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

Robert H Byrne

doi:10.1186/1467-4866-3-11

Abstract

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 Kn or pKn* = -log Kn* respectively, where Kn and Kn* are equilibrium constants. For equilibria involving carbonate complexation, the constant C is written as pQ = -log(K2lKn [HCO3-]) where K2l is the HCO3 - dissociation constant, Kn is 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.

Highlights

Solution speciation exerts important controls on chemical behavior
Compilations of Principal Species[2] dominantly involved (a) simple hydrated cations and anions (e.g. Na1, Ca21, Cl2, F2), (b) ion pairs with sulfate (e.g. MgSO40 and CaSO40), (c) fully hydrolyzed elements (e.g. HmPO4m23, HmAsO4m23, MoO422 and WO422) and (d) chloride complexes (e.g. AuCl22, HgCl422)
Hydrogen ions are an exception to this generalization

Summary

Introduction

Solution speciation exerts important controls on chemical behavior. Speciation is known to influence solubility, membrane transport and bioavailability, adsorptive phenomena and oceanic residence times, volatility, oxidation/reduction behavior, and even physical properties of solutions such as sound attenuation. Compilations of Principal Species[2] dominantly involved (a) simple hydrated cations and anions (e.g. Na1, Ca21, Cl2, F2), (b) ion pairs with sulfate (e.g. MgSO40 and CaSO40), (c) fully hydrolyzed elements (e.g. HmPO4m23, HmAsO4m23, MoO422 and WO422) and (d) chloride complexes (e.g. AuCl22, HgCl422) While it was noted[1,2] that hydroxide complexes were important for all ions with oxidation numbers greater than two, hydroxide complexes were notably absent in Principal Species tabulations until the following decade. The thermodynamic data compilations of Sillen and Martell[3,4] catalyzed rapid advances in equilibrium models of seawater speciation These works were followed by additional compilations[5,6,7] that were critically important to modern seawater speciation assessments. As the result of approximately twenty years of progress in seawater speciation, the Principal Species assessment of Bruland[12] listed seventeen elements with carbonate-dominated Principal Species

Speciation calculations

Discussion

Equilibrium data

Findings

Overview of speciation in seawater