Welcome to the WG 145 site


Our Working Group is dedicated to advancing the ability of chemical oceanographers to model chemical speciation in seawater and other natural waters. We are collaborating with a group of national metrological institutes to carry out new laboratory measurements to characterise the thermodynamic properties and speciation in the major and minor components of seawater, and in the aqueous buffers used to calibrate instruments for measuring pH. Concurrently, we are also working on an uncertainty analysis of currently available data and “Pitzer” speciation models.

The aim of the Working Group is, over a period of years, to develop a user-friendly comprehensive chemical speciation model of seawater and related natural waters. The model will include a full treatment of uncertainties. Our efforts are supported by grants from the Natural Environment Research Council in the UK, and the National Science Foundation in the USA. This Working Group is sponsored by the Scientific Committee on Oceanic Research.

On this website we describe our activities and achievements, and current activities of interest to the chemical oceanography community. For further information, please contact either David Turner (david.turner@marine.gu.se), or Simon Clegg (s.clegg@uea.ac.uk).

Latest News (January 2021)

Our paper describing measurements of Tris buffer solubilities in various salt solutions, and measurements of water activities of aqueous Tris, has just been published. The citation and abstract are given further below.

Working Group members Simon Clegg, Andrew Dickson, and David Turner, and Jason Waters of NIST (USA), are currently working on a manuscript describing insights from an uncertainty analysis of the current best Pitzer speciation model of the Tris buffers used in seawater pH measurements. Also, associate member Frank Bastkowski of PTB (Germany) has begun Harned Cell measurements to characterise the thermodynamic properties of aqueous solutions containing dissolved equimolal TrisH+ and Tris in an NaCl medium. The results will be used to improve the speciation model of Tris buffer in artificial seawater (in which the major solute is NaCl).

P. Lodeiro, D. R. Turner, E. P. Achterberg, F. K. A. Gregson, J. P. Reid, and S. L. Clegg (2021) Solid-liquid equilibria in aqueous solutions of Tris, Tris-NaCl, Tris-TrisHCl, and Tris-(TrisH)2SO4 at temperatures from 5 to 45 oC. J. Chem. & Eng. Data 66, 437-455. (https://dx.doi.org/10.1021/acs.jced.0c00744)

Abstract: The substance Tris (or THAM, 2-amino-2-hydroxymethyl-1,3-propanediol) is used in the preparation of pH buffer solutions for applications in natural water chemistry, including seawater. The development of a chemical speciation model of buffer solutions containing Tris, TrisH+, and the major ions of seawater is desirable, so that the effects of changes in the composition and concentration of the medium on pH can be calculated. The Pitzer activity coefficient equations, commonly used in such speciation models, describe the thermodynamic properties of solutions in terms of interactions between dissolved ions and uncharged solute species. To determine some of these interactions, we have measured solubilities of Tris(s) in water and aqueous solutions of NaCl, TrisHCl, and (TrisH)2SO4 and the solubility of NaCl(s) in aqueous Tris(aq), from 5 to 45°C. We report measurements of the water activities of Tris solutions at 293.5 K to high supersaturation with respect to the solid. Using the Pitzer equations, we compare our results to literature data yielding stoichiometric dissociation constants of TrisH+ in aqueous NaCl, and to electromotive forces of cells containing dissolved Tris, TrisHCl, and NaCl. Values of parameters for the interactions of Tris with the ions TrisH+, Na+, and SO42- at 25°C are determined.

News (October 2020)

Much has happened in the world since our last news item in March. However, we have been able to continue our work, even experiments, and have made good progress over the spring and summer.

First, a manuscript describing measurements of Tris buffer solubilities in various salt solutions, and measurements of water activities of aqueous Tris, is in the final stages of review by J. Chem. and Eng. Data. These measurements, carried out by Pablo Lodeiro at GEOMAR and Flo Gregson at the Bristol Aerosol Research Centre, provide important information about the interactions of the buffer substance Tris with other salts. The results are bring used to improve the chemical speciation model of Tris buffer solutions (for the calculation of pH) that we are developing. In a separate study, water activities of Tris solutions at various temperatures have just been measured by our collaborators Tian Xiaomeng and Prof. Chak Chan at City University of Hong Kong. A programme of Harned Cell measurements to be carried out by WG 145 associate members Regina Easley (NIST) and Frank Bastkowski (PTB, Germany) is being finalised.

