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Spectrofluorimetric Determination of Silicon by Flow Injection Analysis
Analytica Chimica Acta; 177 (1985) 263-266 Elsevier Science Publishers B. V., Amsterdam- Printed in The Netherlands
Short Communication SPECTROFLUORIMETRIC DETERMINATION OF SILICON BY FLOW INJECTION ANALYSIS
P. LINARES, M. D. LUQUE DE CASTRO and M. VALCARCEL*
Department of Analytical Chemistry, Faculty of Sciences, University of Cordoba, Cordoba (Spain) (Received 15th May 1985)
Summary. The sensitive (3Q-600 "'g 1-' Si0 2 ), precise (r.s.d. = 0.25%), fast (120 h-') and selective method is based on the oxidation of thiamine by molybdosilicic acid to highly fluorescent thiochrome. The method is applied to synthetic water samples.
The determination of dissolved silicate as means of analyzing for silicon requires the prior conversion of other chemical forms of this element to silicate. Silicon is determined in a large variety of samples such as semiconductors, metallurgical products, waste waters, rocks and pottery [1, 2]. The determination of silicon in earthenware is particularly interesting [3]. Commonly used methods for determining silicon are gravimetric (either as an organic molybdosilicate or as potassium hexafluorosilicate) or spectrophotometric based on heteropolyacid formation with molybdate [1, 2, 4]. Other heteropolyacid-based procedures have involved voltammetry [5] and fluorimetry [6, 7]. Flow injection analysis (f.i.a.) has been used to ,determine silicon. The method proposed by Israel and Barnes [ 8] for direct determination of silicon involves atomic emission. Other methods involve the formation of molybdosilicate with voltammetric [9, 10] or spectrophotometric [11] detection or monitoring of the molybdenum blue formed by reduction of the heteropoly anion [12, 13]. Fluorimetric detection, however, has not so far been utilized in a flow-injection method for the determination of silicon. The method proposed herein is based on the oxidation of thiamine to highly fluorescent thiochrome by molybdosilicate. This reaction has previously been used for the determination of phosphate [14], in which silica interfered seriously.
Experimental Reagents. Stock aqueous solutions were 0.06 M ammonium heptamolybdate, 1 X 10-2 M silicate and 1 g 1-1 EDTA. Aqueous thiamine solutions of suitable concentration were prepared daily. The buffer solution contained 3.814 g of sodium tetraborate decahydrate and 37.4 ml of 5 M ammonia 0003·2670/85/$03.30
© 1985 Elsevier Science Publishers B.V.
264
adjusted to the desired pH with concentrated hydrochloric acid and diluted to 100 ml with distilled water. Apparatus. A Perkin-Elmer LS-1 LC fluorescence detector was used with a 1.5-cm square flowcell and a Perkin-Elmer 56 recorder. A Gilson Minipuls-2 or Ismatec S-840 peristaltic pump, a Tecator Ll00-1 injection valve and a Selecta S-382 thermostat were also used. The manifold and recommended conditions are shown in Fig. 1.
Results and discussion The reaction sequence yielding the fluorescent product (thiochrome) entails formation of molybdosilicic acid in a perchloric acid medium and oxidation of thiamine in a basic medium. Therefore the manifold designed to implement the method (Fig. 1) consists of a channel of sodium molybdate in an acidic medium into which the sample is injected, with heteropolyacid formation taking place in L 1• The instability of thiamine in a basic medium requires its mixing with the ammoniacal borate solution almost immediately before the point of confluence with the molybdate channel. Thiamine is oxidized in reactor L 2 • The chemical variables were optimized by a modified simplex method, and the system variables and the temperature by the univariant method. The values obtained are those shown in Fig. 1. The calibration graph obtained under these conditions was linear in the range 3(}-600 p.g l-1 silica (5 X 10-71 X lo-s M) (r = 0.9997, 7 points). When 11 identical samples of 4 X 10- 6 M silica were injected in triplicate, the relative standard deviation was 0.25%. The sampling rate was 120 samples per hour. Interferences. Interfering species which may be present in water samples were studied thoroughly. Samples containing 240 p.g l- 1 Si0 2 were used, adding a 100-fold greater weight concentration of the other ion. The results are shown in Table 1. Co(II), Fe(III), Fe(II) and Cu(II) also interfered, but their effect was eliminated by introducing a further channel containing 1 g 1-1 EDTA to mix with the molybdate stream before the injection valve. Attempts to remove the serious interference of phosphate and sulphide by adding oxalate, tartrate or citrate to the samples, as in other methods [1, 9] ml -min-1 Sample
3.0
0.0136 M Moof·
2.0
0.34M HCI 04 SM NH3 0.1MBorax
1.0 1.0
0. 029M Thiamine
4s•c
Fig. 1. Manifold used for determination of silicon and optimum values of the variables. The boxed section is thermostatted.
