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)₂·12H₂O, 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.