pH
meter is an electronic device used for measuring the pH (acidity or
alkalinity) of a liquid. A pH meter is made of a special measuring probe
(usually a glass electrode) attached to an electronic meter that measures
and displays the pH reading. To measure the pH of semi-solid substances,
special probes are used. A pH meter uses glass electrodes for measuring the
pH of a solution. The glass electrode is made of thin glass, which permits
the h+ ions to pass through it. The meter evaluates the electrical potential
and changes this data into a pH reading for a sample. pH meters are used in
school and college laboratories to quickly measure the pH value of any
substance, specially liquids and solutions. They are widely used in
different types of industries.
Parts of pH Meter
The probe
A pH probe is a combination of both the glass and reference electrodes into
one unit. The bottom of the electrode balloons out into a round thin glass
bulb. The probe is best thought of as a tube within a tube. The inner most
contains an unchanging saturated chloride solution. Inside the inner tube
there is the cathode terminus of the reference probe. Both the outer tube
and the inner tube contain a reference solution but only the outer one has
contact with the solution.
The Meter
The meter used in the pH meter is a specific type of voltmeter, having very
high impedance of input channels due to the high resistance of the glass
electrode probes typically used with pH meters. The voltage produced by the
pH electrode is directly proportional to logarithm of the h+ activity. The
display of the meter is scaled in such a way that the displayed result of
measurement is just the pH of the solution. The circuit in a pH meter mainly
consists of operational amplifiers in an inverting configuration. The
inverting amplifier change the small voltage produced by the probe into pH
units, which are then offset by seven volts to give a reading on the pH
scale.
Working of pH Meters
As
a metal comes in contact with another, then due to the differences in
electron mobility, a voltage difference occurs. A similar electric potential
is created when a metal is brought in contact with a solution of salts or
acids, which was the basis of invention of batteries. In the same manner,
when a liquid is brought in contact with another, an electric potential
develops. But in this case a membrane is required to keep such liquids
apart.
A pH meter measures the electro-chemical potential developed between a
known liquid inside the glass electrode and an unknown liquid outside. The
thin glass bulb only allows the active and small hydrogen ions to interact
with the glass. In this way the glass electrode measures the
electro-chemical potential of hydrogen ions. A reference electrode is needed
to complete the electrical circuit. It is important to note that the
instrument does not measure a current but only an electrical voltage. A pH
meter must not be used for measuring the pH OF moving liquids of low
conductivity.
Features of pH Meters
- pH Meter is the most suitable equipment for measuring the pH.
- PH meters are compact, easy to operate, and economical instruments.
- They display the results either in pH units or in some devices in
millivolts.
Types of pH Meters
pH meters covers a wide range of products, from simple and inexpensive
pen-like devices to the complex and expensive laboratory instruments. The
complex pH meters come with computer interfaces and various inputs for
indicator, temperature sensors, and reference electrodes. The cheaper models
require that temperature measurements be entered to adjust for the slight
variation in pH caused by temperature. Specialty probes and meters are used
for special applications or harsh environments. Small devices like pocket pH
meter are also found that automatically compensate for temperature.
Pen Type pH Meter
The pen type pH meters are designed and manufactured for easy operations.
They are low cost devices. They have a wide range of applications including
laboratories, school science departments, aquariums, swimming pools, etc.
- The automatic temperature compensation (ATC) assures correct pH
readings for temperature offsets.
- Some pen type pH meters have dual display showing pH reading and
temperature.
- They are offered with one touch auto calibration and 3 point
measurement.
- The temperature display is switchable between °C and °F.
- They are offered with optional auto power off feature.
- They come with low battery indication feature.
- Usually the pen type pH meters are waterproof.
Pocket pH Meter
The Pocket pH meter is easy to use and durable. It provides fast, accurate
and repeatable measurement of pH values. They are easy to carry from one
place to another. The pocket pH meters are helpful for the students as well
as for the industrial applications. They come with an unique feature of a
retractable electrode. The pocket pH meters come with a protective cap
having a moisture sponge, which keeps the electrode wet and sustains the
products life.
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