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
liter 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 Ω 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.
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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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