United States
Environmental Protection
Agency
Municipal Environmental Research ^
Laboratory i^J-.
Cincinnati OH 45268 f~\l
Research and Development
EPA-600/S2-84-036 Mar. 1984
c/ERA Project Summary
Improved Techniques for
Residual Ozone
Gilbert Gordon and Joyce Grunwell
This project responds to the need for
accurate, acceptable analytical proce-
dures for residual ozone in treated
water. The standard iodometric method
is compared with three iodometric
modifications: The amperometric
method, the arsenic(lll) back titration
method, and the N,N-dietnyl-p-phenvtene-
diamine (DPD) method. In addition, the
study evaluates four analytical methods
based on the reaction of ozone with a
reductant other than iodide ion: The
indigo method, the arsenic(lll) direct
oxidation method, the delta electrode,
and the direct measurement of the UV
absorption at 259 nm. Comparisons are
made of all eight methods regarding the
validity of the ozone titer, the stability
of the titer of the ozonated reagent
solutions, and the ease of calibration of
the reagent solutions.
Also discussed are four exploratory
methods based on newozone-reductant
reactions: The chlorite ion method, the
iodate method, the iron(ll) terpyridine
method, and the cerium method.
Results were that no iodometric
method was recommended. The indigo
method and arsenic(lll) direct oxidation
were the methods of choice and were
recommended as replacements for the
standard iodometric method currently
in wide use. None of the four exploratory
methods gave reliable residual ozone
values.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory, Cincinnati, OH,
to announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
In the past 80 years, chlorine has been
widely used for disinfection of municipal
and industrial wastewaters in the United
States. But recent concerns over the toxic
effects of chlorinated organic byproducts
produced during chlorination of potable
water and wastewater have renewed
interest in the use of ozone for water
treatment. Experience in Europe has
shown that in addition to serving as an
effective disinfectant, ozone acts as an
oxidant to remove taste, color, odor, and
organic matter from water. If ozonation is
to achieve wide application for water
purification in the United States, accurate
analytical procedures acceptable to
technicians and water treatment plant
operators are necessary.
The nature of ozone makes its residual
analysis difficult. The element is usually
generated by passing a stream of dry
oxygen through an electrical discharge
that converts only 2 to 5 percent of the
oxygen to ozone. The instability of pure
ozone precludes the preparation of
weighted reference samples. Large
analytical errors can result from volatili-
zation of ozone from solution, its rapid
decomposition in water, and its reaction
with any trace contaminant in the water,
the reagents, or the glassware.
In the current standard method for the
determination of ozone in water, ozone
oxidizes iodide ion to iodine in a 2-percent
neutral, phosphate-buffered potassium
iodide solution; then the pH is adjusted to
2 with sulfuric acid, and the liberated
iodine is titrated with sodium thiosulfate
to a starch endpoint. The ozgneiiodine
stoichiometry has been extensively
studied and found to range from 0.65 to
1.5. The factors affecting the stoichiome-
-------
try include pH, buffer composition, buffer,
concentration, iodide ion concentration,
sampling techniques, and reaction time.
The pH during the initial ozone-iodide
reaction and the pH during the iodine
determination can alter the ozone:iodine
stoichiometry. The formation of iodate ion
and hydrogen peroxide have been specifi-
cally implicated as factors affecting the
ozone: iodine stoichiometry. Modifications
in the iodine determination include
changes in endpoint detection, pH, and
back titration techniques.
This project compares the standard
iodometric method with three iodometric
modifications: The amperometric method,
the arsenic(lll) back titration method, and
the DPD method. Also evaluated are four
analytical methods based on the reaction
of ozone with a reductant other than
iodide ion: The indigo method, the arsenic
(III) direct oxidation method, the delta
electrode, and the direct measurement of
the UV absorption at 259 nm.
All eight methods are compared as to
the validity of the ozone titer, the stability
of the titer of the ozonated reagent
solutions, the stability of the reagent
solutions, and the ease of calibration of
the reagent solutions.
Four exploratory methods based on
new ozone-reductant reactions are
discussed: The chlorite ion method, the
iodate method, the iron(ll) terpyridine
method, and the cerium method. The
failure of these potential new methods is
discussed.
Experimental
Ozone was generated from dry oxygen
by an OREC Model 03V9-0 ozonator.*
Ozone solutions were prepared in a
3000-ml contactor (Figure 1) equipped
with a medium-porosity glass frit and a
sampling stopcock with a Leur tip.
Additional stopcocks allowed the inclusion
of a closed-flow loop for mixing and for
the Delta electrode. A Cole-Parmer
Model 7149-10 all-Teflon pump provided
circulation.