265
TABLE 1 Study of interferences in the determination of silicon Tolerated wt. ratio Foreign ion: Si0 2
Foreign ion
100
Acetate, EDTA, citrate, Cl-, co;-, oxalate, Cro~-. I-, NO;, N03, SO!-, tartrate, Br-, F-, NH:, Na+, K+, Ca 2 +, Mg 2 +, Cd 2 +, Cr 3 +, NF+, Zn 2+, Hg 2 +, Pb 2 +, Al3+, co•+a, Fe•+a, Fe 3 +\ Cu 2 +a Aso:-. so;IClOPO!-, s•-
50 15 5
<1 a In the presence of EDTA. TABLE 2
Synthetic water samples used for method evaluation Sample No.
Composition (J.Lg ml- 1 )
1
5.0 SiO., 15 Cl-, 10 co;-, 5 NO;, 2 NO;, 5 so~-. 100 Ca 2 +, 100 Mg 2 +, 1 Fe 3+, 2 Cu 2+. 20.0 Si0 2 , 5 PO!-, 20 Cl-, 20 co;-, 10 NO;, 5 SO~-. 20 Ca 2 +, 10 Mg 2 +, 5 Fe 3 +, 2 Fe 2 +, 10 Pb 2 +. 15.0 Si0 2 , 40 c1-, 50 co;-, 10 N03, 40 so~-. 15 F-, 5 cro~-. 50 Ca 2 +, 20 Mg 2 +, 10 Fe 3+, 10 Pb 2 +, 20 Cu 2 +. 20.0 Si0 2 , 2 PO!-, 50 Cl-, 10 co;-, 10 NO;, 10 SO~-. 10 so;-, 20 F-, 20 Br-, 200 Ca 2 +, 100 Mg 2 +, 20 AP+, 5 Co 2 +. 10.0 Si0 2 , 20 CI-, 100 co;-, 20 NO;, 5 NO:!, 50 so~-. 50 I-, 10 Ca 2 +, 50 Mg 2 +, 10 Fe 2+, 10 Hg2+. 15.0 Si0 2 , 200 c1-, 5 co;-, 10 so~-. 20 so;-, 5 Br-, 20 I-, 20 Ca 2 +, 20 Mg 2 +, 10 Fe 3 +, 10 NF+. 20.0 Si0 2 , 5 Po:-. 20 c1-, 20 co;-, 10 NO;, 10 so~-. 20 ca2+, 10 Mg•+, 5 Fe 3+, 5 Fe 2+, 10 Pb 2+, 10 Co 2 +. 20.0 Si0 2 , 5 Po:-. 40 c1-, 10 co;-, 20 No;, 5 NO;, 20 so~-. 10 so;-, 50 Ca 2 +, 100 Mg2+, 20 Hg 2 +, 10 Cu 2 +.
2
3
4 5
6 7
8
or by adding precipitants such as calcium, mercury(II) or silver were unsuccessful. Determination of silicon in synthetic samples. In order to test the applicability of the proposed method, a series of synthetic water samples was prepared, containing various amounts of common cations and anions appropriate to several types of waters. The results were compared with those obtained by applying the standard spectrophotometric method based on the formation of molybdosilicic acid in hydrochloric acid medium and monitoring the absorbance at 410 nm [2]. The composition of the samples is shown in Table 2, and the results obtained are given in Table 3. The results indicate the useful-
266 TABLE 3 Comparison of results obtained with the flow injection system and the standard spectrophotometric procedure [ 2] Sample (Table 2)
Si0 2 added (tJg ml- 1 )
1 2 3 4 5 6 7 8
5.0 20.0 15.0 20.0 10.0 15.0 20.0 20.0
Si0 2 found (tJg ml- 1)
Error(%)
Standard method
F.i.a.
