Anal. Chem. 1995,67,535-540
Direct and Sequential Potentiometric Determination of Hypochlorite, Chlorite, and Chlorate Ions When Hypochlorite Ion Is Present in Large Excess Luke C. Adam and Gilbert Gordon* Department of Chemisty, Miami University, Oxford, Ohio 45056
A selective potentiometric technique has been developed for the determinationof OC1- at high concentration (e.g., 1-3 M) and C102- and c103- at comparatively low concentrations (e.g., 50-150 mg/L) when these species are present together in solution. The method uses sulfite ion as a mask to selectively and quantitatively remove OC1- at pH 10.5. Thus, OC1- is measured and masked at the same time. At pH 10.5, s0s2- does not react with C102- or C103-. The remaining SO& is quantitatively removed with triiodide ion. Following the masking procedure, the sequential determinationof Cl02- and C103is carried out either by iodometric titration at the appropriate pH or by ion chromatography. The results from the direct potentiometric titration using SO& are compared to those of an indirect reference method. Oxidizing agents such as chlorine (Clz) and hypochlorite ion are widely used in drinking water disinfection1s2in the United States. Recently, chlorate ion and possibly chlorite ion have been suggested to exist in drinking water at measurable levels when hypochlorite ion is used as the sole chlorinating agent. Chlorite and chlorate ions may be a potential health hazard, and their lowlevel toxicity is controversial. The US.EPA may regulate3s4the amount of ClOz- and C103- permitted in drinking water by the year 2000. The most likely s ~ u r c e of ~ ' ~these ions is the decomposition of concentrated aqueous sodium hypochlorite (NaOC1) stock solutions or solid calcium hypochlorite (Ca(OC1)z) during storage before being applied as the disinfectant. Thus, an analytical methodology is needed for the direct and selective determination of OC1-, ClOz-, and C103- in concentrated NaOCl or Ca(OC1)2 solutions. In solution, OC1- and ClOz- have similar reactivities, and both ions react with the reagents typically used for their determination, (1) White, G. C. Handbook of Chlorination, 2nd ed.; Van Nostrand Reinhold Co. Inc.: New York, NY,1986. (2) Gordon, G.; Cooper, W. J.; Rice, R G.; Pacey, G. E. Disinfectant Residual Measurement Methods, 2nd ed.; AWWA Research Foundation: Denver, CO, 1992; pp 9-10.39-41. 53-55, 150-151. (3) Regli, S. EPA Presentation to AWWA Technical Adisory Workgroup, November 11, 1990. EPA Discussion of Strawman Rule for Disinfection Byproducts. Presented to EPA Science Advisory Board Drinking Water Committee, September 22, 1989. (4) EPA Priority List of Substances Which May Require Regulation Under the Safe Drinking Water Act. Fed. Regist. 1991, 56 (9), 1470-1474. (5) Bolyard, M.; Fair, P. S.; Hautman, D. P. Enuiron. Sci. Technol. 1992,26, 1663-1665. (6) Gordon, G.; Adam, L. C.; Bubnis, B. P.; Hoyt, B.; Gillette, S. J.; Wilczak, A J. Am. Water Works Assoc. 1993,9, 89-97. 0003-2700/95/0367-0535$9.00/0 0 1995 American Chemical Society
such as I- or S Z O ~ ~The - . various difficulties of other methods used to analyze solutions containing Cl02- and OC1- have been summarized? Freshly produced concentrated solutions of commercial grade NaOCl (Le., 5-15 wt % measured as Cl2) contain levels of chlorite and chlorate ions which range from 200 to 2000 times less than the concentration of OC1-. Due to this large concentration difference, the analysis of each ion is not straightforward. The primary objective of this work is to demonstrate a selective technique for the determination of OC1- at high concentration and ClOz- and C103- at low concentrations when these species are present in solution. EXPERIMENTAL SECTION
