Alkalinization of Thin Layer Samples with a Selective Proton Sink Membrane Electrode for Detecting Carbonate by Carbonate-Selective Electrodes

Potentiometry is known to be sensitive to so-called free ion activity and is a potentially valuable tool in environmental speciation analysis. Here, the direct detection of free and total carbonate is demonstrated by alkalinization of a thin layer sample (∼100 μm), which is electrochemically triggered at a pH responsive membrane placed opposite a carbonate-selective membrane electrode. The concept may serve as a promising future methodology for in situ environmental sensing applications where traditional sampling and pretreatment steps are no longer required. The possibility of increasing the pH of the sample was demonstrated first with a proton selective membrane (pH readout at zero current) placed opposite the thin layer gap. An optimal applied potential (600 mV) for 300 s resulted in a pH increase of 4 units in an artificial sample, with a relative standard deviation (RSD) of ∼2%. The pH probe was subsequently replaced by a solid contact carbonate selective electrode for the determination of carbonate species (4.17 μM) in a sample of 1 mM NaHCO3. Increasing the pH to 12.1 by the electrochemically controlled proton sink allowed one to convert bicarbonate to the detectable carbonate species. Initial bicarbonate concentration (∼1 mM) was obtained as the difference between the converted bicarbonate and the initial carbonate concentration. An initial application of this concept was illustrated by the speciation analysis of an unfiltered sample from the Arve river (12.3 ± 0.2 μM and 22.5 ± 0.3 mM carbonate and bicarbonate, respectively). The values were confirmed by volumetric titration.