REVIEW

Jour. Functional Water Vol.7(1), pp.1-13, 2012

Electrolyzed Oxidizing Water Generation Methods
Brian WATERS1, Marcio AMAZONAS2, and Yen-Con HUNG1
1

Department of Food Science and Technology, The University of Georgia,
2

General Manager, AMZ Resulting, USA.
2011.9.1

Received

Electrolyzed oxidizing (EO) water, also known as

Ultimately, the concentration and ratio of the species ofDenkaisui in Japan, or electro-chemically activated water

free chlorine, combined chlorine and chloride ion are(ECA or ECAW), is a solution generated by passing adependent on method of generation, and pH of EO water.

dilute salt solution (NaCl and KCl are commonly used)

In its pure form, chlorine (Cl2) is a poisonous

through an electrolytic cell.

yellow-green

The anode side of angas.

However,chlorine

exhibitsaelectrolytic cell, from which acidic EO water is obtained,

pH-dependent chemistry.

produces various chlorine compounds and ions such as

solution that chlorine is dissolved in will play a dominanthypochlorous acid (HOCl), hypochlorite ion (OCl-) androle in what compound chlorine will assume.

chlorine gas (Cl2), which are all collectively known as

3 to 7, HOCl is the dominant chlorine species. HOCl is afree chlorine.

strong oxidant (ORP > 900 mV), and is the preferred free

HOCl is the main antimicrobial agent

present in EO water.

In other words, the pH of the
From pHEO water generally has a low pH

chlorine species for disinfection. HOCl is generated by(2.3 – 2.7) and a high oxidation – reduction potential

the hydrolysis of dissolved Cl2, as demonstrated in the(ORP) (> 1000 mV).

following equation:

On the cathode side of the electrolytic cell, a diluteCl2 + H2O

solution of NaOH is generated, and this is sometimes

HOCl + Cl- + H+

(1)

referred to as electrolyzed reducing (ER) water. Unlike

Due to the high molar concentration of water in

EO water, the principal use of ER water is not in the

aqueous solutions, this reaction proceeds via 1st order

reduction of microbial numbers. Instead, ER water iskinetics (k1 = 22.3s-1) 4).

1)

Disregarding the effects of all

mainly used as a degreaser and cleanser . Additionally,other factors (i.e. product concentrations, temperature,

work has been done on the health benefits of ER water incatalysts), the concentration of Cl2 is the most important

terms of cancer prevention, wound care, skin care andfactor in determining forward reaction rate in equation 1.

also as drinking water for improving digestion and cow

Below pH 3, Cl2 becomes increasingly dominant.

milk production2). Chemically, ER water is characterized

One major cause of free chlorine loss from water at low

as having a high pH (10-11.5) and a low ORP (-800 to

pH (< 4) is due to off-gassing of Cl2. The solubility of

3)-900 mV) .

chlorine gas at 20ºC is about 7g/l pure water, and it isreasonable to assume that this solubility can be altered bytemperature changes and changes in partial pressure over

Properties of EO water
EO water is primarily a solution consisting of HOCl,

the surface of the liquid (as Henry's Law states). At-

OCl , and dissolved Cl2, that when in contact withlow pHs, the reverse reaction of equation 1 predominates,organic materials, variable amounts of organic chlorides,

and it is a 3rd order reaction (k-1 = 4.3 x 104 M-2s-1) 4).

such as trihalomethanes (THMs), chloramines andAs evident, this reaction proceeds at a much greater ratehaloacetic acids (HAAs), all often referred to as boundthan(or combined) chlorine, are formed.

The sum total of of

However, it is important to understand that since thisHOCl, OCl-, Cl2, and chloramines is known as the total

reaction relies on the availability of free protons andchlorine concentration of an EO water solution.

hypochlorous acid, a low pH is necessary to shift thetheforwardreaction

described

Yen-Con Hung: The University of Georgia, 1109 Experiment St., Griffin, GA. 30223, USA. yhung@uga.edu

1previously.

equilibrium to the left in equation 1.

species below pH 1 is Cl2.

A low pH is

The dominant species

necessary for this because both hypochlorous acid andbetween pH 1 and 7.5 is HOCl, and the dominant species

free protons exist in greater quantities at low pH, thereby

above pH 7.5 is ClO-.

increasing the likelihood of the simultaneous collision ofStandard reduction potential is a measure of how

the three species required for the reverse reaction ineasily a chemical species acquires electrons, becoming

equation 1 to predominate.

reduced in the process and oxidizing the species theelectrons were acquired from.

Above pH 7.54 (the pKa of HOCl at room-

temperature), OCl is the dominant free chlorine species.

The larger and morepositive the number is, the greater ability as an oxidant-

However, OCl- has a standard

OCl is not as strong an oxidant as HOCl is (ORP ~the species has.

600-700 mV), but it is more chemically stable. The pKa

reduction potential of 0.9 V, and HOCl has a standardof HOCl can be determined experimentally using a

reduction potential of 1.49 V, making HOCl a stronger

variety of methods and the Henderson-Hasselbalchoxidant than OCl- is7).
Electrochemical reactions within an EO waterequation below:
pH = pKa + log ([OCl-] / [HOCl])generator cause the formation of the chlorine species(2)

observed in EO water.

The reactions in an EO water

generator can be divided into 2 different categories:

At pH 7.54 (at 25ºC), 50% of HOCl is deprotonatedto OCl-. Therefore, above pH 7.54, the dominant chlorine

electrode localized reaction and reactions in solution.

