Robert H. Byrne

Spectrophotometric seawater pH measurements: total hydrogen ion concentration scale calibration of m-cresol purple and at-sea results

Spectrophotometric open-ocean seawater pH measurements are simple, fast and precise. The sulfonephthalein indicator m-cresol purple (mCP) is recommended for open-ocean surface-to-deep pH measurements. The salinity- and temperature-dependence of such measurements (293⩽T⩽303 and 30⩽S⩽37) is given as: pHT(sw) = 1245.69T + (2.11 × 10−3)(35 − S) + log(R − 0.00692.2220 − R0.1330) on the total hydrogen ion concentration scale ([H+]T = [H+]f + [HSO4−]), in units of mol·(kg-soln)−1. R is the ratio of indicator absorbances at molar absorptivity maxima (i.e. R + 578A/434A). The at-sea analytical precision of the technique, evaluated on two recent NOAA cruises, is approximately 0.0004 pH units.

Rare earth element scavenging in seawater

Abstract Examinations of rare earth element (REE) adsorption in seawater, using a variety of surface-types, indicated that, for most surfaces, light rare earth elements (LREEs) are preferentially adsorbed compared to the heavy rare earths (HREEs). Exceptions to this behavior were observed only for silica phases (glass surfaces, acid-cleaned diatomaceous earth, and synthetic SiO2). The affinity of the rare earths for surfaces can be strongly affected by thin organic coatings. Glass surfaces which acquired an organic coating through immersion in Tampa Bay exhibited adsorptive behavior typical of organic-rich, rather than glass, surfaces. Models of rare earth distributions between seawater and carboxylate-rich surfaces indicate that scavenging processes which involve such surfaces should exhibit a strong dependence on pH and carbonate complexation. Scavenging models involving carboxylate surfaces produce relative REE abundance patterns in good general agreement with observed shale-normalized REE abundances in seawater. Scavenging by carboxylate-rich surfaces should produce HREE enrichments in seawater relative to the LREEs and may produce enrichments of lanthanum relative to its immediate trivalent neighbors. Due to the origin of distribution coefficients as a difference between REE solution complexation (which increases strongly with atomic number) and surface complexation (which apparently also increases with atomic number) the relative solution abundance patterns of the REEs produced by scavenging reactions can be quite complex.

The influence of temperature and pH on trace metal speciation in seawater

Abstract Using available data we have constructed complexation schemes which depict the influence of temperature and pH on metal speciation in seawater. Our calculations show that the extent of complexation of strongly hydrolyzed metals in seawater is strongly temperature and pH dependent. The extent of complexation of the twenty or more metals present in seawater predominantly as carbonate complexes is substantially influenced by pH and temperature, but to a much smaller degree than strongly hydrolyzed metals. For the small group of metals extensively complexed with chloride ions in seawater, temperature and pH appear to be variables of minor significance. We have identified the following concerns. (i) In general, previous experimental work does not provide well defined descriptions of metal hydrolysis under weakly alkaline conditions. (ii) Very few studies have been devoted to characterizing the temperature dependence of carbonate complexation equilibria. (iii) There are major uncertainties in the primary data upon which characterizations of Pd (II), Pt (II), Au (I), Ag (I) and Cu (I) chemistry are based.

Complexation of trivalent rare earth elements (Ce, Eu, Gd, Tb, Yb) by carbonate ions

Abstract Carbonate stability constants for five rare earth elements (Ce3+, Eu3+, Gd3+, Tb3+, and Yb3+) have been determined at t = 25°C and 0.70 ± 0.02 M ionic strength through solvent exchange techniques. Estimated stability constants for Ce, Eu, and Yb are in close agreement with previous work. Analyses using Gd and Tb provide the first carbonate stability constants for these elements based on direct measurements. Our measured stability constants were used to estimate carbonate stability constants for the entire suite of REEs. Our Eu, Gd, and Tb carbonate stability constants demonstrate the existence of a “Gd-break”: Carbonate stability constants for Gd are smaller than those for Eu and Tb. In analogy to Gd concentration anomalies reported in field observations, Gd stability constant anomalies have been defined in terms of the difference log L β n (Gd) − log { ( L β n (Eu) + L β n (Tb)) 2 } , where Lβn(M) = [MLn][M3+]−1[L]−n. Examinations of REE-organic stability constants demonstrate that 106 out of 125 organic ligands have negative Gd anomalies in their first stability constants. The magnitudes of negative Gd anomalies generally become greater with increasing magnitude in Gd-ligand stability constants. Field observations of positive anomalies in shale-normalized Gd concentrations can be explained in terms of REE scavenging by organic surface ligands, such as polyaminocarboxylic acids, which possess a more negative Gd anomaly than carbonate ligands. Our modeling efforts indicate that a mixture of strongly complexing organics such as polyaminocarboxylic acids, and weakly complexing organics such as mono- and dicarboxylic acids is consistent with the pattern of REE scavenging by marine particulate matter.

