Specifications

pH can be dened by the following equation:
pH = -log [H
3
O
+
]
Where [H
3
O
+
] = The concentration in Moles/litre of the hydronium ion
in solution.
In pure water, the H
3
O
+
ion concentration is 1 x 10
-7
Moles/litre. Using the
formula denition above, pH value of pure water is derived as follows:
pH value of pure water = -log [1 x 10
-7
] = 7
There is an equal concentration of H
3
O
+
ions and OH
-
ions in pure water
at 25 °C. The equilibrium constant, Kw of water, is the product of the H
3
O
+
ion and OH
-
ion concentrations:
Kw = [H
3
O
+
] [OH
-
]
Hence,
Kw of pure water is = [1 x 10
-7
] [1 x 10
-7
] = 1x10
-14
The Kw of water must remain constant. Therefore, an increase in either
the H
3
O
+
ions or OH
-
ions will result in a decrease of the other ions,
resulting in acidity (increased H
3
O
+
) or alkalinity (increased OH
-
).
pH Measurement
pH is always measured across a medium.
Although the litmus paper is one of the most common methods of pH
measurement, it can only provide a rough indication which might be
insucient in most applications.
The more accurate method involves the use of a measurement system
that consists of a pH meter and a pH electrode that has a hydrogen ion
sensitive glass bulb. The most common sensing element used in the
electrode is the glass membrane as it is selective for H+ ions i.e. H+ ions
can permeate through the hydrated layer of glass membrane. However
the electrode body may not necessarily be glass.
The movement of ions into the hydrated membrane changes the
electrochemical eect inside the glass which is measured in mV and then
converted via the pH meter to be reected as a pH value.
Hence depending on the concentration of ions in the solution, the mV
and hence pH varies.
The performance of an electrode is dependent on two parameters
– Oset and Slope.
Oset in pH Electrode
Theoretically, when placed in pH 7.00 buer at 25 °C, a pH electrode
produces 0 mV which the pH meter reads as 7.00 pH. The dierence
between 0 mV and the electrode’s actual reading is called the oset error
which can be as high as ±25 mV.
In other words, when the electrode is not in measurement or in pH 7
buer solution, the output (or reading) will be known as the oset.
While in theory, the mV value should be zero, however in practice this is
rarely the case because of the following reasons:
• Liquid dierence
• Bulb composition
• Wire geometry dierence and other factors
pH/ORP | About pH/ORP Measurement
About pH Measurement
Why is pH Important?
pH is the measure of acidity and alkalinity of a liquid. It is one of the most
common parameters measured in a wide variety of industries ranging
from water and wastewater treatment, chemical production, agriculture
research and production, environmental monitoring, chemical and life
sciences research, biochemical and pharmaceutical research, electronics
production to food processing and other industrial applications.
For instance, the level of pH is crucial during the manufacture of sugar.
An inappropriate pH level can result in the formation of unwanted acids
and very little sugar. pH control is also important in sewage treatment
management as it is undesirable to pollute sewers with highly acidic or
alkaline spills.
Here are examples of pH in a few common industrial and household
products:
pH Theory
pH is the measure of how acidic or alkaline a substance is on a scale of 0
to 14. pH values below 7 represent the acidic half of the scale; pH values
above 7 represent the alkaline or basic half of the scale. The pH value 7
is neutral, i.e. neither acidic nor alkaline.
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10
Battery acid [0.3]
Vinegar [2.9]
Pickle processing [3.5]
Boric acid [5.0]
Corn [6.2]
Milk [6.7]
Fresh water aquarium [7.1]
Household dye [13.6]
[13.1] Bottle washing
Copper plating
[12.8]
[12.6] Bleach
Ammonia
[11.4]
[11.3] Brass plating
Milk of magnesia
[10.3]
[9.4] Lime-soda softening
Borax
[9.3]
[8.4] Baking soda
Saltwater aquarium [8.0]
[8.0] Seawater
Blood [7.5]
[7.4] Swimming pool water
[7.0] Distilled water
[6.3] Brewing process
[5.8] Nickel plating
[4.3] Orange juice
[3.2] Photo engraving
[2.3] Lemon juice
[5.0] Food processing
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10
9
8
7
6
5
4
3
2
1
0
13