-

species is OCl .
The relative distribution of the three forms of freePrinciple of EO water generation

chlorine as they relate to pH can be visualized on Figure

Electrode localized reactions

5)

1 .

Electrode localized reactions are simply reactions
that occur in the double layer region surrounding
electrodes. The double layer region consists ofspecifically-adsorbed ions (the inner Helmholtz plane)
followed by a layer of solvated ions (the outer Helmholtz
plane). In terms of EO water generators, the reactions

pKa1

pKa2

can be divided into two different types of reactions: anoxidation of chloride ion at the anode and a reduction ofwater at the cathode.
The anode side of an electrolytic cell, from which EO
water is obtained, carries out the following reaction:
2Cl-

Cl2 + 2e-(3)Electrons are abstracted from chloride ions (Cl-) in

Fig. 1. Distribution of chlorine species from pH 0 to 12
Adapted from Deborde and vonGunten’s review (2008).

water by the anode to form chlorine gas.

Figure 1 assumes a chloride concentration of 5 mM.

is generated on the cathode side of an electrolytic cell

The label “pKa1” on the graph denotes the pKa of theused in EO water production. The following reactions

As mentioned previously, a dilute solution of NaOH-

OCl / HOCl couple. This pKa does not appear to beoccur in the cathode side:

concentration dependent, but it is pH dependent. The

2H2O + 2e-“pKa2” label denotes the apparent pKa of the Cl2 / HOCl
couple. Unlike pKa1, pKa2 appears to shift based on
chloride concentration.
increasewith

2OH- + H2(4)

Water is reduced at the cathode to form hydrogen gas

To be more specific, pKa2 will

and hydroxide ions in (4). In contrast to EO water, ER

6)

water is characterized by a high pH (10-11.5) and a low

increasing

chlorideconcentration .

ORP (-800 to -900 mV)3).

According to the graph, the dominant free chlorine2

feed solution.

Reactions in solution

Another important concern is corrosion.

Cl- is a significant catalyst of corrosion11).

Besides the solvation of chloride compounds in the

Therefore,-

feed solution, there are other chemical reactions

lowering the concentration of Cl in EO water throughoccurring in solution which play an important role in theuse of generation technologies with greater conversion

function of an EO water generator. The solution reactions

efficiency is one possible strategy to limit corrosion of

in EO water are dependent on the electrode-localized

EO water.

reactions.
HOCl + Cl- + H+

Cl2 + H2O
HOCl

EO water generators
In theory, an EO water generator is a type of(5)OCl- + H+(6)electrochemical cell that is used for the separation andDissolved chlorine gas either leaves solution oroxidation state change of Cl- and its counter ion. A

undergoes a rapid reaction with water to form

generalized sketch of an electrolytic cell is shown in

hypochlorous acid (HOCl), protons and more chloride

Figure 2 below:

ions, as shown in (5). In (6), HOCl can lose a proton toform its conjugate base hypochlorite (OCl-) in a pH –dependent equilibrium.
Other reactions also occur in solution, and thesereactions may contribute to the unique properties of EO
water, despite being low in concentration.

Like theprimary reactions, these reactions are pH-dependent as
well. Many of these reactions are involved in the slow
decomposition of HOCl.

For example, in the pH rangeof 5-8, the mechanism proposed by Adam et al.8)
involves the synthesis of chlorate (ClO3-) from HOCl.

Fig. 2. Typical electrolytic cell
adapted from Bard and Faulkner12)

These findings support much earlier work done by
9)

Lister .

-The decomposition of HOCl to ClO3 is thirdorder, and the values of ΔH* and ΔS* are 64.0 ± 0.6 kJ /mol and -67.0 ± 2 J / mol K respectively.
10)

An electrolytic cell requires two electrodes and aAnother

power supply.

-

The polarity of electron flow from theclaims that Cl catalyzes

power source in Figure 2 indicates that the electrode on

the decomposition of OCl in the pH 9-10 range.

the left is a cathode, and the electrode on the right is an

Decomposition of HOCl at lower pH values is difficult toanode.

measure because any decomposition is overshadowed byand positively charged in the figure, respectively) are

the off-gassing of Cl2 at these pHs.

best described by the types of redox reactions that

paper by Adam and Gordon-

Also of importance in EO water generation is the

The cathode and anode (which are negatively

happen around them.

Reduction reactions happen at theconcept of conversion efficiency. Conversion efficiency,

cathode, and oxidation reactions happen at the anode.

or the percentage of salt that can be converted into HOCl,

Both the power supply and electrodes will be covered in

is an important concern in EO water generators.

greater detail later in this review.

Conversion efficiency is defined as the following:

As mentioned previously, an EO water generator
functions as an electrolytic cell. Due to the variation in% conversion efficiency =[free chlorine] / [initial salt] x 100

EO water generator design, there is a great deal of(7)difference in the operating parameters normally used by

Conversion efficiency is important for a number ofa generator. As such, the most essential parameters arereasons. Obviously, greater conversion efficiency leads

the chemical requirements need for the desired reactions

to a higher concentration of free chlorine in EO waterto occur.

while using a lower concentration of electrolyte in thelargely on conditions such as amount and direction of the3

The functioning of the cell will depend

electrical current and electrolyte concentration13).

Also, precious metals such as platinum are sometimes
added as part of the surface coating as well 19).

