protonation equilibria of marine dissolved organic matter

Protonation Equilibria of Marine Dissolved Organic MatterAuthor(s): Douglas L. Huizenga and Dana R. KesterSource: Limnology and Oceanography, Vol. 24, No. 1 (Jan., 1979), pp. 145-150Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2835522 .

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Limnol. Oceanogr., 24(1), 1979, 145-150 ? 1979, by the American Society of Limnology and Oceanography, Inc.

Protonation equilibria of marine dissolved organic matter'

Douglas L. Huizenga and Dana R. Kester Graduate School of Oceanography, University of Rhode Island, Kingston 02881

Abstract Dissolved organic matter isolated by activated charcoal adsorption from river, estuarine,

coastal, and open ocean waters was examined by acid-base titration in 0.7 molal NaClO4 medium. Two types of sites were present in the pH range 2-8. One set of sites with an inflection point at pH 3.5 amounted to 11 gmol of sites per milligram of organic carbon. These sites were present in all samples and showed uniform properties among the samples from different marine environments. A second set of sites with an inflection point at pH 6.5 was present in some samples and amounted to <1 ,umol of sites per milligram of organic carbon. These results provide a characterization of the acid-base equilibria of marine organic matter which is required in an assessment of possible trace metal-organic interactions in seawater.

Only 10-20% of dissolved organic matter (DOM) in seawater has been identi- fied in terms of specific molecular substances. The remaining uncharacterized fraction probably is a complex mixture of compounds which are relatively stable. DOM may play important roles in disso- lution and precipitation kinetics of solid phases, in hydrocarbon solubilization, and in trace metal speciation. The chemical interactions of dissolved organic matter with other seawater constituents must be understood before a predictive model of seawater chemistry can be made. The acid-base properties of DOM yield information on the types and numbers of pH- active functional groups present on the organic matter that determine many of the chemical properties of DOM.

In order to achieve adequate sensitiv- ity, dissolved organic matter must be extracted from seawater and concentrated before studies can be made of its acid- base properties. The isolation technique may alter properties of the dissolved organic material. Yields may vary for different organic compounds so that the resulting isolate may not be representative of the original DOM, or conditions dur- ing isolation may cause chemical altera- tion of the organic matter functional groups. Amberlite XAD-2 resin has been used to isolate humiclike materials from freshwater and seawater (Mantoura and

1 This work was supported by the Office of Naval Research (contract N00014-76-C-0226).

Riley 1975; Stuermer and Harvey 1974). When elution conditions are relatively mild, overall yields of seawater DOM are generally only 10% (Stuermer and Har- vey 1977). Isolation with activated charcoal adsorption imposes more severe elution conditions but gives higher yields, ranging from 30-70% (Jeffrey 1969; Kerr and Quinn 1975). Ultrafiltration followed by dialysis has been used to concentrate and desalt organic matter from seawater (Wilson and Kinney 1977), but the low molecular weight materials are lost with this technique.

The acid-base behavior of seawater DOM has been little studied. Acid titration of a Sargasso Sea fulvic acid isolated with XAD-2 resin by Stuermer and Har- vey (1974) showed the presence of 4 umol of sites mg-' org C. The dissociation of hydrogen ions from organic acid groups occurred over a wide pH range with most of the sites dissociated above pH 5 (Stuermer 1975). Organic matter concentrated from the Gulf of Alaska by ultrafiltration and desalted by dialysis showed a carboxylic intrinsic pK of 3.9 (Wilson and Kinney 1977). Gamble (1970, 1972) presented a theory of fulvic acid dissociation in which the members of a group type (sites that are chemically similar and exhibit one inflection point on the titration curve) were not assumed to be identical. Variation in the simple acid dissociation constant was considered to arise from both site differences and charge interaction. The charge interaction theory

145



146 Huizenga and Kester

Table 1. Collection information for organic matter samples (more complete information can be found in Kerr 1977).

Depth Sample Location (m)

1 Narragansett Bay A 41-34'N,71024'W 16 Mar 76 surface 2 Narragansett Bay B 41?34'N,71?24'W 13 Apr 76 surface 3 Mixed river water * 14 Feb 77 4 Block Island Sound water, surface f 9 Mar 77 surface 5 Sargasso Sea, intermediate 36?35'N,66?03'W 14 May 74 1,000 6 Equatorial Atlantic, intermediate 6?24'N,25?36'W 12 Jun 76 1,500 7 Coastal equatorial Atl., surface 4?18'N, 7?00'W 5 Jun 76 25 8 Equatorial Atlantic, surface 2?33'S, 7?00'W 7 Jun 76 25 9 Coastal Peru, low productivity 15?20'S,78?30'W 13 Apr 75 25-50

* Sample was a mixture of water from Blackstone, Pawtuxet, and Taunton Rivers. f Sample collected 5.5 km south of Point Judith, R.I.

developed by Wilson and Kinney (1977) for humic materials considers all sites of a given type to be identical, so that titration curve broadness is caused by charging effects.

