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iodide with spectrofluorimetric and spectrophotometric detection, respectively
Analyrrcu Clmmca Acm, 248 (1991) 219-224 Elsevier Science Publishers B.V., Amsterdam
219
Flow-injection procedures for determination of iodide and iodate/iodide with spectrofluorimetric and spectrophotometric detection, respectively M. Yaqoob
* and M. Masoom
Instmte of BmchemzstTy, Universzty of Baluchwan, Quetta (Pakistan)
A. Townshend School of Chemstry, Unwemty of Hull, Hull HlJ6 7RX (UK) (Recerved 21st August 1990)
AbSbCt
I&de was determined using a flow-mlecuon system based on the catalytic reductton of cerium(IV) to fluorescent cermm(II1) [X(ex) = 260 nm, h(em) = 350 nm]. Iodate and iodrde were detenmned spectrophotometrically by a sensitive colour reacuon with p-aminophenol. Iodide was ox&red by hydrogen peroxide to mdate in the presence of ~ammophenol. Iodate in turn oxrdized p-aminophenol to an indamine dye which has an absorbance maximum at 540 nm.
Keywords. Flow system;
Fhronmetry;
Spectrophotometry;
Flow-injection analysis (FIA) was originally used to automate serial assays, but is now a general solution-handling and data-gathering technique. It is finding increasing applications 111 routine analysis, research, teaching, monitoring of chemical processes and for enhancing the performance of various instruments. The underlying theory and applications of FIA have been described by Ruzicka and Hansen [l]. Some procedures for iodide determination have been .reported, based on flow-injection analysis coupled with spectrophotometry [2-41, potentiometry [5] and amperometry [6,7]. The catalytic properties of iodide have been largely explored as a means for its quantification. The method reported by Iwasaki et al. [8] is based on the catalytic effect of iodide on the reaction between thiocyanate and nitrite ions. This method has been improved and 0003-2670,'91/$03.50
0 1991 - Ekevrer Scrence Pubhshers B.V.
Iodate;
Iodide
applied to the determination of iodate and iodide in sea water [9] and potable water [lo]. Iodide has also been determined by its catalysis of the reduction of cerium(IV) by arsenious acid [ll], by the use of iodide-iodine as a catalyst for the iron(III)-arsenic(II1) system [12] and by liquidliquid extraction with methylene blue [13]. The fluorimetric method reported here is based on the work of Deguchi et al., [14] on the iodidecatalysed reduction of cerium(IV) to fluorescent cerium(II1) [X(ex) = 260 nm; X(em) = 350 rim]: 2 Ce(IV)
+ As(II1) i%e2
Ce(II1)
+ As(V)
The rate of colour formation increases in proportion to the amount of iodide present, allowing the determination of trace amounts of iodide. The second procedure reported is based on that of Fuchs et al. [15], involving the reaction of
220
M YAQOOB
iodate with p-aminophenol. Iodate oxidizes paminophenol to a quinoneimine, which immediately condenses with a second p-aminophenol molecule to give a blue indamine dye: J=&zJ
+ HOGNH,-
Cenum
(IV)
0.8
sulphate
acid -
Fig. 1. Flow-mpxtion bon of iodide. Indamine dye
In the case of iodide, the reaction becomes catalytic with the mtroduction of an oxidizing agent (hydrogen peroxide), which causes continued oxidative regeneration of iodate [16]. It provides simplicity, high precision and low reagent consumption, which are combined with the benefits of amplification for the spectrophotometric determination of iodate and iodide. Iodide ions are oxidized by hydrogen peroxide to iodine and iodate. However, in the presence of p-aminophenol, the equilibrium is shifted towards the formation of iodate, because of the redox reaction between iodate and p-aminophenol [14]: I-+
3 H202 p-aminophenol~IO; + 3 H,O
IO,
+ p-aminophenol += I- + indamine dye
ET AL
mamfold for the fluorimetnc determma-
Aqueous p-ammophenol
solutions, 0.05% (w/v)
for iodide and 0.3% (w/v) for iodate. These solutions are not light stable and therefore fresh solutions were prepared every 2 h, filtered and stored in brown bottles. Acetate buffer, 0.5 M (pH 5.0). Acetate buffer was prepared by dissolving 10.25 g of sodium acetate in 250 ml of water and adjusting the pH to 5.0 with 2 M acetic acid. Iodide and iodate standard solutions, 0.1 M. Stock solutions were prepared separately by dissolving oven-dried potassium iodide (1.66 g) and potassium iodate (2.14 g) in 100 ml of water. Working standard solutions of iodide and iodate were prepared by appropriate dilution of the stock standard solutions with water.