Reagents. All solutions were prepared using deionized, triply distilled water from a Barnstead/NANOpure water purification system. The resulting water was of the highest quality and did not have a measurable chlorine demand. The preparation of highpurity sodium hypo~hlorite~~~ has already been described. In some cases, NaOCl was prepared by slowly bubbling Clz gas (Matheson) through 0.1 M NaOH at 0 "C. The 0.1 M SOs2- solutions were prepared from ACS reagent grade crystalline Na2S03. The Nar SO3was dissolved in Orfree water and kept in the dark. Cooling the solution had no effect on S032- stability and was therefore unnecessary. Chlorite ion standards were prepared from triply recrystallized1° sodium chlorite with a purity of 99.64%. Chlorate ion standards were prepared from ACS reagent grade potassium chlorate with a purity of 299.99%. All other reagents were prepared from ACS reagent grade chemicals. Mixed standards of OC1-, ClOz-, and C103- were prepared by adding known amounts of dilute ClOz- and C103- to a solution of OC1- at pH 12.5. Mixed standards were used immediately following preparation. Instrumentation. The laboratory and commercial NaOCl solutions were analyzed for OC1-, ClOz-, and C103- by potentiometric titration (VIT 90 video titrator; PlOl platinum K401 SCE electrode pair, Radiometer, Copenhagen, Denmark). Solutions of C10~-and C103- were also analyzed by ion chromatography (IC) @ionex Dx-100, Dionex Corp., Sunnyvale,CA) following the removal of OC1- by S032-or ethylenediamine5 (en). (7) Suzuki, K; Gordon, G. Anal. Chem. 1978,50, 1596-1597. (8) Adam, L C.; Suzuki,IC;Fhbiin, I.; Gordon, G. Inorg. Chem. 1992,31,35343541. (9) Cady, G. H. In Inorganic synthesis; Moeller, T., Ed.; McGraw-Hill Co.: New York, NY,1957; Vol. 5, pp 156-165. (10) Fibiin, I.; Gordon, G. Complex Formation Reactions of the Chlorite Ion. Inorg. Chem. 1991,30, 3785-3787.
Analytical Chemisfry, Vol. 67,No. 3, February 7, 7995 535
Standardization of S032- and OC1-. This method used S032- (from NazS03) as a standard reagent. The S0s2- solution was standardized by the potentiometric titration of a standard solution of 13-. The inflection point in the electrode potential vs titrant volume curve was used as the end point. The 13- solution was produced by the addition of a known volume of standard potassium iodate to 0.3 M KI at pH 1.3. Iodate ion reacts with Iat pH 1.3 to produce stoichiometric amounts of &-. The standardized S O P solution was used to titrate samples of unknown OC1concentration. The direct potentiometric titration using SO+- was initially developed by titrating solutions of known OC1- concentration. A 0.05 M OC1- solution prepared in our laboratory was free of ClOzfor several days when kept cold and in the dark. The OC1solution was standardized potentiometrically at pH 1.3 by iodometric titration, which is described later as the “reference method. Direct Potentiometric73trationUsing SOs2-. The following reaction scheme illustrates the reaction steps for the simultaneous measurement and removal of OC1- by titration with SO+?- for the sequential measurement of ClOz- and C103-. The effect of pH on the stoichiometry for each reaction step is as follows: OC1-
I,-
+ SO;-
-
SO-:
+ SO-: + 20H- - SO-:
+ C1- pH 10.5 + 31- + H 2 0
(1)
pH 10.0-10.5 (2)
I,-
+ 2S20,2- - 31- + S,O;-
pH 9-10.5
(3)
A known volume of sample containing ClOz- and C103- in an excess of OC1- was added to 10 mL of 0.4 M borate buffer (PH 10.5). More borate buffer was used if necessary to control the pH of the solution because OC1- should be maintained at pH 10.5. If a large excess of OH- was present, dilute HCl was added to adjust the pH. The SO+- was used as the titrant in this procedure. At this point, the reactive species in solution were OC1- and S032-, as shown by eq 1. The titration was carried out potentiometrically, and the inflection point in the electrode potential vs titrant volume curve was used as the end point. At pH 10.5, S O P reacts with OC1- (eq 1) but does not react with the ClOz- or C103- present in concentrated