pH Measurement Principles¶
A typical pH probe
pH measurement is fundamental in a wide variety of applications, including agriculture, wastewater treatment, industrial processes, environmental monitoring, and research and development. A standard pH measuring system consists of three core elements:
- pH Electrode (Probe): The sensor that generates a voltage proportional to the pH of the solution.
- Temperature Compensation Element: A sensor to correct for the temperature-dependency of the pH electrode.
- pH Meter/Controller: The instrument that processes the electrode's voltage and temperature to display a calibrated pH reading.
What is pH?¶
pH is the measure of the acidity or alkalinity of a solution. The pH value quantifies the concentration of hydrogen ions (H+).
- A higher concentration of H+ results in a more acidic solution and a lower pH value.
- A lower concentration of H+ results in a more alkaline solution and a higher pH value.
Most pH readings range from 0 to 14. Solutions with a pH less than 7 are acidic, while solutions with a pH greater than 7 are basic or alkaline. A pH of 7 is considered neutral.
Specifically, the pH value of a solution is the negative logarithm of its hydrogen ion activity (α), which is the product of the hydrogen ion concentration [H+] and its activity coefficient (γH+) at that concentration:
pH Electrodes¶
Construction¶
The modern pH electrode is a combination electrode, which integrates a glass electrode and a reference electrode into a single body.
Cross-section of a combination pH probe
pH is determined by measuring the potential difference (voltage) between these two electrodes.
- The glass electrode features a thin glass membrane at its tip, which is selectively permeable to hydrogen ions. This is the sensing element that responds to the pH of the test solution.
- The reference electrode provides a stable, constant potential. It contains a reference element (e.g., Ag/AgCl) in contact with a fill solution of a known, fixed pH (typically pH 7).
High-Impedance Nature of pH Probes¶
A critical characteristic of pH electrodes is their very high source impedance, resulting from the large electrical resistance of the glass bulb (typically 10 MΩ to 1000 MΩ). This necessitates the use of a high-impedance measuring device to avoid drawing current from the electrode, which would polarize it and lead to inaccurate readings.
Ultra-Low Input Bias for Maximum Accuracy
The uThing::iPH™ analog front-end is meticulously designed for this challenge. It features an input bias current as low as 20 fA (femtoamperes), corresponding to an input impedance well into the TeraOhm (TΩ) range. This ensures a negligible offset error (e.g., ±0.2 mV for a 1 GΩ probe), enabling highly accurate measurements even with the most demanding probes.
Transfer Function & Temperature Dependence¶
The voltage output of a pH electrode follows the Nernst equation, which defines its transfer function:
Where:
- E is the voltage from the probe.
- E₀ is the standard potential at pH 7 (ideally 0 mV).
- R is the universal gas constant.
- T is the absolute temperature in Kelvin.
- F is the Faraday constant.
The term (2.303 * R * T / F) is the Nernst slope, which represents the electrode's sensitivity in mV per pH unit. As the equation shows, this slope is directly proportional to temperature.
Temperature is Critical
This temperature dependence is significant. For example:
- At 25°C, the slope is 59.16 mV/pH. The output swings from +414 mV (pH 0) to -414 mV (pH 14).
- At 100°C, the slope increases to 74.04 mV/pH. The output swings from +518 mV (pH 0) to -518 mV (pH 14).
Failure to compensate for temperature variations could lead to significant measurement errors depending on the temperature of the medium.
Automatic Temperature Compensation (ATC)¶
The uThing::iPH™ performs continuous Automatic Temperature Compensation (ATC) by measuring the solution's temperature and applying the correct Nernst slope in the pH calculation. The dongle uses two possible temperature sources:
1. On-Board Temperature Sensor¶
The dongle integrates a calibrated on-board sensor to measure ambient temperature. This is the default source and provides acceptable correction when the solution temperature is close to ambient.
Info
For best results with the on-board sensor, connect the dongle via a USB extension cable. This isolates it from host computer heat, ensuring the sensor accurately reflects the ambient temperature.
2. External Temperature Probe¶
For the highest accuracy, especially when the solution's temperature deviates significantly from ambient, an external temperature probe is recommended. The uThing::iPH™ supports an isolated, waterproof DS18B20 temperature probe (sold separately).
The dongle automatically detects and uses the external probe for ATC when connected.
BNC Connector¶
The uThing::iPH™ uses a standard BNC connector, ensuring compatibility with a wide range of consumer and lab-grade pH and ORP (Oxidation-Reduction Potential) probes.
BNC connector and compatible probes
Probe Calibration¶
Ideal vs. Real Probes
An ideal pH electrode has a 0 mV offset at pH 7.00 and a theoretical slope of -59.16 mV/pH at 25°C. In reality, all probes deviate from this ideal response. Furthermore, their characteristics change over time due to aging, contamination of the glass membrane, and depletion of the reference electrolyte.
To ensure accurate and repeatable measurements, regular calibration is essential.
Calibration Frequency¶
The need for calibration depends on the application and electrode condition:
- A new probe must always be calibrated.
- For high-accuracy applications or when measuring aggressive solutions, daily or weekly calibration may be necessary.
- For general water-quality monitoring, monthly calibration usually suffices.
Verifying Calibration
To quickly check if calibration is needed, immerse the probe in a known pH buffer. If the reading deviates from the buffer's value by more than your required accuracy (e.g., > 0.05 pH), a new calibration is recommended.
Please refer to the Calibration section for detailed instructions on the procedure.