Chemical Speciation

Chemical speciation is defined as the distribution of a chemical element between  different molecular and ionic forms in seawater.  It determines the reactivity and bioavailability of the elements in seawater, and is key to our understanding of biogeochemical and acidification processes in the ocean. It is necessary to model speciation in order to predict how the rate and extent of chemical reactions in the global ocean will be affected by increasing temperature and decreasing pH.

The form in which a trace element or other component of seawater is present, and its tendency to react, depends on its activity. This is the product of its concentration (usually molality) and an activity coefficient (γ) which is a complex function of temperature, pressure, and salinity (or, more generally, solution composition). Many of the important reactions in seawater involve acid-base equilibria, which introduces pH as a further variable. Changing pH is also at the heart of the process and effects of CO₂ uptake by the oceans, and of the speciation of dissolved inorganic carbonate. The definitions of pH and the use of buffers to calibrate pH instruments, and the relationship of measured pH to that calculated using thermodynamic models of seawater, are complex and not always appreciated.

It is desirable to be able to calculate pH, and the activities and speciation of all seawater components, within a unified framework that, (i) includes the major and trace elements in seawater and its mixtures with freshwaters, (ii) encompasses the buffers that are used to calibrate pH and other instruments, and (iii) can be extended to include other saline environments such as brines and pore waters. Progress has been made towards this goal, mainly in the 1980s and 1990s, and today the principal chemical speciation model of seawater is that of Millero and co-workers at the University of Miami.The model uses the equations of Pitzer to calculate activity coefficients, and is applicable primarily to major ion seawater (from 0 to 50 ^oC, and 0 to >40 salinity) containing the species H⁺, Na⁺, K⁺, Mg²⁺, Ca²⁺, Sr²⁺, Cl^–, Br^–, OH^–, HCO₃^–, B(OH)₄^–, HSO₄^–, SO₄^2-, CO₃^2-, CO₂, B(OH)₃, and H₂O.

Why is a Working Group Needed?

The ability to model the biogeochemistry and especially the carbonate chemistry of the world’s oceans, seas, and estuaries is required to undertstand and to manage environmental change due to human activity. However, the numbers of new studies yielding the activity, thermal, and volumetric data and stability constants needed to develop the models, and our quantitative understanding of speciation in the oceans, have been in decline for many years. The numbers of skilled experimenters and modellers have also fallen. Furthermore, there is no comprehensive evaluation that relates the capabilities of speciation models, and the measurements upon which they are based, to current and future needs in chemical oceanography as exemplified in current programmes such as GEOTRACES.

What This Working Group Will Do

The first objective of this working group is to document the current status, and basis in laboratory measurements, of Pitzer models of seawater and estuarine water and the complexation of key trace metals including Fe, Cu, Mn, Cd, Mn, and Zn. We will define current capabilities and limitations for oceanographic and biogeochemical calculations, and establish what is needed (in both laboratory measurements and modelling) to meet future requirements. A second, associated, objective is to provide a database of Pitzer model parameters and equilibrium constants for seawater (and their variation with temperature and pressure), including trace metal complexation, which can be used by skilled practitioners. The uncertainties, and the effects on calculated properties such as pH, will be evaluated.

Members of the Working Group have been awarded grants from the Natural Environment Research Council in the UK, and National Science Foundation in the USA, to carry out the work above, including experiments to measure thermodynamic properties in support of model development. Collaborations with the national metrological institutes of Germany, France, and Japan (who will also carry out experiments) have enabled us to focus initially on a chemical speciation model of the Tris/TrisH⁺ buffers used to calibrate pH instrumentation, with particular emphasis on the ‘traceability’ of the model and the propagation of measurement uncertainties. Our activiies and results are described elsewhere on this website. Details of the formation of the Working Group, sponsored by SCOR (the Scientific Committee on Oceanic Research), are below.

Formation of the Working Group

The SCOR Working Group 145 on Modelling Chemical Speciation in Seawater to Meet 21st Century Needs (MARCHEMSPEC) was approved September 2014 and had a proposed duration of 4 years with financial Sponsors being SCOR and NSF. The duration of the Working Group has since been extended, most recently to late 2021. The terms of reference are found below. A first meeting was held 12-13 April 2015 in Sibenik, Croatia. The second meeting was held on 21st February 2016 in New Orleans, USA (preceding the AGU/ASLO Ocean Sciences Meeting).

Terms of Reference

1. To document the current status, and basis in laboratory measurements, of Pitzer models of seawater and estuarine water focusing on the chemistry of ocean acidification and micronutrient trace metals (including, but not limited to, Fe, Cu, Cd, Co, Mn, and Zn). Current capabilities and limitations for oceanographic and biogeochemical calculations will be defined, and future needs established. Important gaps in knowledge, which should have high priority for new measurements, will be identified. The components to be covered will include the seawater electrolytes, the selected trace metals, and buffer solutions and key organic ligands such as those used in CLE-CSV titrations.
2. To publish the results of the first term of reference in the refereed scientific literature, and to introduce the conclusions and recommendations to the oceanographic community at a “town hall” event or special session at an international ocean sciences meeting.
3. To specify the functions and capability for a web-based modelling tool that will make chemical speciation calculations easily accessible for a wide range of applications in oceanography research and teaching, and thus improve understanding and spread best practice in modelling.
4. To implement the web-based tool for chemical speciation calculations, based upon the specification developed in the third term of reference which will also be used to obtain external funding to develop the programs, documentation, and site.