OC1-. For this reason, the borate buffer and precise pH control were essential to the titration. Following the initial potentiometric titration to measure OC1-, 13- was added to the solution until the first perceptible yellow color was visually observed in order to remove the excess S032-in the solution, as shown by eq 2. If any excess 13- remained in the solution, it was removed by the addition of thiosulfate ion to a colorless visual end point, as shown in eq 3. After completion of the OC1- removal steps (eqs 1-3), the sample contained ClOz- and C103- (as well as Sod2-, C1-, I-, and possibly Sz032-and S40.2-) and was suitable for analysis by IC following appropriate dilution. Chlorite and chlorate ions were also determined sequentially7 by iodometric titration at the appropriate pH and in an 0.3 M excess of KI. The iodometric titration of ClOz- and C103- involves the following stoichiometric relationships: C10,-
+ 4H’ + 61- - 21,- + C1- + 2 H 2 0
pH 1.3 (4)
536 Analytical Chemistty, Vol. 67,No. 3,February 1, 1995
Table I. Ion Chromatographic Experimental Conditions
experimentalparameter
experimentalcondition
guard/separator columns detection suppressor suppressor regenerant regenerant flow rate sensitivity background eluent eluent flow rate system back-pressure injection loop volume
Dionex MFC/AGS/ASS Dionex CDM-2 conductivity Dionex AMMS anion micromembrane 25 mN HzS04 10 mL/min 1pS full-scale
ClO,-
6- 10 pS 20 mM NaOH and 100 mM 1.0 mL/min 950-1150 psi 25 p L
+ 6H+ + 91- - 31,- + C1- + 3H20
6M
Hf (5)
After the addition of an appropriate amount of dilute HCl, the stoichiometric amount of Is- formed due to the ClOz- in the sample (eq 4) was titrated potentiometrically with thiosulfate ion, as shown by the following equation:
2S20,2- + I,-
- 31- + S,O;-
(6)
The inflection point in the electrode potential vs titrant volume curve was used for the end point of eq 6. After ClOz- was measured and a sufficient amount of nitrogen-purged 9-10 M HCl was added, the stoichiometric amount of 13- formed due to the C103- in the sample (eq 5) was titrated with thiosulfate ion to a colorless end point, as shown by eq 6, using visual end point detection. Measurement of C102- and C103- by Ion Chromatography. Hypochlorite ion solutions were analyzed for ClOz- and c103- by ion chromatography. The chromatographic conditions6J1J2are noted in Table 1. Prior to injection, NaOCl samples containing OC1-, ClOz-, and C103- were diluted to 0.06 M OC1-, and 0.10 M S032-was used to mask the OC1- at pH 10.5 (eq 1). The remaining sulfite ion was quantitatively removed with 13- (eq 2). Any remaining Is- was removed with SO& (eq 3). After completion of the OC1- removing steps (eqs 1-3), the sample contained ClOz- and C103- (as well as SO?-, C1-, I-, and possibly s03’- and S4O&). When en was used as an OC1- masking agent, the following procedure was followed. Prior to injection, NaOCl samples containing OC1-, ClOz-, and C103- were diluted to 0.0012 M OC1-. The OC1- was masked by the addition of 0.5 mL of 0.40M en per 100 mL of sample. While en is an effective OC1- masking agent, it is not known if the reaction between en and OC1- involves the formation of an en-OC1- complex or the formation of an organic chloramine. Regardless of the method used to mask OC1-, an additional dilution was necessary for the C103- measurement if the NaOCl sample was more than a few days old, due to the formation of C103- from decomposition6J3 of OC1-. Chloride ion was removed prior to injection using Ag+ cartridges (Dionex). The use of these cartridges had no adverse effect on the calibration curves for C l o y or C103-, which ranged from 0.1 to 1.0 mg/L of each ion. (11) Bubnis, B. P. Private communication, 1992. (12) EPA Method 300.0 (Revision 2.2); Environmental Monitoring Systems Laboratory: Cincinnati, OH, April, 1993. (13) Lister, M. W. Can. /. Chem. 1956,34, 465-478.
