Myron L ARH1 - Manual (Page 14)

their solution, like the KCl favored by chemists for its stability.
Users dealing with sea water, etc., use NaCl as the model for their
concentration calculations. Users dealing with freshwater work with
mixtures including sulfates, carbonates and chlorides, the three
predominant components (anions) in freshwater that Myron L Company
calls “natural water”. These are modeled in a mixture called “442” which
the Myron L Company markets for use as a calibration standard, as it does
standard KCl and NaCl solutions.
C. When does it make a lot of difference?
First, the accuracy of temperature compensation to 25°C determines the
accuracy of any TDS conversion. Assume we have industrial process
water to be pretreated by R.O. Assume it is 45°C and reads 1500 µS
uncompensated.
1. If NaCl compensation is used, an instrument would report 1035
µS compensated, which corresponds to 510 ppm NaCl.
2. If 442 compensation is used, an instrument would report 1024
µS compensated, which corresponds to 713 ppm 442.
The difference in values is 40%.
In spite of such large error, some users will continue to take data in the
NaCl mode because their previous data gathering and process
monitoring was done with an older NaCl referenced device.
Those who want true TDS readings that will correspond to evaporated
weight will select the correct Solution Type.
The ARH1 contains circuitry for the 3 most commonly referenced
compounds — KCl, NaCl and 442. In the LCD display, the solution type
being used is shown on the left.
XIV. TEMPERATURE COMPENSATION (Tempco)
and TDS DERIVATION
When making conductivity measurements, the Solution Selection
determines the characteristic assumed as the instrument reports what a
measured conductivity would be if it were at 25°C. The characteristic is
represented by the tempco, expressed in %/°C. If a solution of 100 µS at
25°C increases to 122 µS at 35°C, then a 22% increase has happened
over this change of 10°C. The solution is said to have a tempco of 2.2
%/°C.
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 ARH1 uses mathematically
generated models for known salt characteristics that vary with
concentration and temperature.
The ARH1 contains circuitry for characteristics of the 3 most commonly
referenced compounds — KCl, NaCl and 442. In the display, the solution
type being used is shown on the left.
X V . pH MEASURING
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 (0H ) 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.
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
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