MASH 210, 311
T.Elsarnagawy
A pH meter is an electronic instrument used to measure the
pH (acidity or alkalinity) of a liquid
A typical pH meter consists of a special measuring probe (a
glass electrode) connected to an electronic meter that
measures and displays the pH reading
The pH term translates the values of the hydrogen ion
concentrationinto numbers between 0 and 14.
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.
consequently, pH is a logarithmic measurement of the
number of moles of hydrogen ions (H+) per liter of solution
pH = log10
1
+
+
(H )
= −log10 (H )
Specially-prepared electrode are
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
a measuring electrode
must be constructed of special glass
to screen out hydrogen ions from all
the other ions floating around in the
solution
The measuring electrode, develops a
potential (voltage) directly related to
the hydrogen ion concentration of
the solution
▪ Note the thin lithium-doped glass
membrane across which the pH
voltage is generated
a reference electrode
The reference electrode
provides a stable
potential against which
the measuring electrode
can be compared
The porous junction
shown at the bottom of
the electrode is where the
potassium chloride buffer
and process liquid
interface with each other
a battery
where the positive terminal is the measuring electrode and the
negative terminal is the reference electrode.
a temperature sensor
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 preamplifier
is a signal-conditioning device
It takes the high-impedance pH electrode signal and changes it into a
low impedance signal which the analyzer 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
The analyzer or transmitter
has a man machine interface for calibrating the
sensor and configuring outputs and alarms
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)
voltage will be developed of one
polarity
At a high pH (alkaline)
a voltage will be developed of the
opposite polarity
Most often used pH electrodes are glass
electrodes.
Typical model is made of glass tube ended with
small glass bubble.
Inside of the electrode is usually filled with
buffered solution of chlorides in which silver
wire covered with silver chloride is immersed.
pH of internal solution varies - for example it
can be 1.0 (0.1M HCl) or 7.0 (different buffers
used by different producers).
A typical modern pH probe is a combination
electrode, which combines both the glass
and reference electrodes into one body.
It consists of
1.
2.
3.
4.
5.
6.
7.
a sensing part of electrode, a bulb made from specific
glass
sometimes electrode contain small amount of AgCl
precipitate inside the glass electrode
internal solution, usually 0.1M HCl for pH electrodes
internal electrode, usually silver chloride electrode or
calomel electrode
body of electrode, made from non-conductive glass or
plastics.
reference electrode, usually the same type as 4
Junction , because we need to measure difference of
potentials between sides of glass in the glass
electrode. the junction closes the circuit through
the solutions - internal and external - and the pH
meter. It is usually made from ceramics or capillary
with asbestos or quartz fiber
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
The circuit of a simple pH meter usually
consists of operational amplifiers in an
inverting configuration, with a total voltage
gain of about -17.
The inverting amplifier converts the small
voltage produced by the probe (+0.059
volt/pH) into pH units, which are then offset
by seven volts to give a reading on the pH
scale.
For example:
At neutral pH (pH 7) the voltage at the probe's output is 0
volts. 0 * 17 + 7 = 7.
At basic pH, the voltage at the probe's output ranges from
+0 to +0.41 volts (7 * 0.059 = 0.41).
▪ So for a sample of pH 10 (3 pH units above neutral), 3 * 0.059 = 0.18
volts), the output of the meter's amplifier is 0.18 * 17 + 7 = 10.
At acid pH, the voltage at the probe's output ranges from -
0.41 volts to -0.
▪ So for a sample of pH 4 (3 pH units below neutral), -3 * 0.059 = -0.18
volts, the output of the meter's amplifier is -0.18 * 17 + 7 = 4.
The two basic adjustments performed at calibration set
the gain and offset of the inverting amplifier
The calibration process correlates the voltage produced by
the probe (approximately 0.06 volts / pH unit) with the pH
scale.
Calibration should be done before each measurement
For normal use: calibration at the beginning of each day
The pH meter has three controls
1st control (calibrate): to set the meter reading equal to the
value of the first standard buffer
2nd control (slope): which is used to adjust the meter reading to
the value of the second buffer.
3rd control allows the temperature to be set
Calibration should be performed with at least two
standard buffer solutions that span the range of pH values
to be measured.
Place the probe in a sample of pH 7.00 calibration
buffer. Adjust the meter for a pH of 7.00.
Rinse the probe with distilled water and then gently
blot the tip with a tissue to prevent solution carryover.
Immerse the probe in pH 4.00 buffer and adjust the
pH meter so it gives a reading of 4.00.
Rinse the probe and blot, repeating steps 1 to 3 until
readings are stable and no further adjustments are
necessary. Usually a couple of repetitions are
sufficient.
After each single measurement, the probe is
washed with distilled water or deionized water to remove
any traces of the solution being measured,
blotted with a clean tissue to absorb any remaining water
which could dilute the sample and thus alter the reading,
and then quickly immersed in another solution
About once a month, the probe may be cleaned using pH-
electrode cleaning solution; generally a 0.1 M solution of
hydrochloric acid (HCl) is used, having a pH of about one.
Storing: The probe tip must be kept wet at all times and
should be stored in buffer solution when not in use
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.
The basic pH scale extends from 0 (strong acid) to 7 (neutral, pure
water) to 14 (strong alkaline).
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 zerovoltage electrical connection to the liquid
http://www.ph-meter.info/pH-electrodeconstruction