1430
Anal. Chem. 1900, 52, 1430-1433
Quantitative Determination of Chloride, Chlorite, and Chlorate Ions in a Mixture by Successive Potentiometric Titrations Tsung-Fel Tang and Gilbert Gordon Department of Chemistry, Miami University, Oxford, Ohio 45056
Samples are titrated wlth standard silver nitrate solution in 70% (v/v) methanol and a combination chloride Ion selective electrode Is empioyed for the potentlometric end-point detection. The chloride ion concentration corresponds to the first potentlometrlc end point. Chlorite ion Is reduced to form chloride ion in the neutral pH range by using As(II1) and 0.005% OsO, as the catalyst. The chloride ion formed is titrated to the second potentiometric end point. Finally, chlorate ion Is reduced to chloride ion by further acidification to 1 M sulfuric acid and by using 0.1 % OsO, as the catalyst. The chloride ion formed is titrated to the third potentlometric end point. The precision of this method is better than 2 % (2-15 kmoi). The results are in good agreement with other methods. The advantages of this method include elimination of air oxidation and direct comparison of the chlorine balance with the redox balance in solutions containing mlxtures of oxychlorine species.
During the studies of various reactions and interactions of the oxychlorine species, it was necessary to develop a simple, direct method for the quantitative determination of chloride, chlorite, and chlorate ions in the presence of each other. In a previous study, Prince ( I ) reported that chlorite ion interfered with the determination of chloride ion in Volhard titrations. Prince’s method also requires a t least a 4-h reaction period for the determination of chlorate ion. Another method which was described by Chen (2) utilized either ferrous ion or iodide ion as a reductant for the determination of chlorite and chlorate ions. In this method, the chloride ion was determined spectrophotometrically by the mercuric chloranilate technique (3). Chen reported, however, that chlorite ion also interfered with this technique by liberating chloranilic acid in a nonquantitative way. Therefore, in the presence of chlorite ion, Chen found it necessary to determine chloride ion indirectly by a modified procedure ( 2 ) . In a latter report by Hong and Rapson ( 4 ) ,the iodometric determination of chlorate ion in a strongly acid medium was suggested, although it is well-known that air oxidation of iodide also may take place under these conditions ( 5 ) . Even Chen’s work suggests that serious air oxidation of iodide ion takes place in a 0.11 N sulfuric acid medium. Thus, the iodometric determination of chlorate ion requires carefully controlled conditions to maintain the absence of atmospheric oxygen. Norkus reported that relatively high concentrations (0.1 N) of both chlorite (6) and chlorate (7) ions can be determined by reduction with As(II1) in the presence of Os04 as a catalyst, but no method was described for the determination of chloride ion. In a later report, Yamasaki, Ohura, and Nakamori (8) utilized potassium borohydride to reduce chlorite ion to chloride ion. In a separate sample, sulfurous acid was used t o reduce both chlorite and chlorate ions to chloride ion. The chloride ion formed in each sample was determined by titration with a standard silver nitrate solution using short0003-2700/80/0352-1430$01 .OO/O
circuit amperometry for the end-point detection. In the present study, we found that in a 70% (v/v) methanol medium, chlorite ion and chlorate ion can be selectively reduced to form chloride ion by As(II1) in the presence of Os04. Under these conditions, the reaction can be followed by using a chloride ion selective electrode (ISE) with the chloride ion concentration being determined by potentiometric titration using a standard silver nitrate solution as the titrant. This method allows the direct determination of chloride, chlorite, and chlorate ions in a mixture by means of three successive titrations. The determination of chlorate ion takes less than 20 min and no interference from air oxidation is observed. This procedure results in the direct analysis of chlorine species, thus, providing for a comparison of the total chlorine balance with the redox balance in solutions containing mixtures of oxychlorine species.
