Understanding pH measurement

In the process world, pH is an important parameter to be measured and controlled.

The pH of a solution indicates how acidic or basic (alkaline) it is.
The pH term translates the values of the hydrogen ion concentration
- which ordinarily ranges between about 1 and 10 x -14 gram-equivalents
per litre - into numbers between 0 and 14.

On the pH scale a very acidic solution has a low pH value such as 0, 1, or 2 (which corresponds to a large concentration of hydrogen ions;
10 x 0, 10 x -1, or 10 x -2 gram-equivalents per litre) while a very basic
solution has a high pH value, such as 12, 13, or 14 which corresponds
to a small number of hydrogen ions (10 x -12, 10 x -13, or 10 x -14
gram-equivalents per litre).
A neutral solution such as water has a pH of approximately 7.

A pH measurement loop is made up of three components, the pH
sensor, which includes a measuring electrode, a reference electrode,
and a temperature sensor; a preamplifier; and an analyser or transmitter.
A pH measurement loop is essentially a battery where the positive
terminal is the measuring electrode and the negative terminal is the
reference electrode. The measuring electrode, which is sensitive to
the hydrogen ion, develops a potential (voltage) directly related to
the hydrogen ion concentration of the solution. The reference
electrode provides a stable potential against which the measuring
electrode can be compared.


Typical pH sensor

When immersed in the solution, the reference electrode potential
does not change with the changing hydrogen ion concentration.
A solution in the reference electrode also makes contact with the
sample solution and the measuring electrode through a junction,
completing the circuit. Output of the measuring electrode changes
with temperature (even though the process remains at a constant pH),
so a temperature sensor is necessary to correct for this change in
output. This is done in the analyser or transmitter software.

The pH sensor components are usually combined into one device
called a combination pH electrode. The measuring electrode is
usually glass and quite fragile. Recent developments have replaced
the glass with more durable solid-state sensors. The preamplifier
is a signal-conditioning device. It takes the high-impedance pH
electrode signal and changes it into alow impedance signal which
the analyser or transmitter can accept. The preamplifier also
strengthens and stabilizes the signal, making it less susceptible
to electrical noise.

The sensor's electrical signal is then displayed. This is commonly
done in a 120/240 V ac-powered analyser or in a 24 V dc loop-powered
transmitter.

Additionally, the analyser or transmitter has a man machine
interface for calibrating the sensor and configuring outputs
and alarms, if pH control is being done.

Keep in mind, application requirements should be carefully
considered when choosing a pH electrode. Accurate pH measurement
and the resulting precise control that it can allow, can go a long
way toward process optimisation and result in increased product
quality and consistency. Accurate, stable pH measurement also
controls and often lowers chemical usage, minimising system
maintenance and expense.

Keeping the system up and running.
A system's pH electrodes require periodic maintenance to clean
and calibrate them. The length of time between cleaning and
calibration depends on process conditions and the user's accuracy
and stability expectations. Overtime, electrical properties of the
measuring and reference electrode change. Calibration in
known-value pH solutions called buffers will correct for some
of these changes. Cleaning of the measuring sensor and reference
junction will also help. However, just as batteries have a limited life,
a pH electrode's lifetime is also finite. Even in the "friendliest"
environments, pH electrodes have to be replaced eventually.

From an article by Tom Griffiths, Honeywell


pH measurement
A very important measurement in many liquid chemical processes
(industrial, pharmaceutical, manufacturing, food production, etc.)
is that of pH: the measurement of hydrogen ion concentration in a
liquid solution. A solution with a low pH value is called an "acid,"
while one with a high pH is called a "caustic." The common pH scale
extends from 0 (strong acid) to 14 (strong caustic), with 7 in the
middle representing pure water (neutral):

pH is defined as follows: the lower-case letter "p" in pH stands for the
negative common (base ten) logarithm, while the upper-case letter
"H" stands for the element hydrogen. Thus, pH is a logarithmic
measurement of the number of moles of hydrogen ions (H+) per litre
of solution. Incidentally, the "p" prefix is also used with other types of
chemical measurements where a logarithmic scale is desired, pCO2
(Carbon Dioxide) and pO2 (Oxygen) being two such examples.

The logarithmic pH scale works like this: a solution with 10-12 moles
of H+ ions per liter has a pH of 12; a solution with 10-3 moles of H+
ions per liter has a pH of 3. While very uncommon, there is such a
thing as an acid with a pH measurement below 0 and a caustic with
a pH above 14. Such solutions, understandably, are quite concentrated
and extremely reactive.

