MarChemSpec Paper Wins a Prize!
Co-authors Frank Bastkowski and Beatrice Sander of the National Metrology Institute of Germany (PTB), and Simon Clegg of the University of East Anglia, were recently awarded “Best paper of 2025” by CITAC (Cooperation on International Traceability in Analytical Chemistry).
The title of the paper is: Activity Coefficients of HCl in Solutions Related to “Tris” Buffers in Artificial Seawater. III. Tris Buffer + NaCl + H2O, from 0.2 to 3.25 mol kg-1 Ionic Strength and from 5 to 45 °C, J. Chem. Eng. Data 71, 33-45 (2026), doi.org/10.1021/acs.jced.5c00450
The work contributes to the body of thermodynamic data we are accumulating for components of the Tris buffer solutions used to calibrate seawater ‘total’ pH measurements. These data are needed to extend the scale and improve metrological traceability and therefore accuracy. They also underpin our current studies into pressure effects on buffer pH and the ability to measure seawater pH at in situ pressures in the oceans.
The picture shows author Simon Clegg receiving the award certificate on behalf of all three co-authors from CITAC Chair Zoltan Mester of the National Research Council of Canada.
An article in the CITAC newsletter for 2026 (see page 61) describes why our award winning study is important for the measurement of seawater pH. It is Part III of a continuing series by the scientists at PTB and also with Igor Maksimov and colleagues at the National Metrology Institute of Japan. The MarChemSpec group is very pleased to acknowledge these essential contributions from both institutes and the encouragement of CITAC and the BIPM (International Bureau of Weights and Measures) for our work.
Measurements are continuing at the two national metrology institutes this year. We anticipate that the next publication in our data series will be of osmotic coefficients of aqueous solutions of Tris and TrisHCl (the hydrogen chloride salt of Tris) with colleagues in the Department of Inorganic Chemical Technology at the University of Belgrade.
The CITAC initiative aims to foster collaboration between existing organisations to improve the international comparability of chemical measurements. CITAC develops new guidelines in the field of metrology in chemistry in cooperation with international sister organizations such as Eurachem and IUPAC. Recent publications of CITAC include the EURACHEM /CITAC Guide: Evaluation of measurement uncertainty from in-house precision and recovery data, 1st Edn., edited by Ricardo Bettencourt da Silva, Eurachem, 2026.
Ocean Sciences 2026
MarChemSpec scientists David Turner and Simon Clegg will be present at the meeting in Glasgow, and available for discussions. They can be found at the Ocean Carbon and Biogeochemistry (OCB) booth in the Exhibition Hall on the Tuesday (10 – 11 am), and Wednesday (11 am to noon). Please email if you want to see us at some other time.
David’s poster OB14B-0473 describes a thorough update of our trace metal complexation model (Modelling the Inorganic Complexation of the GEOTRACES Core Elements From Freshwater to Seawater, Focusing on Hydroxide and Carbonate Ligands and a Revised Treatment of Fe(III) Hydrolysis). The revised treatment of Fe(III) – hydroxide complexation is a particular focus, and has implications for ocean chemistry.
We are now updating the MarChemSpec software for the revised trace metal treatment, and this should be available for distribution in the spring of this year. Please contact us if you want to know more (s.clegg@uea.ac.uk, david.turner@marine.gu.se).
New Postdoc position available
We have a 2 year position at the University of East Anglia available now, to work on a model of pressure effects on the Tris buffers used to calibrate total pH for seawater measurements. The successful applicant will work with Simon Clegg using data from the literature and also results of new measurements being made at the UK’s National Oceanography Centre. The position is described here.
We have almost completed moving the MarChemSpec website to a new host (the address will remain the same), and will soon be updating the content to reflect changed affiliations of our taskgroup, new papers published in 2025, and a new 3 year measurement project just begun. Please stay tuned…
Capacity Building in Physical Chemistry for Oceanography
New survey highlights emerging skills gaps in the physical chemistry of natural waters
Earlier this year we conducted an online survey and consultation with the ocean science community to assess what we perceive as emerging skills gaps in basic physical chemistry training and expertise in several areas of chemical oceanography, especially (but not exclusively) including the ocean carbonate system. In the survey, we asked for this information:
- Expertise, applications, and professional roles
- Opinions concerning skills gaps in physical chemistry for different areas of oceanography and needs for capacity building
We received well over 100 responses, with very many insightful observations and answers to our questions. We note a few highlights below, and invite you to read the Survey Report describing the skills gap survey results and associated community feedback on recommended paths forward. The great majority of respondents expressed their opinions as to the nature of the skills gaps, sometimes at length, and we believe the report provides important insights into the current state of chemical oceanography, its gaps and blind spots, and novel ideas for moving forward.
Survey feedback highlights:
- Participation was representative across career stages (almost evenly split among PhD/postdoc, mid-career, and late career).
- Top participant research areas: Ocean carbonate system, including marine carbon dioxide removal (mCDR); laboratory measurements and instrumentation; trace metals; chemical oceanography/aqueous geochemistry; chemical sensors; standards and reference materials.
- Geographical spread of respondents: 42% North America; 32% Europe; 11.6% Far East (including Australia and New Zealand); 6.7% Central and South America; 8% rest of the world.
Responses across all career stages cited a lack of appropriate training (both undergraduate and PhD level), leading to inadequate research skills, with many specific examples given. There was universal support for a workshop to discuss the issues and to recommend next steps.
Join us for a virtual community discussion at OA Week in November
To follow up on this survey, we are convening an online community discussion on Tuesday 19 November at 1600-1730 GMT/1100-1230 ET as part of the Global Ocean Acidification Observing Network (GOA-ON) Ocean Acidification (OA) Week 2024. The purpose of this discussion will be to decide next steps for a community activity (most likely a Workshop) including its focus, content, participants, and outcomes to help address the emerging skills gap identified in the survey. Please register here. We encourage you to attend even if your professional interest is not ocean acidification, as this discussion and its applications are much broader than OA.
We look forward to meeting you in the discussion session. It will be important both as a source of new ideas, priorities, and the consensus that we need to convince others – funding organisations, for example – that we have an issue that is essential to address.
If you would like further information, or you represent an organisation that would like to participate in this effort, please get in touch with either Heather Benway (hbenway@whoi.edu) or Simon Clegg (s.clegg@uea.ac.uk).
MarChemSpec has been accepted as a project within the UN Ocean Decade, which runs from 2021 – 2030. Joining the UN Ocean Decade is part of our efforts to gain improved international visibility and encourage greater use of the modelling tools.
Our Ocean Decade webpage, which is under construction, can be found at https://forum.oceandecade.org/ventures/106957.
Our modelling posters for Ocean Sciences 2024 can now be viewed here: dissolved CO2 and mineral solubilities, and inorganic complexation of GEOTRACES trace metals.
Other activities: we are making improvements to the MarChemSpec trace metal model, and have begun work on manuscripts describing thermodynamic measurements that will support development of a MarChemSpec model of the Tris buffer solutions used for calibration of pH. The measurements have been made by our collaborators at the national metrological institutes of the USA, Japan, and Germany over the last several years.
MarChemSpec at Ocean Sciences 2024, and New Release of Modelling Software
MarChemSpec is the Marine Chemical Speciation Model for acid-base speciation, trace metal complexation, and saturation with respect to solid phases in natural waters containing the inorganic species present in seawater. Project leaders Simon Clegg and David Turner will be present at Ocean Sciences and, to coincide with the meeting, we have a new release of the modelling software. The improvements make it easier to use for many oceanographic applications, and we have also updated the trace metal model to our most recent work. These are the new features:
- The software is now able to automatically adjust an input solution (seawater or some other natural water) to fixed single values, or pairs, of these quantities: total dissolved inorganic carbon, total alkalinity, total or free pH, and partial pressure or fugacity of CO2. This makes the model simpler to use for many oceanographic and mCDR (marine CO2 removal) applications.
- Our model of trace metal complexation has been improved for Fe(III) hydrolysis, based on work described in one of our posters (see below). This model does not yet have the new capabilities noted above.
Talk to Us About MarChemSpec Applications
Simon Clegg and David Turner will present posters on MarChemSpec applications to marine carbon dioxide removal (CM24A-1140) and trace metal complexation (OB34C-0880).
Meet Simon and David at their posters, and visit the Ocean Carbon & Biogeochemistry (OCB) Exhibit Booth (BOOTH 512, OSM Exhibit Hall, MAP) to test drive the new software and discuss applications. Their availability at the OCB booth is likely to be broadly:
- Tuesday: 10 am – 6 pm
- Wednesday: 10 am – 4 pm
- Thursday: about 10 am, or 12 noon – 1 pm
To arrange a specific time to meet, please email Simon (s.clegg@uea.ac.uk) and David (david.turner@marine.gu.se), or talk to Heather Benway or Mai Maheigan at the booth.
Simon will also be attending the Workshop to Build Consensus Toward Science-Led Protocols for Ocean Alkalinity Enhancement (Monday morning, February 19), and the mCDR Networking Event (Monday evening, February 19).
Software Downloads
The new releases of the software can be downloaded (as zip files) from the links below. At this time both the seawater and trace metal models are available only as standalone Windows programs, and as a Windows MATLAB function. (After the meeting we will prepare Linux and macOS standalone models, and the Python function. The software page on this website will also be updated later.)
Unzip the archives to your computer, and follow the instructions in the \docs subdirectories. In the cases of the seawater model and MATLAB function we have provided revised versions of the original instruction manuals, plus shorter Supplements that describe how to use the new features. For the trace metal model, there is just a revised version of the original manual. If you have any problems installing or using the software, please contact us.
Chemical Speciation Modelling Software Version 1.01, and Tutorials
The MarChemSpec modelling software and its capabilities are described in our June 2023 post, and also under the new menu item ‘Software’ (see above). The models were released on June 15-16 at an event following the Ocean Carbon and Biogeochemistry Summer Workshop. Our tutorials were recorded, and can be viewed on YouTube here.
The new version 1.01 of the software can now be downloaded. This differs from our initial release in June in these respects:
- An error in the model for trace metal complexation has been corrected. This affects only Cu2+ complexation at temperatures other than 25 oC.
- The documentation of the MarChemSpec Python function has been amended to describe better how directory paths should be specified to the function.
Please go to the software page on this website for downloads and a link to our Zenodo archive.
There will be a further release of the software, with greatly increased capabilities, to coincide with AGU/ASLO Ocean Sciences in February 2024. Our group will be attending the meeting.
If you would like to be added to our email list for future announcements, please send a request to Heather Benway (hbenway@whoi.edu).
Chemical Speciation Modelling Software Now Available for Download
For all potential users of the MarChemSpec models, especially attendees (both in-person and virtual) at the Tutorial and Launch event at Woods Hole on 15th and 16th June: Downloads of the standalone models for execution from a command prompt (and from Excel, for Windows only), and MATLAB and Python functions, are available for Windows and Linux (and now Apple macOS) at the links below.
Attendees at the Tutorials: you will receive an email with some further instructions about what we would like you to do.
These are the programs and functions that can be downloaded:
- The standalone seawater model (MCS_sea): calculate seawater state parameters (pCO2, fCO2, total pH, carbonate and borate equilibrium constants), with estimated uncertainties, for natural waters containing the species of reference seawater but of arbitrary composition.
- The standalone seawater model for the effects of composition change (MCS_delta): calculate the change in seawater state parameters, with estimated uncertainties, corresponding to a change in natural water composition (typically from reference seawater to something with a different major ion composition).
- The standalone model for the calculation of the inorganic complexation of GEOTRACES trace metals (MCS_trace) in natural waters containing the species of seawater but of arbitrary composition.
- MATLAB and Python functions that can carry out several different types of calculation using the seawater and trace metal complexation models (and also one for artificial seawater and Tris buffers).
Functions for the R language will be available later this year.
The downloads are in the form of zip files (Windows) and zipped tar files (Linux and macOS):
- Standalone programs (Windows)
- Standalone programs (Linux)
- Standalone programs (Apple macOS)
- MATLAB and Python functions (Windows)
- MATLAB and Python functions (Linux)
- Standalone programs used from Excel worksheet (Windows)
Questions and comments: email Simon Clegg (s.clegg@uea.ac.uk) for general matters, David Turner (david.turner@marine.gu.se) for MATLAB issues, and Terra Ganey (tganey@ucsc.edu) for Python.