Following sample dilution, addition of the masking agent (eqs 1-3 or using en), and removal of C1-, the sample was injected directly onto the chromatographic column. The standard curves for Cl02- and C103- had a minimum correlation coefficient of 0.9990. To ensure calibration curve integrity, a quality control (QC) standard containing both ions was analyzed at a minimum of every 10 sample injections. The relative chromatographicareas for the QC standards deviated less than *3%. Reference Method for the Determination of OC1-, ClOz-, and C103-. The reference method is a variation of the iodometric method14 for chlorine and was used for comparison to the direct potentiometric titration using so32-.The addition of an aliquot of NaOCl containing OC1-, ClOz-, and ClO3- to a solution of 0.3 M KI and 0.05 M HCl produced a stoichiometric amount of 13-, as shown by the following stoichiometric equations:
OC1C10,-
+ 31- + 2H' - I,- + C1- + H,O + 61- + 4H' - 21,- + C1- + 2H,O
pH 1.3
(7)
pH 1.3
(8)
The stoichiometric amount of 13- formed due to the OC1- and Cl02- in the sample was titrated potentiometricallywith thiosulfate ion, as shown by eq 6. The idection point in the electrode potential vs titrant volume curve was used as the end point. In a separate sample of OC1-, ClOz-, and C103-, the Cl02- and C103concentrations were determined by ion chromatographyafter the addition of en as the OC1- masking agent. Once the Cl02concentration was determined from the ion chromatographic analysis, the following equation, which was derived from the stoichiometric relationships of eqs 6-8, was used to obtain the OC1- concentration:
[OCl-I = DF{ (0.5[S20,2-1V,/VJ - 2[C10,-]}
(9)
where DF is the dilution factor, V, is the volume of thiosulfate ion titrant (in mL), and V, is the volume of hypochlorite ion sample (in mL). Figure 1 is a flow diagram summarizing the analytical methodologies employed. Clearly, Figure 1 shows that the OC1species is quantitated by difference using two independent measurements in the indirect reference method, while each species of interest is determined directly in the method involving S032-. RESULTS AND DISCUSSION
Stability of Cl02- and C l o y after Masking of OC1- with S032- or en. While it has been shown5that samples containing Cl02- and C103- in the presence of 1.2 and 2.4 mg/L of OC1with 50 mg/L of en are stable for several weeks, it is not known if masking '100 mg/L of OC1- with en has any effect on the stability of Cl02- and C103-. It is also not known if samples containing OC1-, Cl02-, and C103- produce stable concentrations of ClOz- and C103- when treated with S032-. Thus, it is important to determine if solutions of Cl02- and C103- are stable when OC1is masked with S032- or en because, in practice, the analysis of samples containing Cl02- and C103- may not necessarily be carried out immediately following treatment with the masking agent. (14) Standard Methods for Drinking Wuter and Waste Water, 16th ed.; APHA, AWWA, and WPCF Washington, DC,1989; pp 298-300.
Sample containing OCI-. CIO,-. and 00,-
I Potentiometric titration of the rum of OCI- t ci0,urmg S,O,'-
Potentiometric titration and removal of OCI- with SO,'followed by the addition of 1,- and
so,'-
Add en followed by IC to measure CIOI- and CI0,-
............Reference Method ..-----.-.---
..
Potentiometric titration of remaining CI0,-at pH 1.3 with SIO;followed by visual endpoint titration of CIO1- in 6M H' with S,O?'