In the operation of an EO water generator, a powerTheresource provides the electron flow that allows an

are several reasons for coating electrodes.

electrical potential to develop in the cell.

electrode helps to protect it from corrosion as well as

The size andCoating an

increases the activity of the electrode.

direction of the potential will determine the nature of thereactions that happen at the electrodes. A relationship

Pure metal electrodes are not often used in

between potential and current is usually best described

applications involving chlorine solutions due to the risk

by a current-potential curve. A current-potential curveof decreased output over time due to electrode surface

shows the potential needed to drive a reaction at a certain

corrosion. Oxide coatings reduce the oxidation of therate, which is known as the overpotential. In EO water

metal electrode core by forming a passive barrier.

generators, a potential of 2 V or greater is sufficient tocoatings also increase the interfacial area of the electrode,cause the oxidation of chloride at the anode because theallowing for greater reaction rates.

The

potential (vs. standard hydrogen electrode) of equation(3) above is 1.3612).

Cell potentials in an EO water

generator

dependent

are

set

on

the

Separatory membranes in EO water generationelectrolyte

Membranes are other factors which can affect the13)

concentration, for the purpose of avoiding arcing .

function of an EO water generator. The purpose of

Several studies have confirmed that the amount of

membranes in EO water generators is to divide the water

chloride present in the feed solution of an EO water

to anode and cathode chambers and/or allow the selective

generator will relate to the amount of free chlorine

movement of ions into the anode and cathodeproduced by the generator

1,14). The consensus is that ancompartments of the electrolytic cell.

increase in chloride concentration in solution will lead toan increase in free chlorine concentration.

There are several different membrane types to

However,consider: diaphragms, monopolar membranes and bipolar

there are other factors, such as hydraulic regime andmembranes. A typical diaphragm cell is pictured in Figure

solution residence time, that could have an impact on the3.

-value of Cl concentration as a variable for free chlorine
formation.
Electrodes in EO water generation
Since electrodes are the sites where
significant reduction of Cl- to Cl2 takes
place, the electrodes are arguably the mostimportant factors in EO water generators,
as well as electolytic cells in general.
Electrolytic cells in general have 2
electrodes made from metal, typicallytitanium and stainless steel for the anodeandcathode,respectively

semiconductors.

15,16)orIn addition to a metal

core construction, electrodes are often
coated with oxides16).

Oxides made fromiridium and ruthenium are the mostcommonly used, although studies havebeen

undertaken

with

mixed

oxide

Fig. 3. Diaphragm Cell Modelcoatings, such as tin and iridium oxide 17)
as well as nickel and cobalt oxides 18).

4

Diaphragm cells make use of an asbestos “diaphragm”

allow the transport of negatively-charged cations whilethat resists the flow of hydroxide ions to the anode.

prohibiting the transport of positively-charged anions. In

Sodium ions are able to freely pass through the diaphragm.

general, exchange membranes must have the following

Additionally, some 7 chloride ions are able to passproperties in order to be effective in EO water generation

through the diaphragm as well. In a sense, the diaphragm21):1) A membrane must have the physical and chemicalfunctions not as an impermeable wall (as the nameability to withstand NaOH and Cl2.

diaphragm implies), but as a selectively-permeable
membrane. Solvent is able to freely pass through the2) A membrane must have low electrical resistance.

diaphragm but certain charged species are not. This

3) A membrane must allow transport only Cl- from thecatholyte to the anolyte.

method of separation of the cathode and anodecompartments of the electrolytic cell is being replaced by

4) A membrane must operate at high current density.

monopolar and bipolar membranes, mainly due to5) A membrane must be immune to impurities (i.e.

environmental concerns associated with the asbestos

heavy metals) in the solution.

diaphragm found in diaphragm cells.

In contrast to the single layer design of monopolar
membranes, dipolar membranes employ two different

Monopolar and bipolar membranes both have a
The difference between a diaphragm

membranes with a hydrophilic layer sandwiched between

cell and a membrane cell lies in the behavior of thethem. An example of a dipolar membrane is shown inpartition (membrane) separating the anode and cathodeFigure 5.

standard design.

chambers. Another difference lies in the composition of
the cathode, which is usually nickel in membrane cells.
Ion exchange membranes allow the flow of ions in a
specific direction, dependent on the nature of themembrane, but not vice-versa. Generally, there are cationexchange membranes, which permit the flow of positive
ions, and anion exchange membranes, which permit theflow of negative ions. Figure 4 is a model of an anion
exchange membrane.

Fig. 5. Dipolar membrane
adapted from Pab by et al. 20)

Figure 5 depicts a dipolar membrane that consists of acation and anion selective membrane. Generally, ions in
the transition region between the two membranes travel
through the membranes in the direction of their charges 22).
Fig. 4. Anion Exchange Membrane

A special type of EO water generator with three chamber

adapted from Pabby et al. 20)

cells has electrolyte flowing through the middle chamber
between the cation and anion membranes. Very pure EO

Monopolar membranes are single-layered membranes

water can be produced at the anode chamber.
+

The

-

that allow the selective transport of ions in the direction of

membranes separate Na and Cl in the direction of

charge. Specifically, anion exchange membranes only

current flow towards the cathode and anode, respectively.

5This design is considered economical because of low

desired product15).

power consumption, the ability to operate at high current

involved in chlorine production by salt electrolysis are

densities; high acid and base concentrations can bemostly titanium coated with oxides from the platinum

produced, a high current efficiency and low maintenancegroup.

