The following article significantly influenced the design of Powell ORP Electrodes.
- Made in United States of America
- Reprinted from JOURNAL OF THE ELECTROCHEMICAL SOCIETY Vol. 97, No. 8, August 1950.
- Written by D.J. Pye, Great Western Division, The Dow Chemical Company, Pittsburg, California.
- Originally prepared for delivery before the Cleveland Meeting, April 19 to 22, 1950.
The reaction of chlorine and caustic soda used in the manufacture of sodium hypochlorite
bleach liquors is usually controlled by manual titration but may be better controlled
by oxidation potential or pH methods. The oxidation potential method is preferred
for reasons of simplicity and accuracy. Proper control results in a product of maximum
stability, improved bleaching speed, and more efficient use of raw materials. Curves
are presented showing the correlation of alkalinity and oxidation potential as measured
by platinum-calomel and platinum-silver electrode pairs. Data on the effect of oxidation
potential on stability and bleaching speed are also shown. An automatically controlled
continuous process for manufacture on this basis is suggested.
The extensive manufacture of sodium hypochlorite bleach liquors for laundry, household,
and industrial uses involves the relatively simple reaction in solution:
2 NaOH + Cl2 → NaOCl + NaCl + H2O
The reaction is usually carried out in agitated glass-lined or rubber-lined tanks
as a batch process. Gaseous or liquid chlorine is sparged into dilute caustic soda
of such concentration as to yield the desired bleach strength at the end point.
Although most of the material is marketed as 5% NaOCl for household use, higher
concentrations of 10% and 16% bleach are prepared for laundry and other commercial
uses where long-term stability is not a problem.
While the purification of the caustic solution to remove decomposition catalysts
and other impurities is quite well controlled by carbon filtration and water softening
prior to chlorination, the control of the bleach batch end point is critical and
requires appreciable attention by the operator. This end point is determined by
destroying the hypochlorite in a grab sample with neutral hydrogen peroxide and
titrating the excess alkalinity with standard acid. The end point is quite critical
in that a product with excess alkalinity greater than the minimum amount required
for stability is wasteful of caustic soda, undesirably alkaline, and slow in bleaching
speed. On the other hand, over-chlorinated material is seriously unstable in an
autocatalytic manner which can result in a boil over releasing large quantities
of chlorine into the atmosphere. Just beyond the end point the pH decreases, producing
HOCl and the reaction takes place forming more acid, thus further reducing the pH
until Cl2 is liberated.
2HOCL + NaOCl → NaClO3 + 2HCl
The simple nature of the desired reaction makes the process ideally suited to electrometric
control by either pH or oxidation potential methods. The neutralization of the caustic
with chlorine exhibits a marked pH inflection point at the reaction end point which
can itself be used as a control system. For practical reason, however, the control
based on oxidation potential is to be preferred.
The potential of a platinum electrode in alkaline chlorine solutions is given by
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
The substitution of continuous electrometric control for intermittent manual testing
then makes possible the design of a fully automatic continuous reactor control system
since the measuring instrument can be made to control the chlorine flow.
In order to correlate the oxidation potential of the hypochlorite solutions with
excess alkalinity, emf measurements were made on a box-type portable 1.1 volt potentiometer
using a platinum-saturated calomel cell electrode pair. For laboratory experiments
the electrode pair consisted of a 2 cm2 piece of platinum foil and a
standard sleeve type calomei cell.
For control of commercial batches, a more rugged assembly was constructed as shown
in Figure 1. The measuring electrode was formed by firing several layers of platinum
platina on the end of the glass tube having a platinum wire sealed in its end. The
porous disc liquid junction of the calomel cell was found less troublesome than
the usual loose sleeve.
Figure 1. Platinum-calomel electrode assembly for tank immersion.
More recently it has been found that a simple silver foil or rod electrode may be
substituted for the calomel cell as a reference electrode, thus avoiding the annoyance
of maintaining a KCl solution level in the cell.
Reaction Control by Oxidation Potential
The oxidation potential was correlated with alkalinity in the laboratory by chlorinating
5-gallon quantities of 2, 5, and 16 percent bleach having high excess alkalinity.
The decrease in alkalinity was followed by titration with standard acid after destroying
the hypochlorite with neutral H2O2. The relationship between oxidation potential
and excess alkalinity near the end point is shown in Figure 2.
Figure 2. Potential of platinum saturated calomel electrodes in hypochlorite solutions.
It will be noted that the true electrometric end point falls short of the zero alkalinity
ordinate by the amount of alkali attributed to the hydrolysis of the basic NaOCl.
The curves are very steep at the true end point and exhibit a relatively large voltage
shift. For this reason it is almost impossible to stop the reaction at the exact
inflection point and it was thus found more practical to slow the chlorine flow
at about 0.50 volt and stop the reaction at 0.62 volt. Although this end point has
been found to be sufficiently stable for production purposes, it has been observed
that more stable end point is obtained if the reaction is allowed to go slightly
over and the back to the control point with caustic.
