The principles by which glass electrodes work, which are in turn how pH meters work, were investigated by a German chemist called Fritz Haber and his student: Zygmunt Klemensiewicz. Their findings were published in 1909.
The modern definition of pH was only set in 1924 and so before this pH testing did not really exist, even though people were aware of general concepts of acids and alkalis.
Even after 1924, pH testing was something that was left to professionals and scientists, as there was little need for it in everyday life.
The glass electrodes were turned into pH meters in around 1934/5 by American chemist Arnold Beckman. He attached the electrodes to an amplifier and voltmeter to make it more sensitive.