A set of sites about a single inflection point on a titration curve for organic matter often exhibits dissociation over a wider pH range than that expected for a simple acid. The simple acid dissociation constant, K, is defined by:

K = [H][A]/[HA]. (1)

For an organic matter titration curve, K varies with the degree of dissociation. The broadness of the curves may indicate either the presence of different sites with similar pK values or an interaction among adjacent sites of a single type. For ex- ample, there may be a variety of carboxylic acid groups associated with different structures in the organic matter, or the electrostatic charge resulting from one dissociated carboxylic acid may suppress the ionization of neighboring carboxylic acid groups.

With a synthetic polymeric acid, the simple acid dissociation constant, K, varies as charge builds up on the polymer, and a more complicated equilibrium relationship, the modified Henderson-Has- selbach relationship, is obtained due to charging effects (Gregor et al. 1955):

Ka = [H]([A]/[HA])n, (2) pH = pKa + n log([A]/[HA]), (3)

where Ka and n are constants. Acid-base

properties of humic materials from soils have been examined and generally fol- low the modified Henderson-Hasselbach relationship with n values between 1 and 2 (Gamble 1970; Stevenson 1976). With natural organic matter, where the structure is unknown, the modified Hender- son-Hasselbach relationship may reflect charge interaction effects and chemical differences in the sites.

Experimental

Organic matter was isolated by adsorption onto activated charcoal. Acidified seawater (pH 2) was passed over activated charcoal which was then sequentially eluted with 7 M NH40H (to yield fraction 1), methanol (to yield fraction 2), and a 50-50 mixture by volume of 14 M NH40H and methanol (to yield fraction 3). Elution of charcoal which had no ad- sorbed organic matter provided a blank. No attempt was made to put the organic matter in the fully protonated form. The isolation procedure, carried out by R. A. Kerr, is discussed in detail elsewhere (Kerr 1977). Organic carbon analyses were performed by the method of Men- zel and Vaccaro (1964) using a 2-h oxi- dation at 105?C. Samples were stored in glass bottles at 20C with the headspace in the bottle purged with nitrogen gas.

Titrations were done in 0.7 m (molal) NaClO4 medium to maintain the ionic strength of seawater but to avoid possible effects of calcium and magnesium ions. Potentiometric measurements were done



Protonation equilibria 147

10

TITRAT ION CURVES

8

pH 6

4 - BLANK ORGANIC SAMPLE

2 0 40 80 120 160 200

OH- ADDED, ,u mol Fig. 1. Titration data for an organic sample

(sample No. 2, fraction 1) and blank.

with a digital pH meter (Corning model 112). The calomel reference electrode (filled with 3.5 m KCI) was separated from the NaCl04 medium by a fiber- tipped salt bridge (filled with 4.0 m NaNO3) to prevent precipitation of KC104. Solutions were kept at 25.00? + 0.050C and stirred constantly. The airspace was continuously purged with nitrogen pre- saturated with water vapor.

A sample containing about 1 mg org C was prepared in the NaCl04 medium and the pH adjusted to 2. The sample solution was then titrated with base (0.05 m NaOH, 0.65 m NaCl04). The concentration of DOC in the test solution was about 70 mg liter-'. Each of the three fractions eluted from charcoal was titrated separately.

The electrodes were calibrated in terms of the hydrogen ion concentration by titration of standardized 0.7 m HC104 into 0.7 m NaCl04 medium, so that

pH = -log[H+]. (4) The dilute buffer NBS pH scale, PHNBS,

0 I. 12 0

E o

F- 4 - 0~~~~~

F .

o cc0

H ~ ~ ~ p

H- 2..H

Fig. 2. Titration curves (with blank subtracted) for forward and back titration of organic matter functional groups (sample No. 4, fraction 1). *- Forward titration with base; 0-back titration with acid.

can be related to the hydrogen ion concentration scale (McBryde 1969):

10-PHNBs = F [He]. (5)

For the electrodes used in this work, r =

1.37.

Results The samples examined in this study

were a subset of those considered by Kerr (1977), collected by him and supplied to us for this study. Collection information on the samples is given in Table 1. Fur- ther information about these samples is given by Huizenga (1977, 1978) and Kerr (1977).

