Image credit to Kellie Griffen
I'm wondering how the very first measurement of pH was conducted.
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Arnaldus de Villa Nova around the year 1300 is credited with developing a litmus test for pH, (Chemical Education Digital Library), utilizing a blue dye derived from lichens in the Netherlands. A good chronology of the advancement of pH testing technology from 1906-2009 can be found in John Buie's 2010 article in Lab Manager, October 07, 2010, "Evolution of the pH Meter: Tracing the origins of the pH meter."