Apparatus EXPERIMENTAL
Reagents and solutions All reagents were of analytical-reagent
grade (Merck, BDI-I) and deionized, distilled water was used throughout. Cerrum(IV) sulphate solution, 0.005 M. This solution was prepared by dissolving 0.51 g of cerium(IV) sulphate in 24.3 ml of concentrated sulphuric acid and diluting to 250 ml with water. Arsenious acid solution, 0.01 M. A 0.5-g amount of arsenious oxide was dissolved in 200 ml of water containing 3.0 g of sodium hydroxide, acidified with 10 ml of concentrated sulphuric acid and diluted to 250 ml with water. Hydrogen peroxide solution, 2 M. This solution was prepared from commercially available hydrogen peroxide [30% (w/v), 9.8 M] by taking 20.4 ml and dilutmg it to 100 ml with water.
For the spectrofluorimetric determination of iodide, the flow-injection manifold shown in Fig. 1 was used. Samples were injected using a rotary valve (Rheodyne 5020) with a sample loop of 20 ~1. A peristaltic pump (Ismatec Mini S820) was used for propelling the carrier and reagents (0.8 ml mix-‘). PTFE tubing (0.5 mm i.d.) was used throughout the manifold. A spectrofluorimeter (Perkin-Elmer Model 3000) with a flow cell (25 ~1) consisting of a silica tube was used to record the fluorescence of cerium(II1) produced at A(ex) = 260 nm and X(em) = 350 nm; the excitation and emission slits were set at 5 nm. The response was fed directly to a strip-chart recorder. For the spectrophotometric determination of iodate and iodide, the single-channel FIA manifold shown in Fig. 2 was used. In this instance 25+1 samples were injected. The absorbance of both species was monitored at 540 nm using a spectrophotometer (LKB, Novaspec II) with a
FLOW
INJECTION
PROCEDURES
S
FOR
IODIDE
AND
221
IODATE
I25 /II)
mljnin -
Reagents
1
0.43
W 01
300 cm
-
Fig. 2 Flow-iqection and Iodide
flow-through recorder.
b
*-a-*--
manifold for the determmation
cell (30 ~1) connected
of iodate
RESULTS AND DISCUSSION
Fluonmetric determinatron of iodide In order to obtain a system with high accuracy, VENOUS factors controlling the catalytic action were studied, including concentration of reagents, flow-rate, mixing coil length and the presence of foreign ions. The effects of 1 x 10e3-50 X 10B3 M cerium(IV) sulphate and arsenious acid concentrations on the reduction process were determined by mixing the various concentrations in the reservoir in a single-channel FIA system. The signals obtained decreased gradually with time. The solution in the reservoir turned deep yellow, showing instability of the reagents when mixed in this manner. Therefore, a dual-channel system was used and the solutions were mixed before the injection point. As shown in Fig. 3, the highest signals were ob-
Concentration
(x lo-
l
to a chart
3 M)
Fig. 3. Effect of (a) cerium(IV) sulphate and (b) arsemous acid concentrations on the reduction of cerium(IV) to cermm(II1).