Titrations were performed over the pH range 2 to 8, so any sites already dissociated at pH 2 are not reflected in the titration data. Titration data for an organic sample (sample No. 2, fraction 1) and the blank are shown in Fig. 1. The blank titration curve was subtracted from the organic matter sample titration curve to obtain a measure of organic matter de- protonation exclusive of the medium and blank as the pH increased. Titration results for the pH range 8-10 showed a




0

E 8

E

04 LiJ

I-

z 0 I- o~~~~~~~~ I 0-2 E l l l : 2 4 6 8 a.. pH Fig. 3. Organic matter titration curves for dif-

ferent concentrations of organic matter (sample No. 4, fraction 1) in test solution. 0-30 mg kg-1 H20; *-88 mg kg-1 H20.

high and variable blank, so that results above pH 8 could not be used. Tests were performed to confirm that the titration curves represented reversible equilibria and that, when normalized per milligram of organic carbon, they did not depend on the concentration of organic carbon in solution. Figure 2 illustrates the titration of an organic sample from pH 2 to pH 7 followed by a back titration to pH 3. The results show that the dissociation reactions are reversible. Figure 3 demonstrates that the titration curves expressed on a per milligram of organic carbon basis do not vary for bulk organic concentrations between 30 and 90 mg C per kg H20. This observation indicates that the acid dissociation properties of the DOM functional groups are determined primarily by their inherent characteristics and not by interactions among the organic constituents.

Titration curves for the three fractions of a Narragansett Bay DOM sample (sample No. 2) are given in Fig. 4, normalized to the amount of organic carbon in the test solution. The three titration curves are quite similar in shape but differ in the number of sites present. The first fraction, which was eluted with NH40H,

a~ NARRAGANSETT BAY DOM 016 TITRATION CURVES

E FRACTION 1

0 i2

o FRACTION 3

p8

(sampFRACTION 2 Z 4 0

xx

0 2 4 6p8

Fig. 4. Titration curves for the three fractions of a Narragansett Bay dissolved organic matter sample (sample No. 2).

shows the greatest number of sites per unit of organic carbon. The second fraction, which was eluted with methanol, shows the smallest number of sites. The weighted average of the titration curves for the three fractions yields a combined titration curve given in Fig. 5. The average is obtained by weighting each fraction according to the amount of organic carbon obtained from charcoal in that fraction. The combined titration curve should represent the acid-base behavior of the organic matter originally present in seawater.

Discussion In the pH range 2-6, all samples

showed titration curves similar to Fig. 5 with an inflection at pH 3 to pH 4. These are probably carboxylic groups and we have termed these the type 1 sites. Two out of nine samples (No. 5 and No. 9) showed the presence of a second inflec-



Protonation equilibria 149

2. NARRAGANSETT BAY D O M 0 12

FE

E8

w 14

_ / DATA* MODEL-

zo0 0 F - 0

L4*

2 4 p 6 8 p H8

Fig. 5. Combined titration curve for a Narragan- sett Bay dissolved organic matter sample (sample No. 2). Symbols represent data; solid line represents model fit to type 1 sites.

tion point at pH 6.5 (see Fig. 6). We have termed these the type 2 sites.

The modified Henderson-Hasselbach equation was used as a model for a non- linear least-squares fit of the type 1 site titration curve. Because we are applying the modified Henderson-Hasselbach relationship to a natural organic sample of unknown and possibly variable structure, the parameters provided by the model may account for a variety of chemical interactions and their theoretical signifi- cance is not assured. Data in the pH range 3-6 were used for the calculations with the SAS 76 NLIN program by the Marquardt method (Barr et al. 1976). The total number of type 1 sites, pKa, and n were determined and the results are given in Table 2. Figures 5 and 6 show the model fit to the data for two samples. While there are differences in pKa and n among the samples, the overall variation is small. The mean and standard devia-

g SARGASSO SEA INTERMEDIATE 12 WATER DOM

E

0 E8 _....

0

H-4-

F- DATA

Zn O /MODEL z o 0

0

4'

2 4 6 8 pH

Fig. 6. Combined titration curve for Sargasso Sea intermediate water sample (sample No. 5). Symbols represent data; solid line represents model fit to type 1 sites.

tion of parameters for the eight samples of marine dissolved organic matter are:

Number of type 1 sites = 11.0 + 1.8 Amol mg-' org C;

PKa = 3.6 0. 1; n = 2.0 0.2.

The type 1 sites of dissolved organic matter isolated from different marine envi-

Table 2. Model parameters and asymptotically valid standard errors for type 1 sites.

Number of sites Sample (/mol-mg-' org C) pK. n

1 13.82 ? 0.27 3.51 ? 0.03 1.85 ? 0.03 2 13.51 ? 0.28 3.33 ? 0.03 1.96 ? 0.03 3 10.33 ? 0.13 3.67 ? 0.02 1.96 ? 0.03 4 10.03 ? 0.16 3.56 ? 0.02 1.98 ? 0.03 5 11.62 0.28 3.56 0.03 2.38 ? 0.05 6 9.60 ? 0.25 3.75 ? 0.03 1.78 ? 0.05 7 9.73 ? 0.21 3.61 ? 0.03 1.92 ? 0.04 8 10.89 ? 0.31 3.57 ? 0.04 1.98 ? 0.05 9 8.96 0.12 3.67 0.02 1.98 ? 0.03




ronments show small differences in char- acter and numbers. Dissolved organic matter from river water entering Narra- gansett Bay yields acid-base parameters within the range of those observed for marine samples.

