A new approach to measuring pH in brines and other concentrated electrolytes

We propose an operational and theoretical approach to dealing with pH in brines and other concentrated aqueous solutions. The operational method is analogous to the standard procedure for measuring pH in dilute solutions, but uses a liquid-junction-free cell consisting of an ion specific electrode (ISE) for H + and another for a different ion, such as Cl −. The latter ISE replaces the standard reference electrode used in the usual procedure. The potential measured is proportional to pHCl = pH + pCl, or any corresponding quantity, such as pH/Na = pH − pNa, depending on the type of second ISE. The procedure also requires the separate measurement of the total concentration of the ion to which the second ISE responds, and input of these and other solution data to a geochemical modeling code (EQ3NR), which uses the Pitzer equations (Pitzer, 1973) to compute a thermodynamic model of the solution. High ionic strength standard solutions, whose compositions are calculated using the same code, are used to calibrate the liquid-junction-free cell. A pH scale convention is then used to decouple pH from pCl, pNa, pBr, etc. We investigated ISE couples pairing cations (K + and Na +) as well as anions (Br −) against H + to demonstrate the general applicability of this approach to measuring pH. The effects of these and other potentially interfering ions (Ca 2+, Mg 2+, BO 3− 3, SO 2− 4, and HCO − 3) on the various ISE couples were studied. By making a HCl measurements in sample solutions equilibrated with calcite we have used the solubility of calcite in high ionic strength solutions (determined using the Harvie et al., 1984, derived thermodynamic data consistent with the Pitzer formulation) as an independent verification of the method.