\
a
60
120
180
240
300
03
08
15
25
Imllmln)
(cm)
Fig. 4 Effect of (a) flow-rate and (b) nuxmg coil length on the rate of reduction of cenum(IV) to cenum(II1)
tamed with 5 x 1O-3 M cerium(IV) sulphate and 10 x lop3 M arsenious acid, and these were therefore used in subsequent investigations. The effect of flow-rate on sensitivity, sampling rate and reagent consumption were investigated. As shown in Fig. 4, a decrease in fluorescence was observed as the flow-rate was increased. At lower flow-rates (0.3 ml mm-‘) there was a gradual increase in the peak height but peak broadening and a low sample throughput were a problem and therefore a flow-rate of 0.8 ml mm’ was selected for acceptable sensitivity and a reasonable sample throughput. The effect of mixing coil length on the sensitivity was also investigated. There was a slight increase in the peak height when the mixing coil length was increased from 60 to 300 cm. A mixing coil length of 180 cm was found to be optimum and was therefore selected. Calibration. A series of iodide standard solutions were injected into the optimized system. A linear calibration graph was obtained covering the range 1.0 x 10e5-3.0 x lop5 M. The resulting graph had a correlation coefficient of 0.992 (n = 6). The detection limit (twice the blank noise) was 5.0 x lo-’ M. The sampling rate was 45 h-‘. The relative standard deviation (r.s.d,) was 0.8% for ten injections of 3.0 x 10K5 M iodide. Effect of forezgn ions. The effect of various foreign ions was studied and it was found that
222
M
YAQOOB
El-AL
bromide and chloride ions at a concentration 60 times in excess had no effect at the level of 3 X lo- 5 M iodide. Nitrate ions had a slight effect when added in an equimolar ratio. Azide ions interfered seriously in an equimolar ratio and also gave a response when injected into the system in the absence of iodide. Spectrophotometric determination of iodate / iodide Determrnation of iodate. Preliminary studies on the composition of the carrier solution for the oxidation of p-aminophenol with iodate showed measurable signals using a mixture of paminophenol (0.3%, w/v) and acetate buffer (0.5 M, pH 5.0). The effects of p-aminophenol solution (0.05% 0.48, w/v) and acetate buffer (0.05-1.0 M) were examined. The results obtained are shown in Fig. 5. An acetate buffer concentration of 0.5 M (pH 5.0) and a p-aminophenol concentration of 0.3% (w/v) were chosen for further studies. The effect of the ratio of acetate buffer to p-aminophenol was also investigated. A reasonable and reproducible absorbance was obtained at a ratio of 1: 1. The effect of acetate buffer pH was investigated and the results are shown in Fig. 5. The absorbance mcreased with pH up to pH 5.0 and decreased thereafter. Above pH 5.0 less indamine dye formation took place, which caused a decrease
04 1
0 z d R 2
03-
oz-
Ol-
00 01
02
03
0.4
(%)
005
01
05
10
CM)
10
30
50
70
(PHI
Fig 5 Effects of (0) p-aminophenol, (A) acetate buffer and (B) pH of acetate buffer on complex formation.
0
(cm1
80
160
240
300
360
02
043
0 73
12
(mllmln)
Fig. 6. Effects of (0) mixing coil length and (m) flow-rate on the rate of complex formation.