Two samples showed the presence of type 2 sites. These sites were present ex- clusively in the first fraction. The Sargas- so Sea intermediate water sample (No. 5) contained 0.8 ,mol mg-' org C of type 2 sites. The coastal Peru sample (No. 9) showed 0.5 ptmol mg-' org C of type 2 sites. The nature of these sites and their origin are unknown. These sites do show variation in numbers present for dissolved organic matter obtained from different environments, as opposed to the uniformity of the type 1 sites.

Conclusions The acid-base characteristics of dis-

solved organic matter (DOM) extracted from seawater show systematic features in a wide range of marine samples. The necessity to extract and concentrate DOM from seawater produces an inherent limitation to the relationship between the observed acid-base properties and those present in the initial seawater. We chose to use an extraction procedure which would provide a maximum yield of DOC.

The acid-base titration behavior of the organic matter in seawater between pH 2 and 8 suggests that carboxylic acid functional groups play a major role in the interaction of DOM with other chemical systems. On the basis of these titration curves, between pH 6 and 8 seawater DOM has a relatively pH independent capacity of dissociated functional groups of about 10 ,mol of sites per liter of seawater for a DOC concentration of 1 mg liter-'. Below pH 6 the concentration of dissociated functional groups decreas- es with pH. References BARR, A. J., J. H. GOODNIGHT, J. P. SALL, AND J. T.

HELWIG. 1976. A user's guide to SAS 76. Sparks.

GAMBLE, D. S. 1970. Titration curves of fulvic acid: The analytical chemistry of a weak acid polyelectrolyte. Can. J. Chem. 48: 2662-2669.

. 1972. Potentiometric titration of fulvic acid: Equivalence point calculations and acidic functional groups. Can. J. Chem. 50: 2680- 2690.

GREGOR, H. P., L. B. LUTTINGER, AND E. M. LOEBL. 1955. Metal-polyelectrolyte complexes. 1. The polyacrylic acid-copper complex. J. Phys. Chem. 59: 34-39.

HUIZENGA, D. L. 1977. Protonation characteristics of the dissolved organic matter in seawater. M.S. thesis, Univ. Rhode Island. 86 p.

. 1978. Supplemental data report on inves- tigations of marine dissolved organic matter protonation equilibria. Grad. School Oceanogr. Univ. Rhode Island Tech. Rep. 78-1. 96 p.

JEFFREY, L. M. 1969. Development of a method for isolation of gram quantities of dissolved organic matter from sea water and some chemical and isotopic characteristics of the isolated material. Ph.D. thesis, Texas A&M Univ. 152 p.

KERR, R. A. 1977. The isolation and partial characterization of the dissolved organic matter in seawater. Ph.D. thesis, Univ. Rhode Island. 181 p.

, AND J. G. QUINN. 1975. Chemical studies on the dissolved organic matter in seawater. Isolation and fractionation. Deep-Sea Res. 22: 107-116.

McBRYDE, W. A. 1969. The pH meter as a hydrogen-ion concentration probe. Analyst 94: 337- 346.

MANTOURA, R. F., AND J. P. RILEY. 1975. The analytical concentration of humic substances from natural waters. Anal. Chim. Acta 76: 97-106.

MENZEL, D. W. AND R. F. VACCARO. 1964. The measurement of dissolved organic and partic- ulate carbon in seawater. Limnol. Oceanogr. 9: 138-142.

STEVENSON, F. J. 1976. Stability constants of Cu2+, Pb2+, and Cd2+ complexes with humic acids. Soil Sci. Soc. Am. J. 40: 665-672.

STUERMER, D. H. 1975. The characterization of humic substances in seawater. Ph.D. thesis, Mass. Inst. Technol. Woods Hole Oceanogr. Inst. 188 p.

I AND G. R. HARVEY. 1974. Humic substances from seawater. Nature 250: 480-481. * AND . 1977. The isolation of humic

substances and alcohol soluble organic matter from seawater. Deep-Sea Res. 24: 303-309.

WILSON, D. E., AND P. KINNEY. 1977. Effects of polymeric charge variations on the proton- metal ion equilibria of humic materials. Lim- nol. Oceanogr. 22: 281-289.

Submitted: 5 May 1978 Accepted: 7 August 1978



protonation equilibria of marine dissolved organic matter

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