Standard method
5.3 21.3 16.3 19.8 11.2 15.3 23.1 20.3
5.0 19.9 16.2 19.9 10.0 14.9 19.7 20.2
5.4 6.3 8.8 -1.2 12.4 1.9 15.5 1.7
F.i.a.
0.2 -0.5 8.3 -0.6 0.1 -0.7 -1.3 1.1
ness of the f.i.a. method, which has the following improvements over the standard procedure. The standard method requires the use of 50 ml of sample solution, in addition to another 50 ml of solution for the blank, whereas the f.i.a. method requires only. 0. 7 5 ml of test solution, which is diluted with distilled water to 25 ml; blanks are unnecessary. A minimum time of 8 min is needed for complete colour development in the standard method; the flowinjection technique permits determining up to 120 samples per h. Samples containing <2 Jl.g ml-1 Si0 2 cannot be determined by the standard method, whereas the flow procedure allows the determination of ;;:.:30 Jl.g I-1 • Finally, the flow-injection procedure is more accurate (Table 3). REFERENCES 1 W. J. Williams, Handbook of Anion Determination, Ellis Horwood, Chichester, 1977. 2 American Public Health Association, American Water Works Association and Water Pollution Control Federation, Standard Methods for the Examination of Water and Wastewater, 15th edn., Am. Public Health Assoc., New York, 1980, p. 429. 3 H. Bennett, Analyst (London), 102 (1977) 153. 4 L. G. Hargis, Anal. Chem., 42 (1970) 1494. 5 A. G. Fogg, N. K. Bsebsu and B.·J. Birch, Talanta, 28 (1981) 473. 6 K. Krzysztof, Chem. Anal., 14(6) (1969) 1325. 7 G. Ellion and J. A. Radle, Anal. Chem., 33 (1961) 1623. 8 Y. Israel and R. M. Barnes, Anal. Chem., 56 (1984) 1188. 9 A. G. Fogg and N. K. Bsebsu, Analyst (London), 106 (1981) 1288. 10 A. G. Fogg and G. C. Cripps, Analyst (London), 108 (1983) 1485. 11 Y. Hirai, T. Yokoma, N. Yoza, T. Tarutani and S. Ohashi, Bunseki Kagaku, 30 (1981) 350. 12T. Yokoyama, Y. Hirai, N. Yoza, T. Tarutani and S. Ohashi, Bull. Chem. Soc. Jpn., 55 (1982) 3477. 13J. Thomsen, K. S. Johnson and R. L. Petty, Anal. Chem., 55 (1983) 2378. 14J. Holzbecher and D. E. Ryan, Anal. Chim. Acta, 64 (1973) 151.
Short Communication SPECTROFLUORIMETRIC DETERMINATION OF SILICON BY FLOW INJECTION ANALYSIS
P. LINARES, M. D. LUQUE DE CASTRO and M. VALCARCEL*
Department of Analytical Chemistry, Faculty of Sciences, University of Cordoba, Cordoba (Spain) (Received 15th May 1985)
Summary. The sensitive (3Q-600 "'g 1-' Si0 2 ), precise (r.s.d. = 0.25%), fast (120 h-') and selective method is based on the oxidation of thiamine by molybdosilicic acid to highly fluorescent thiochrome. The method is applied to synthetic water samples.
The determination of dissolved silicate as means of analyzing for silicon requires the prior conversion of other chemical forms of this element to silicate. Silicon is determined in a large variety of samples such as semiconductors, metallurgical products, waste waters, rocks and pottery [1, 2]. The determination of silicon in earthenware is particularly interesting [3]. Commonly used methods for determining silicon are gravimetric (either as an organic molybdosilicate or as potassium hexafluorosilicate) or spectrophotometric based on heteropolyacid formation with molybdate [1, 2, 4]. Other heteropolyacid-based procedures have involved voltammetry [5] and fluorimetry [6, 7]. Flow injection analysis (f.i.a.) has been used to ,determine silicon. The method proposed by Israel and Barnes [ 8] for direct determination of silicon involves atomic emission. Other methods involve the formation of molybdosilicate with voltammetric [9, 10] or spectrophotometric [11] detection or monitoring of the molybdenum blue formed by reduction of the heteropoly anion [12, 13]. Fluorimetric detection, however, has not so far been utilized in a flow-injection method for the determination of silicon. The method proposed herein is based on the oxidation of thiamine to highly fluorescent thiochrome by molybdosilicate. This reaction has previously been used for the determination of phosphate [14], in which silica interfered seriously.