in absorbance, and therefore pH 5.0 was selected for subsequent use. Flow-rates and mixing coil length were optimized for the manifold (Fig. 6). There was a slight increase in the peak height as the mixing coil length was increased from 80 to 300 cm, but further increase did not result in an increase in sensitivity. Therefore, a coil length of 300 cm was adopted. During studies on the effect of flow-rate on response, a flow-rate of 0.43 ml min-’ were found to be suitable and was therefore used in further studies. Calibration A series of standard solutions of iodate covering the range 5.0 x 10-4-13.0 x low4 M were injected using the optimized conditions. A linear calibration graph was obtained with a correlation coefficient of 0.9993 (n = 6) and a regression equation y = 0.740x - 0.003 [y = absorbance; x = concentration (mM)]. The limit of detection was 5.0 x 10e6 M iodate. The sampling rate was 25 h-l. The r.s.d. for ten determination of 5.0 x lop4 M iodate was 1.5%. Determination of iodide. Various concentrations of the carrier stream consisting of hydrogen peroxide (0.1-5.0 M) and p-aminophenol (O.Ol-0.2%, w/v) were used to establish the optimum carrier stream composition. Measurable signals were obtained when a mixture of 2.0 M hydrogen peroxide and 0.05% (w/v) p-aminophenol was used as the carrier stream (Fig. 7). A non-reproducible and unsteady baseline was observed with further
FLOW
INJECTION
PROCEDURES
FOR
IODIDE
AND
223
IODATE
TABLE
1
Effect of foreign Ions (5 X10m3 (5 X 10e4 M) determinatron
OD 01 001
I
I
I
1
05
10
20
50
(Ml
IJl
015
02
I%)
005
Fig 7. Effects of(e) hydrogen peroxrde and (m) p-ammophenol concentratrons on complex formation
increases in hydrogen peroxide and p-aminophenol concentrations. Owing to this deleterious effect, 2.0 M hydrogen peroxide and 0.05% (w/v) p-ammophenol were used in further studies. The effect of the ratio of hydrogen peroxide and p-aminophenol was also examined. The optimum peak-height absorbance was achieved at a ratio of p-aminophenol to hydrogen peroxide of 1 : 4. The effects of mixing coil length (90-450 cm) and flow-rate (0.2-1.2 ml ruin-‘) were studied for the system (Fig. 8). Maximum absorbance signals were obtained at a mixing coil length of 270 cm
M) on mdate and iodide
Forergn ion
Peak height (mm) a Iodate
Iodide
None Acetate Bromtde Chloride Fluonde Nrtrate Oxalate
48 NDb 45 44 48 37 45
77 76 75 73 74 80 NDb
a All peak-height values are means of three readings. b Not deter-nutted.
and a flow-rate of 0.43 ml mm’, and these values were used in further studies. Calzbration. A linear calibration graph was obtained for iodide in the range 5.0 x 10-4-13.0 x 10e4 M with a correlation coefficient of 0.999 (n = 6) and a regression equation y = 0.43x 0.008 [ y = absorbance; x = concentration (mM)]. The detection limit was 5.0 X 10e5 M and the sample throughput was 30 h-‘. The r.s.d. for ten injections of 5.0 X lop4 M iodide was 0.8%. Effect of foreign ions. The effect of foreign ions was studied by addition to iodate and iodide standards. The results are summarized in Table 1. Conclusions
The fluorimetric and spectrophotometric methods described show excellent sensitivity and selectivity coupled with the high speed of the FIA technique. 8 Ol-
8
The authors thank the Pakistan Atomic Energy Commission for financial assistance. 8/
\
.-b-b_
.‘8
REFERENCES
b /
\
‘\ 1
1
I
1
90
180
270
360
02
0 43
073
12
1
450
kml
(ml/mm)
Ftg 8 Effects of (0) mrxmg cod length and (m) flow-rate on the rate of complex for-matron.
J Ruvcka and E.H. Hansen, Flow InJectron Analysis, Wiley, New York, 2nd Edn , 1988. A Al. Wehard and A Townshend, Anal Chrm Acta, 198 (1987) 45 M. Miyazakt, N. Okubo, K. Hayakawa and T. Umeda, Chem. Pharm Bull., 32 (1984) 3702. O.F. Kamson, Anal. Chrm. Acta, 179 (1986) 475. J. Slamna, W.A. Lmgerak and F. Bakker, Anal. Chrm. Acta, 117 (1980) 91
224 6 D. Mackoul, D.C Johnson and K.G. Schick, Anal. Chem., 56 (1984) 436. 7 H. Man and H. Yan, K’o Hsueh Tung Pao, 27 (1982) 959. 8 1. Iwasaki, S Utsunu and T. Gzawa, Bull. Chem. Sot. Jpn., 26 (1953) 108. 9 N. Yonehara, Bull. Chem. Sot. Jpn., 37 (1964) 1101. 10 R.E. Moxon, Analyst, 109 (1984) 425. 11 E.B Sandell and I.M. Kolthoff, J. Am. Chem. Sot., 56 (1934) 1426; Micro&m Acta, 1 (1951) 9.