54)

As mentioned previously, electrodes

Ruthenium oxide, the most common coating, isoften used with oxides of titanium, tin and iridium 28-30).

costs .

For
Generator parts in EO water generation

chlorineproduction

involvinghydrolysis

ofhydrochloric acid (HCl), carbon anodes are preferred.

The power source of an EO water generator is one ofElectrode material will also influence other aspects of thethe most basic parts of the generator that is not subject toprocess, such as overpotential, selectivity, lifetime as

modification.

well as behavior under nonoptimal conditions15).

The power source provides an external

voltage that is greater than the open circuit potential of

The surface area and geometry of an electrodethe cell. In practice, a 120V power supply is commonly

directly influence the function of EO water generators inused.

several ways.

Some models of generators with adjustable

First, the geometry of an electrode can

23)

influence energy consumption in the electrolytic cell.

The body of an EO water cell must be constructed

Both electrolysis and the movement of solution throughvoltage can range from 0-18V .
with several considerations.

First, the body must be

the cell represent most of the energy consumption in an

water-tight to prevent the leakage of electrolyte solution

electrolytic cell15).

onto open circuitry.

Because of the corrosive nature of

can be minimized by the electrode having an open,

the solutions being used and produced, EO water

porous construction. This type of electrode construction

generator parts are often made of plastics such as PVC.

imparts less turbulence to solution flow. In addition to

The energy cost of pumping solution

In addition to concerns about the solutions being

energy consumption, electrode geometry can influencehandled by the EO water generator, movement ofreaction rate. A porous electrode has a higher massdifferent fluids through the generator is also important.

transfer coefficient, allowing for a greater rate of

The body must be constructed to allow the efficient

reaction15). Of particular note is that both desirable andimport of electrolyte solution and the efficient export ofundesirable reactions can have their rates influenced by

EO water, ER water (if there is a separation between theelectrode geometry. For example, cylindrical and coaxial

anode and cathode), and gases such as oxygen andconfiguration of electrodes are claimed to have a higherhydrogen.

salt conversion rate than parallel plates. Also, coaxial

The accumulation of these gases in thecells use a ceramic membrane, which has thegenerator could lead to an explosion hazard.

disadvantage of being fragile during transportation, butwith a longer life span in use, compared with "plastic"

Factors affecting EO water generation

membranes or diaphragms.

There are many factors both internal and external to

Surface condition, in this case, refers to corrosion,an EO water generator that affect performance of the
One of the most important sets of factors are thosescaling or some other type of fouling of the electroderelating to the electrodes. Electrode material, surface

surface. These surface conditions of an electrode canarea, electrode geometry and surface condition must all

be negatively affected by the presence of impurities inbe considered when addressing the contribution of thesolution.

cell.

electrodes to an EO water generator's performance.
Internal factors affecting EO water generation

Electrode material has been the focus of several

Mass transfer factors refer to the movement ofstudies in EO water generation, such as the development
16,24), mixed metal oxides

molecules in bulk solution to the electrode surface region.

coating metal18,25) and doped carbon-based electrodes26,27).

Figure 6 below illustrates the concepts of the electrode

Electrode material was found to influence material yieldsurface region and the bulk solution:of metal oxide coated metal(referred to as conversion efficiency), which is the

The greater the rate of mass transfer, the more

fraction of the starting material converted into theefficient the electrolytic process is. There are several

6

Fig. 6. Electrode surface region and bulk solutionadapted from Bard and Faulkner 12)
factors of mass transfer to be considered: mode ofincreasing the concentration of chloride ion will increasetransfer, surface concentrations and adsorption.

production of HOCl under many circumstances13,14).

Mode of transfer refers to the method by which

Other factors worthy of consideration are so-called “loss”

solvated ions move from the bulk solution phase to thereactions in electrolytic generators. Depending on theelectrode surface region. Diffusion and convection are

generator design, the following reactions could occur to a

two examples of how transfer of solvated ions could

greater or lesser degree 31):

occur.

The mode of transfer applicable to particular

2ClO3- + 4Cl- + 6H+ + 3/2 O2 + 6e- (8)6ClO- + 3H2O

electrolytic generator is usually dependent on factors

ClO- + H2O + 2e-

relating to construction of the generator. Surface

2HOCl + ClO

concentration refers to the density of solvated ions in the

2ClOelectrode surface region, and adsorption is defined as the--Cl- + 2OH--

(9)

ClO3 + 2Cl + H

+

(10)

-

O2 + 2Cl( 11 )

method and rate which solvated ions adhere to theReaction (8) takes place at the anode. OCl- is oxidized

electrode surface. Both surface concentration andalong with water to produce chlorate (ClO3-) ions. At theadsorption are influenced by electrode-specific factors

cathode, ClO- is reduced to Cl-, as shown in (9). Reactions(as discussed previously in this review) as well as factors(10) and (11) both take place in solution. Overall, these

relating to the solution itself.

reactions are said to be more prevalent when solutions
from the cathode and anode are allowed to mix, as is the

Solution variables, such as bulk concentration ofcase for some EO water generation technologies.

electroactive species, concentrations of other species as

Electrical factors, such as potential, current andwell as the nature of the solvent, all influence an EO
12)water generator's function .