EXPERIMENTAL For the electrochemical experiments, an Orion model 96-17 combination chloride ion selective electrode (ISE) was used. The potentiometric titration system included a Metrohm Herisau Model E 536 potentiograph and a Metrohm Herisau Model E 535 auto-buret. The titrant delivering speed was 4 min/mL for all of the sample runs. The potential was monitored via the chloride ISE and the titration end point was taken as the inflection point in the titration curve. The inflection point was determined by the use of a Metrohm Herisau EA 893 evaluating rule. Reagent grade chemicals and distilled deionized water were used throughout. Potassium chlorate (Matheson, Coleman and Bell) was recrystallized from triply distilled water. The titrant was nominally 9.8 X lo4 M silver nitrate in a 70% (v/v) methanol medium which was standardized with standard sodium chloride solutions immediately prior to use. The As(II1) solutions (0.2 N) were prepared by dissolution of reagent grade arsenic(II1) oxide (Mallinkrodt) in basic solution followed by adjustment to pH 6.5 by the addition of dilute sulfuric acid. The osmium tetraoxide solution (approximately 0.1% by weight) was prepared in 0.1 N sulfuric acid medium, followed by dilution to 0.005% with distilled deionized water. The nominally 0.05 M sodium chlorite (Matheson, Coleman and Bell) stock solutions were freshly prepared prior to each experiment and stored in an aluminum foil covered bottle. In a separate set of experiments, the sodium chlorite was shown to have greater than 99% sodium chlorite and 0.47% sodium chloride. The additional impurity was presumed to be sodium perchlorate (9).
RESULTS AND DISCUSSION Determination of Chloride Ion in the P r e s e n c e of a Large Excess of Chlorite Ion. The sample was prepared by delivering known volumes (10) of the sodium chlorite stock solution and the sodium chloride stock solution into a 50-mL beaker. The sample was adjusted to 70% (v/v) methanol followed by titration with standard silver nitrate solution to the potentiometric end point using the chloride ISE as an indicator electrode. Blank corrections were made for the chloride ion impurities contained in the sodium chlorite stock solution. The accuracy and precision of the titration was examined by sequential addition of standard sodium chloride solution. The amount of sodium chloride was titrated after 0 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
Table I. Determination of Chlorite Ion' iodometric method, sample pmol C10,1 2 3 4
13.00 13.04 13.04 13.05
mean :
13.03
was prepared by delivering known volumes of the sodium chloride stock solution and the sodium chlorite stock solution into a 50-mL beaker. The sample was adjusted to 70% (v/v) methanol and titrated with the standard silver nitrate solution to the first potentiometric end point which corresponds directly to the micromoles of chloride ion. Following the first end point, approximately 0.4 mL of 0.2 N As(II1) solution and 5 drops of OsOl (0.005%) were added to the sample. After approximately 5 min, the sample was titrated with standard silver nitrate solution to a second potentiometric end point. The difference between the two end points corresponds to the micromoles of chlorite ion. These results and a direct comparison with the iodometric method are shown in Table 11. Determination of t h e C h l o r a t e Ion. The samples were prepared by delivering known volumes of the potassium chlorate stock solution into 1 M sulfuric acid in a 70% (v/v) methanol medium. Approximately 0.5 mL of the 0.2 N As(II1) and 3 drops of 0.1 70OSO, stock solution were added to reduce the chlorate ion to chloride ion in the sample. After 20 min, the sample was titrated with standard silver nitrate solution to the potentiometric end point. The results using 8.517 pmol of C103- by weight were 8.59 f 0.04 pmol of C103- when using the modified iodometric method and 8.51 f 0.02 (individual results were 8.524, 8.514, 8.487, and 8.524) Kmol of C10< using the method reported here. Determination of Chloride, Chlorite a n d Chlorate Ions i n Synthetic Mixtures. Synthetic samples were prepared by mixing known volumes of the chloride, chlorite, and chlorate stock solutions and adjusting to 70% (v/v) methanol by the addition of reagent grade methanol. The sample was titrated with standard silver nitrate solution to the first potentiometric end point which corresponds to the micromoles of chloride ion. Following the first end point, a n excess of the 0.2 N As(II1) stock solution and 3-6 drops of Os04 (o.OC)5%) were added to reduce the chlorite ion to chloride ion in the sample. After approximately 5 min, the resulting solution was titrated with silver nitrate solution to the second potentiometric end point. The difference between the two end points corresponds to the micromoles of chlorite ion. Finally, approximately 2-3 mL of 9 M sulfuric acid and 6 drops of OsO, (0.1%) were added to reduce the chlorate ion to chloride ion in the sample. After 20 min, the resulting solution was titrated to the thud potentiometric end point The difference between the third and second end points corresponds to the micromoles of chlorate ion. Titrations were also carried out in a buffered system. Approximately 0.01-0.02 mL of 0.25 M sodium bicarbonate solution (pH 8.5) was added t o the sample before the first titration. Other procedures were the same as the unbuffered system. For either the buffered or unbuffered systems, a blank chloride ion concentration was determined in the absence of added chloride ion. This blank corresponds t o the chloride
present work, pmol C10,12.99 12.99 13.00 13.01
i
0.02
13.00
*
0.01
a The uncertainties represent standard deviation from the mean for replicate titrations.