M YAQOOB
ET AL.
12 R. Gmelin and AI. Virtanen, Acta Chem. Stand., 13 (1959) 1469 13 T. Koh, M. Gno and I. Makino, Analyst, 113 (1988) 945. 14 T. Deguclu, A Tanaka, I. Sanemosa and H. Nagi, Bunseki Kagaku, 32 (1983) 23. 15 J. Fuchs, E. Jungresis and L. Ben-Dor, Anal. Chim. Acta, 31 (1964) 187 16 D.F Boltz and J.A. Howell, Colorimetnc Determination of Nonmetals, Wiley, New York, 2nd edn., 1978.
219
Flow-injection procedures for determination of iodide and iodate/iodide with spectrofluorimetric and spectrophotometric detection, respectively M. Yaqoob
* and M. Masoom
Instmte of BmchemzstTy, Universzty of Baluchwan, Quetta (Pakistan)
A. Townshend School of Chemstry, Unwemty of Hull, Hull HlJ6 7RX (UK) (Recerved 21st August 1990)
AbSbCt
I&de was determined using a flow-mlecuon system based on the catalytic reductton of cerium(IV) to fluorescent cermm(II1) [X(ex) = 260 nm, h(em) = 350 nm]. Iodate and iodrde were detenmned spectrophotometrically by a sensitive colour reacuon with p-aminophenol. Iodide was ox&red by hydrogen peroxide to mdate in the presence of ~ammophenol. Iodate in turn oxrdized p-aminophenol to an indamine dye which has an absorbance maximum at 540 nm.
Keywords. Flow system;
Fhronmetry;
Spectrophotometry;
Flow-injection analysis (FIA) was originally used to automate serial assays, but is now a general solution-handling and data-gathering technique. It is finding increasing applications 111 routine analysis, research, teaching, monitoring of chemical processes and for enhancing the performance of various instruments. The underlying theory and applications of FIA have been described by Ruzicka and Hansen [l]. Some procedures for iodide determination have been .reported, based on flow-injection analysis coupled with spectrophotometry [2-41, potentiometry [5] and amperometry [6,7]. The catalytic properties of iodide have been largely explored as a means for its quantification. The method reported by Iwasaki et al. [8] is based on the catalytic effect of iodide on the reaction between thiocyanate and nitrite ions. This method has been improved and 0003-2670,'91/$03.50
0 1991 - Ekevrer Scrence Pubhshers B.V.