Second, at the Ocean Sciences meeting in San Diego in February we showed draft software for the calculation of the pH of Tris buffers in artificial seawater, and the K* of the carbonate system in standard seawater. The novel features of these models are: (i) the ability to vary the composition of the solution medium from seawater stoichiometry; (ii) the calculation of uncertainty contributions from all the individual elements of the model. The draft software is still available to try here. Since the meeting we have refined the uncertainty treatment, and a manuscript describing the methods and results is currently in preparation. Here is a result we’d like to show you: ranked uncertainty contributions (as percentages of the total calculated variance) to a model-calculated pH of Tris buffer in artificial seawater of salinity 35.

This bar chart shows that the main contributor to the uncertainty of a calculated pH (‘total’ scale) is the uncertainty of the thermodynamic equilibrium constant for bisulphate dissocation (top of chart). After this come the interactions between the hydrogen ion and chloride ion, and the TrisH+ ion and chloride ion.

The key finding is that, of the very many interactions represented by parameters in the chemical speciation model, only a very few matter. This is extremely helpful for model development.

What about dissolved organic matter, and trace metals? Due to its polydisperse nature, the thermodynamics of natural organic matter (NOM) cannot be treated using the Pitzer approach that we are applying to standard seawater and Tris buffers. However, NOM is important as a trace metal complexant, and also as a contributor to titration alkalinity in lower salinity waters.  Martha Gledhill and Pablo Lodeiro at GEOMAR  have begun using the NICA-Donnan model, developed for freshwater NOM, to characterise the chemistry of marine NOM. Together with David Turner at the University of Gothenburg, they are assessing the use of a novel model code that combines the Pitzer and NICA-Donnan approaches. The first application to be studied is a new data set from the Amazon estuary.

At the recent SCOR Meeting the lifetime of WG 145 was extended for at least a further year in recognition of what we have achieved so far and of our current and planned activities.

News (March 2020)

The Working Group met for the fourth time on February 16th in San Diego, just before  AGU/ASLO Ocean Sciences in San Diego. This meeting, including notes summarising our discussions, is described under the Meetings link above. We were particularly pleased
to talk with Associate Member Regina Easley about the possible participation of NIST in our experimental programme.

The solubility measurements carried out by Pablo Lodeiro work (GEOMAR), and studies of the statistics of our Pitzer-based chemical speciation model “MarChemSpec” (by Matthew Humphreys, UEA), were presented at the meeting. At our lunchtime event, and in the SCOR booth in the exhibition hall, we demonstrated our draft speciation models showing the calculation of uncertainty contributions to the predicted pH of Tris buffers, and K* of the carbonate system in seawater. This was successful and attracted a good number of participants. The models will remain available here, and we encourage you to try them. Questions and feedback are welcome.

Our email list of scientists who would like to be kept up to date with our progress now
has over 100 names. If you would like to be added to it, please contact Heather Benway (hbenway@whoi.edu).

News (February 2020)

Draft software for the calculation of pH in Tris buffers in artificial seawater and other solutions, and carbonate system K* in standard seawater and other natural waters, is now available here: MarChemSpec. It is easy to use, with both user input and results displayed on a single web page. Try it!

The chemical speciation model used in this software yields estimates of the total uncertainties in the calculated quantities, and also the individual contributions of all thermodynamic equilibrium constants and Pitzer ion-interaction coefficients to those totals. We believe this is unique in this type of model, and a valuable research tool.

We will be talking about the model, and demonstrating the software, at the Ocean Sciences meeting in San Diego this month (see the post below). Please join us, if you can.

