The slope and response characteristics of the ORP sensor can be tested in quinhydrone-saturated pH 4 and 7 solution.  A properly functioning ORP electrode should exhibit a difference of 90 ± 40 mV when submersed in the two solutions of quinhydrone-saturated pH 4 and pH 7 solutions.

Prepare two beakers, one with pH 4 solution and the other with pH 7 solution.  Add a “pinch” of quinhydrone crystals to each of the buffers while stirring gently until saturated (no more crystals dissolve). Then stir for 30 seconds to finish preparation of the standards.

Rinse the electrode with distilled or de-ionized water, blot dry, and then place the electrode into a beaker containing quinhydrone saturated pH 7 buffer. Stir gently, and wait for readings to stabilize.

When the readings stabilize record the mV value.  The mV reading should be 85 ± 20 mV.

Rinse the electrode with distilled or de-ionized water, blot dry and place in the quinhydrone-saturated pH 4 buffer. Stir gently and wait for the readings to stabilize.

When readings stabilize, record the mV value.  The reading should be 175 ± 20 from the reading in the quinhydrone-saturated pH 7 solution.

If the difference in potentials is not 90 ± 40 mV, follow the instructions in the “Cleaning and Reconditioning” section. Please note that the quinhydrone-saturated pH standard should be used within a few hours of preparation.

If electrode performance is still not restored, the sensor may need to be returned to the NexSens Service Department for assessment.


The quality of results depends greatly upon the quality of the standards. Use fresh standards when problems arise; it could save hours of troubleshooting. Errors may result from the contamination or degradation of prepared standards.

Quinhydrone-saturated pH 4 and pH 7 solutions should be prepared fresh and used within a few hours.

Zobell solution comes in powdered form and is mixed with distilled or deionized water.  After mixing the solution, it should be refrigerated and used within 6 months.

Keep all standards tightly covered when not in use.

See WQ-ORP Smart USB Sensor Measurement Techniques for additional details on proper measurement techniques to be used with the sensor.

REV: 13H24

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