Chemical speciation models based upon the Pitzer activity coefficient equations, including the propagation of uncertainties. II. Tris buffers in artificial seawater at 25 °C, and an assessment of the seawater ‘Total’ pH scale

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 seawater chemistry. development an acid-base chemical speciation model containing Tris, major ions desirable so that: (i) effects changes composition medium on can be calculated; (ii) free (a measure [H+]) total ([H+] + [HSO4?])) scales interconverted; (iii) approximations inherent definition scale quantified; (iv) electrode pairs such as H+/Cl? H+/Na+ more easily calibrated measurement pH. As a first step towards these goals we have extended Pitzer-based Waters Millero (Mar. Chem. 149, 8–22, 2013) artificial to include at 25 °C. Estimates variances covariances additional interaction parameters were obtained by Monte Carlo simulation. This enables uncertainty any model-calculated quantity (e.g., pH, speciation) estimated, well individual contributions all equilibrium constants. important development, because it allows key interactions identified. was tested against measured EMFs cells °C, mean deviation found 0.13 ± 0.070 mV salinities 20 40. Total calculated electromotive forces are dominated from just few parameters, making likely that readily improved. quantify difference between various definitions –log10([H+] [HSO4?]) time (item above). results suggest low using established approach substituting TrisH+ Na+ solutions, especially if effect substitution. relationships force (EMF), scale, with molality constant salinity shown linear above about 0.01 0.02 mol kg?1 molality. buffers ratios vary unity very simply. Technical aspects extrapolation zero (at salinity), clarified. Recommendations made further work extend temperature range 0–45 improve accuracy, requirements fully met.