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Application Note
pH Theory
pH is the measurement of the acidity or alkalinity of a solution, calculated by the relative
number of hydrogen ions (H+) or hydroxyl ions (OH-) present in a solution. Acidic
solutions have a higher relative number of hydrogen ions, while alkaline (basic)
solutions have a higher relative number of hydroxyl ions. Acids either dissociate (split
apart) to release hydrogen ions or react with water to form hydrogen ions. Bases are
substances that dissociate to release hydroxyl ions or react with water to form hydroxyl
ions.
pH is defined as the negative logarithm of the hydrogen ion activity: pH = - log [H+]
At 25°C, a reading of 7.0 pH is neutral, solutions with a pH value less than 7 are acidic
and solutions with pH value above are alkaline. The typical pH range is 0 to 14,
although solutions containing non-water solvents can have pH values outside this
range. pH is also logarithmic with each pH unit has 10 as many hydrogen ions and the
unit above it. At pH 3 there are ten (10) times as many hydrogen ions present in the
solution that at pH 4 and one hundred (100) time more than at pH 5.
pH measurement is based on the use of a pH sensitive electrode (typically glass),
reference electrode, and a temperature probe to provide a temperature signal to the pH
analyzer. Most electrodes available in today’s market are “combination” electrodes – a
combination of the measurement electrode and reference electrode. Also available are
Combination pH/ATC (or 3-in-1) Electrodes which combines the measurement
electrode, reference electrode and temperature compensation.
pH Values (typical) of Common Substances:
ACIDS
Hydrochloric Acid (HCl)
0.0
Lemon Juice
2.2
Vinegar
3.0
Wine & Beer
4.0
Coffee (Black)
5.0
Milk
6.6
NEUTRAL
Pure Water
7.0
BASES
Blood
7.4
Baking Soda (Sodium Bicarbonate) 8.5
Milk of Magnesia
10.6
Ammonia
11.0
Lye
13.0
Sodium Hydroxide (NaOH)
14.0
pH electrodes use a specially formulated pH sensitive glass and when in contact with a
solution develops a voltage potential proportional to the pH of the solution. The reference
electrode is designed to maintain a constant potential at any given temperature, and
serves to complete the pH measuring circuit within the solution. It provides a known
reference potential for the pH electrode. The difference in the potentials of the pH and
reference electrodes provides a millivolt signal proportional to the pH value. pH electrodes
are designed to produce a 0 mV signal at 7.0 pH, with a slope (theoretical) of -59.16 mV /
pH at 25°C.
For additional information or assistance please contact BioExpress.