Below: Previous MarChemSpec announcement, with further details
“MARCHEMSPEC” stands for Marine Chemical Speciation, and will be the name for the models and software tools produced by our project. The software implements our models of natural waters containing the ions of seawater, artificial seawater, and traces metals including the GEOTRACES core species.
The models and software will be launched with talks and demonstrations at Woods Hole Oceanographic Institution, following the June 2023 Ocean Carbon and Biogeochemistry Summer Workshop. These demonstrations, both in-person and online, will take place on Thursday 15th June (afternoon), and Friday the 16th June (morning), US Eastern time.
Please go to this link for more details, and to express your interest in attending. The talks will be recorded. Downloads of the software will be available here (at crystalspring.whoi.edu/marchemspec/) from early June.
These easy-to-use models are for the calculation of:
- Acid-base equilibria, and CaCO3 saturation in natural waters containing the ions of seawater.
- Inorganic complexation of trace metals Al, Cd, Co, Cu(II), Fe(II), Fe(III), Mn, Ni, Pb and Zn in natural waters.
What does the software consist of? First, there are two standalone programs that can be run from the command prompt. The first one takes a file of natural water compositions, and temperatures, as input. The outputs are the equilibrium speciation and the calculated values of pH (three different measures), stoichiometric equilibrium constants for the carbonate system, borate, fluoride, and water. These equilibrium constants are expressed on the same basis as those described in, for example, chapter two of Dickson et al. (2007) Guide to Best Practices for Ocean CO2 Measurements, North Pacific Marine Science Organisation, PICES Special Publication 3, IOCCP Report No. 8. Estimates of uncertainties in the calculated values of these quantities are provided. They are also presented on both an ‘amount content’ basis (moles per kg of solution), and a molality basis (moles per kg of pure water). This is true of all our programs.
The other standalone program is very similar, except that it takes pairs of natural water compositions as inputs and calculates the changes in pH and the equilibrium constants mentioned above between the two solutions. This program could be used to calculate the change in these key properties associated with a change in composition from that of standard seawater to one with a different ionic composition, for example. The model also provides estimates of the uncertainties in the calculated differences in pH and pK.
These programs are the two most flexible that we can provide, and can easily be used to process large numbers of compositions. Output as .csv files (comma separated values) is provided, and these can be read directly into spreadsheet programs. Current limitations: the compositions of the natural waters are input either as practical salinities or as total amount contents (or molalities) of all solute species, including H+. We have not yet added the capability to equilibrate a solution to user-specified values of pairs of the four key variables total pH, alkalinity, DIC, and pCO2. This will be implemented later in the year.
Second, we have also produced versions of the model that can be called, in a simple way, from within MATLAB, Python, and R. These have essentially the same capabilities as the standalone programs but must be called once for each individual solution being processed. For calculations including trace metals these versions of the model also output the proportions of the total amounts of each dissolved metal that are complexed by each of the inorganic anions.
The software has been developed mostly on Windows machines, which we are most familiar, but should be available for Linux and perhaps for Apple machines by the time of the launch in June. More information will be provided here (and directly, by email, to those who sign up for the presentations and tutorials on the OCB Summer Workshop site.
Simon Clegg (s.clegg@uea.ac.uk) and David Turner (david.turner@marine.gu.se) will give the presentations and demonstrations at the launch event. Please contact one of us if you have questions.
The third of our papers on chemical speciation modelling is now at the pre-proof stage of publication. It describes our model of acid-base equilibria in natural waters containing the solutes present in standard seawater: the major ions (Na+, Mg2+, Ca2+, K+, Sr2+, Cl–, and SO42-) plus H+, OH–, HSO4–, and the components of dissolved carbonate (CO2*, HCO3–, CO32-), borate (B(OH)4– and B(OH)3), and fluoride (F– and HF):
• S. L. Clegg, J. F. Waters, D. R. Turner, and A. G. Dickson (2023) Chemical speciation models based upon the Pitzer activity coefficient equations, including the propagation of uncertainties. III. Seawater from the freezing point to 45 °C, including acid-base equilibria. Mar. Chem. (pre-proof), art. no. 104196, https://doi.org/10.1016/j.marchem.2022.104196
The paper, and its extensive supporting information, includes detailed comparisons of model calculations with the measured carbonate system constants K1, K2, and KB and also the dissociation product of water (KW) and saturation of seawaters with respect to calcite. Please get in touch with Simon Clegg (s.clegg@uea.ac.uk) or David Turner (david.turner@marine.gu.se) if you would like to know more.
