For long term health of your elctrode use pH storage solution. It is a mixture of Potassium Chloride, Postassium Hydrogen Phthalate, Silver Nitrate, and Sodium Hydroxide.
Volume: 230 ml
Electrode storage solution is designed specifically for maintaining pH meter electrodes in optimal condition. Before storage, add a small amount of storage solution to the cup/cover of the pH meter (enough to submerge the glass electrode). This will maintain hydration of the electrode, increasing its life and its ability to be calibrated.