In the pH measurement process, using the reference electrode, in most cases the reference electrode with salt solution. The reference electrode through the salt bridge solution and the solution to be measured and communicated in the solution to be measured and the reference solution of salt bridge is formed between the liquid junction, the liquid junction ion in the solution on both sides continuously across the interface diffusion on the opposite side, due to the diffusion of ions at different speeds, and the liquid junction on both sides with a charge of opposite sign, generating potential the difference, which is called the liquid junction potential, also known as the diffusion potential, it will affect the measurement accuracy of electrode. In order to reduce the liquid junction potential difference or to keep it stable and reproducible, you must use the KCl solution with high concentration (more than 3mol/L) as the reference electrode salt solution, it has high ionic strength enough, 5~10 times larger than the measured medium, the maximum possible ionic strength in the liquid junction the interface of K+ and Cl-, always spread outward, and because the K+ and Cl- mobility are almost equal, positive and negative charge distribution, thus forming a small liquid junction potential, which is why the reference electrode in the reference solution must use high concentration KCl solution.