The concentration ofquantityofelectricity,also

affect

the

rate

of

electroactive species (in this case, the concentrations ofelectrolysis12).

chloride ion and its oxidized electrolysis product, Cl2)

potential is supplied by a 120V power supply in many

will influence the rate of electrolysis based on simple

cases.

equilibrium mechanics. Low concentrations of chloride

voltage was an important contributor in the production of

ion will cause a decreased production of Cl2 (which

free chlorine.

iscorresponding increases in free chlorine.

subsequently

converted

toHOCl).

Likewise,7

As mentioned previously, electricalWork by Ezeike and Hung13) concluded that
Generally, increases in voltage led toExternal factors affecting EO water generation

generation technologies are compared by type of feed

In addition to factors within the generator itself,

solution used, membrane type, the ability for a user tovariables external to the generator, such as temperature,control pH and/or current, and the need for dilution before

pressure and time, can also influence the efficiency of theuse.

electrolysis process.

Differences in feed solution among common generator

The role of temperature in EO

water generators has been explored previously

1,14)

.

It

types

was determined that temperature was significant in theBrine solutions are the most common types of feed

determination of the magnitude of DC current andsolutions used in EO water generation. The chlor-alkali

reaction rates.

However, when compared with otherprocess in industry utilizes an estimated 90 million tons

variables such as salt concentration, the role ofof salt per year35), and the manufacture of chlorine is thetemperature was determined as not significant.

For thislargest single use of salt. Of the salts that can be used inreason, temperature is not regarded as an important

EO water production, NaCl is the most commonly-used1)

salt36-38).

factor in generator function .

Other salts, such as KCl can also be used, but

In addition to temperature, pressure has been

NaCl is more readily available and more economical.

determined to play a role in function of a EO water

Since both the salts are readily soluble in water (both32,33). The role of fluid pressure in EO waterover 30g/100g of water at room temperature) and thegenerators is on gas solubility and concentration of Cl2 in

potentials required for Cl- oxidation at the anode are

solution.

In particular, increases in pressure lead tothe same for both salts (-1.36 V), there is essentially nogreater solubility of Cl2 in solution. Decreases in theelectrochemical difference between the two. EO watersolubility of chlorine gas from a sample of EO water leadgenerators that use brine feed solutions are therefore the34)

to decreases the level of free chlorine in the solution .

most common types of generators available, and the

The effect of pressure on Cl2 concentration of EO water isfunction of these types of generators is the most studied.

only positive to a point, however. It is expected that

In contrast to salt, some EO water generators use HCl

Cl2 concentration will provide a

as a source of Cl- ions. A few EO water generators are

generator

eventuallytheconcentration barrier to the production of more Cl2.

commercially available, especially for producing near

The discussion of time as a variable is limited toresidence time of feed solution.

neutral pH EO water. Outside of EO water generation,

It is expected thatstudies have been conducted to convert gaseous HCl

residence time of feed solution will be directly

waste into chlorine in order to address environmental

proportional to the concentration of free chlorine

concerns39).

produced through electrolysis.

NaCl and HCl is solubility.

One large difference that exists between
At room temperature, thesolubility of NaCl is about 38 g / 100 g of water. In
Common types of EO water generators
There are a wide variety of EO water generators

contrast, the solubility of HCl is approximately 70 g /100

available today. In Table 1 below, several EO watersolubility of HCl can lead to more Cl- ions in solution for

g of water at room temperature. Theoretically, the higher

Table 1. Comparison of Common EO water generation technologies
Generator

System 1

System 2

System 3

System 4

System 5

Feed solution

NaCl

NaCl

HCl

NaCl

NaCl

Membranearrangement

singlenon-selective

singlenon-selective

none

singleion selectivenone

pH control

yes

no

no

yes

no

Current control

yes

yes

no

no

no

Need for dilution

no

no

no

yes

no

*All systems are available in Dr. Hung’s lab at the University of Georgia

8electrolysis. Also, EO and ER water generated using
feed

HCl lacks Na+ ions. This is beneficial for operation thatcaustic soda (NaOH) is not used.

NaOH may also

contribute to the production of oxygen radicals, like OH.

anode

and O., which represent a more rapid path to corrosion

cathodethan Cl-.
Differences in pH control among common generator
EO wateroutlettypes
In cases where pH control is needed, one method of

ER wateroutletpH control involves the restriction of ER water exit fromthe generator. Some generators are constructed in a way
Fig. 8. pH control by alkaline water recycling

such that the back pressure created from doing this allows
ER water to mix with EO water inside the cell. This isoutlined in Figure 7 below:

The arrows in Figure 8 represent the flow of alkalinefeedwater from the cathode to the anode, which results in an
increase in sodium and hydroxide ions to produce an EO
water solution of a higher pH.

anode

Generators that produce acidic EO water have acathodedivision between the anode and cathode regions of thecell.

This is done in order to prevent the anolyte frommixing with the catholyte.

Another way to generate

near neutral EO water is to not have a division between
the anode and cathode 42). This type of EO water has a

ER waterEO water

pH typically between 5 and 7 and a lower ORP (~850mV at pH 5) 40,41).

clamp

Table 2 summarizes the properties ofacidic EO water and near neutral EO water.

Fig. 7. pH control by back pressure

Table 2. Comparison of acidic EO water andnear neutral EO water properties
In Figure 7, feed solution enters the generator, and theAcidic EO

Near Neutral EO

pH

2.3-2.7 3,59)

5.0-6.5 42,43)

by a clamp). This causes a buildup of pressure which

ORP> 1000 mV 3) 800-200 mV 40,41,43)

forces some ER water into the anode chamber to mix withPrimary chlorine species

electrochemical reactions occur in the same way as other

General chemical properties

EO water generators. The difference is that ER water flowis restricted as it exits the machine (in the case of Figure 7,

HOCl, Cl2 10)

HOCl, OCl- 8,58)

EO water. The near neutral pH EO water exits themachine from the EO water outlet. Essentially, the pH of

The properties of acidic and near neutral EO water

the EO water can be adjusted by controlling the backcontribute to their stability and antimicrobial activity.

pressure at the ER side.