News (January 2020)

The Working Group will hold a meeting at Ocean Sciences 2020 in San Diego (16-21 February), and will also host a lunchtime session on Thursday 20 February, 12.45-1.45pm. At this session we will describe our latest results, including some software that we have developed for the calculation of pH in Tris buffers in artificial seawater, and carbonate system K* in standard seawater. One-on-one demonstrations of the software will be given at the SCOR Exhibit Booth (#341). Please see this announcement.

After Ocean Sciences, the links to the two web-based demonstration programs that we showed at the meeting will be made available on this site.

At the meeting, members of the Working Group will be giving the following presentations:

Finally, we are pleased to announce that Working Group member Andrew Dickson (Scripps Institution of Oceanography) is leading a newly formed IAPSO-sponsored Best Practices Study Group in Seawater pH Measurements.

News (July 2019)

Further experiments

This month, Pablo Lodeiro and Eric Achterberg (GEOMAR, Kiel) are starting further solubility experiments to characterise the thermodynamic properties of the buffer substance Tris in aqueous solutions of the components of seawater. The work on Tris/NaCl solutions (referred to in the previous news item) is complete, and the new experiments will focus on the interaction of Tris with sulphate ions. They will be carried out by Lucía González, a Chemistry student at the Universidad Autónoma de Madrid, under Pablo’s supervision. We are very pleased that Lucía has chosen to join us for the summer and take part in the important work being carried out at GEOMAR.

A further contribution is also likely to be made by Professor Jonathan Reid and Florence Gregson of the Bristol Aerosol Research Centre (BARC). Professor Reid’s group specialises in measurements of the behaviour and properties of single, suspended, aerosol droplets. The techniques that have been perfected at BARC can be used to make accurate measurements of thermodynamic properties, and Florence will be determining the wateractivity / molality relationship of aqueous Tris solutions to much higher concentrations than has been done in the past. Although, of course, the molality of Tris in seawater pH buffers is low, the high concentration data will enable us to more precisely determine the values of the interaction parameters for the Pitzer speciation model
of the buffer that we are developing.

Florence’s work will be immediately valuable in helping us analyse the results of the experiments at GEOMAR, and we are grateful to her and to Jonathan for their contribution.

Speciation model development

David Turner (University of Gothenburg) has completed, with Pablo Lodeiro, a preliminary analysis of Pablo’s experiments on Tris/NaCl aqueous solutions. This will probably be finished after Florence’s planned work at Bristol has been done (see above).

Matthew Humphreys and Simon Clegg at UEA have recently completed the coding of the chemical speciation model for seawater, and Tris buffers in artificial seawater, that is the core of this project. We have also partially completed sensitivity testing, which has already enabled us to rank the influence of uncertainties in values of equilibrium constants and Pitzer parameters to model-calculated quantities such as pH. This will allow us to focus future experimental and modelling work, aimed at improving the accuracy of the model,
on the key systems (individual electrolytes, and simple mixtures) that have the greatest influence. It is also a necessary step towards the quantitative estimation of uncertainties in model-calculated quantities. Seawater pH is the most important of these, to be followed by carbonate speciation in the future when we extend the model to standard seawater.

Harned cell studies

At the beginning of this year we completed an intercomparison of measurements of two HCl/NaCl aqueous solutions, one very dilute and one concentrated (5 mol kg-1 ionic strength). The participating laboratories were NIST (USA), PTB (Braunschweig, Germany), LNE (Paris, France), NMIJ (Japan), and SIO (USA). We discovered that the concentrated solution, which contained a much higher Cl. molality than any of those routinely measured by the laboratories, caused electrode degradation in some cases. This month a further intercomparison is being done. We expect to avoid the degradation problem by reducing the ionic strength of the more concentrated solution to 1 mol kg-1.

The results of this second intercomparison, in combination with what has already been achieved, will yield a quantitative measure of the mutual consistency of HCl activities determined by the different laboratories. This is important for the project, because measurements from all of these laboratories will be used to improve the speciation model (Pitzer model interaction parameters will be determined from the data).

News (February 2019)

New member of our team

We are delighted to welcome postdoctoral scholar Dr Ellen Briggs to our group. She is employed at the Scripps Institution of Oceanography (SIO), and will work with Andrew Dickson on the Harned cell experiments, and heat capacity determinations, that are a part of the joint project with Simon Clegg and Matthew Humphreys at the University of East Anglia.

