pH meter and it's working principle

1. pH METER
DESIGN AND WORKING

2. • A pH meter provides a value as to how acidic or alkaline a liquid is. The basic principle of the pH meter is to measure the
concentration of hydrogen ions. Acids dissolve in water forming positively charged hydrogen ions (H+). The greater this
concentration of hydrogen ions, the stronger the acid is. Similarly alkali or bases dissolve in water forming negatively
charged hydrogen ions (OH-). The stronger a base is the higher the concentration of negatively charged hydrogen ions
there are. The amount of these hydrogen ions present solution is dissolved in some amount of water determines the pH.
• A pH value of 7 indicates a neutral solution. Pure water should have a pH value of 7. Now pH values less than 7 indicate an
acidic solution while a pH value greater than 7 will indicate an alkaline solution. A solution with pH value of 1 is highly acidic
and a solution of pH value of 14 is highly alkaline.
WHAT IS pH??

3. • A pH meter will be made up of a probe, which itself is made up of two electrodes. This
probe passes electrical signals to a meter which displays the reading in pH units. The
glass probe has two electrodes because one is a glass sensor electrode and the other is
a reference electrode. Some pH meters do have two separate probes in which case one
would be the sensor electrode and the other the reference point.
• Both electrodes are hollow bulbs containing a potassium chloride solution with a silver
chloride wire suspended into it. The glass sensing electrode has a bulb made up of a very
special glass coated with silica and metal salts. This glass sensing electrode measures
the pH as the concentration of hydrogen ions surrounding the tip of the thin walled
glass bulb. The reference electrode has a bulb made up of a non-conductive glass or
plastic.
• When one metal is brought in contact with another, a voltage difference occurs due to
their differences in electron mobility. Similar is the case with two liquids. A pH meter
measures essentially the electro-chemical potential between a known liquid inside the
glass electrode (membrane) and an unknown liquid outside. Because the thin glass bulb
allows mainly the agile and small hydrogen ions to interact with the glass, the glass
electrode measures the electro-chemical potential of hydrogen ions or the potential of
hydrogen. To complete the electrical circuit, also a reference electrode is needed.
BASIC PRINCIPLE OF A pH METER
GLASS BULB

4. A typical modern pH probe is a combination electrode, which combines both the glass
and reference electrodes into one body. The combination electrode consists of the
following parts :
• A sensing part of electrode, a bulb made from a specific glass
• Internal electrode, usually silver chloride electrode or calomel electrode
• Internal solution, usually a pH=7 buffered solution of 0.1 mol/L KCl for pH
electrodes
• Reference electrode, usually the same type as 2
• Reference internal solution, usually 0.1 mol/L KCl
• Junction with studied solution, usually made from ceramics or capillary
with asbestos or quartz fiber.
• Body of electrode, made from non-conductive glass or plastics.
The bottom of a pH electrode balloons out into a round thin glass bulb. The pH
electrode is best thought of as a tube within a tube. The innermost tube (the inner
tube) contains an unchanging 1×10−7 mol/L HCl solution. Also inside the inner tube is the
cathode terminus of the reference probe. The anodic terminus wraps itself around the
outside of the inner tube and ends with the same sort of reference probe as was on the
inside of the inner tube. It is filled with a reference solution of 0.1 mol/L KCl and has
contact with the solution on the outside of the pH probe by way of a porous plug that
serves as a salt bridge.
HOW IS THE pH PROBE DESIGNED??

5. • A silver chloride electrode is a type of reference electrode, commonly used in electrochemical measurements. For
example, it is usually the internal reference electrode in pH meters. The reaction is between the silver metal (Ag) and
its salt — silver chloride (AgCl, also called silver(I) chloride).
The corresponding equations can be presented as follows:
• This reaction is characterized by fast electrode kinetics, meaning that a sufficiently high current can be passed
through the electrode with the 100% efficiency of the redox reaction (dissolution of the metal or cathodic deposition
of the silver-ions). The reaction has been proved to obey these equations in solutions of pH values between 0 and 13.5.
SILVER CHLORIDE ELECTRODE

6. The pH meter measures the potential difference and its changes across the glass membrane. The potential difference must
be obtained between two points; one is the electrode contacting the internal solution. A second point is obtained by
connecting to a reference electrode, immersed in the studied solution. Often, this reference electrode is built in the glass
electrode (a combination electrode), in a concentric double barrel body of the device.
WORKING OF A pH METER

7. • Ag/AgCl | HCl | glass || probed solution | reference electrode)
• AgCl(s) | KCl(aq) || 1×10-7M H+ solution || glass membrane || Test Solution || ceramic junction || KCl(aq) | AgCl(s) | Ag(s)
• The potential difference relevant to pH measurement builds up across the outside glass/solution interface marked ||
• The bulb is sealed to a thicker glass or plastic tube, and filled, for example, with a solution of HCl (0.1 mol/dm3). In this solution is
immersed a silver/silver chloride electrode with a lead to the outside through a permanent hermetic seal. The filling solution has
constant Cl- concentration, which keeps the Ag/AgCl inner electrode at fixed potential.
• The pH sensing ability of the glass electrode stems from the ion exchange property of its glass membrane.
• Glass is mostly amorphous silicon dioxide, with embedded oxides of alkali metals. When the surface of glass is exposed to water,
some Si–O- groups become protonated
• Si-O- + H3O+ ≡ Si-O-H+ + H2O (2)
• The exchange of hydronium (or written as proton, H+) between the solid membrane and the surrounding solution, and the
equilibrium nature of this exchange, is the key principle of H3O+ sensing. As with any interface separating two phases between
which ionic exchange equilibrium is established, the glass membrane/solution interface becomes the site of a potential difference
• Eglass electrode = E ‘ + RT/2.303F log a(H3O+)
Where E’ represents the sum of the constant offset potentials of the inner glass surface/solution and the two
Ag/AgCl electrodes. At 30°C the potential of the glass membrane changes by about 60 mV for each one unit of pH