In this video, we will calibrate the EXO Nitrate Ion Selective Electrode, or ISE, in mg/L using Kor Software. You can also use the EXO Handheld or the Kor Mobile App.
Before calibrating, we want to make sure our nitrate sensor has been soaked overnight in 100 mg/L nitrate standard solution. New sensors are shipped dry, so this step will hydrate the sensor and get it ready for calibration. It is also a good idea to soak sensors that have been used in the field because this can help remove interfering ions that might have been picked up during use. After soaking, rinse and dry the sensor before proceeding with the calibration.
As a best practice, make sure all the sensors, probe guard, cal cup, and wiper are clean. It may be helpful to have a dedicated probe guard and cal cup just for calibration. I also suggest removing the bottom plate from the guard for ISE calibrations to minimize the chance of fluid transfer between standards.
We will perform a 2-point calibration for this sensor. We’ll use a 1 mg/L nitrate standard solution as the first point and a 100 mg/L nitrate standard as the second point. These solutions can be purchased from YSI. If you expect significant temperature variation during deployment, you may choose to do a 3-point calibration. You can find instructions for this in the EXO manual.
So let’s begin. Fill the cal cup to the first line with the 1 mg/L nitrate standard and immerse the sonde.
In Kor, we will select nitrate to calibrate. Kor Software will indicate when the readings are stable in real time on this graph. Note that the nitrate millivolt value or raw sensor value should be around 200 mV +/- 20 mV for a new sensor. An older sensor may fall outside of this range for the first point, but if it has a good slope on the second point, it will still provide accurate measurements. Once readings stabilize, we’ll click apply and proceed to the next standard. Click to add another cal point. Now we’ll remove the sonde from the standard and rinse and dry the sensor guard and sensors.
For Cal Point 2, we will use the 100 mg/L nitrate standard. Fill the cal cup to the first line and immerse the sonde. You should see readings around 90 to 130 millivolts below the millivolt value of the first cal point. This will indicate a good slope and that the sensor is performing within spec.
Wait for readings to stabilize before applying the calibration. Once readings are stable, click apply.
Now you will see the calibration summary screen where you can choose to redo the cal point or complete the calibration. I’ll choose to complete the calibration.
Our EXO Nitrate ISE is now calibrated! Please check out our other EXO University videos, and thanks for watching.