Immerse a bulb G made from a specific glass membrane with solution B, in which the pH is proven, into test solution A; then, electromotive force is generated on both sides of the glass membrane. Immerse appropriate electrodes E1 and E2 into both solution A and B, and measure the electric potential difference between the two electrodes using a voltmeter V to acknowledge the electromotive force generated on the glass membrane. At this time, single-electrode potentials e1 and e2 are generated on the electrodes E1 and E2, and these potentials will be e1 + E - e2 from the voltmeter (E is the electromotive force of the pH to be measured.). e2 is constant, as electrode E2 and solution B are invariant; e1 varies depending on solution A (test solution) and pH cannot be calculated. As shown in Figure 2, by immersing electrodes E1 and E2 into the same KCl-saturated solution, e1 = e2 and e1 + E - e1 = E, pH can be calculated. When two different types of solutions with different properties make contact, an electric potential difference particular to the property and type of solution, the so-called “liquid junction electromotive force,” is generated. KCl solution has such a property that its electric potential difference is constant to any solution. Therefore, a KCl solution is generally used as the internal liquid of a reference electrode.