Ion selective electrode
ID: ion-selective-electrode
An ion selective electrode is tool used in analytical chemistry used to determine the concentration of a given ion in a solution.
The method can determine the concentration of one ion even if there are multiple different types of ion in the solution, although in some cases this can alter the results. This is done with the help of a selective membrane that only allows certain ions through.
One cool example application mentioned on LibreTexts[ref] is the measurement of fluorine concentration in water. They explain that fluorine is added to the drinking water supply in some countries to help protect people's teeth, and so you want to be able to measure as part of quality control to make sure it is being added in correct the quantities.
The method requires calibrating with calibrating solutions which takes one hour, but once you are calibrated you just stick the sensor into the analyte and get readings in a minute without any expendables. The membrane seems to be a bit fragile which requires care, but overall it looks like a convenient method.
Ion selective electrode schematic
. Source. 
Sample calibration curve for an ion selective electrode
. Source. The curve is simple and log linear, so once you have that it is easy to fit new measurements to the curve.Ion selective electrodes Tech Tips by Vernier
. Source. The plump Asian American lady with a PhD explains how to use the thing.
It seems like a proprietary training video given with the product that this Peruvian university decided to upload to YouTube. Heroes.
Using an Ion Selective Electrode by University of Alberta
. Source. New to topics? Read the docs here!