Arduino pH sensor

Introduction

3 major commercially available or mass-produced solutions:

1. Atlas Scientific and Gravity analog pH Kit with board, sensor and buffer solutions. Hiqh quality, not open source.
2. DFRobot Gravity: Analog pH Sensor Kit
3. Other (Chinese) manufacturers.
4. Seeed Studio

If the pH electrode is labotorary grade, it is not designed for submerging into measuring solution permanently. Thus, use an industrial-grade pH electrode (lifecycle about 3 years), but calibrate it at least twice every year.

Theory

Briefly

An ideal pH sensor because H⁺ ions. Ranging linearly:

• U = 0 @ pH 7
• U > 0 @ pH < 7
• U < 0 @ pH > 7

A high quality op amp is needed to amplify the voltage.

The pH electrode

• a passive sensor

See more about impedance and Arduino ADC at https://www.e-tinkers.com/2020/07/a-review-on-seeed-studio-ph-and-ec-sensor-kits-part-1/

Nernst Equation

The Gibbs free energy is related to the free energy change under standard state by

$\Delta G=\Delta G^{0}+RT\ln Q$

where $Q$ is the reaction potential, $R$ is the universal ideal gas constant. Also

$\Delta G=-zFE$ where $F=N_{A}q$ is the Faraday constant, $z$ is the number of electrons transferred in cell reaction, and thus we have

${\begin{aligned}E_{\text{cell}}&=E_{\text{cell}}^{0}-{\frac {RT}{zF}}\ln Q\\&=E_{\text{cell}}^{0}-{\frac {2.3RT}{nF}}\ln Q\end{aligned}}$

pH electrode

Usually made of a glass. Glass membrane is filled with a buffer solution of known pH. When the probe is dipped into the solution, hydrogen ions in the test solution start exchanging with other positively charged ions on the glass membrane, which creates an electrochemical potential across the membrane

References

https://www.e-tinkers.com/2019/11/measure-ph-with-a-low-cost-arduino-ph-sensor-board/