In terms of stability, near neutral EO water is much more

Another method of pH control involves a recycling

stable due to no loss of Cl2 through evaporation and themechanism that allows alkaline water to be mixed withrelatively slow rate of ClO- decomposition than acidic

EO water, as illustrated by Figure 8.

EO water 43,44,45), which is more prevalent at lower pH
values 44,45). On the antimicrobial property, near neutral

9

EO water has a high percentage of chlorine in the form

utilized sulfonic groups, however problems at highof hypochlorous acid and can compensate for its low

NaOH

ORP values.

Eventually, the single cation exchange membranes

concentrations

evolvedinto

led

modifications53).

tocomposite membranes.

Composite

Differences in current control and membrane technology

membranes have a thin layer of carboxylate polymer onamong common generator types

the cathode side of the membrane.

The layer prevented

Some EO water generation technologies allow thethe back migration of hydroxide (OH-) ion, and it wasuser to control the magnitude of the current through thestill sufficiently thin to keep membrane resistance low53).

electrolytic cell.

Bipolar membranes (Figure 9) are cation and anion

As explained previously, increases inelectrical current in an EO water generator result in

exchange membranes joined closely together.

increased concentrations of free chlorine.

chlor-alkali

Besides this,industry,

bipolarIn themembranesarecurrent control can also allow control the rate of

manufactured to be immersed in an electrolyte flow that

undesired reactions 46).

runs perpendicular to the membranes below.

Current control allows for the47)

The design in Figure 9 enables single units of bipolar

as well as the efficiency of the process . There are amembranes to be used modularly as components in a

few options available for current control.

rack.

prediction of energy consumption in a specific process48)

One suchThis design saves space and allows the individual

option is the use of amplifier instruments to regulate

units to be connected electrically in a series. Feed pipeschanges in potential or current49).

carry brine to a distributor (not pictured) in a direction

Membrane technology is not just limited to thespecificity of the membrane.

perpendicular to plane of view.

Often, the number ofmembranes and their arrangement can influence theDifferences in solution flow among common generator

efficiency of an EO water generator. Semipermeable,

types

or selectively permeable, ion exchange membranes havealready been introduced in this review.

The flow of feed solution and product solution in an

Cationic

EO water generator is another factor worthy ofmembranes, in particular, have been studied in somedetail either alone

50,51)

consideration.

or in comparison to other types ofmembranes, such as diaphragms52).

For the chlorine manufacture industry,

there is a compelling need to increase variables, such as

In the chlor-alkali

mass transfer, that lead to higher product yields55).

industry, diaphragm cells are being phased out in favor ofPrevious work by Hsu 1) indicated that increases in waterion exchange membranes.

Ion exchange membranes

flow rate increased electrolysis current. Other work bycarry an advantage over diaphragms because the

Hsu14) as well as Ezeike and Hung13) indicated that

membranes have less of an environmental impact
in terms of construction materials used inmembraneproduction

consumption.

as

well

as

Membrane/electrode

power

There are two types of ion

exchange membrane configurations to consider:
single and bipolar.
Single ion exchange membranes, as discussed
earlier, allow a specific charge of ion to passbipolar membraneelectrodes

through while excluding counter charged ions.
The membrane is used to physically separate thecathode and anode in an electrolytic cell.

In thechlor-alkali industry, cation exchange membranes
are most often used53).

constructed of perfluorinated polymers (Nafion
polymers).

feed pipe

Cationic membranes are®

Fig. 9. Bipolar membrane unit configuration

The original Nafion® polymers

adapted from Schniders 57)

10increases in water flow rate decreased the level of freechlorine in EO water.

6)

Both groups concluded that

Durik SE, Cherney DP, Tarr CJ, el al.: Determining
activeoxidant

speciesreacting

withlower residence time of electrolyte within the cell led toorganophosphate pesticides in chlorinated drinking

a decreased level of free chlorine. However, generator

water. U. S. Environmental Protection Agency. pg.

design changes to increase residence time must not

45. 2006.

reducetheproduction

rate/capacity.

Increased

7)

residence time can be achieved through modification of

Dowd MT: Assessment of THM formation with
MIOX. UNC Master’s Thesis. 1994.

speed that electrolyte passes through the cell or

8)Adam LC, Fábián I, Suzuki K, et al.: Hypochlorous

configuring the flow pattern in the cell to allow foracid decomposition in the pH 5-8 region. Inorg.

multiple passes.

Chem., 31, 3534-3541, 1992.

Another way to modify residence timeis to induce turbulence.

56)

Subbaiah and coworkers

9)reported that increasing turbulence enhances the mass

Lister MW: The decomposition of hypochlorous
acid. Can. J. Chem., 30, 879-889, 1952.

transfer coefficient at the membrane surface, leading to10) Adamhigher yields in product.

LC,

GordonG:

Hypochlorite

ion

Effectsoftemperature,ionic

decomposition:

strength and chloride ion. Inorg. Chem., 38,
1299-1304, 1999.

