Chemical Engineering May 2022 - 38

Feature Report
A Primer on
pH Measurement
Understanding the basic principles of operation and construction designs of glass
membrane sensors can help in choosing the right pH sensor
Colin Ziegler,
Endress+Hauser Liquid
Analysis
IN BRIEF
BACKGROUND
BASIC PRINCIPLES OF
pH MEASUREMENT
FACTORS AFFECTING pH
MEASUREMENT
DIGITAL VERSUS
ANALOG pH SENSORS
FUTURE TRENDS IN pH
SENSORS
Background
The glass electrode pH meter, as we know it
today, was first invented by Arnold Beckman
in 1934 [1]. As with many scientific breakthroughs,
the impetus for the development
of a new technology came from a specific
market need. In this case, it was the citrus
industry of California that called for an accurate
measure of acidity that would not be
influenced by the presence of sulfur dioxide
preservatives. Beckman's invention was
based on the principle that measuring the
activity of H+ ions in a solution informs us
about its concentration.
Beckman's glass-tube electrode method
of measuring pH is still used today. However,
several developments have helped to improve
the reliability and longevity of modern devices.
pH measurement is not only critical in the citrus
industry, other industries like pharmaceuticals,
food and beverages, and water treatment
all rely on accurate pH measurement
(Figure 1). Understanding the functional principles
of pH measurement and developments
in the technology is key to choosing the most
suitable pH sensor for each application.
Basic principles of pH measurement
In pure water, a small number of molecules
dissociate into H+ and OH- ions:
H2O I H+ + OH-
J
38
O
ne of the most critical measurements
in liquid chemical processing
is pH. In this article, the
importance of the sensor construction
to ensure quality measurements is
discussed, as well as the critical factors to
consider when choosing a pH sensor. This
article also explores the key differences between
analog and digital sensors. Finally,
readers will learn how digital sensors can
improve workers' safety while decreasing
the maintenance efforts and lowering the
possibility of measurement value drift.
Endress+Hauser
FIGURE 1. The measurement of pH is important for many sectors
of the chemical process industries, including pharmaceuticals,
food-and-beverages and water/wastewater treatment
The extent of this dissociation can be described
by the self-ionization constant of water,
Kw, which depends only on the temperature.
(1)
At 25°C, Kw = 1.01 ×10-14 for pure water.
When acids or bases are added to the water,
the number of ions change, but Kw remains
the same, which allows us to use a consistent
pH scale for all aqueous solutions. An acid
has a higher activity of H+ ions than OH- ions,
while a neutral solution has an equal activity
of both. An alkaline solution has a lower activity
of H+ ions than OH- ions. The relationship
of pH and H+ activity, a(H+), is defined by a
logarithmic equation:
(2)
In pure water, there is an equilibrium between
H+ and OH-, so at 25°C, the activity
of H+ is nearly 10-7. From Equation (2), the
pH of pure water is -log10(10-7) = 7, which is
defined as neutral. For concentrations of H+
greater than 10-7, the pH will be less than 7
(acidic) and for concentrations less than 10-7,
the pH will be greater than 7 (alkaline).
A pH sensor is effectively an electrochemical
cell that changes its electrical potential on
a glass membrane based on the activity of
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