30
A 4% solution of acetic acid (pH 4, vinegar) can be quite palatable, but
a 4% solution of sulfuric acid (pH 0) is a violent poison. pH provides the
needed quantitative information by expressing the degree of activity of
an acid or base.
In a solution of one known component, pH will indicate concentration
indirectly. However, very dilute solutions may be very slow reading, just
because the very few ions take time to accumulate.
B. pH Units (TPH1)
The acidity or alkalinity of a solution is a measurement of the relative
availabilities of hydrogen (H
+
) and hydroxide (OH
-
) ions. An increase in
(H
+
) ions increases acidity, while an increase in (OH
-
) ions increases
alkalinity. The total concentration of ions is fixed as a characteristic of
water, and balance would be 10
-
7
mol/liter (H
+
) and (OH
-
) ions in a neutral
solution (where pH sensors give 0 voltage).
pH is defined as the negative logarithm of hydrogen ion concentration.
Where (H
+
) concentration falls below 10
-7
, solutions are less acidic than
neutral, and therefore are alkaline. A concentration of 10
-9
mol/liter of
(H
+
) would have 100 times less (H
+
) ions than (OH
-
) ions and be called
an alkaline solution of pH 9.
C. pH Sensor (TPH1)
The active part of the pH sensor is a thin glass surface that is selectively
receptive to hydrogen ions. Available hydrogen ions in a solution will
accumulate on this surface and a charge will build up across the glass
interface. The voltage can be measured with a very high impedance
voltmeter circuit; the dilemma is to connect the voltmeter to solution on
each side.
The glass surface encloses a captured solution of potassium chloride
holding an electrode of silver wire coated with silver chloride. This is the
most inert connection possible from a metal to an electrolyte. It can still
produce an offset voltage, but using the same materials to connect to the
solution on the other side of the membrane causes the 2 equal offsets
to cancel.
The problem is, on the other side of the membrane is an unknown test
solution, not potassium chloride. The outside electrode, also called the
Reference Junction, is of the same construction with a porous plug in
place of a glass barrier to allow the junction fluid to contact the test solution
without significant migration of liquids through the plug material.
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