In the process of trying to calibrate a number of hygrometers, with Sensirion MEMS electronic sensors, I came across the method of using wet salt: an airtight container will stabilize at a predictable relative humidity over time, given a particular salt and temperature.
For a list of salts and their RH values at particular temperatures see this page on the website of Vernier, a maker of STEM education equipment. The various salts suggested for this method repeatably produce a range of low to high relative humidities, also allowing for two-point calibration.
I require my hygrometers be calibrated close to 40% RH. With a predictable 43% RH that makes potassium carbonate the ideal candidate. However, when I wettened potassium carbonate with water it underwent an moderate exothermic reaction. Googling what was going on, I read "potassium carbonate dissociates completely in water into potassium (K+) and carbonate ions (CO32-)."
Which makes me wonder:
Can potassium carbonate be used at all for the wet salt calibration method, or does its swift dissociation render it useless for this application?