Zimmer and Peacock provide a pack of calibration solutions for testing your potassium sensors.
Potassium Calibration Solution Kit - ZP1000985
The product allows you to test your potassium sensor in clinically relevant concentrations.
The kit contains different concentration solution: 0 mM, 2mM, 4mM, 6mM and 8 mM - 10 ml of each concentration.