LMP91200

+

± pH

BUFFER

VCM

BUFFER

VDD

VDD

GND

GND

MCU

VOUT

ADC

VOCM

VREF

GND

VCMHI

NC

NC

GUARD1

GUARD2

INP

VCM

GND

pH Electrode

15

LMP91200

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SNAS571E – JANUARY 2012 – REVISED FEBRUARY 2016

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Copyright © 2012–2016, Texas Instruments Incorporated

8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

8.1 Application Information

8.1.1 Theory of pH Measurement

The 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. These days, pH electrodes are composed of a sensing electrode with the reference

electrode built into the same electrode body, and 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).

(1)

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.

Because 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.

8.2 Typical Application

Figure 34. Typical Application