Datasheet

LMP91200
SNAS571C –JANUARY 2012–REVISED MARCH 2013 www.ti.com
Application Information
Theory of pH measurement
pH electrode measurements are made by comparing the readings in a sample with the readings in standards
whose pH has been defined (buffers). When a pH sensing electrode comes in contact with a sample, a potential
develops across the sensing membrane surface and that membrane potential varies with pH. A reference
electrode provides a second, unvarying potential to quantitatively compare the changes of the sensing
membrane potential. Nowadays pH electrodes are composed of a sensing electrode with the reference electrode
built into the same electrode body, they are called combination electrodes. A high input impedance meter serves
as the readout device and calculates the difference between the reference electrode and sensing electrode
potentials in millivolts. The millivolts are then converted to pH units according to the Nernst equation.
Electrode behavior is described by the Nernst equation:
E = Eo + (2.3 RT/nF) log aH+, where
E is the measured potential from the sensing electrode,
Eo is related to the potential of the reference electrode,
(2.3 RT/nF) is the Nernst factor,
log aH+ is the pH, (aH+ = activity of Hydrogen ions).
2.3 RT/nF, includes the Gas Law constant (R), Faraday’s constant (F), the temperature in degrees Kelvin (T) and
the stoichiometric number of ions involved in the process (n). For pH, where n = 1, the Nernst factor is 2.3 RT/F.
Since R and F are constants, the factor and therefore electrode behavior is dependent on temperature. The
Nernst Factor is equivalent to the electrode slope which is a measure of the electrode response to the ion being
detected. When the temperature is 25 °C, the theoretical Nernst slope is 59.16 mV/pH unit.
LMP91200 in pH meter with ATC (Automatic Temperature Compensation)
The most common cause of error in pH measurements is temperature. Temperature variations can influence pH
for the following reasons:
• the electrode slope will change with variations in temperature
• buffer and sample pH values will change with temperature
Measurement drift can occur when the internal elements of the pH and reference electrodes are reaching thermal
equilibrium after a temperature change. When the pH electrode and temperature probe are placed into a sample
that varies significantly in temperature, the measurements can drift because the temperature response of the pH
electrode and temperature probe may not be similar and the sample may not have a uniform temperature, so the
pH electrode and temperature probe are responding to different environments.
The pH values of buffers and samples will change with variations in temperature because of their temperature
dependent chemical equilibria. The pH electrode should be calibrated with buffers that have known pH values at
different temperatures. Since pH meters are unable to correct sample pH values to a reference temperature, due
to the unique pH versus temperature relationship of each sample, the calibration and measurements should be
performed at the same temperature and sample pH values should be recorded with the sample temperature.
The LMP91200 offers in one package all the features to build a pH meter with ATC. Through the SPI Interface is
possible to switch from pH measurement mode to temperature measurement mode and collect both temperature
and potential of sensing electrode.
pH measurement
The output of a pH electrode ranges from 415 mV to −415 mV as the pH changes from 0 to 14 at 25°C. The
output impedance of a pH electrode is extremely high, ranging from 10 MΩ to 1000 MΩ. The low input bias
current of the LMP91200 allows the voltage error produced by the input bias current and electrode resistance to
be minimal. For example, the output impedance of the pH electrode used is 10 MΩ, if an op amp with 3 nA of
Ibias is used, the error caused due to this amplifier’s input bias current and the source resistance of the pH
electrode is 30 mV! This error can be greatly reduced to 1.25µV by using the LMP91200.
20 Submit Documentation Feedback Copyright © 2012–2013, Texas Instruments Incorporated
Product Folder Links: LMP91200