While pH can be measured by color changes in certain chemical powders
(the "litmus strip" being a familiar example from high school chemistry
classes), continuous process monitoring and control of pH requires a
more sophisticated approach. The most common approach is the use
of a specially-prepared electrode designed to allow hydrogen ions in
the solution to migrate through a selective barrier, producing a
measurable potential (voltage) difference proportional to the solution's
pH:

The design and operational theory of pH electrodes is a very complex
subject, explored only briefly here. What is important to understand is
that these two electrodes generate a voltage directly proportional to
the pH of the solution. At a pH of 7 (neutral), the electrodes will produce
0 volts between them. At a low pH (acid) a voltage will be developed
of one polarity, and at a high pH (caustic) a voltage will be developed
of the opposite polarity.

An unfortunate design constraint of pH electrodes is that one of them
(called the measurement electrode) must be constructed of special
glass to create the ion-selective barrier needed to screen out hydrogen
ions from all the other ions floating around in the solution. This glass is
chemically doped with lithium ions, which is what makes it react
electrochemically to hydrogen ions. Of course, glass is not exactly what
you would call a "conductor;" rather, it is an extremely good insulator.
This presents a major problem if our intent is to measure voltage between
the two electrodes. The circuit path from one electrode contact, through
the glass barrier, through the solution, to the other electrode, and back
through the other electrode's contact, is one of extremely high resistance.

The other electrode (called the reference electrode) is made from a
chemical solution of neutral (7) pH buffer solution (usually potassium
chloride) allowed to exchange ions with the process solution through a
porous separator, forming a relatively low resistance connection to the
test liquid. At first, one might be inclined to ask: why not just dip a metal
wire into the solution to get an electrical connection to the liquid? The
reason this will not work is because metals tend to be highly reactive
in ionic solutions and can produce a significant voltage across the
interface of metal-to-liquid contact. The use of a wet chemical interface
with the measured solution is necessary to avoid creating such a voltage,
which of course would be falsely interpreted by any measuring device
as being indicative of pH.

Here is an illustration of the measurement electrode's construction.
Note the thin, lithium-doped glass membrane across which the pH
voltage is generated:

Here is an illustration of the reference electrode's construction. The
porous junction shown at the bottom of the electrode is where the
potassium chloride buffer and process liquid interface with each other:

The measurement electrode's purpose is to generate the voltage
used to measure the solution's pH. This voltage appears across the
thickness of the glass, placing the silver wire on one side of the voltage
and the liquid solution on the other. The reference electrode's purpose
is to provide the stable, zero-voltage connection to the liquid solution
so that a complete circuit can be made to measure the glass electrode's
voltage. While the reference electrode's connection to the test liquid
may only be a few kilo-ohms, the glass electrode's resistance may
range from ten to nine hundred mega-ohms, depending on electrode
design! Being that any current in this circuit must travel through both
electrodes' resistances (and the resistance presented by the test liquid
itself), these resistances are in series with each other and therefore
add to make an even greater total.

An ordinary analog or even digital voltmeter has much too low of an
internal resistance to measure voltage in such a high-resistance circuit.
The equivalent circuit diagram of a typical pH probe circuit illustrates
the problem:

Even a very small circuit current traveling through the high resistances
of each component in the circuit (especially the measurement electrode's
glass membrane), will produce relatively substantial voltage drops
across those resistances, seriously reducing the voltage seen by the
meter. Making matters worse is the fact that the voltage differential
generated by the measurement electrode is very small, in the millivolt
range (ideally 59.16 millivolts per pH unit at room temperature). The
meter used for this task must be very sensitive and have an extremely
high input resistance.

The most common solution to this measurement problem is to use an
amplified meter with an extremely high internal resistance to measure
the electrode voltage, so as to draw as little current through the circuit
as possible. With modern semiconductor components, a voltmeter with
an input resistance of up to 1017 O can be built with little difficulty.

Another approach, seldom seen in contemporary use, is to use a
potentiometric "null-balance" voltage measurement setup to measure
this voltage without drawing any current from the circuit under test.
If a technician desired to check the voltage output between a pair of
pH electrodes, this would probably be the most practical means of
doing so using only standard benchtop metering equipment:

As usual, the precision voltage supply would be adjusted by the
technician until the null detector registered zero, then the voltmeter
connected in parallel with the supply would be viewed to obtain a
voltage reading. With the detector "nulled" (registering exactly zero),
there should be zero current in the pH electrode circuit, and therefore
no voltage dropped across the resistances of either electrode, giving
the real electrode voltage at the voltmeter terminals.

