Application of potentiometric method using a cell without liquid junction to underway pH measurements in surface seawater

► We evaluated a method to carry out underway pH measurements of surface seawater. ► It works by means of a cell without liquid junction using electrodes for hydrogen and sodium ions. ► It was shown that the cell has long-term stability and can be calibrated only once a day. ► A buffer solution, TRIS-HCl-TRIS-NaCl-H2O, is recommended for calibration. ► This approach has been applied to a cruise in the Sea of Okhotsk during September and October 2007.