We carry pH/ORP Storage Solution with buffer preservative (mold inhibitor) added. Each bottle contains 500 mL of pH/ORP Storage Solution. We sell this as: pH Storage Solution, order code PH-SS, $18.
If you wish to prepare your own storage solution, you can do so by adding 10g of solid potassium chloride (KCl) to 100 mL of a buffer pH 4 solution. It is also a good idea to add some buffer preservative (add 3 drops preservative/100 mLs solution). The preservative can be purchased from Flinn Scientific (1-800-452-1261) if needed (order code BO175, 60-mL bottle). We currently ship our electrodes in a buffer storage solution that already has Buffer Preservative added to it.
If you do not have pH 4 buffer solution, we do offer pH Buffer Capsule Kit, order code PH-BUFCAP, $29. If you don't have these available, you can prepare your own pH 4 buffer using the following recipe: Add 2.0 mL of 0.1 M HCl to 1000 mL of 0.1 M potassium hydrogen phthalate.