Wiring requirements for pH electrodes tend to be even more severe
than thermocouple wiring, demanding very clean connections and
short distances of wire (10 yards or less, even with gold-plated
contacts and shielded cable) for accurate and reliable measurement.
As with thermocouples, however, the disadvantages of electrode
pH measurement are offset by the advantages: good accuracy and
relative technical simplicity.

Few instrumentation technologies inspire the awe and mystique
commanded by pH measurement, because it is so widely misunderstood
and difficult to troubleshoot. Without elaborating on the exact chemistry
of pH measurement, a few words of wisdom can be given here about
pH measurement systems:

All pH electrodes have a finite life, and that lifespan depends greatly
on the type and severity of service. In some applications, a pH electrode
life of one month may be considered long, and in other applications the
same electrode(s) may be expected to last for over a year.

Because the glass (measurement) electrode is responsible for generating
the pH-proportional voltage, it is the one to be considered suspect if
the measurement system fails to generate sufficient voltage change
for a given change in pH (approximately 59 millivolts per pH unit), or
fails to respond quickly enough to a fast change in test liquid pH.


If a pH measurement system "drifts," creating offset errors, the
problem likely lies with the reference electrode, which is supposed to
provide a zero-voltage connection with the measured solution.
Because pH measurement is a logarithmic representation of ion
concentration, there is an incredible range of process conditions
represented in the seemingly simple 0-14 pH scale. Also, due to the
nonlinear nature of the logarithmic scale, a change of 1 pH at the top
end (say, from 12 to 13 pH) does not represent the same quantity of
chemical activity change as a change of 1 pH at the bottom end (say,
from 2 to 3 pH). Control system engineers and technicians must be
aware of this dynamic if there is to be any hope of controlling process
pH at a stable value.

The following conditions are hazardous to measurement (glass)
electrodes: high temperatures, extreme pH levels (either acidic or
alkaline), high ionic concentration in the liquid, abrasion, hydrofluoric
acid in the liquid (HF acid dissolves glass!), and any kind of material
coating on the surface of the glass.

Temperature changes in the measured liquid affect both the response
of the measurement electrode to a given pH level (ideally at 59 mV
per pH unit), and the actual pH of the liquid. Temperature measurement
devices can be inserted into the liquid, and the signals from those
devices used to compensate for the effect of temperature on pH
measurement, but this will only compensate for the measurement
electrode's mV/pH response, not the actual pH change of the process
liquid!

Advances are still being made in the field of pH measurement, some
of which hold great promise for overcoming traditional limitations of
pH electrodes. One such technology uses a device called a field-effect
transistor to electrostatically measure the voltage produced by a
ion-permeable membrane rather than measure the voltage with an
actual voltmeter circuit. While this technology harbors limitations of
its own, it is at least a pioneering concept, and may prove more
practical at a later date.

REVIEW:
pH is a representation of hydrogen ion activity in a liquid. It is the
negative logarithm of the amount of hydrogen ions (in moles) per
liter of liquid. Thus: 10-11 moles of hydrogen ions in 1 liter of liquid
= 11 pH. 10-5.3 moles of hydrogen ions in 1 liter of liquid = 5.3 pH.

The basic pH scale extends from 0 (strong acid) to 7 (neutral, pure
water) to 14 (strong caustic). Chemical solutions with pH levels
below zero and above 14 are possible, but rare.

pH can be measured by measuring the voltage produced between
two special electrodes immersed in the liquid solution.

One electrode, made of a special glass, is called the measurement
electrode. It's job it to generate a small voltage proportional to pH
(ideally 59.16 mV per pH unit).

The other electrode (called the reference electrode) uses a porous
junction between the measured liquid and a stable, neutral pH buffer
solution (usually potassium chloride) to create a zero-voltage
electrical connection to the liquid. This provides a point of continuity
for a complete circuit so that the voltage produced across the
thickness of the glass in the measurement electrode can be measured
by an external voltmeter.

The extremely high resistance of the measurement electrode's glass
membrane mandates the use of a voltmeter with extremely high
internal resistance, or a null-balance voltmeter, to measure the
voltage.


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