Hach MP-4, Руководство пользователя

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Appendix D Additional information on pH
and ORP (MP-6 and MP-6p models)
D.1 pH
D.1.1 pH as an indicator
pH measures the acidity or alkalinity of an aqueous solution. Another way to
describe pH is as the hydrogen ion activity of a solution.
pH measures the effective, not the total acidity of a solution. 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 indicates concentration directly. Very
dilute solutions may be very slow reading because very few ions take time to
accumulate.
D.1.2 pH units
The acidity or alkalinity of a solution measures the relative availability of hydrogen
(H
+
) and hydroxide (OH
-
) ions. An increase in H
+
ions increases acidity, while an
increase in OH
-
ions increases alkalinity.
pH is defined as the negative logarithm of hydrogen ion concentration. Where
H
+
concentration falls below 10
-7
mol/liter, solutions are less acidic than neutral, and
therefore are alkaline. A concentration of 10
-9
mol/liter of H
+
has 100 times less H
+
ions than OH
-
ions and is an alkaline solution of pH 9.
D.1.3 pH sensor
The active part of a pH sensor is a thin glass surface that is selectively receptive to
hydrogen ions. Available hydrogen ions in a solution accumulate on this surface
and a charge builds up across the glass interface. The voltage can be measured
with a high impedance voltmeter circuit.
The glass surface encloses a captured solution of potassium chloride that holds 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 cancels the two equal offsets.
The other electrode, also called the reference junction, allows the junction fluid to
contact the test solution, without significant migration of liquids, through the plug
material.
The pH/ORP sensor in the MP series meters (MP-6 and MP-6p) ( Figure 10 ) is a
single construction in an easy-to-replace package. The sensor body holds an
oversize solution supply for long life. The reference junction is a wick that is porous
so that it can provide a stable, low, permeable interface. It is located under the
glass pH sensing electrode.