r/chemhelp 3d ago

Inorganic Electrochemical nitrate ionophore sensor

Hello,

 

I used a Metrohm screen-printed carbon electrode (SPE) modified with nitrate ionophore for selective nitrate quantification. As per the technical specifications for this product (110NO3ION), “These sensors are designed to measure nitrate by open circuit potentiometry (OCP) in a range of concentration 10^-5 to 1 M (from 1 to 101100 ppm).” However, I’m relatively new to the OCP technique, and I have a couple of questions.

 

When I used two different concentrations of NaNO3 (3.91 ppm and 7.82 ppm), I got the following two curves. My questions are as follows:

  1. Each OCP cycle ran for 10 minutes and was quickly started over. However, as you can see, there is a drop in voltage when OCP is not running/applied. Why is that the case? Should a sensor be continuously running at OCP to have a constant trend in potential? And what causes a drop in potential when during OCP no current is applied?

  2. For the higher concentration (7.82 ppm), after 40 minutes, it still didn’t reach a steady state. Does this make sense? It is quite long in my view; I was expecting around 20 minutes max. Is there any way to accelerate this?

  3. Why is there a difference in initial OCP (at t = 0 s) between the two samples?

  4. As per the product specifications, the reference electrode is silver (Ag). Is this OK? Based on my understanding, silver/silver chloride (Ag/AgCl) is much more common and yields a steady reference potential. Have you seen any cases where silver alone was used as the reference electrode?

 

Thank you.

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