Examination of comparative rare earth element complexation behavior using linear free-energy relationships

The comparative behavior of rare earth formation constants, Lβn(M), where M a trivalent rare earth element (REE), has been assessed using compilations of rare earth-organic ligand stability constants. Linear free-energy relationships of the form log Lβn(M) = In(M) + Sn(M) log Lβn(Gd), where In(M) and Sn(M) are constants, are used to quantitatively describe the relative magnitudes of Lβn(M) for rare earth complexation by a given ligand L. Our characterizations provide a means of estimating formation constants for the entire suite of rare earths when formation constants Lβn(M) = MLnM3+−1 [L]−n) have been determined experimentally for at least one element. We have used the results of our analyses to calculate rare earth phosphate stability constants for the entire suite of rare earths. Estimated stability constants for each rare earth have been used to assess the relative importance of phosphate complexes, carbonate complexes, and hydrolyzed rare earths in groundwaters. Speciation calculations for representative groundwater environments indicate a strong dependence of Speciation on REE atomic number and pH. Between pH 7 and 9, heavy REE complexes, MPO40, can be important compared toMCO3+ andM(CO3)2−. ThecomplexesM(PO4)23− become significant at higher phosphate concentrations than those considered in our model groundwater. For the light REEs at pH ≤9, the concentrations ofMCO3+ complexes can exceed the concentrations of MPO40 complexes. For pH ≤ 6, M3+ concentrations exceed the concentrations of complex ions. Phosphate complexes exhibit an insignificant role in REE Speciation in seawater. Using3 ×103 as a carbonate/phosphate ion concentration ratio representative of seawater ([CO32−[PO43−] ≈3103) calculations indicate that, for all rare earths, [MCO3+][MPO40] 100, and [CeCO3+[CePO40] ≈ 450.

Solubility of hydrous ferric oxide and iron speciation in seawater

Iron solubility equilibria were investigated in seawater at 36.22‰ salinity and 25°C using several filtration and dialysis techniques. In simple filtration experiments with 0.05 μm filters and Millipore ultra-filters, ferric chlorides fluorides, sulfates, and FeOH2+ species were found to be insignificant relative to Fe(OH)2+ at p[H+] = −log [H+] greater than 6.0. Hydrous ferric oxide freshly precipitated from seawater yielded a solubility product of ∗Kso = [Fe3+][H+]−3 = 4.7 · 105. Solubility studies based on the rates of dialysis of various seawater solutions and on the filtration of acidified seawater solutions indicated the existence of the Fe(OH)30 species. The formation constant for this species can be calculated as ∗β3 = [Fe(OH)30] [H+]3/[Fe3+] = 2.4 · 10−14. The Fe(OH)4− species is present at concentrations which are negligible compared to Fe(OH)2+ and Fe(OH)30 in the normal pH range of seawater. However, there is at least one other significant ferric complex in seawater above p[H+] = 8.0 (possibly with bicarbonate, carbonate, or borate ions) in addition to the Fe(OH)2+ and Fe(OH)30 species.

Direct observations of basin-wide acidification of the North Pacific Ocean.

[1] Global ocean acidification is a prominent, inexorable change associated with rising levels of atmospheric CO 2 . Here we present the first basin-wide direct observations of recently declining pH, along with estimates of anthropogenic and non-anthropogenic contributions to that signal. Along 152°W in the North Pacific Ocean (22-56°N), pH changes between 1991 and 2006 were essentially zero below about 800 m depth. However, in the upper 500 m, significant pH changes, as large as -0.06, were observed. Anthropogenic and non-anthropogenic contributions over the upper 800 m are estimated to be of similar magnitude. In the surface mixed layer (depths to ~100 m), the extent of pH change is consistent with that expected under conditions of seawater/ atmosphere equilibration, with an average rate of change of -0.0017/yr. Future mixed layer changes can be expected to closely mirror changes in atmospheric CO 2 , with surface seawater pH continuing to fall as atmospheric CO 2 rises.