1
Figure 1. Flow diagram of analytical methodologies employed. Table 2. Stability of Samples Containing C102- and CI03- after Masking OCI- with s0a2-
ClOz- content (mg/L)
A
days
B
C
0 7 14 28
0.070 0.080 0.059 0.072
0.126 0.123 0.129 0.119
0.196 0.195 0.190 0.187
aV
0.070 f 0.009
0.124 f 0.004
0.192 f 0.004
c103- content (mg/L)
B
A
days
C
0 7 14 28
0.487 0.508 0.470 0.479
0.652 0.660 0.649 0.640
0.800 0.793 0.816 0.794
aV
0.486 & 0.016
0.650 f 0.008
0.801 f 0.011
The f refers to one standard deviation from the mean.
A stability study was carried out for several solutions of highpurity NaOCl treated with S032- or en. The solutions were stored at 5 "C in brown poly(tetrafluoroethy1ene) (PTFE) bottles. The results are presented in Tables 2 and 3. In Tables 2 and 3, the letters A-G refer to discrete solutions. The standard deviations of Cl02- and C103- samples for both OC1- masking techniques over the given storage period are equivalent to the standard deviations obtained from the ClOz- and C103- calibration curves. Also, there are no increasing or decreasing trends in the data 6.e. the individual points appear to be randomly distributed). Thus, it is concluded that the samples are stable over the period of the above study. Determination of OC1-, ClOz-, and C103-. There are two problems involved in the use of S032- as a standard reagent for the titration of OC1-. First, S032- reacts with oxygen15 in the air to form sulfate ion:
2so;-
+ 0, - 2s0,2-
(10)
Even when Orfree HzO is used to prepare the 0.10 M S032Analytical Chemistry, Vol. 67, No. 3, February 1, 1995
537
Table 3. Stability of Samples Containing C102- and C103- after Masking OCI- with en
ClOz- content (mg/L) E F
D
days
G
0 14 27 76
0.012 0.016 0.010 0.011
0.037 0.035 0.044 0.046
aP
0.012 f 0.003
0.041 & 0.005 0.062 k 0.003
0.080 f 0.003
D
c103- content (mg/L) E F
G
days 0 14 27 76 aP
0.063 0.066 0.061 0.059
0.079 0.083 0.081 0.076
0.176 0.180 0.145 0.181
0.187 0.185 0.162 0.185
0.243 0.247 0.233 0.240
0.437 0.442 0.416 0.435
0.171 & 0.18
0.180 f 0.012
0.238 f 0.011 0.432 f 0.011
The f refers to one standard deviation from the mean.
p5
5
I
A
4 N
$
vr 1
E
I
3
3
2 J
2
2
1
1
E
0
0
0
1
2
3
4
5
m i OCIFigure 2. Required volumes of 13- (m) necesary for complete reaction with excess SOs2- added to OCI-. Required volumes of S032- (0) necesary for complete reaction with OCI-.
reagent and the solution is stored in an air-tightbrown container, the solution of S032- decomposes 0.1-0.15%/h. This suggests that solutions of S032- may also undergo intrinsic decomposition in addition to reacting with 02. Open containers of s03’decompose 2-3 times faster due to the reaction with 02 from the air. The second problem which must be overcome when using S032- is that the reaction of OC1- and S O P does not involve a color change at the equivalence point. The three most straightforward methods of end point detection in this case are to use a redox indicator, to measure the oxidation-reduction potential of the solution during the titration, or to add excess S O P and backtitrate with 13- to a yellow end point. In preliminary experiments, the back-titrationwas tested. The solid squares in Figure 2 demonstrate the volumes of 0.1 M 13needed to reach the end point after the addition of 5 mL of 0.1 M S032-to various volumes of 0.1 M OC1-. The dashed line in Figure 2 is the theoretical results that should be obtained for the stoichiometry of reaction 1. The volumes of 13- above -3 mL deviate significantlyand are lower than predicted. This is due to the fact that when small volumes of OC1- are used, larger concentrations of s03’- remain, and the S032- has more time to react with 02 from the air during the back-titration. (15) Halperin, L.; Taube, H. /.Am. Chem. SOC.1952, 74, 380-387.
