Use of a simple empirical model for the accurate conversion of the seawater pH value measured with NIST calibration into seawater pH scales

The seawater pH measurement is usually quite complicated because that matrix is characterized by a high ionic strength leading to calibration errors if NIST standards are used. For this matrix, different pH scales like the "total hydrogen ion concentration scale" (TOT) and the "seawater scale" (SWS), are defined, and suitable synthetic seawater solutions must be prepared according to standard procedures to calibrate the glass electrode. This work provides a new approach to make seawater pH measurements by using the glass electrode calibrated with the NIST standards (pH_NIST) converting the pH_NIST into the right TOT or SWS scales by using empirical equations derived from theoretical thermodynamic data: pH_TOT=pH_NIST+0.10383+4.33⋅10^-5TS+3.633⋅10^-5T²-4.921⋅10^-5S², and pH_SWS=pH_NIST+0.097733+4.1059⋅10^-5TS+3.5437⋅10^-5T²-4.941⋅10^-5S², for the TOT and SWS scales, respectively. These equations are functions of two simple experimental parameters, namely, T = temperature (°C) and S = salinity (PSU, (g/L), Practical Salinity Units). These equations were experimentally validated and the uncertainty of pH_TOT and pH_SWS was demonstrated to have no statistical difference with the corresponding values obtained following the standard operative procedure (SOP) using commercially unavailable seawater-like buffers. The proposed method has therefore the same performances and it is largely preferable as it avoids long and tedious procedures of the synthetic seawater preparations.