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Sodium Hydroxide Control With ORP
Powell Oxidation Reduction Potential Instrumentation (ORP) is the best available
technology for the indication and control of sodium hydroxide concentrations for
scrubbers on the market today. When the correct electrode installation has been
made using the Powell ORP electrodes,
the silver/platinum electrode pair produce a millivoltage signal output according
to the following formula:
Where E0 is the standard oxidation potential for the reaction.
Since the electrode potential is primarily a function of the hydrogen ion concentration,
a sharp inflection is noted at the reaction end point similar to that obtained by
pH measurements.
Experience has shown that typical millivoltages in a
chlorine scrubber starting with a typical 21% sodium hydroxide and reacting
with chlorine will produce the following approximate mVDC signals with respect to
the remaining sodium hydroxide versus the weight percent sodium hypochlorite.
Table 1: Remaining Sodium Hydroxide vs. Wt % Sodium Hypochlorite
|
Wt % NaOH
|
Wt % NaOCl
|
ORP mVDC
|
|
21.34
|
0.00
|
0
|
|
19.90
|
1.12
|
288
|
|
18.47
|
2.31
|
311
|
|
16.93
|
3.45
|
330
|
|
15.49
|
3.50
|
345
|
|
14.22
|
5.54
|
358
|
|
12.88
|
6.81
|
372
|
|
11.44
|
8.01
|
384
|
|
9.90
|
8.88
|
396
|
|
8.49
|
10.25
|
410
|
|
7.00
|
11.27
|
424
|
|
5.38
|
12.67
|
441
|
|
3.95
|
13.59
|
460
|
|
2.31
|
15.13
|
488
|
|
1.48
|
15.58
|
507
|
|
0.81
|
15.99
|
535
|
|
0.42
|
16.44
|
561
|
*Millivoltage Direct Current
ORP vs. Wt% NaOH
Using the data in Table 1, the ORP Curve with the 21% by weight sodium hydroxide
starting point can be represented as a graph, as shown in Figure 1 below. Using
the proprietary Powell ORP electrodes, any chlorine scrubber can be continuously
or batch controlled at high sodium hydroxide levels as required by individual design
of the scrubber.
Powell customers have successfully controlled sodium hydroxide concentrations by
using continuous and automatic control sodium hydroxide levels at typical levels
such as 12% by weight with excellent results. Other customers choose to operate
their scrubbers at much lower levels of sodium hydroxide such as 1.5% using Powell
ORP electrodes either by batch or continuous. If the system is automated with the
application of automatic sodium hydroxide supply valves in conjunction with the
Powell ORP electrodes, process upsets can be easily and quickly managed. On batch
chlorine scrubbers, the Powell ORP electrodes are used to provide information and
alarms to inform the operators when to change batch sodium hypochlorite tanks in
the scrubbing operation.
For More Information
Contact Powell today to
discover how you can operate your emergency scrubbers automatically and continuously
with the confidence necessary in today’s environmental standards.