A multi-stop Silverman syringe was
designed to minimize sampling time for
multiple analyses (Figure 2).
A solution of ozone was prepared, and
the ozone concentration was determined
at known time intervals by two or more
analytical methods. The resulting time
and concentration profile for each
analytical method was graphed and fitted
to the generalized rate law
OVO,
dt
=k, [03] [OrT] +k2 [03]
Pump
Figure 1. Contactor with flow cell.
using FIT 80. The apparent rate constants
were calculated and compared within
each kinetic run. Identical kinetic param-
eters between methods indicated that
each method gave the same result. This
kinetic technique minimized sampling
errors resulting from concentration
changes and allowed a direct comparison
of methods under conditions of rapidly
changing ozone concentration.
Conclusions
Stability of the Titer of the
Ozonated Reagent Solutions
Convenient laboratory analysis demands
stable ozonated reagent solutions. With
the DPD method, the ozone titer changed
so rapidly with time (both for ozone in
A B C
purified water and for ozone solutions
with added hydrogen peroxide) that the
method cannot be recommended for
routine ozone analysis (Figure 3). The
arsenic(lll) back titration titer steadily
increased for ozone solutions with added
hydrogen peroxide (Figure 4). The ozone
titer by the amperometric method with
excess sodium thiosulfate increased 4
percent in 9 min with ozone in purified
water. The ozone titer determined by the
arsenic(lll) direct oxidation method and
the indigo method varied less than 3
percent over 3 hr, even with added
hydrogen peroxide.
Stability and Calibration of the
Reagent Solutions
The arsenic(lll) stock solutions are
stable, standard solutions readily prepared
by weight. Dilute working solutions should
be prepared daily. Indigo disulfonate stock
solution would need replacement at least
every 4 weeks. The more stable indigo tri-
sulfonate stock solution would require re-
placement only every 10 weeks. Calibra-
tion of the indigo dyes is essential,
time consuming, and based on iodometry.
These problems could be avoided if high-
er-purity dye were readily available and
calibration could be based on weight.
Accuracy of the Ozone Titer
The ozone tilers differed among methods
only when changes in the ozone reductant
reaction were involved. Differences between
iodometric and noniodometric methods
were not directly caused by iodate ion
formation or hydrogen peroxide formation.
Conditions that reduced ozone decay
before reaction with reductant reduced
the scatter observed within a single
method and reduced the differences ob-
served among the analytical methods. All
methods occasionally give a point that is
30 to 50 percent removed from that
calculated on an otherwise smooth decay
1
1
> J J s / / /
\
\
PI
' / X / y
y / / /
1
- 1 — ^ . — .
//////,
1
/(^
"Mention of trade names or commercial products
does not constitute endorsement or recommendation
for use.
2
A. Teflon adaptor
B. 50-ml syringe barrel
C. "Five-step" brass rod
Figure 2. Multi-stop calibrated syringe.
D. Plexiglas guide
E. Syringe plunger
F. Brass cap
-------
18
17
-^
\ jg
1
1 '5
S 14
c
u
o 1^
O
4)
1 '2
0
77
• NoHzOz
• 3 ppm HiOi
' * \
r
« • • ^
• • • i
• * **
• ?
• . ?
• 8
• 0
• u
• . §
• •
-------
ozone decay products. This tendency
produced significantly low ozone tilers.
The failure of the iodate method was
the most puzzling and troublesome. At low
concentrations, iodine did not quantita-
tively disproportionate to iodate ion. Since
valid ozone titers demand 100-percent
conversion of iodine to iodate ion, the
iodate method failed.
Purging Technique
The purging technique is unreliable
because of ozone decomposition during
the purge and reabsorption steps.
Recommendation
No iodometric method is recommended.
The indigo method and arsenic(lll) direct
oxidation are the methods of choice. The
Standard Methods committee should con-
sider replacing the standard iodometric
method currently in wide use with either
the indigo or the arsenic(lll) direct
oxidation method.
The full report was submitted in ful-
fillment of CR-806302 by Miami Univer-
sity under the sponsorship of the U.S.
Environmental Protection Agency.
Gilbert Gordon and Joyce Grunwellare with Miami University, Oxford. OH 45056.
Edward J. Opatken is the EPA Project Officer (see below).
The complete report, entitled "Improved Techniques.for Residual Ozone," (Order
No. PB 84-151 224; Cost: $11.50, subject to change) will be a vaitable only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
U S GOVERNMENT PRINTING OFFICE, 1984 - 759-015/7637
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
u
------- |