Ellen completed her Ph.D at SIO with Todd Martz in 2017, and the theme of her dissertation was the expansion of marine biogeochemical observations through the creation of novel autonomous sensors for constraining the aqueous carbon dioxide system. She focused on the development of solid state sensors capable of rapid (<60 s) and simultaneous measurement of pH and AT (total alkalinity) of seawater for monitoring the aqueous carbon dioxide system. Initial results with these ISFET devices indicate precision of 2-10 μmol kg-1 for AT and 0.005 for pH. They require no external reagents, have low power consumption, and meet the rugged demands required for integration with autonomous platforms. The prototype sensor has undergone preliminary field testing, and been deployed at Kaneohe Bay, Hawaii. Ellen also served as Co-Chief Scientist, in 2018, on the US GO-SHIP repeat hydrography cruise S04P. With her considerable laboratory and instrumental skills she will be a valuable contributor to our project, and to the work of SCOR WG145.


Pablo Lodeiro and Eric Achterberg (GEOMAR, Kiel) have almost completed a large series of solubility measurements of the uncharged buffer substance Tris (or THAM, tris(hydroxymethyl)aminomethane) in aqueous NaCl solutions from 5 oC to 45 oC, and of NaCl in aqueous Tris. These measurements will help us to quantify the interactions in artificial seawater solutions containing TrisH+/Tris pH buffer that control the activity coefficients of the Tris and the pH. We will fit the results to obtain the relevant interaction parameters in the Pitzer model. This work will contribute to the accurate model of the buffer system that we are developing in order to calculate pH, and chemical speciation, for applications involving both seawater and other natural waters (of varying composition).

We are also currently analysing the results of the laboratory intercomparison exercise in which the electromotive forces of a series of test solutions were measured using Harned Cells at the national metrology laboratories of Germany, France, and Japan; the National Institute of Standards and Technology; and the laboratory of WG 145 member Andrew Dickson at Scripps Institution of Oceanography. One of the important findings, so far, is that the different methods of electrode construction employed at the labs, and the varying lengths of time each set of electrodes is used, can lead to significant variations in performance. In particular, the test solution containing the highest Cl- molality could not be reliably measured in some cases. We are discussing how to solve this problem, and to what extent the design of the solutions to be measured in our experimental programme needs to be altered.

News (August 2018)


We have now mostly completed a laboratory intercomparison exercise in which the electromotive forces of a series of test solutions are being measured using Harned Cells at the national metrology laboratories of Germany, France, and Japan; the National Institute of Standards and Technology; and the laboratory of WG 145 member Andrew Dickson at Scripps Institution of Oceanography (SIO) . The intercomparison involves two test solutions, one of very low ionic strength (0.1 mol kg-1) and one of high ionic strength (5.0 mol kg-1). Samples of both solutions were prepared at SIO and then distributed to all participants for measuring. Also, each laboratory prepares and measures their own samples.

Most of these laboratories are also making Harned cell measurements to contribute to the development of the speciation model. The purpose of the intercomparison is to quantify the consistency that can be expected of data from the different labs. Our first finding has been that the measurements on the very concentrated solution have highlighted differences in electrode preparation and treatment by the laboratories, which are important for our future work.


Our draft manuscript describing the needs of oceangraphers for chemical speciation modelling tools, the results of surveys of the chemical oceanography community, and reviews of modelling tools by Working Group members, is now in the final stages of preparation. It has been commented on by Working Group members, and will be submitted to Best Practices in Ocean Observing.

This month Andrew Dickson will be advertising a postdoc position at his laboratory at Scripps. The researcher will be to carry out Harned cell measurements on salt solutions, and develop novel methods for the accurate measurement of heat capacities of solutions using a differential scanning calorimeter. If you are interested, please contact adickson@ucsd.edu.

Working Group member Martha Gledhill has been awarded a grant from the German Research Council DFG to characterise the chemistry of marine dissolved organic matter using the NICA-Donnan modelling approach. The results from this project will represent an important step in extending seawater speciation models to include dissolved organic matter. The project will start in January 2019.


