Latest News (July 2022)

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.


News (August 2021)

We have just submitted our first two modelling papers to Marine Chemistry (the abstracts are shown below).

The first manuscript concerns the model of artificial seawater, which is the core of any speciation model of seawater and related natural waters. It particularly focuses on acidified artificial seawater, because measurements of these solutions are essential to the definition of the ‘total’ pH scale for seawater, developed by WG 145 member Andrew Dickson.

The second manuscript describes the first speciation model of the Tris buffer solutions (equimolal Tris and TrisH+ in artificial seawater) used to define the total pH scale. We examine the assumptions inherent in the definition of the scale, the relationship of total pH to true total hydrogen ion concentration, and prospects for the extension of the scale to low salinities. There may be implications for the calculation of carbonate system equilibria from measured total pH.

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: Artificial Seawater from 0 to 45 oC

Matthew P. Humphreys, Jason F. Waters, David R. Turner, Andrew G. Dickson, and Simon L. Clegg

ABSTRACT: Accurate chemical speciation models of solutions containing the ions of seawater have applications in the calculation of carbonate system equilibria and trace metal speciation in natural waters, and the determination of pH. Existing models, based on the Pitzer formalism for the calculation of activity coefficients, do not yet agree with key experimental data (potentiometric determinations of H+ and Cl activity products in acidified artificial seawaters) and, critically, do not include uncertainty estimates. This hampers both applications of the models, and their further development (for which the uncertainty contributions of individual ion interactions and equilibrium constants need to be known). We have therefore implemented the models of Waters and Millero (Mar. Chem. 149, 8-22, 2013) and Clegg and Whitfield (Geochim. et Cosmochim. Acta 59, 2403-2421, 1995) for artificial seawater, within a generalised treatment of uncertainties, as a first step towards a more complete  model of standard seawater and pH buffers. This addition to the model enables both the total uncertainty of any model-calculated quantity (e.g., pH, speciation) to be estimated, and also the contributions of all interaction parameters and equilibrium constants. Both models have been fully documented (and some corrections made). Estimates of the variances and covariances of the interaction parameters were obtained by Monte Carlo simulation, with simplifying assumptions.  The models were tested against measured electromotive forces (EMFs) of cells containing acidified artificial seawaters. The mean offsets (measured – calculated) at 25 oC for the model of Waters and Millero are: 0.046±0.11 mV (artificial seawater without sulphate, 0.280 mol kg-1 to 0.879 mol kg-1 ionic strength);  and -0.199±0.070 mV (artificial seawater, salinities 5 to 45). Results are similar at other temperatures. These differences compare with an overall uncertainty in the measured EMFs of about 0.04 mV. Total uncertainties for calculated EMFs of the solutions were dominated by just a few contributions: mainly H+-Cl, Na+-Cl, and H+-Na+-Cl ionic interactions, and the thermodynamic dissociation constant of HSO4. This makes it likely that the accuracy of the models can readily be improved, and recommendations for further work are made. It is shown that calculated standard EMFs used in the definition of the marine ‘total’ pH scale can be accurately predicted with only slight modification to the original models, suggesting that they can contribute to the extension of the scale to lower salinities.


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 the Marine ‘Total’ pH Scale

Simon L. Clegg, Matthew P. Humphreys, Jason F. Waters, David R. Turner, and Andrew G. Dickson

ABSTRACT: The substance Tris (or THAM, 2-amino-2-hydroxymethyl-1,3-propanediol, CAS 77-86-1), and its protonated form TrisH+, is used in the preparation of pH buffer solutions for applications in seawater chemistry. The development of an acid-base chemical speciation model of buffer solutions containing Tris, TrisH+, and the major ions of seawater is desirable so that: (i) the effects of changes in the composition and concentration of the medium on pH can be calculated; (ii) pH on the free (a measure of [H+]) and total (a measure of ([H+] + [HSO4])) scales can be interconverted; (iii) approximations inherent in the definition of the total pH scale can be quantified; (iv) electrode pairs such as H+/Cl and H+/Na+ can more easily be calibrated for the measurement of pH. As a first step towards these goals we have extended the Pitzer-based speciation model of Waters and Millero (Mar. Chem. 149, 8-22, 2013) for artificial seawater extended to include Tris and TrisH+, at 25 oC. Estimates of the variances and covariances of the additional interaction parameters were obtained by Monte Carlo simulation (Humphreys et al., submitted to Mar. Chem.). This enables both the total uncertainty of any model-calculated quantity (e.g., pH, speciation) to be estimated, and also the individual contributions of all interaction parameters and equilibrium constants. This is important for model development, because it allows the key interactions to be identified. The model was used to quantify the difference between the operationally defined total pH scale and true -log10([H+] + [HSO4]) in Tris buffer solutions at 25 oC, for the first time. The results suggest that the total pH scale can readily be extended to low salinities using the established approach for substituting TrisH+ for Na+ in the buffer solutions, especially if the speciation model is used to quantify the effect on pH of the substitution. The relationships between electromotive force (EMF), and pH on the total scale, with buffer concentration artificial seawater at constant salinity are shown to be linear over a reasonable range of buffer molality. The pH of Tris buffers containing ratios of TrisH+ to Tris that vary from unity can be very simply calculated (not requiring a model). Other technical aspects of the total pH scale, such as the extrapolation of pH to zero buffer (at constant salinity), are examined. The model was tested against measured EMFs of cells containing Tris buffer in artificial seawater at 25 oC, and the mean deviation (measured – calculated) was found to be 0.13±0.070 mV for salinities 20 to 40, using TrisH+-Cl interaction parameters revised in this work. Total variances for calculated electromotive forces of the buffer solutions are dominated by contributions from just a few parameters, making it likely that the model can readily be improved with respect to accuracy. Recommendations for further work are made in order to extend the model to the 0 – 45 oC, and reduce errors to within or close to the experimental uncertainties of the data upon which the total pH scale is based.

News (July 2021)

We presented a poster at the Ocean Carbon and Biogeochemistry Summer Workshop (2021), entitled “Chemical Speciation Models Including the Propagation of Uncertainties: Application to the Marine ‘Total’ pH Scale“. This describes our project overall: our collaborators, the ‘timeline’, some key results, and future plans. Download and view the pdf below (click on the green section headers to move between pages).

Our next posts, which will appear soon, will describe the contents of the first two of the major results of the project – evaluated and tested chemical speciation models of Tris buffers in artificial seawater, and acidified artificial seawater, and implications for the marine total pH scale.

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.

Experiments

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.