pHT measurements of TRIS buffer solutions in artificial seawater matrix in the salinity range 5–40 and temperature range 5–40°C. Part 2: Uncertainty of pHT values

In the first part of this study, we determined a fit function which allows the calculation of the total pH (pH T ) value of an artificial seawater/2-amino-2-(hydroxymethyl)-1,3-propanediol (TRIS) buffer, extrapolated to 0 mol kg⁻¹ H 2 O TRIS molality (i.e. to a true pH T scale), in terms of temperature and practical salinity. In this second part, we present a method to quantify the uncertainty of that function. The uncertainty plays an important role in the characterization of the indicator dye used for the routine spectrophotometric pH T measurement in seawater. However, it has not been determined previously. We assigned uncertainties to measured input quantities, and propagated them through the calculation steps described in the first part. Both, analytical uncertainty calculation and Monte Carlo simulations have been applied to adequately consider the various steps of the procedure needed to calculate pH T values of artificial seawater. The combined standard uncertainty of pH T ( S , T , b ( TRIS ) = 0 mol kg −1 H 2 O ) for artificial seawater/TRIS buffers was 0.0011 over the practical salinity range 5 to 40 and temperature range 5 °C to 40 °C. This uncertainty can now be used to determine, the uncertainties of the characterization of the indicator dye used for seawater pH T measurements.