Simon Clegg, Andrew Dickson, and members of the national metrology laboratories of German and France will be attending the 17th ICPWS (the International Conference on the Properties of Water and Steam) in Prague at the beginning of September. There are sessions on the thermodynamic properties of seawater, and marine chemistry and seawater pH, which are very relevant to the activities of WG 145. We will be discussing our future work and its relationship to the concerns of the Joint Committe on Seawater, and particularly the development of marine pH and metrological traceability to SI units.

David Turner will be representing the Working Group at the Advanced Workshop on the Solution Chemistry of TCEs (technology-critical elements), to be held in January 2019 in Bialstock, Poland. The workshop includes consideration of  trace element speciation calculations, and the measurement of equilibrium constants and their incorporation into databases.


News (May 2018)


At GEOMAR the measurements of the solubility of Tris in aqueous NaCl solutions from 25 oC to 45 oC have now been completed, and the remaining set of experiments (for 0 oC to 20 oC) will begin shortly. As described in the news entry for April, the data will be used to help quantify the effects of Na+ and Cl ions on Tris activity coefficients and to determine the values of interaction parameters in the Pitzer speciation model we are developing.


We have just uploaded a report describing the results of our surveys of chemical oceanographers to assess their needs for chemical speciation modelling tools. This report can be found on the Publications page of this website. The survey results will inform the design of the modelling programs that are one of the outputs of the Working Group. This design is being outlined, to encourage further input from the community, in the manuscript mentioned in last month’s post.

News (April 2018)

This is a summary of news about our current activities.


Pablo at GEOMAR has just completed the first of a series of measurements of the solubility of Tris in aqueous NaCl solutions at different temperatures. The data, together with the results of Harned Cell experiments to be carried by other WG 145 members, will be used to quantify the effects of Na+ and Cl- ions on Tris activity coefficients and to determine the values of interaction parameters in the Pitzer speciation model we are developing.

At Scripps Institution of Oceanography Andrew Dickson will very shortly have a new technician “in post”, who will be carrying out many of the Harned Cell experiments that we have planned for our project. One of his first tasks, after training, will be to take part in an inter-laboratory comparison of Harned cell measurements of emfs of a small number of standard solutions. Andrew Dickson’s laboratory will produce one set of these standard solutions, batches of which will be distributed to all laboratories. In addition, each laboratory will prepare samples of the same solutions, and measure both sets. The following laboratories are taking part: Scripps Institution of oceanography, NIST (Gaithersburg, USA), PTB (Braunschweig, Germany), LNE (Paris, France), NMIJ (Japan).

We are currently discussing our project with Jelena Miladinovic and her colleagues at the Faculty of Technology and Metallurgy, University of Belgrade. They are one of relatively few groups who carry out isopiestic experiments (very accurate measurements of water activity), which would be a valuable complement to Harned Cell data (which yield HCl activity products). We hope that Jelena and her group will be able to join our collaboration and, later this year, measure water activities of some of the key mixtures of Tris and seawater salts.


David, Heather, Simon, and other members of WG 145 are preparing a manuscript describing the results of our online survey of chemical oceanographers regarding their usage/needs for chemical speciation models, and the reviews of current models carried out by WG 145 members early in the life of this Working Group. The manuscript will also contain our detailed proposals for the speciation modelling tools that will be produced by WG 145 (chemical species includes, inputs and outputs, links with “core” chemical oceanography models such as CO2SYS, etc.). It is expected that our survey results will be placed on this website, both in order to be referenced by the manuscript and for general comment.


At the beginning of September 2018, in Prague, there is an ICPWS conference (17th International Conference on the Properties of Water and Steam). Andrew Dickson will be participating in the session and workshop “Marine chemistry and seawater pH”. It is likely that Simon Clegg will attend too, in order to talk about our project. We hope other members of the collaboration will also be able to come to the meeting. The detailed programme of the conference isn’t available yet, but the website is http://www.icpws2018.com/.