Conclusions
There is a wide variety of EO water generation

11) Chin RJ, Nobe, K.: Electrodissolution kinetics oftechnologies commercially available. Due to differences

iron in chloride solutions. J. Electrochem. Soc., 119,in construction and operation, the exact definition and1457-1461, 1972.

quality of EO water can be significantly different.

12) Bard AJ, Faulkner, LR: Electrochemical methods:

Understanding EO water generators, in particular their

Fundamentals and applications. Wiley, New York,

design, components and factors effecting EO water

NY. 2001.

generator performance can lead to the research and13) Ezeike GOI, Hung Y-C: Acidic electrolyzed water

manufacture of safer and more effective EO water

properties as affected by processing parameters andgenerators.

their response surface models. J. Food. Proc. Prot.
28, 11-27, 2004.

References
1) Hsu SY: Effects of flow rate, salt concentration and14) Hsu SY: Effects of flow rate, salt concentration andwater temperature on chemical and physicaloxidizing water generator. J. Food Eng., 60,

properties of electrolyzed oxidizing water. J. Food

469-473, 2003.

water temperature on efficiency of an electrolyzed15) Couper M A, Pletcher D, Walsh, FC: ElectrodeEng., 66, 171-176, 2005.
2)

Ye J, Li Y, Hamasaki T, et al.: Inhibitory effect ofmaterials for electrosynthesis. Chem. Rev., 90,

electrolyzed reduced water on tumor angiogenesis.

837-865, 1990.
16) Kraft A, Blaschke M, Kreysig D, et al.:Biol. Pharm. Bull. 31, 19-26, 2008.
3)

4)

Huang Y-R, Hung Y-C, Hsu S-Y, et al.: Application

Electrochemicalof electrolyzed water in the food industry. Food

Hypochlorite production from potable water,Con., 19, 329-345, 2008.

chlorine consumption and the problem of calcerous

Wang TX, Margerum DW: Kinetics of reversible

deposits. J. Appl. Electrochem., 29, 895-902, 1999.

chlorine hydrolysis: temperature dependence and17) Balko EN, Nguyen PH: Iridium-tin mixed oxide

general-acid/base-assisted

mechanisms.

disinfection

part

II:

Inorg.

anode coatings. J. Appl. Electrochem., 21, 678-682,

Deborde M, von Gunten U: Reactions of chlorine

18) Suffredini HB, Cerne JL, Crnkovic FC, et al.:with inorganic and organic compounds during water

Recent developments in electrode materials fortreatment-Kinetics and mechanisms: a criticalwater electrolysis. Int. J. Hydro. Ener., 25, 415-423,

review. Water Res., 42, 13-51, 2008.

2000.

1991.

Chem., 33, 1050-1055, 1994.
5)

water

11

19) Takei T, Laitinen HA: The effect of platinumof proton exchange membrane fuel cells. In Fullcoatings on the catalytic activities of SnO2/Si

Cell Technologies: State and Perspectives, N.

electrodes for oxygen reduction. Surf. Technol., 19,Sammes et al. (eds.), Springer, 205-210. 2005.

273-281, 1983.

34) Len S-V, Hung Y-C, Chung D, et al.: Effects of20) Pabby AK, Rizvi SSH, Sastre AM: Handbook of

storage conditions and pH on chlorine loss inmembrane separations: chemical, pharmaceutical,

electrolyzed oxidizing (EO) water. J. Agric. Food

food and biotechnological applications. Taylor andChem., 50, 209-212, 2002.

Francis Group, L.L.C., Boca Raton, FL. 2009.

35) Sedivy VM: Economy of salt in chloralkali21) Davis TA, Genders JD, Pletcher, D: A first course in

manufacture. From the National Salt Conference,ion permeable membranes, The electrochemical

Grandidham. 2008.

consultancy, UK, pg 199, 1997.

36) Stan

SD,

Woods

JS,

and

Daeschel

MA:22) Strathmann H, Krol JJ, Rapp H-J, et al.: Limiting

Investigation of the presence of OH radicals incurrent density and water dissociation in bipolarelectrolyzed NaCl solution by electron spin

membranes. J. Mem. Sci., 125, 123-142, 1997.

resonance spectroscopy. J. Agri. Food Chem., 53,
4901-4905, 2005.

23) Unknown: Hoshizaki water electrolyzer: Model37) Hricova D, Stephan R, Zweifel, C: Electrolyzed

ROX-20TA-U. Service Manual. 2000.
24) Trasatti S. Electrochim. Acta, 32, 369, 1987.

water and its application in the food industry. J,25) De Faria LA., Boodt JFC, Trasatti S: Eletrocatalytic

Food Prot., 71, 1934-1947, 2008.

properties of Ru + Ti + Ce mixed oxide electrodes38) Kubota A, Nose K, Yonekura T, et al.: Effect offor the Cl2 evolution reaction. Electrochim. Acta, 42,

electrolyzed strong acid water on peritoneal

3525-3530, 1997.

irrigation of experimental perforated peritonitis.

