Emerson Process Management 3081 pH/ORP instruction manual Calibration, 124

MODEL 3081 pH/ORP	SECTION 14.0
	ORP MEASUREMENTS

The Nernst equation for reaction 3 is:

0.1987 (t + 273.15)		log		[Cr+3] 2
E = E°-	6		[Cr O -2] [H+]14 (4)
			2	7

Note that the hydrogen ion factor in the concentration ratio is raised to the fourteenth power. The table shows the expected effect of changing pH on the measured ORP at 25°C.

pH changes	ORP changes by
from 2.0 to 2.2	7 mV
from 2.0 to 2.4	35 mV
from 2.0 to 1.8	47 mV
from 2.0 to 1.6	75 mV

The Nernst equation can be written for any half reaction. However, not all half reactions behave exactly as predict- ed by the Nernst equation. Why real systems do not act as expected is beyond the scope of this discussion. The potential of chromium (VI) - chromium (III) couple used as an example above does not perfectly obey the Nernst equation. However, the statement that pH has a strong effect on the electrode potential of the couple is true.

• As mentioned earlier, ORP is best suited for measur- ing changes, not absolute concentrations. If ORP is used to determine concentration, great care should be exercised. An example is the determination of chlorine in water. When water is disinfected by treatment with chlorine gas or sodium hypochlorite, free chlorine forms. Free chlorine is a mixture of hypochlorous acid (HOCl) and hypochlorite ions (OCl-). The relative amount of hypochlorous acid and hypochlorite present depends on pH. For disinfection control, total free chlo- rine, the sum of hypochlorous acid and hypochlorite ion, is important. Equation 5 shows the half reaction for hypochlorous acid:

HOCl + H+ + 2e¯ = Cl¯ + H2O				(5)
The Nernst equation is
E = E° -	0.1987 (t + 273.15)	log	[Cl-]	(6)
			[HOCl] [H+]
2

Only the concentration of hypochlorous acid appears in the Nernst equation. To use ORP to determine total free chlorine, equation 6 must be rewritten in terms of

free chlorine. Although the details are beyond the scope of this discussion, the result is shown in equation 7:

0.1987 (t + 273.15)		log	[Cl-] {[H+] + K}
E = E° -				(7)
			C [H+] 2
	2
			a

where K is the acid dissociation constant for hypochlor- ous acid (2.3 x 10-8) and Ca is the total free chlorine concentration. As equation 7 shows the measured ORP depends on the hydrogen ion concentration (i.e., pH), the chloride concentration, the free chlorine con- centration, and temperature. Therefore, for ORP to be a reliable measurement of free chlorine, pH, chloride, and temperature must be reasonably constant.

Assume the free chlorine level is 1.00 ppm and the chloride concentration is 100 ppm. The table shows how slight changes in pH influence the ORP.

pH changes	ORP changes by
from 8.0 to 7.8	3.9 mV
from 8.0 to 7.6	7.1 mV
from 8.0 to 8.2	4.4 mV
from 8.0 to 8.4	9.2 mV

Around pH 8 and 1.00 ppm chlorine, a change in ORP of 1.4 mV corresponds to a change in chlorine level of about 0.1 ppm. Therefore, if pH changed only 0.2 units and the true chlorine level remained constant at 1.00 ppm, the apparent chlorine level (determined by ORP) would change about 0.3 ppm.

14.8 CALIBRATION

Although there is no internationally recognized ORP calibration standard, the iron (II) - iron (III) couple enjoys some popularity. The standard is a solution of

0.1M iron (II) ammonium sulfate and 0.1 M iron (III) ammonium sulfate in 1 M sulfuric acid. The solution has good resistance to air oxidation. If stored in a tight- ly closed container, the shelf life is one year. Because the standard contains equal amounts of iron (II) and iron (III), the ORP does not change appreciably if the solution becomes slightly diluted. In addition, minor variability in actual concentration does not affect the standard ORP.

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