- What is pH?
- Why is pH important?
- Measuring pH
- Typical applications
What is pH?
1e-14 1e-13 1e-12 1e-11 1e-10 1e-09 1e-08 1e-07 1e-06 1e-05 1e-04 0.001 0.01 0.1 1.
Hydrogen Ion Concentration [H+]
Wastewater (before treatment)
Wastewater (after treatment)
A neutral solution is a solution in
which there are equal concentrations
of hydrogen ions (H+) and hydroxide
A neutral solution, such as water, has
a pH of 7.
pH is defined as the activity of hydrogen
ions in a solution, as shown by the equation:
It is used to determine how acidic or basic a solution is.
pH = - log10 aH+
In dilute solutions, activity is equivalent
to concentration, so it is effectively a
measurement of the concentration of
hydrogen ions in solution.
The more hydrogen ions present
in solution, the lower the pH, and
therefore the more acidic the solution.
Why measure pH?
Properties of products, such as
appearance and taste, will be affected
by acidity during production. In order
to produce the optimal product, the
pH needs to be monitored during
production and adjusted to the
appropriate pH. E.g. in the beverage
industry, wine will taste different
depending on its pH, and will taste sour
if the product is too acidic.
Costs of production can be
minimised, as having the optimal pH
during growth of a product will result
in a maximum yield of the product. E.g.
in hydroponics, the pH of the solution
the plants are grown in should be kept
at a certain pH in order to optimise the
growth of crops, and so produce the
Sometimes, if the pH goes outside
the optimal range, the product can
become harmful to the consumer, so it
is essential to maintain an optimal pH
to prevent this. E.g. chlorine levels in
swimming pools need to be high
enough to kill any micro-organisms, but
not so high that they cause irritation to
the eyes of those swimming. This can be
determined by the pH of the pool.
Equipment can be damaged by
corrosion from chemicals if they are
outside the desired pH range, so the pH
must be monitored and maintained.
Legally, some industries have to
keep pH levels within a certain range.
E.g. wastewater must be at a suitable
pH before it enters the sewage system.
The most accurate
method of measuring pH
of a solution is using a pH
meter and a pH electrode.
The pH electrode must
be used alongside a
as a standard. This can
be using two separate
electrodes, or a
which includes both.
The electrode measures
the activity of hydrogen
ions in the sample
solution and produces a
potential output in mV.
The hydrogen ion activity
is measured by the
migration of hydrogen
ions into or out of a gel
layer that forms between
the outer surface of the
glass electrode and the
sample solution being
If the sample is acidic,
the hydrogen ions will
migrate into this gel layer,
resulting in a positive
charge on the outer
surface of the electrode,
and hence a positive
If the sample is alkaline,
hydrogen ions will
migrate from the gel layer
into the sample, resulting
in a negative charge
forming on the outer
surface of the electrode,
and hence a negative
This potential output is
converted to the pH scale,
to give a reading in pH
units, which is the more
common scale used to
Inner Tube Housing
the Membrane Sensing
- pH 7
- pH before treatment typically 6.5-8.5
- pH after treatment 7.0-7.2
- pH depends on what you are growing,
best pH range for most plants is 6.0-7.0
Food/dairy products (meats, cheeses, milk etc.)
- pH of milk is 6.3-8.5
- pH of cheese varies depending on type of cheese,
but typically around 5.0-6.0
- pH of meat depends on type of meat,
but typically between 5.0 and 7.0
Alcoholic beverages (wine, beer)
- pH of white wine typically between 3.0-3.4
- pH of red wine typically around 3.3-3.6
- pH of beer typically about 4.0