(a) ZA bears two exchangeable protons, one in a carboxy group with an acid dissociation constant pKa1=2.35 and a second one in an enolic hydroxy group with pKa2=6.90. (b) Spectra of hyperpolarized ZA and urea in buffer phantoms at different pH values. The pH sensitivity in the physiological range is due to pKa2 and results in a change of the chemical shifts of the two 13C-labelled carbon positions with respect to the pH insensitive 13C urea peak as a function of pH. Additional peaks of a decay product of ZA, PP hydrate (PPH), can be identified. (c) The fit of a scaled logistic function to the data from b results in a calibration curve with a direct dependence between chemical shift and pH.