Arduino pH sensor

Introduction

"The probe generates millivolt signals that drift with temperature, the signal conditioning board needs proper calibration, and those cheap pH electrodes from online marketplaces degrade faster than you’d expect." https://pcbsync.com/ph-sensor-arduino/

Some 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. Seeed Studio
4. DIY MORE
5. Other (Chinese) manufacturers.

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.

The probe:

• Keep wet (store in KCl solution).
• Don’t put in hot/corrosive stuff.
• Rinse with distilled water after use.

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

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

The voltage generated across the probe's glass membrane follows the Nernst equation and changes by approximately 59.16 mV/pH at 25°C.

pH electrode

A pH electrode is a voltmeter that measures the electrical potential difference created by hydrogen ions interacting with a glass membrane. Is is usually made of a glass (typically lithium-doped silicate glass). Glass membrane is filled with a buffer solution (usually potassium chloride, pH=7) of known pH. When the probe is immersed into the solution, hydrogen ions in the test solution interact with other positively charged ions on the glass membrane, which creates an electrochemical potential across the membrane

Temperature effects

1. The electrode’s voltage response changes with temperature (the 59.16 mV/pH value varies).
2. the actual pH of solutions changes with temperature independent of measurement effects.

Add a DS18B20 waterproof temperature sensor.

Calibration

Electric

https://www.electroniclinic.com/ph-meter-arduino-ph-meter-calibration-diymore-ph-sensor-arduino-code/

• short the external part and the center of the probe connector: 2.5V output.

References

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