United States
Environmental Protection
Agency
Environmental Monitoring and
Support Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S4-85/074 Dec. 1985
v>EPA Project Summary
Improved Methods of
Analysis for Chlorate,
Chlorite, and Hypochlorite
Ions at the Sub-mg/L Level
Gilbert Gordon
A comparison is given of existing
analytical methods for the oxyhalogen
species, including chlorate ion, chlorite
ion, and hypochlorous acid/chlorine.
Known limitations of the methods and
lower limits of analysis are discussed.
New and improved precise methods of
analysis for low level (<1 mg/L)
measurement of oxychlorine species
including chlorate and chlorite ions are
given. Advantages and disadvantages
of speciation are noted.
A modified iodometric method is
reported to measure chlorate ion con-
centration down to the sub-mg/L level
with high precision and accuracy. Hex-
ane and nitrogen are used as shielding
agents to isolate the sample solution
from the air phase to prevent air oxida-
tion of iodide ion. Chlorate ion is re-
duced by iodide ion in 6M hydrochloric
acid. By the addition of saturated
sodium phosphate, the hydrogen ion
concentration is lowered to approxi-
mately 3M, and the liberated iodine is
titrated with standard sodium thiosul-
fate solution. The endpoint is detected
using Thyodene as the indicator or by
the color of iodine in the hexane layer. A
potentiometric endpoint is recommend-
ed for the determination of chlorate ion
at concentrations less than 1.0x10~4 N
(3.5 mg/L reported as chlorine). By this
modified iodometric method, the chlo-
rate ion down to the sub-mg/L level can
be determined with a precision of better
than ±1 % with a lower detection limit of
0.1 mg/L. The method is proposed as a
standard baseline method of analysis.
An analytical method is reported to
determine hypochlorite, chlorite, and
chlorate ions in a mixture at the sub-
mg/L level. Chlorate ion is determined
directly by a modified iodometric
method in which hexane, along with
nitrogen gas, is used as a shielding
agent to isolate the sample solution
from the air phase to prevent air oxida-
tion of iodide ion. A saturated sodium
phosphate solution is used to partially
neutralize the acidic sample solution
just before titrating. Chlorite ion is
determined directly by trie modified
Suzuki-Gordon method where free re-
sidual chlorine (chlorine and hypochlo-
rous acid) is converted to hypochlorite
ion which in the mixture, is reduced to
chloride ion by sulfite ion at pH 1O.5.
Following this oxidation, the chlorite
ion followed by the oxidation of excess
sulfite ion by iodine is reduced to
chloride ion by iodide ion at pH 2.
Hypochlorite ion is determined indirect-
ly from the results of the modified iodo-
metric method and the modified Suzuki-
Gordon method. The titration endpoint
is determined directly from the first
derivative titration curve obtained by a
potentiometric method. The overall
precision is ±3% for oxychlorine species
at concentrations in the range of 0.3-
100 mg/L with lower detection limits
of 0.1 mg/L.
This Project Summary was developed
by EPA's Environmental Monitoring
and Support Laboratory, Cincinnati,
OH, to announce key findings of the
-------
research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering infor-
mation at back).
Introduction
Methods for the chemical analysis of
solutions of the chlorine-containing
oxidizing agents above the 3 mg/L appear
to be well defined in the chemical litera-
ture. In the 1979 update of "Drinking
Water and Health," the National Academy
of Science estimated acceptable exposure
values of 0.38 mg/L for chlorine dioxide
and 0.21 mg/L for chlorite ion. Later in
1979, the Federal Register published
restrictions which should be placed on
the level of residual oxidants such as
chlorine dioxide and the chlorite and
chlorate ions which result from the
chlorine dioxide treatment of water to be
used as drinking water.
In early 1980 none of the analytical
techniques in the literature for the anal-
ysis of chlorine-containing oxidizing
agents were capable of routinely deter-
mining chlorate ion concentrations down
to the sub-mg/L level.
Results
Determination of Chlorate
Ion at the Sub-mg/L Level
This is made possible by a modified
iodometric method in which preferably
hexane and nitrogen gas are used as
shielding agents to prevent air oxidation
of iodide ion. Chlorate ion is reduced by
the iodide ion in 6M hydrochloric acid. In
addition, saturated sodium phosphate
solution is used to partially neutralize the
acidic sample solution prior to the actual
titration. The liberated iodine is titrated
with standard thiosulfate solution. The
endpoint is detected potentiometrically
using the homemade combination Redox
electrode as shown in Figure 1. The
precision is better than ±5% down to the
sub-mg/L level.
The results are shown in Table 1 and
indicate that the determination of chlorate
ion in concentrations greater than 1.0 x
10~4 is possible by using this modified
iodometric method—with appropriate
blank corrections. In these experiments,
time-dependent measurements show that
chlorate ion is quantitatively reduced by
iodide ion within 20 minutes. The repro-
ducible blank volume correction is neces-
sary since it corresponds to 0.0-5.0% of
the sample titration volume. This result
indicates that the air oxidation of iodide
ion is effectively minimized by using
hexane as a shielding agent and by
Cork-
1 MKCI
Solution
Ag/AgCI -
Heat Shrinkable
Teflon
Pyrex Glass Tubing
O.D. 6 mm.
Hole for Buret
Pyrex Glass Tubing
O.D. 2 mm.
Vycor Tip
Pt Wire
Figure 1. Combination redox electrode.
neutralization of the sample solution by
the addition of a deaerated saturated
solution of sodium phosphate.