Figure 3 shows the curve obtained using the platinum-silver electrode pair and includes
the potential of the silver electrode vs. a normal calomel electrode as a measure
of the constancy of silver as a reference electrode. While such a variation would
be unsatisfactory for theoretical work, it is quite reproducible and is satisfactory
for production purposes.
Figure 3: Potential of platinum-silver electrodes in hypochlorite solutions.
Large-scale production batches have been satisfactorily controlled by this method
on an experimental basis giving results as predicted form the laboratory experiments.
Reaction Control by pH Methods
The reaction may also be controlled by pH methods since the pH changes rapidly at
the reaction end point giving a curve similar in shape to the emf curves. Theoretically,
then, pH control would be as useful as oxidation potential control but, from the
practical standpoint, oxidation potential control has several real advantages.
- Oxidation potential measurements require only simple low impedance measuring devices,
whereas pH control by glass electrode requires complex high impedance electronic
- Glass electrodes, being subject to asymmetry potential errors, require occasional
buffer checks to maintain accuracy.
- A standard glass electrode is subject to sodium ion errors at high pH values in
the presence of high sodium ion concentrations which tend to decrease its sensitivity.
Special low sodium ion error electrodes are not satisfactory due to an inherently
slow speed of response.
- Oxidation potential electrodes are instantaneous in response so that the tendency
to over-chlorinate at the sensitive end point is minimized.
From the standpoint of practical plant operation, the oxidation potential control
is to be preferred if for no other reason than that the instrumentation is lower
The stability of 16% bleach solutions as a function of oxidation potential and excess
alkalinity is shown in Fig. 4.
Figure 4. The stability of 16% bleach solutions as a function of oxidation potential
and excess alkalinity.
It will be noted that although the effect is not pronounced, the solution is actually
more stable at the true end point than in more alkaline solutions. It is also apparent
that the oxidation potential near the end point is a much more sensitive measure
of the stability than titration values since the three samples titrating 3.6 g/l
NaOH showed marked differences in potential and stability. Thus, these successive
very small increments of chlorine added to the bleach caused a marked increase in
oxidation potential, while the change in alkalinity was so slight as to fall within
the limits of error of the titration method. For this reason, the product may be
finished close to the end point with much more confidence using electrometric control
Close finishing practice, providing it can be done with confidence, is desirable
not only from the standpoint of increasing caustic efficiency, but also for its
effect on the product quality.
Since the amount of excess alkalinity allowed to remain in the bleach liquor will
affect the pH and the oxidation potential of the diluted bleaching solution, it
is important to consider its effect on the rate of bleaching of colored compounds.
In the course of some recent work concerning the bleaching of stains from walnut
shells, tests were made on the effect of alkalinity and oxidation potential on the
rate of de-colorization on an air-oxidized pyrogallol solution with 2% hypochlorite.
Fig. 5 shows the rate of color change of the dye solution at three different oxidation
potentials obtained by adjusting the potential with chlorine prior to adding the
dye. Pyrogallol is a moderately resistant color and its pronounced response to increased
oxidation potential may be considered typical. The potential of 0.62 volts indicates
a stable bleach that could be produced under proper control in either a batch or
Figure 5. Effect of Oxidation Potential on Bleaching Rate
While the electrometric control of a batch chlorination is a considerable improvement
over titration methods, the maximum savings in operating labor and increase in chlorination
efficiency will be realized with a continuous process.
Although, to the author’s knowledge, oxidation potential control has not yet been
used as a basis for a continuous process, experience with batch processes permit
several statements to be made concerning the design of such a system. Assuming that
a stable bleach solution of minimum alkalinity is to be manufactured, the critical
end point requires that some attention be given to the instrumentation features.
Transport lag to the electrodes should be avoided so that direct tank immersion
electrodes should be provided. Trouble may be encountered with high local chlorine
concentrations unless good agitation is provided. This is particularly true if liquid
chlorine feed is anticipated.
The demands on the automatic chlorine control from the electrodes should be minimized
by providing a flow controller for the caustic feed. A better system which has proved
adequate for similarly critical systems is to provide a flow controller for the
caustic, and a ratio controller for the chlorine operated form the caustic control,
the set point of which is controlled by the emf recorder. Such a system would provide
a highly uniform product even under adverse conditions.
The experiments on laboratory and plant scale on the control of the hypochlorite
reaction by means of oxidation potential lead to the conclusions that electrometric
control is reliable, efficient, and practical. By this means, a product can be manufactured
which has maximum stability and optimum bleaching speed under conditions of maximum
caustic utilization. The method will reduce the time required to finish a batch
of bleach by elimination of repeated titration tests and thus reduce operating labor
and increase the daily reactor capacity. The control method may be adapted to permit
a fully automatic continuous reaction system which would still further reduce the
cost of manufacture.
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