Specifications

MODEL 30 pH/DO SECTION 5.0

PROGRAMMING THE ANALYZER

5.5.3 Definitions — pH/ORP

1. ORP. ORP is oxidation-reduction potential. It is the voltage difference between a noble metal (usually platinum)

indicator electrode and a silver/silver chloride reference electrode.

2. REDOX. Redox is redox potential. Redox potential is measured the same way as ORP. The sign of the redox

potential is the negative of ORP.

3. SIGN CONVENTION. The ORP of a solution of 0.1 M iron (II) ammonium sulfate and 0.1 M iron (III) ammoni-

um sulfate in 1 M sulfuric acid is positive. The redox potential is negative.

4. GLASS IMPEDANCE FAULT. The Model 30 can be used with pH sensors having advanced diagnostic features

that permit the continuous determination of glass and reference electrode impedance. When the analyzer

detects low glass impedance, indicating a broken or cracked glass membrane, it automatically displays a

fault message. If the Model 30 is to be used with non-glass pH electrodes, the glass impedance fault should be

disabled.

5. SOLUTION TEMPERATURE CORRECTION. The pH of a solution, particularly an alkaline one, is a function

of temperature. If the temperature changes, so does the pH, even though the concentration of the acid or base

causing the pH remains constant. Solution temperature compensation converts the pH at the measurement

temperature to the pH at a reference temperature. The Model 30 corrects the pH to a reference temperature

of 25°C.

For an accurate solution temperature correction, the exact composition of the solution must be known. The

Model 30 has built-in temperature corrections for high purity water and for dilute solutions of a strong base.

The analyzer also accepts a custom correction.

6. CUSTOM TEMPERATURE CORRECTION. The custom temperature correction has units of ∆pH/°C. The

example shows how the analyzer uses the custom correction. Suppose the pH of the solution being measured

decreases 0.022 units for every Celsius degree rise in temperature. The temperature correction is

-0.022pH/°C. If the pH at 20°C is 8.95, the corrected pH (at 25°C) is 8.95 + (-0.022)(25 - 20) or 8.84.

7. ISOPOTENTIAL pH. The isopotential pH is the pH at which the cell voltage (the cell is the pH electrode, ref-

erence electrode, and solution being measured) is independent of temperature. Most pH cells have isopoten-

tial pH reasonably close to 7.00, so the Model 30 assumes the cell isopotential pH is 7.00. However, certain

specialized electrodes have isopotential pH different from 7.00.

NOTE

Do NOT change the isopotential pH of the transmitter unless you are thoroughly

familiar with the role of sensor and analyzer isopotential point in pH measure-

ment, OR the sensor operating instructions specifically state the isopotential pH

is a value other than pH 7.00.