538 Analytical Chemistry, Vol. 67, No. 3, February 1, 1995
Significant improvements in the determination of OC1- with S032- are observed through the use of automated potentiometric titration. The problem of the colorless equivalence point is avoided, and measurement of the redox potential eliminates any differencewhich would exist between the h-ue equivalence point produced by S032-oxidation and OC1- reduction and the end point of a redox indicator. In Figure 2, five volumes of OC1- are titrated with standard S032- to demonstrate the stoichiometry of the reaction between S032- and OC1-. The titrations are carried out with the buret tip below the solution surface to minimize the interference from 02. In Figure 2, the volume of titrant required to reach the equivalence point is plotted as a function of the volume of OC1- used. The calculated slope of 0.997 f 0.007 demonstrates the expected reaction stoichiometry for reaction 1. Figure 2 demonstrates that reaction 1 is very reproducible, even when the OC1- concentration in the titration flask is varied from 0.002 to 0.017 M. The results agree to within f0.3% with iodometric titration values. These results show that the reaction of 02 with S032-does not interfere under these experimental conditions. The reaction of 02 with the SO+- titrant is minimized because the buret system of the automatic titrator is completely sealed. It is also possible to allow only nitrogen gas to enter the buret refilling solution by placing a nitrogen-purged plastic bag over the air vent of the refilling solution container. A single automated potentiometric titration for OC1- takes only 4 min. The next titration is set up and initiated in less than 60 s. Thus, the throughput for the determination of OC1- is 12 titratiodh. The S O P reagent is restandardized in triplicate at 2 h intervals, which allows the concentration of SO+- to be known to 3~0.2%while consuming only 13%of the sample analysis time. The next step in testing the method was to titrate mixed standards containing OC1-, ClOz-, and ClOs-. Once the OC1- is removed from the sample (eqs 1-3), a sufficient amount of 0.1 M HCl is added to acidify the sample to pH 1.3. At this pH, the ClOz- present in the solution reacts with I- to form 13-, which is titrated with standard sZ032-.Preliminary experiments with mixed standards produced results with poor reproducibility and were consistently 10-20% high for Cl02-. Because the reaction of S032with OC1- is well-behaved, the reaction of S032-with 13- (eq 2) was studied in detail. The standardization of S032- is carried out with 13- at pH 1.3. However, in the method being tested, this same reaction is carried out at pH 10-10.5. One explanation for the error in the ClOzdetermination is that the 13- undergoes disproportionation16at pH 10.5, as described by the following series of reactions: pH 10.0-10.5
I,
+ 2 0 H - - 01- + I- + H,O 301- - IO,- + 21-
(12) (13)
pH 1.3 IO3-
+ 6H+ + 81- - 31,- + 3H,O
(14)
The triiodide ion is in rapid equilibrium with 12. Iodine disproportionates to form hypoiodite ion (IO-) in the presence of base.
,L
t
Table 4. Standardization of 0.100 M 803-* Uslng 13and OCI- at the pH Values Qiven pH pH
reagent
= "3 10.5
pH
[S03-21 (M)
SDa
error (%)
Variable Reagent Volume 1s1sOCI-
1.3
0.1000
10.5 10.5
0.0986 0.0985
zkO.0004 10.0007 10.0007
0.40 0.71
0.71
Constant Reagent Volume 13-
0
5
15
10
a
mL 13Figure 3. Reaction of 0.1 M SOs2- and 0.03 M 10.5. 7r
0
10
1.3
0.0997
10.5
0.1007
&0.0001 10.0002
0.10 0.20
The 1refers to one standard deviation from the mean.
at pH 1.3 and
13-
,
5
oc1-
I
15
mL 13Figure 4. Reaction of 0.1 M SOs2- containing 0.2 M KI with 0.05 M 13- at pH 10.5.
