 4: 289-297. ARENA, G., E. RIZZARELLI, S. SAMMARTANO, AND C.

RIGANO. 1979. A non-linear least squares ap- proach to the refinement of all parameters in- volved in acid-base titrations. Talanta 26: 1-14.

BARBERO, J.A.,K.G. MCCURDY,AND P.R. TREMAINE. 1982. Apparent molal heat capacities and vol- umes of aqueous hydrogen sulfide and sodium hy- drogen sulfide near 25°C: The temperature depen-

274 Notes

dence of H2S ionization. Can. J. Chem. 60: 1872- 1880.

BATES, R. G. 1973. Determination of pH: Theory and practice. Wiley.

CULBERSON, C., AND R. M. PYTKOWICZ. 1973. Ioni- zation of water in seawater. Mar. Chem. 1: 309- 316.

DICKSON, A. G. 1984. pH scales and proton-transfer reactions in saline media such as sea water. Geo- chim. Cosmochim. Acta 48: 2299-2308.

p, AND J. P. RILEY. 1979. The estimation of acid dissociation constants in seawater media from potentiometric titrations with strong base 1. The ionic product of water-KK,. Mar. Chem. 7: 89- 99.

DYRSSEN D., AND I. HANSSON. 1973. Ionic medium effects in seawater-a comparison of acidity con- stants of carbonic acid and boric acid in sodium chloride and synthetic seawater. Mar. Chem. 1: 137-149.

GOLDHABER M. B., AND I. R. KAPLAN. 1975. The apparent dissociation constants of hydrogen sul- fide in chloride solutions. Mar. Chem. 3: 83-104.

HANSSON, I. 1973. A new set of pH scales and stan- dard butlers for seawater. Deep-Sea Res. 20: 479- 491.

HERSHEY, J. P., M. FERNANDEZ, P. J. MILNE, AND F. J. MILLERO. 1986. The ionization of boric acid in NaCl, Na-Ca-Cl and Na-Mg-Cl solutions at 25°C. Geochim. Cosmochim. Acta 50: 137-148.

-, T. PLESE, AND F. J. MILLERO. 1988. The pK,* for the dissociation of H,S in various ionic media. Geochim. Cosmochim. Acta. In press.

KHOO, K. H., C. H. CULBERSON, AND R. G. BATES. 1977. Thermodynamics of the dissociation of ammonium ion in seawater from 5 to 40°C. J. Solut. Chem. 6: 281-290.

MEHRBACH, C., C. H. CULBERSON, J. E. HAWLEY, AND R. M. PYTKOWICZ. 1973. Measurement of the apparent dissociation constants of carbonic acid in seawater at atmospheric pressure. Limnol. Oceanogr. 18: 897-907.

MILLERO, F. J. 1979. The thermodynamics of the carbonate system in seawater. Geochim. Cosmo- chim. Acta 43: 1651-1661.

~ 198 1. The ionization of acids in estuarine waters. Geochim. Cosmochim. .4cta 45: 2085- 2089.

---. 1986~. The thermodynamics and kinetics of the hydrogen sulfide system in natural waters. Mar. Chem. 18: 121-147.

___ 1986b. The pH of estuarine waters. Limnol. Oceanogr. 31: 839-847.

-,AND V. THURMOND. 1983. The ionization of carbonic acid in Na-Mg-Cl solutions at 25°C. J. Solut. Chem. 12: 401-4 12.

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Submitted: 22 April 1987 Accepted: 21 June 1987

Revised: 15 December 1987

Limnol Oceanogr, 33(2), 1988, 274-280 0 1988,by the American Society of Limnology and Oceanograph), Inc

Laboratory studies on core sampling with application to subtidal meiobenthos collection

Abstract -Three aspects of subtidal meio- benthic sampling were examined through labo- ratory simulation experiments with Sephadex gel beads as an epibenthic meiofaunal mimic. Re- sults indicate that corer diameter (i.d. from 2.6 to 10.5 cm) does not affect sampling efficiency if slow corer penetration takes place. Disturbance in cores mimicking conditions during retrieval by a diver resulted in a bias when these cores were subsampled after collection. Such effects may also be present in larger (e.g. box) corers when a distinct flocculent layer is present. A dramatic effect on vertical profile was found when cores were preserved by fast-freezing. Cores should be of a size that can be analyzed in their entirety, subsampled as a homogenate, or subsampled with a device in place as the corer enters the sediment.

An awareness of the ecological impor- tance of meiofauna has heightened the need for adequate field sampling. Unbiased pop- ulation estimates are necessary to underpin virtually all meiofaunal studies, including those on pollution effects, community struc- ture, and ecosystem function. As a conse- quence, many sampling techniques have been used in the field to attempt quantita- tive sampling (McIntyre and Warwick 1984). Unfortunately, the proliferation of techniques has not been accompanied by a program of testing and evaluation. Only a few studies (McIntyre 197 1; Elmgren 1973;