On the basis of these results, the
determination of chlorate ion at the level
of 1.0 x 10"5 N (0.35 mg/L) was studied by
using the same procedure. However, the
color of iodine in the hexane layer is a
very pale pink at this concentration level;
hence the endpoint could not be detected
correctly and reproducibly.
In order to improve the disadvantages
inherent in the optical endpoint method
described above for more concentrated
solutions containing chlorate ion «1.0 x
10"4 M CI03 ), the potentiometrictitration
method was modified.
With lower chlorate ion concentrations,
the reaction between chlorate ion and
iodide ion requires 40 minutes for com-
pletion. The endpoint was detected from
the first derivative titration curve. The
results for solutions containing 1.011 x
10~5 equivalents per liter of chlorate ion
were determined with a precision of ±1 %.
Determination of Mixtures of
Chlorate and Chlorite Ions
The concentrations of chlorite and
chlorate ions are determined directly and
-------
continuously on a single sample thus
minimizing cumulative errors. Two suc-
cessive titrations are followed potenti-
ometrically with an internal precision and
accuracy of better than ±2%. Typical
results are shown in Table 2.
Determination of Hypochlorite,
Chlorite, and Chlorate Ions in
Mixtures at the Sub-mg/L Level
An analytical method is reported to
determine hypochlorite, chlorite, and
chlorate ions in a mixture at the sub-
mg/L level. Chlorate ion is determined
directly by a modified iodometric method
in which hexane along with nitrogen gas
is used as a shielding agent to isolate the
sample solution from the air phase to
prevent air oxidation of iodide ion. A
saturated sodium phosphate solution is
used to partially neutralize the acidic
sample solution just before titrating.
Chlorite ion is determined directly by the
modified Suzuki-Gordon method where
free residual chlorine(chlorine and hypo-
chlorous acid) is converted to hypochlorite
ion which in the mixture is reduced to
chloride ion by sulfite ion at pH 10.5.
Following this oxidation, the chlorite ion
fol lowed by the oxidation of excess sulfite
ion by iodine is reduced to chloride ion by
iodide ion at pH 2. Hypochlorite ion is
determined indirectly from the results of
the modified iodometric method and the
modified Suzuki-Gordon method. The
titration endpoint is determined directly
from the first derivative titration curve
obtained by a potentiometric method. A
derivative endpoint curve is shown in
Figure 2. The overall precision is ±3% for
oxychlorine species at concentrations in
the range of 0.3-100 mg/L with lower
detection limits of 0.1 mg/L. Typical
results are shown in Table 3.
Endpoint Determination
The potentiometric titration method is
recommended for each of the determina-
tions at low concentration levels (especi-
ally below 1 mg/L) because the blank is
relatively small and no subjectivity by the
investigator is involved.
Table 1. Determination of Chlorate Ion by the Modified Iodometric Method Using Hexane as a
Shielding Agent
Sample
1
2
3
4
5
6
Equiv/L
CIO3~ added
1.002 x 10'^
1.031 x 10'2
5.035 xW'3
2.609 x 10~3
6.194x10'*
1.011 xW~4
Equiv/L
CIO3' found*
(1.007± 0.001 )x10~1
(1. 033 ± 0.005) xlO"2
^5.03 ±0.08) x10~3
(2.60 ±0.03) x10'4
(6.21 ±0.07) xlO"*
(1.00 ±0.02) x10~"
% £ rror
0.5 ±0.1
0.2 ± 0.5
0.1 ±0.6
0.4 ± 0.5
0.2 ± 0.8
1.1 ±1.0
'The uncertainties represent the standard deviation from the mean for eight replicate samples
except for samples 3 and 5 which consisted of nine replicates.
Table 2. Determination of Chlorite and Chlorate Ion Concentrations in Synthetic Mixtures
mg/L ClOi
added x10~sN
Single Sample
Method
0.9613
3.384
0.9025
1.763
mg/L CJO2~
founcfx W's N
0.955 ± 0.024
3.41 5 ±0.072
0.911 ±0.021
1.751 ±0.036
mg/L CI03~
added x 10~SN
0.9957
0.9194
3.948
1.873
mg/L CI03~
foundtx 10~*N
0.983 ±0.031
0.911 ±0.028
3.93o ±0.08o
1.89, ± 0.04s
The uncertainties represent the standard deviation from the mean for eight replicate deter-
minations.
AII numerals dropped below the line indicate less than total confidence in their absolute value, as
determined.
7r-
1
3.5 4.0
Titration Volume, mL
Figure 2. Derivative endpoint curve.
4.5
-------
Table 3. Determination of Various Oxychlorine Species in Synthetic Mixtures
added
found
% error
added
found
% error
Equiv/L HOC!
x10~*N
100.5
99.6
0.9%
8.741
8.60,
1.6%
Equiv/L CIOz
x W~eN
119.1
120.0
0.8%
9.613
9.81 „
2.1%
Equiv/L ClOi
xW'a/V
100.3
101.7
0.4%
9.957
9.78e
1.7%
Aggregate
Oxid. Power
319.9
321.3
0.4%
28.311
28.20
0.4%
Gilbert Gordon is with Miami University, Oxford, OH 45056.
Daniel Bender is the EPA Project Officer (see below).
The. complete report, entitled "Improved Methods of Analysis for Chlorate,
Chlorite, andHypochlorite Ions at the Sub-mg/L Level,"(Order No. PB 86-118
684/AS; Cost: $9.95, subject to change) will be available 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:
Environmental Monitoring and Support Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Official Business
Penalty for Private Use $300
EPA/600/S4-85/074
0000329 PS
U S ENVIR PROTECTION AGENCY
REGION 5 LIBRARY
230 S DEARBORN STREET
CHICAGO It 60604
------- |