Rare earth and yttrium phosphate solubilities in aqueous solution

Rare earth and yttrium phosphate solubility products range over more than 1 order of magnitude. Minimum solubilities are observed for light rare earths between Ce and Sm. For the elements Ce, Pr, Nd, and Sm solubility products (log Ksp0 (M) = log ([Mi3+] [PO43−])) at zero ionic strength and 25°C can be approximated as log Ksp0,(M) = −26.3 ± 0.2. Rare earth phosphate solubility products for well-aged, coarse precipitates increase substantially between Sm and Lu, with log Ksp0(Lu) estimated as −24.7. The solubility product of Y is similar to that of Ho (log Ksp0 (Y) = −25.0) and is much higher than those of all light rare earths. The solubility product of La is substantially larger than that of Cc (log Ksp0(La) − log Ksp0 (Ce) ≈ 0.5). Solubility products are strongly dependent on the conditions of solid phase formation. Fresh precipitates are much more soluble than slowly formed, well-aged, coarse precipitates. The pattern of rare earth and yttrium phosphate solubility products is generally similar to the fractionation patterns which are developed during phosphate coprecipitation.

Inorganic speciation of dissolved elements in seawater: the influence of pH on concentration ratios

Assessments of inorganic elemental speciation in seawater span the past four decades. Experimentation, compilation and critical review of equilibrium data over the past forty years have, in particular, considerably improved our understanding of cation hydrolysis and the complexation of cations by carbonate ions in solution. Through experimental investigations and critical evaluation it is now known that more than forty elements have seawater speciation schemes that are strongly influenced by pH. In the present work, the speciation of the elements in seawater is summarized in a manner that highlights the significance of pH variations. For elements that have pH-dependent species concentration ratios, this work summarizes equilibrium data (S = 35, t = 25°C) that can be used to assess regions of dominance and relative species concentrations. Concentration ratios of complex species are expressed in the form log[A]/[B] = pH - C where brackets denote species concentrations in solution, A and B are species important at higher (A) and lower (B) solution pH, and C is a constant dependent on salinity, temperature and pressure. In the case of equilibria involving complex oxy-anions (MOx(OH)y) or hydroxy complexes (M(OH)n), C is written as pKn= -log Knor pKn* = -log Kn* respectively, where Knand Kn* are equilibrium constants. For equilibria involving carbonate complexation, the constant C is written as pQ = -log(K2lKn[HCO3-]) where K2lis the HCO3- dissociation constant, Knis a cation complexation constant and [HCO3-] is approximated as 1.9 × 10-3 molar. Equilibrium data expressed in this manner clearly show dominant species transitions, ranges of dominance, and relative concentrations at any pH.

Carbonate Complexation of Yttrium and the Rare Earth Elements in Natural Waters

Abstract Potentiometric measurements of Yttrium and Rare Earth Element (YREE) complexation by carbonate and bicarbonate indicate that the quality of carbonate complexation constants previously obtained via solvent exchange analyses are superior to characterizations obtained using solubility and adsorptive exchange analyses. The results of our analyses at 25°C are combined with the results of previous solvent exchange analyses to obtain YREE carbonate complexation constants over a wide range of ionic strength (0 ≤ I ≤3 molal). YREE carbonate complexation constants are reported for the following equilibria, M3++nHCO3−⇌M(CO3)n3−2n+nH+, where n = 1 or 2. Formation constants written in terms of HCO3− concentrations require only minor corrections for ion pairing relative to the corrections required for constants expressed in terms of CO32− concentrations. Formation constants for the above complexation equilibria, CO3Hβ1=[MCO3+][H+][M3+]−1[HCO3−]−1 and CO3Hβ2=[M(CO3)2−][H+]2[M3+]−1[HCO3−]−2, have very similar dependencies on ionic strength because the reaction MCO3++HCO3−⇌M(CO3)2−+H+ is isocoulombic. Potentiometric analyses indicate that the dependence of logCO3Hβ1 and logCO3Hβ2 on ionic strength at 25°C is given as (A) log CO 3 H β n = log CO 3 H β n 0 −4.088 I 0.5 /(1+3.033 I 0.5 )+0.042 I where CO3Hβn0 denotes a formation constant at 25°C and zero ionic strength. Recommended values for logCO3Hβ1 and logCO3Hβ20, expressed in the form (element, −logCO3Hβ10, −logCO3Hβ20), are as follows: (Y, 2.85, 8.03), (La, 3.60, 9.36), (Ce, 3.27, 8.90), (Pr, 3.10, 8.58), (Nd, 3.05, 8.49), (Sm, 2.87, 8.13), (Eu, 2.85, 8.03), (Gd, 2.94, 8.18), (Tb, 2.87, 7.88), (Dy, 2.77, 7.75), (Ho, 2.78, 7.66), (Er, 2.72, 7.54), (Tm, 2.65, 7.39), (Yb, 2.53, 7.36), (Lu, 2.58, 7.29).