Iodate;
Iodide
applied to the determination of iodate and iodide in sea water [9] and potable water [lo]. Iodide has also been determined by its catalysis of the reduction of cerium(IV) by arsenious acid [ll], by the use of iodide-iodine as a catalyst for the iron(III)-arsenic(II1) system [12] and by liquidliquid extraction with methylene blue [13]. The fluorimetric method reported here is based on the work of Deguchi et al., [14] on the iodidecatalysed reduction of cerium(IV) to fluorescent cerium(II1) [X(ex) = 260 nm; X(em) = 350 rim]: 2 Ce(IV)
+ As(II1) i%e2
Ce(II1)
+ As(V)
The rate of colour formation increases in proportion to the amount of iodide present, allowing the determination of trace amounts of iodide. The second procedure reported is based on that of Fuchs et al. [15], involving the reaction of
220
M YAQOOB
iodate with p-aminophenol. Iodate oxidizes paminophenol to a quinoneimine, which immediately condenses with a second p-aminophenol molecule to give a blue indamine dye: J=&zJ
+ HOGNH,-
Cenum
(IV)
0.8
sulphate
acid -
Fig. 1. Flow-mpxtion bon of iodide. Indamine dye
In the case of iodide, the reaction becomes catalytic with the mtroduction of an oxidizing agent (hydrogen peroxide), which causes continued oxidative regeneration of iodate [16]. It provides simplicity, high precision and low reagent consumption, which are combined with the benefits of amplification for the spectrophotometric determination of iodate and iodide. Iodide ions are oxidized by hydrogen peroxide to iodine and iodate. However, in the presence of p-aminophenol, the equilibrium is shifted towards the formation of iodate, because of the redox reaction between iodate and p-aminophenol [14]: I-+
3 H202 p-aminophenol~IO; + 3 H,O
IO,
+ p-aminophenol += I- + indamine dye
ET AL
mamfold for the fluorimetnc determma-
Aqueous p-ammophenol
solutions, 0.05% (w/v)
for iodide and 0.3% (w/v) for iodate. These solutions are not light stable and therefore fresh solutions were prepared every 2 h, filtered and stored in brown bottles. Acetate buffer, 0.5 M (pH 5.0). Acetate buffer was prepared by dissolving 10.25 g of sodium acetate in 250 ml of water and adjusting the pH to 5.0 with 2 M acetic acid. Iodide and iodate standard solutions, 0.1 M. Stock solutions were prepared separately by dissolving oven-dried potassium iodide (1.66 g) and potassium iodate (2.14 g) in 100 ml of water. Working standard solutions of iodide and iodate were prepared by appropriate dilution of the stock standard solutions with water.
Apparatus EXPERIMENTAL
Reagents and solutions All reagents were of analytical-reagent
grade (Merck, BDI-I) and deionized, distilled water was used throughout. Cerrum(IV) sulphate solution, 0.005 M. This solution was prepared by dissolving 0.51 g of cerium(IV) sulphate in 24.3 ml of concentrated sulphuric acid and diluting to 250 ml with water. Arsenious acid solution, 0.01 M. A 0.5-g amount of arsenious oxide was dissolved in 200 ml of water containing 3.0 g of sodium hydroxide, acidified with 10 ml of concentrated sulphuric acid and diluted to 250 ml with water. Hydrogen peroxide solution, 2 M. This solution was prepared from commercially available hydrogen peroxide [30% (w/v), 9.8 M] by taking 20.4 ml and dilutmg it to 100 ml with water.
For the spectrofluorimetric determination of iodide, the flow-injection manifold shown in Fig. 1 was used. Samples were injected using a rotary valve (Rheodyne 5020) with a sample loop of 20 ~1. A peristaltic pump (Ismatec Mini S820) was used for propelling the carrier and reagents (0.8 ml mix-‘). PTFE tubing (0.5 mm i.d.) was used throughout the manifold. A spectrofluorimeter (Perkin-Elmer Model 3000) with a flow cell (25 ~1) consisting of a silica tube was used to record the fluorescence of cerium(II1) produced at A(ex) = 260 nm and X(em) = 350 nm; the excitation and emission slits were set at 5 nm. The response was fed directly to a strip-chart recorder. For the spectrophotometric determination of iodate and iodide, the single-channel FIA manifold shown in Fig. 2 was used. In this instance 25+1 samples were injected. The absorbance of both species was monitored at 540 nm using a spectrophotometer (LKB, Novaspec II) with a
FLOW
INJECTION
PROCEDURES
S
FOR
IODIDE
AND
221
IODATE
I25 /II)
mljnin -
Reagents
1
0.43
W 01
300 cm
-
Fig. 2 Flow-iqection and Iodide
flow-through recorder.
