Direct Determination of Potassium—Sodium Activity Ratio in Soil Suspension with Two Ion—Selective Electrodes

A K^+-selective electrode and a Na^+-selective electrode were used to construct a measuring cell without liquid-junction for the determination of the ion activity ratio of K^+ to Na^+ in soil suspensions.The measured cell potential was not affected by the total electrolyte concentration when the total cation concentration was 10^-1-10^-3 mol L^-1 and the concentration ratio Ck+/CNa+ was 10:1 to 1:50.When the concentration ratios were equal to 1 and the total electrolyte concentrations were 10^-2 and 10^-3 mol L^-1,the ion activity ratio measurenent would not be affected by pH in the pH range of 3.5 to 11.5 and 4.4 to 11 respectively.Ions other than H^+ have no remarkable influence on the measurement.The ion activity ratio of K^+ to Na^+ measured directly in soil suspension agree well with those in centrifuged supernant solution.The relative deviation was within 4%.From the measured ion activity ratio,the difference of the bonding energies of K^+ and Na^+ ions was calculated.