The hypoiodite ion is very unstable and quickly decomposes16 to form IO3-. When the pH is adjusted to pH 1.3 (as is the case for the titration of ClOz-), IO3- reacts with I- to form 13-. This additional 13- would be titrated along with the 13- produced by the analytical reaction (eq 4). The use of excess I- in the preparation of 13- inhibits the formation of 12, as shown by eq 11. If I2 is not formed, disproportionation cannot occur. The 13- used for all the experiments is prepared with a 3:l molar excess of I-. Figure 3 illustrates the reaction of 0.1 M SOs2- and 0.03 M 13- at pH 1.3 and 10.5. The linear data at pH 1.3 correspond to a 1:l stoichiometryfor the reaction of 13- with S032-. The curved data in Figure 3 illustrate the variable stoichiometry observed at pH 10.5. This suggests that the 13- is undergoing disproportionation at pH 10.5, even in the presence of excess I-. In a separate set of experiments, 0.6 g of KI (0.2 M after mixing) is dissolved in the s03'- solution prior to the addition of 0.05 M 13-, and the reactions are repeated, as shown in Figure 4. Clearly, the addition of the KI before the addition of any 13- is sufficient to stop any disproportionation from occurring on the time scale of the titration. The data in Figure 4, where no KI was added to the S032- solution, demonstrate that some disproportionation occurs instantaneously when small amounts of the 13- titrant are added. Thus, even though the Is- is prepared in a Bfold excess of I-, it is necessary to add 0.2 M KI prior to the addition of 13- in order to stop the disproportionation from occurring. Typical results for the reaction of 0.100 M with standard 13- and standard OC1- for comparison are shown in Table 4. Both (16) Vogel, A I. Quantitative Inorganic Analysis, 3rd ed.; John Wiley: New York, NY. 1969.
variable and constant reagent volumes are used. The reaction of either 13- or OC1- with S032-produces statistically indistinguishable results for the determination of the SO$- concentration. For the data in Table 4,S032-is added to a solution of 13-. Thus, the order of mixing is the opposite of that in Figures 3 and 4. No disproportionation is observed because the I- concentration in this experiment is large throughout the titration. The use of 13at pH 1.3 and a constant 13- volume produces the most reproducible results, with 0.10%error at one standard deviation, and these experimental conditions are recommended for the standardization of S032-. Table 5 shows typical results for the analysis of each ion of interest in a mixed standard. The theoretical values are calculated from the combination of a known value of each separate standard solution. In Table 5, % 0 3 2 - and en refer to the reference method which determines the sum of OC1- and ClOz-. In the reference method, the concentration of ClOz- and C103- are determined by IC following the addition of en. The method using SO? at pH 10.5 is followed by either iodometric titration or IC. The use of for the determination of OC1- results in accuracy and reproducibility similar to those reported for the reference method. Both methods produce statistically equivalent results for ClOzand C103- when IC is used. When S032- is used to remove OC1-, followed by iodometric titration to measure ClOz-, the results are equivalent to those of the reference method, while the iodometric titration for C103tends to give high results with much poorer reproducibility. This is expected due to air oxidation of I- to form Is- in 6 M HCl. This iodometric titration of C103- is very time consuming and labor intensive because all reagents must be degassed with nitrogen for at least 15 min in order to purge any 02 kom the solutions. It is recommended that the 6 M HC1 be degassed with nitrogen or helium for at least 40 min immediately prior to analysis. Small losses of HCl occur during degassing; however, the decrease in the HCl concentration does not interfere with the measurements. Also, the sample must be degassed for 5 min following the ClOzmeasurement. Table 6 contains the results of the analysis of a 0.716 M commercial NaOCl solution, which was analyzed on the day of receipt. Again, the reference and SO? methods give similar results for each ion. The data demonstrate that the use of SO? to directly measure OC1- in the presence of ClOz- is a very accurate method for the analysis of concentrated NaOCl. CONCLUSIONS