26) Ferro S, De Battisti: Electrocatalysis and chlorine

Surg. Today., 39, 514-5177, 2009.

evolution reaction at ruthenium dioxide deposited39) Johnson J, and Winnick J: Electrochemicalon conductive diamond. J. Phys. Chem. 106,

membrane separation of chloride from gaseous

2249-2254, 2002.

hydrogen chloride waste. Sep. Pur. Technol. 15,

27) Zhang Y, Yoshihara S, Shirakashi T: Novel223-229, 1999.

application of boron-doped diamond and related40) Park H, Hung Y-C, Chung D: Effects of chlorinematerial to electrochemical generation of functional

and pH on efficacy of electrolyzed water forwater. Electrochim. Acta., 51,1008-1011, 2005.

inactivating Escherichia coli O157:H7 and Listeria28) Coulter M, Ed,: Modern Chlor-Alkali Technology:

monocytogenes. Int. J. Food Micro., 91, 13-18,

Ellis Horwood: Chichester, 1980, Vol. 1.
29) Jackson C, Ed.:2004.

Modern Chlor-Alkali Technology;41) Xiong K, Liu H-J, Liu R, et al.: Differences in

Ellis Horwood: Chichester, 1983, Vol. 2.

fungicidal efficiency against Aspergillus flavius for30) Wall K, Ed.: Modern Chlor-Alkali Technology; Ellis

neutralized and acidic electrolyzed oxidizing waters.

Horwood: Chichester,, Vol. 3., 1986.

Int. J. Food. Micro., 137, 67-75, 2010.

31) Kristajić N, Nakić V, Spasojević M: Hypochlorite

42) Gómez-López VM, Ragaert P, Ryckeboer J et al.:production II. Direct electrolysis in a cell divided by

Shelf-life of minimally processed cabbage treatedan anionic membrane. J Appl Electrochem., 21,with neutral electrolysed oxidising water and stored637-641, 1991

under equilibrium modified atmosphere. Int. J. Food.

32) Neagu C, Jansen H, Gardeniers H, et al.: The

Microbiol., 117, 91–98, 2007.

electrolysis of water: an actuation principle for43) Cui X, Shang Y, Shi Z, et al.: Physicochemical

MEMS with a big opportunity. Mechatronics, 10,properties and bactericidal efficiency of neutral and571-581, 2000.

acidic electrolyzed water under different storage

33) Grigoriev SA, Kalinnikov AA, Porembskiy VI, et

conditions. J. Food Eng., 91, 582-586, 1990.

al.: Mathematical modeling and experimental study44) Underwood WF, Mack, E: The decomposition of12sodium hypochlorite-an ion reaction. J. Phys.

52) Chen C-P, Tilak BV, Quigley JW: Generalized
expressions for estimating the current efficiency ofChem., 35, 2650-2657, 1931.
45) Wang TX, Margerum DW: Kinetics of reversible

ion-exchange membrane and diaphragm chlor-alkali

chlorine hydrolysis: temperature dependence andcells. J. Appl. Electrochem., 25, 95-105, 1995.

general-acid/base-assisted mechanisms. Inorg.

53) Balster J, Stamatialis DF, Wessling, M: 2004.
Electro-catalytic reactors and the development ofChem., 33, 1050-1055, 1994.
46) Fedkiw PS, Chao JC: Selectivity modification in

bipolar membrane technology. Chem. Eng. Proces.,

electroorganic reactions by periodic current control:

43, 1115-1127, 2004.
54) Nagasubramaniman K, Chlanda FP, Liu K-J: Use of

Electroreduction of nitrobenzene. J. Amer. Chem.

bipolar membranes for generation of acid andEng., 31, 1578-1580, 1985.

base-an engineering and economic analysis. J.

47) Panizza M, Michaud PA, Cerisola G, el al.:Electrochemicaltreatmentofwastewaters

Memb. Sci., 2, 109-124, 1977.

containing organic pollutants on boron-doped55) Schubert M, Kryk H, Hessel G, et al.: Residence

diamond electrodes: Prediction of specific energy

time measurements in pilot-scale electrolytic cells:

consumption

area.

Application of laser-induced fluorescence. Chem.

48) Gabrielli C, Huet M, Keddam M, et al.:56) Subbaiah T, Venkateswarlu P, Das RP, et al.:Investigation of water electrolysis by spectral

Mass-transfer conditions at a cathode support plateanalysis. I. Influence of the current density. J. Appl.

in an electrochemical cell. Chem. Eng. Proces. 34,

Electrochem., 19, 683-696, 1989.

495-501, 1995.

andrequiredelectrodeEng. Comm., 197, 1172-1186, 2010.

Electrochem. Comm., 3, 336-339, 2001.

49) Lauer G, Schlein H, Osteryoung RA:

57) Schneiders K, Zimmermann A, Henssen G.:Amultipurpose electrochemical instrument for control

Membraneelectrolysis-of potential or current. Anal. Chem. 35, 1789–1793,

chlor-alkaliindustry.

Technische

1963.

ThyssenKrupp.
58) Suzuki K, Nakamura T,50) Ogata Y, Uchiyama M, Hayashi M, et al.: Studies ofinnovation

for

the

Mitteilungen

Kokubo S, et al.: The

the pH of the membrane surface in a laboratorychemical properties of slightly acidic electrolyzed

chlor-alkali cell. J. Appl. Electrochem., 20, 555-558,

water prepared with hydrochloric acid as a raw

1990.

material. Bokin Bobai., 33, 63-71, 2005.

51) Liao LB, Chen WM, Xiao XM: The generation and59) Shimizu Y, Furusawa T: Antiviral, antibacterial, andinactivation mechanism of oxidation-reduction

antifungal actions of electrolyzed oxidizing water

potential of electrolyzed water. J. Food Eng., 78,

through electrolysis. Dental J., 37, 1055-1062,

1326-1332, 2007.

1992.

13