This is a 500 mL bottle of 150 µS/cm conductivity standard. Use it to calibrate your conductivity probe.
For most applications, the Conductivity Probe and Go Direct® Conductivity Probe do not need to be calibrated. If you choose to calibrate it, you will need accurate standard solutions. The 150 µ S/cm Standard is a good concentration to calibrate your Conductivity Probe in the low range (0 to 200 µ S/cm). Vernier sells a 500 mL bottle of this solution.