This video is an overview and tutorial for an analog pH sensor that's compatible with both Arduino and Raspberry Pi but I'll also give a general overview of linear functions and interpolation. Plus I'll show you how to write a custom code to use this sensor on an unsupported platform (Espruino).

Measuring pH is pHun! (...I'm here all night folks.)

/* DF Robot Analog pH Sensor */
https://www.dfrobot.com/product-1782....
Arduino Library: https://github.com/DFRobot/DFRobot_PH
Python Library: https://github.com/DFRobot/DFRobot_PH...

/* DF Robot ADC Converter */
https://www.dfrobot.com/product-1730....

/* Pixl.js */
https://shop.espruino.com/pixljs
Espruino IDE: https://www.espruino.com/ide/