Table 11. Determination of Chloride Ion and Chlorite Ion in a Mixturea A. Chloride Ion
by weight, pmol C1-
present work, pmol C1-
4.163
4.153; 4.164; 4.170; 4.164; 4.155
mean: 4.161 B.
sample
k
0.007
Chlorite Ion
iodometric method, pmol C10,-
1 2 3 4 5
3.131 3.113 3.119 3.085 3.121
mean :
3.114
present work, pmol C10,' 3.101 3.116 3.095 3.095 3.103
i
0.02
3.102
i
0.01
The uncertainties represent the standard deviation from the mean for replicate titrations. a
each addition. The plot was linear, corresponding to a slope of 0.961, an intercept of 7.42 x and a correlation coefficient of 0.999. Determination of Chlorite Ion. The sample was prepared by delivering known volumes of the sodium chlorite stock solution into a 50-mL beaker and adjusting to 70% (v/v) methanol by the addition of reagent grade methanol. Approximately 0.25 mL of 0.2 N As(I1I) and 6 drops of OsO, (0.005%) were added to the sample, followed by acidification of the sample by the addition of 3 drops of 1.8N sulfuric acid. Under these conditions, chlorite ion was reduced to chloride ion and the resulting solution was titrated with standard silver nitrate solution to the potentiometric end point. Results of these experiments and comparison with the iodometric method ( 5 ) are shown in Table I. D e t e r m i n a t i o n of Chloride Ion a n d Chlorite Ion i n a M i x t u r e by Successive Titrations. A synthetic mixture
Table 111. Determination of Chloride, Chlorite, and Chlorate Ion in Synthetic Mixturesa c i - x 10'3 M sample added found 10.49 4.98
0.07 0.01
2
10.48 4.988
3 4
5.240 4.988
5.31 i 0.05 4.95 t 0.01
5e 6E 7e
4.988 5.190 6.047
4 . 9 9 t 0.03 5.18 i 0.01 6.06 i 0.02
1
i
*
cio;
x 10'3
1431
M
cia; x 10'3 M addedb found
total C1 added
X
-
M
found
addedC
found
3.033 i 0.004 1.578 i 0.001 1.580 i O . O O l d 1,517i 0.002 1.040 * 0.005 1.048 t O . O O l d 1.595 t 0.005 6.624 i 0.02 3.099 c 0.009
3.02 i 0.01 1.585 * 0.004
3.34 1.67
3.38 1.63
i
0.02 0.02
30.09 15.63
31.0 i 0.1 1 5 . 5 t 0.1
1 . 4 9 ? 0.07 1.05 i 0.01
1.67 1.00
1.64 0.97
t t
0.03 0.01
15.49 10.60
15 3 i 0.4 1 0 . 5 i 0.1
1.57 6.61 3.10
1.657 1.017 1.657
1.62 t 0.01 0.99 k 0.01 1.65 i 0.02
15.65 34.05 22.37
1 5 . 4 t 0.1 33.9 i 0.1 2 2 . 3 i 0.1
i
t i
0.01 0.03 0.02
i
The uncertainties represent the standard deviation from the mean for replicate titrations. Determined by iodometric method. Determined the stock solution by present method. buffered by sodium bicarbonate. a
b e
Calculated by weight. Samples 5, 6, 7 were
1432
ANALYTICAL CHEMISTRY, VOL. 52, NO. 9, AUGUST 1980
600
1
575
-
450-
,
6.0k
5.0-
,/'
,'-
:
