The dielectric constant of the medium at 310 K is assumed equal to 74.31 and is used to calculate the Debye-Hückel terms of 0.52 and 1.33, assuming an ionic radius of 4Å. ZH and ZL are the charges of the indicator species in their protonated and deprotonated states, respectively. Limiting chemical shifts and pKa values for all indicators are provided relative to DSS at 0 ppm (Table ). These limiting values can be related to DMSO or pyrazine as chemical shift references as these compounds are invariant with pH (Figure , c), averaging 2.7141 ppm (DMSO) and 8.6434 ppm (pyrazine). The pH was calculated by taking the sensitivity weighted average of the pH reported by acetate and maleate, as described in our previous work. Table . δH, δL and pKa for determination of pH in cell culture media at 310 K (I = 0.174 M) using Equation . We note that only acetate and maleate were used in this work, however data for other indicators is presented for convenience. All chemical shifts are reported relative to DSS (0.0 ppm).