b
*-a-*--
manifold for the determmation
cell (30 ~1) connected
of iodate
RESULTS AND DISCUSSION
Fluonmetric determinatron of iodide In order to obtain a system with high accuracy, VENOUS factors controlling the catalytic action were studied, including concentration of reagents, flow-rate, mixing coil length and the presence of foreign ions. The effects of 1 x 10e3-50 X 10B3 M cerium(IV) sulphate and arsenious acid concentrations on the reduction process were determined by mixing the various concentrations in the reservoir in a single-channel FIA system. The signals obtained decreased gradually with time. The solution in the reservoir turned deep yellow, showing instability of the reagents when mixed in this manner. Therefore, a dual-channel system was used and the solutions were mixed before the injection point. As shown in Fig. 3, the highest signals were ob-
Concentration
(x lo-
l
to a chart
3 M)
Fig. 3. Effect of (a) cerium(IV) sulphate and (b) arsemous acid concentrations on the reduction of cerium(IV) to cermm(II1).
\
a
60
120
180
240
300
03
08
15
25
Imllmln)
(cm)
Fig. 4 Effect of (a) flow-rate and (b) nuxmg coil length on the rate of reduction of cenum(IV) to cenum(II1)
tamed with 5 x 1O-3 M cerium(IV) sulphate and 10 x lop3 M arsenious acid, and these were therefore used in subsequent investigations. The effect of flow-rate on sensitivity, sampling rate and reagent consumption were investigated. As shown in Fig. 4, a decrease in fluorescence was observed as the flow-rate was increased. At lower flow-rates (0.3 ml mm-‘) there was a gradual increase in the peak height but peak broadening and a low sample throughput were a problem and therefore a flow-rate of 0.8 ml mm’ was selected for acceptable sensitivity and a reasonable sample throughput. The effect of mixing coil length on the sensitivity was also investigated. There was a slight increase in the peak height when the mixing coil length was increased from 60 to 300 cm. A mixing coil length of 180 cm was found to be optimum and was therefore selected. Calibration. A series of iodide standard solutions were injected into the optimized system. A linear calibration graph was obtained covering the range 1.0 x 10e5-3.0 x lop5 M. The resulting graph had a correlation coefficient of 0.992 (n = 6). The detection limit (twice the blank noise) was 5.0 x lo-’ M. The sampling rate was 45 h-‘. The relative standard deviation (r.s.d,) was 0.8% for ten injections of 3.0 x 10K5 M iodide. Effect of forezgn ions. The effect of various foreign ions was studied and it was found that
222
M
YAQOOB
El-AL
bromide and chloride ions at a concentration 60 times in excess had no effect at the level of 3 X lo- 5 M iodide. Nitrate ions had a slight effect when added in an equimolar ratio. Azide ions interfered seriously in an equimolar ratio and also gave a response when injected into the system in the absence of iodide. Spectrophotometric determination of iodate / iodide Determrnation of iodate. Preliminary studies on the composition of the carrier solution for the oxidation of p-aminophenol with iodate showed measurable signals using a mixture of paminophenol (0.3%, w/v) and acetate buffer (0.5 M, pH 5.0). The effects of p-aminophenol solution (0.05% 0.48, w/v) and acetate buffer (0.05-1.0 M) were examined. The results obtained are shown in Fig. 5. An acetate buffer concentration of 0.5 M (pH 5.0) and a p-aminophenol concentration of 0.3% (w/v) were chosen for further studies. The effect of the ratio of acetate buffer to p-aminophenol was also investigated. A reasonable and reproducible absorbance was obtained at a ratio of 1: 1. The effect of acetate buffer pH was investigated and the results are shown in Fig. 5. The absorbance mcreased with pH up to pH 5.0 and decreased thereafter. Above pH 5.0 less indamine dye formation took place, which caused a decrease
04 1
0 z d R 2
03-
oz-
Ol-
00 01
02
03
0.4
(%)
005
01
05
10
CM)
10
30
50
70
(PHI
Fig 5 Effects of (0) p-aminophenol, (A) acetate buffer and (B) pH of acetate buffer on complex formation.
