Another solution would have a different tempco because of its ionization
activity. And, that tempco may be a little different at a different
concentration or temperature. This is why the Ultrameter uses
mathematically generated models for known salt characteristics that vary
with concentration and temperature.
B. Finding the Tempco of an Unknown Solution
One may need to measure compensated conductivity of some solution
unlike any of the 3 standard salts. In order to enter a custom fixed tempco
for a limited measurement range, enter a specific value through the
“USER” function. The tempco can be determined by 2 different
methods:
1.
Heat or cool a sample of the solution to 25°C, and measure its
conductivity. Heat or cool the solution to a typical temperature
where it is normally measured. After selecting USER function,
set the tempco to 0 %/°C as in Disabling Temperature
Compensation, pg. 13 (No compensation). Measure the new
conductivity and the new temperature. Divide the % decrease
or increase by the 25°C value. Divide that difference by the
temperature difference.
2.
Heat or cool a sample of the solution to 25°C, and measure its
conductivity. Change the temperature to a typical measuring
temperature. Set the tempco to an expected value as in User
Programmable Tempco, pg. 12. See if the compensated value
is the same as the 25°C value. If not, raise or lower the tempco
and measure again until the 25°C value is read.
C. Finding the TDS Ratio of an Unknown Solution
Once the effect of temperature is removed, the compensated
conductivity is a function of the concentration (TDS). There is a ratio of
TDS to compensated conductivity for any solution, which varies some
with concentration. The ratio is set during calibration in USER as in
section User Programmable Conductivity to TDS Ratio, pg. 13. A truly
unknown solution has to have its TDS determined by evaporation and
weighing. Then the solution whose TDS is now known can be measured
for conductivity and the ratio calculated. Next time the same solution is to
be measured, the ratio is known.
XVII. pH and ORP MEASURING
A. pH
1. 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
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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.
2. 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 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 , solutions are less acidic than
neutral, and therefore are alkaline. A concentration of 10 mol/liter of (H )
would have 100 times less (H ) ions than (OH ) ions and be called an
alkaline solution of pH 9.
3. 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
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