Question:
I have a customer who is using an electrochemical sensor for CO to comply with NIOSH Method 6604, however they are using Nitrogen instead of air as Zero Gas; they have Nitrogen for Gas Chromatography purposes. This Nitrogen has 99.9% purity. An ISO 17025 auditor told them that this could affect the calibration process for this sensor, so the customer is asking me if this is true.
Do you think that the Nitrogen could affect the sensitivity of CO for this electrochemical sensor? Or is this procedure correct?
Thanks in advance for your help!
Answer:
The first step in the calibration process is the “fresh air calibration” or “fresh air zero adjustment”. In this step the instrument looks at the electrical output of the CO sensor while it is located in contaminant free fresh air, and adjusts the instrument reading to 0 ppm. This provides a point of comparison when the sensor is exposed to atmosphere that contains carbon monoxide. The electrical output of the sensor when it is exposed to CO is higher than the electrical output of the sensor in fresh air. The difference in electrical output is proportional to the concentration. Calibration gas that contains CO is used to adjust the reading of the sensor while it is exposed to gas. This is referred to as the “span calibration” or “gas calibration” step.
The electrochemical reaction used to detect CO requires oxygen. However, the sensor does not require or use O2 during the fresh air zero adjustment step.
Given the tolerances and accuracy of the electrochemical CO sensors used in health and safety instruments, using nitrogen instead of fresh air to adjust the CO sensor should not materially affect the accuracy of the fresh air zero adjustment. However, it is always better to use zero contaminant fresh air, not pure nitrogen for this calibration step.
All electrochemical CO sensors use the same two part reaction to detect gas. The first half-cell reaction occurs at the sensing electrode. The second half-cell reaction occurs at the counter electrode.
Carbon monoxide is oxidized at the sensing electrode. Water from the sensor electrolyte is used up during this first step. The electrolyte in the sensor is an aqueous solution of sulfuric acid.
2CO + 2H2O → 2CO2 + 4H+ + 4e-
The counter electrode acts to balance out the reaction at the sensing electrode by reducing oxygen present in the air to water. The water that is consumed in the first step is regenerated in the second step. The oxygen used in this step comes from O2 dissolved in the electrolyte. The oxygen dissolved in the electrolyte comes from the atmosphere in which the sensor is located.
O2 + 4H+ + 4e- → 4H2O
When you add the two half-cell reactions together you get the overall reaction:
2CO + O2 → 2CO2
For every 2 molecules of CO you detect you use up one molecule of O2.
Note that unless the sensor is in the presence of a high concentration of CO, very little O2 is actually used by the sensor. Even when the atmosphere is oxygen deficient there is usually more than enough oxygen in the air to replace O2 consumed in the detection reaction.
Long term exposure to pure nitrogen eventually depletes the oxygen in the electrolyte. If the oxygen is depleted, the sensor is unable to detect gas. However, this takes quite a while to occur. The electrochemical sensors used in GfG portable instruments are full sized, “4 Series” sensors. Span calibrating or zero-adjusting a GfG electrochemical CO sensor with pure nitrogen, or using CO in nitrogen to span adjust the sensor will not materially affect the accuracy of readings.
The time it takes for the electrolyte to become oxygen depleted is affected by the volume of electrolyte in the sensor. Miniaturized or low volume (flat) CO sensors that contain less electrolyte, (and less dissolved oxygen) can become O2 depleted more quickly.
So, as a general rule, it is better when possible to use contaminant free fresh air to zero-adjust the sensor, and CO calibration gas that includes oxygen when a span calibration adjustment is performed.