0
(cm1
80
160
240
300
360
02
043
0 73
12
(mllmln)
Fig. 6. Effects of (0) mixing coil length and (m) flow-rate on the rate of complex formation.
in absorbance, and therefore pH 5.0 was selected for subsequent use. Flow-rates and mixing coil length were optimized for the manifold (Fig. 6). There was a slight increase in the peak height as the mixing coil length was increased from 80 to 300 cm, but further increase did not result in an increase in sensitivity. Therefore, a coil length of 300 cm was adopted. During studies on the effect of flow-rate on response, a flow-rate of 0.43 ml min-’ were found to be suitable and was therefore used in further studies. Calibration A series of standard solutions of iodate covering the range 5.0 x 10-4-13.0 x low4 M were injected using the optimized conditions. A linear calibration graph was obtained with a correlation coefficient of 0.9993 (n = 6) and a regression equation y = 0.740x - 0.003 [y = absorbance; x = concentration (mM)]. The limit of detection was 5.0 x 10e6 M iodate. The sampling rate was 25 h-l. The r.s.d. for ten determination of 5.0 x lop4 M iodate was 1.5%. Determination of iodide. Various concentrations of the carrier stream consisting of hydrogen peroxide (0.1-5.0 M) and p-aminophenol (O.Ol-0.2%, w/v) were used to establish the optimum carrier stream composition. Measurable signals were obtained when a mixture of 2.0 M hydrogen peroxide and 0.05% (w/v) p-aminophenol was used as the carrier stream (Fig. 7). A non-reproducible and unsteady baseline was observed with further
FLOW
INJECTION
PROCEDURES
FOR
IODIDE
AND
223
IODATE
TABLE
1
Effect of foreign Ions (5 X10m3 (5 X 10e4 M) determinatron
OD 01 001
I
I
I
1
05
10
20
50
(Ml
IJl
015
02
I%)
005
Fig 7. Effects of(e) hydrogen peroxrde and (m) p-ammophenol concentratrons on complex formation
increases in hydrogen peroxide and p-aminophenol concentrations. Owing to this deleterious effect, 2.0 M hydrogen peroxide and 0.05% (w/v) p-ammophenol were used in further studies. The effect of the ratio of hydrogen peroxide and p-aminophenol was also examined. The optimum peak-height absorbance was achieved at a ratio of p-aminophenol to hydrogen peroxide of 1 : 4. The effects of mixing coil length (90-450 cm) and flow-rate (0.2-1.2 ml ruin-‘) were studied for the system (Fig. 8). Maximum absorbance signals were obtained at a mixing coil length of 270 cm
M) on mdate and iodide
Forergn ion
Peak height (mm) a Iodate
Iodide
None Acetate Bromtde Chloride Fluonde Nrtrate Oxalate
48 NDb 45 44 48 37 45
77 76 75 73 74 80 NDb
a All peak-height values are means of three readings. b Not deter-nutted.
and a flow-rate of 0.43 ml mm’, and these values were used in further studies. Calzbration. A linear calibration graph was obtained for iodide in the range 5.0 x 10-4-13.0 x 10e4 M with a correlation coefficient of 0.999 (n = 6) and a regression equation y = 0.43x 0.008 [ y = absorbance; x = concentration (mM)]. The detection limit was 5.0 X 10e5 M and the sample throughput was 30 h-‘. The r.s.d. for ten injections of 5.0 X lop4 M iodide was 0.8%. Effect of foreign ions. The effect of foreign ions was studied by addition to iodate and iodide standards. The results are summarized in Table 1. Conclusions
The fluorimetric and spectrophotometric methods described show excellent sensitivity and selectivity coupled with the high speed of the FIA technique. 8 Ol-
8
The authors thank the Pakistan Atomic Energy Commission for financial assistance. 8/
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REFERENCES
b /
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‘\ 1
1
I
1
90
180
270
360
02
0 43
073
12
1
450
kml
(ml/mm)
Ftg 8 Effects of (0) mrxmg cod length and (m) flow-rate on the rate of complex for-matron.
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