Myron L 729II - Manual (Page 63)

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XI. ADDENDUM
A. pH, ORP and TEMPERATURE RELATIONSHIPS
1. pH
a. pH as an Indicator
pH is the measurement of Acidity or Alkalinity of an aqueous
solution. It is also stated 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 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
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 will increase acidity, while an increase in
(OH-) ions will increase 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 10-9mol/liter of (H+) would have 100 times less
(H+) ions than (OH-) ions and be called an alkaline solution of pH
9.
c. The pH Sensor
The active part of the pH sensor is a thin glass surface which 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 trick 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 coated with silver chloride.
This is as inert a connection as can be made from metal to an
electrolyte. It still can produce an offset voltage, but using the
same materials to connect to the solution on the other side of the
membrane allows the 2 equal offsets to cancel.
The problem is the other side of the membrane is some 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. Migration does occur, and this limits the
lifetime of a pH junction, from depletion of solution inside the
reference junction or from contamination.
d. The Myron L pH Sensor
The pH sensors for the 720 Series II Monitor/controller are a
single construction in an easily replaceable package. The sensor
body holds large solution supply for long life. The reference
junction “wick” is porous to provide a very stable, low permeability
interface. It is located under the glass pH sensing electrode. The
construction combines all the best features of any pH sensor
known.
e. Sources of Error
1. Reference Junction
The most common sensor problem is usually a clogged junction.
The symptom is a drift in the “zero” setting at 7 pH. This is why the
Monitor/controller does not allow more than 1 pH unit of offset
during calibration. At that point the junction is unreliable.
2. Sensitivity Problems
Sensitivity is the receptiveness of the glass surface, which can
be diminished by a film on the surface, or a crack in the glass.
These problems also cause long response time.
3. Temperature Compensation
pH sensor glass changes its sensitivity slightly with temperature,
so the further from pH 7 one is, the more effect will be seen. A pH
of 11 at 40°C would be off by 0.2 units. The sensor senses the
solution temperature, sends the data to the Monitor/controller
which compensates the reading.
2. ORP/Oxidation-Reduction Potential/REDOX
a. ORP as an Indicator
ORP is the measurement of the ratio of oxidizing activity to
reducing activity in a solution. It is the potential of a solution to
give up electrons (oxidize other things) or gain electrons (reduce).
Like acidity and alkalinity, the increase of one is at the expense of
the other, so a single voltage is called the Oxidation-Reduction
Potential, with a positive voltage showing, a solution wants to
steal electrons (oxidizing agent). Chlorinated water will show a
positive ORP value, for instance.
b. ORP Units
ORP is measured in millivolts, with no correction for solution
temperature. Like pH, it is not a measurement of concentration
directly, but of activity level. In a solution of only one active
component, ORP does indicate concentration. Also, as with pH, a
very dilute solution will take time to accumulate a readable
charge.
c. The ORP Sensor
An ORP sensor uses a small platinum surface to accumulate
charge without reacting chemically. That charge is measured
relative to the solution, so the solution “ground” voltage comes
from a reference junction - same as the pH sensor uses.
d. The Myron L ORP Sensor
The ORP sensors for the 720 Series II Monitor/controller are a
single construction in an easily replaceable package. The sensor
body holds large solution supply for long life. The reference
junction “wick” is porous to provide a very stable, low permeability
interface. It is located under the platinum sensing electrode. The
construction combines all the best features of any ORP sensor
known.
Both pH and ORP will indicate 0 for a neutral solution. Calibration
at zero compensates for error in the reference junction.
A zero calibration solution for ORP is not practical, so the
Monitor/controller uses the offset value determined during
calibration to 7 in pH calibration (pH 7 = 0 mV). Sensitivity of the
ORP surface is fixed, so there is no gain adjustment either.