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).