The measurement of OC1- by potentiometric titration with S03z- gives very accurate and reproducible results without Analytical Chemistry, Vol. 67, No. 3, February 1, 1995
539
Table 5. Results for the Selective Measurement of OCI-, Clop-, and Ci03- Concentrationsfrom a Standard Solution containing Each Ion
OC1- method and removal SzO+-, enb sO3'-
s03'-,
so32-,so32-
[OCl-I (M x 102)"
[ClOz-I (M x 104)'
[C103-1 (M x 104)'
4.21 f 0.02 4.20 f 0.01 4.20 f 0.01 4.21
3.03 f 0.03 2.98 f 0.08 2.99 f 0.03 3.05
7.17 f 0.05 7.21 f 0.03 7.34 f 0.40 7.21
ICb IC titration
theoretical valuesC
[cioz-
+ cio3-i(M x 1030 10.20 f 0.08 10.19 f 0.11 10.33 f 0.43 10.26
The f refers to one standard deviation from the mean. b Reference method for the measurement of each ion. Calculated values from the addition of known standards of each separate ion. (I
Table 6. Results for the Selective Measurement of OCI-, Clop-, and cl03- Concentrationsfrom a Commercial Solution of NaOCl
OC1- method and removal %032-,
enb
s03'-,
so$,
s03'-
loa-I ICb IC titration
(M)"
0.716 f 0.02 0.716 f 0.01 0.716 f 0.01
[ClOz-I (M x 102)'
[C103-1 a4 x lo?@
0.0950 f 0.0004 0.0942 f 0.0005 0.0945 f 0.0002
1.37 f 0.01 1.40 f 0.02 1.49 f 0.08
The f refers to one standard deviation from the mean. Reference method for the measurement of each ion.
interference from ClOz- or C103-. In the potentiometric measure ment of ClOz- in OC1- solutions, a sufficientexcess of I- must be added before the addition of 13- (to remove excess so+-)to prevent the disproportionation of 12. The analysis time for the titration of OC1- and ClOz- is 10 min. While ClOz- can be accurately measured by titration or by IC, it is recommended that C103- be measured only by IC following OC1- removal with either S032-or en. Samples containing ClOz- and C103- after masking with S032- or en are shown to be stable throughout the length of each experiment (28 and 76 days). The primary disadvantage of the S103~method is that S032decomposes approximately 0.1-0.15%/h and therefore must be restandardized frequently. However, the standardization of s03'is easily carried out with a potentiometric titrator. As many as 12 titrations can be camed out per hour. Restandardizationevery 2 h requires only 13%of the total analysis time for standards and samples combined. This frequency of restandardizationproduces an uncertainty in the SO+- concentration of &0.2%. This error is equivalent to the standard deviation of a single standardization. The main advantage of this method is that it allows an accurate, selective, and, most importantly, direct measurement of OC1-, CIOz-, and C103-. The primary advantage of the reference method is that the standard thiosulfate ion solution is extremely stable and shows
540 Analytical Chemistry, Vol. 67, No. 3, February 7, 7995
less than 0.1%change in concentration per week. A disadvantage in the reference method is that the OC1- concentration is measured indirectly, which may increase the relative error of the measurement. Also, even if the only species of interest is OC1-, an additional measurement must be carried out to determine the ClOz- concentration. A comparison of results from the direct potentiometrictitration using SO+- and the reference method suggests that the two techniques have equivalent overall accuracies for the determination of OC1-, ClOz-, and C103-, and both methods are satisfactory analytical techniques for the measurement these ions. ACKNOWLEDGMENT Research sponsored by AWWA Research Foundation, 6666 W. Quincy Ave., Denver, CO 80235, as partial fulfillment of Research Contract 833-92. The authors thank Dr. B. P. Bubnis for his many helpful suggestions. Received for review July 19, 1994. Accepted November 15, 1994.@ AC940723Z Abstract published in Advance ACS Abstracts, December 15, 1994.