SCOR Working Group 145 has now transitioned to the Chemical Speciation Task Group of the Joint Committee on the Properties of Seawater which is a permanent committee sponsored by the International Association for the Properties of Water and Steam (IAPWS), SCOR, and by the International Association for the Physical Sciences of the Oceans (IAPSO). Our work will continue under this new affiliation, focusing on the development and application of the models, free software to run the models, and the extension and basis in SI of the total pH scale (in collaboration with national metrology institutes of the USA, Germany, and Japan).
This website will be revised over the following months to reflect these changes, including to become a source for software downloads and online calculations.
Our first two papers on chemical speciation modelling have just been published. They are major outputs of this Working Group, and describe the scientific basis for tools that will be provided to oceanographers later this year. These open access papers are:
• M. P. Humphreys, J. F. Waters, D. R. Turner, A. G. Dickson, and S. L. Clegg (2022) Chemical speciation models based upon the Pitzer activity coefficient equations, including the propagation of uncertainties: Artificial seawater from 0 to 45 °C. Mar. Chem. 244, art. 104095. https://doi.org/10.1016/j.marchem.2022.104095
• S. L. Clegg, M. P. Humphreys, J. F. Waters, D. R. Turner, and A. G. Dickson (2022) Chemical speciation models based upon the Pitzer activity coefficient equations, including the propagation of uncertainties. II. Tris buffers in artificial seawater at 25 oC, and an assessment of the seawater ‘Total’ pH scale. Mar. Chem. 244, art. 104096.
https://doi.org/10.1016/j.marchem.2022.104096
The assessment of the total pH scale in the second paper includes definitions of various forms of pH, a summary of equations relating them, and an explanation of commonly used terms.
The release of software implementing both these models, and another one of solutions containing the solutes present seawater electrolyte (inc. carbonate and borate species), will be announced on this site in due course.
Our manuscript describing the model of seawater electrolyte has just been submitted to Marine Chemistry, and the title and abstract are given below. Please get in touch with Simon Clegg (s.clegg@uea.ac.uk) or David Turner (david.turner@marine.gu.se) if you would like to know more.
Chemical Speciation Models Based Upon the Pitzer Activity Coefficient Equations, Including the Propagation of Uncertainties. III. Standard Seawater from the Freezing Point to 45 °C, Including Acid-Base Equilibria
Simon L. Clegg, Jason F. Waters, David R. Turner, and Andrew G. Dickson
ABSTRACT: A quantitative understanding of pH, acid-base equilibria, and chemical speciation in natural waters including seawater is needed in applications ranging from global change to environmental and water quality management. In a previous study (Humphreys et al., 2022) we implemented a model of solutions containing the ions of artificial seawater, based upon the use of the Pitzer equations for the calculation of activity coefficients and including, for the first time, the propagation of uncertainties. This was extended (Clegg et al., 2022) to include the Tris buffer solutions that are used to calibrate the seawater total pH scale. Here we apply the same methods to develop a model of solutions containing the ions of standard reference seawater, based upon studies by Millero and co-workers. We compare the predictions of the model to literature data for: the dissociation of dissolved CO2 and bicarbonate ion; boric acid dissociation; saturation with respect to calcite, the ion product of water, and osmotic coefficients of seawater. Estimates of the uncertainty contributions of all thermodynamic equilibrium constants and Pitzer parameters to the variance of the calculated quantity are used to determine which elements of the model need improvement, with the aim of agreeing with properties noted above to within their experimental uncertainty. Further studies are recommended. Comparisons made with several datasets for carbonate system dissociation in seawater suggest which are the most reliable, and identify low salinity waters (S < 10) as a region for which dissociation constants of bicarbonate are not yet accurately known. At present, the model is likely to be most useful for the direct calculation of equilibria in natural waters of arbitrary composition, or for adjusting dissociation constants known for seawater media to values for natural waters in which the relative compositions of the major ions are different.