The ideal test solution for an ORP electrode is a commercially-produced standard. Many companies sell ORP standards: Hach, Cole Parmer, and Hanna are three examples.
You can also prepare you own test solution. Here are the recipes for two:
Light's Solution: prepare a 0.1 M solution of both ferric ammonium sulfate (NH4Fe(SO4)2 · 12 H2O, also known as ammonium iron (III) sulfate) and ferrous ammonium sulfate ((NH4)2Fe(SO4)2· 6H2O, also known as ammonium iron (II) sulfate) in 1.0 M sulfuric acid. This solution has a potential of 470 - 480 mV.
1.4080 g K4Fe(CN)6 · 3H20 (Potassium ferrocyanide)
1.0975 g K3Fe(CN)6 (Potassium ferricyanide)
7.4557 g KCl (Potassium chloride)
Dissolve these chemicals in deionized water and dilute the solution to 1.00 L.
This solution has a potential of 225-235 mV.
Connect your ORP sensor to your platform. Immerse the tip of the ORP sensor in a test standard solution. Allow the ORP sensor about a minute to warm up, and check the reading.
You can also use Light's Solution and ZoBell's Solution to calibrate the ORP sensor, but commercial solutions will be more accurate.