Experimental Reagents. Stock aqueous solutions were 0.06 M ammonium heptamolybdate, 1 X 10-2 M silicate and 1 g 1-1 EDTA. Aqueous thiamine solutions of suitable concentration were prepared daily. The buffer solution contained 3.814 g of sodium tetraborate decahydrate and 37.4 ml of 5 M ammonia 0003·2670/85/$03.30
© 1985 Elsevier Science Publishers B.V.
264
adjusted to the desired pH with concentrated hydrochloric acid and diluted to 100 ml with distilled water. Apparatus. A Perkin-Elmer LS-1 LC fluorescence detector was used with a 1.5-cm square flowcell and a Perkin-Elmer 56 recorder. A Gilson Minipuls-2 or Ismatec S-840 peristaltic pump, a Tecator Ll00-1 injection valve and a Selecta S-382 thermostat were also used. The manifold and recommended conditions are shown in Fig. 1.
Results and discussion The reaction sequence yielding the fluorescent product (thiochrome) entails formation of molybdosilicic acid in a perchloric acid medium and oxidation of thiamine in a basic medium. Therefore the manifold designed to implement the method (Fig. 1) consists of a channel of sodium molybdate in an acidic medium into which the sample is injected, with heteropolyacid formation taking place in L 1• The instability of thiamine in a basic medium requires its mixing with the ammoniacal borate solution almost immediately before the point of confluence with the molybdate channel. Thiamine is oxidized in reactor L 2 • The chemical variables were optimized by a modified simplex method, and the system variables and the temperature by the univariant method. The values obtained are those shown in Fig. 1. The calibration graph obtained under these conditions was linear in the range 3(}-600 p.g l-1 silica (5 X 10-71 X lo-s M) (r = 0.9997, 7 points). When 11 identical samples of 4 X 10- 6 M silica were injected in triplicate, the relative standard deviation was 0.25%. The sampling rate was 120 samples per hour. Interferences. Interfering species which may be present in water samples were studied thoroughly. Samples containing 240 p.g l- 1 Si0 2 were used, adding a 100-fold greater weight concentration of the other ion. The results are shown in Table 1. Co(II), Fe(III), Fe(II) and Cu(II) also interfered, but their effect was eliminated by introducing a further channel containing 1 g 1-1 EDTA to mix with the molybdate stream before the injection valve. Attempts to remove the serious interference of phosphate and sulphide by adding oxalate, tartrate or citrate to the samples, as in other methods [1, 9] ml -min-1 Sample
3.0
0.0136 M Moof·
2.0
0.34M HCI 04 SM NH3 0.1MBorax
1.0 1.0
0. 029M Thiamine
4s•c
Fig. 1. Manifold used for determination of silicon and optimum values of the variables. The boxed section is thermostatted.
265
TABLE 1 Study of interferences in the determination of silicon Tolerated wt. ratio Foreign ion: Si0 2
Foreign ion
100
Acetate, EDTA, citrate, Cl-, co;-, oxalate, Cro~-. I-, NO;, N03, SO!-, tartrate, Br-, F-, NH:, Na+, K+, Ca 2 +, Mg 2 +, Cd 2 +, Cr 3 +, NF+, Zn 2+, Hg 2 +, Pb 2 +, Al3+, co•+a, Fe•+a, Fe 3 +\ Cu 2 +a Aso:-. so;IClOPO!-, s•-
50 15 5
<1 a In the presence of EDTA. TABLE 2
Synthetic water samples used for method evaluation Sample No.
Composition (J.Lg ml- 1 )
1
5.0 SiO., 15 Cl-, 10 co;-, 5 NO;, 2 NO;, 5 so~-. 100 Ca 2 +, 100 Mg 2 +, 1 Fe 3+, 2 Cu 2+. 20.0 Si0 2 , 5 PO!-, 20 Cl-, 20 co;-, 10 NO;, 5 SO~-. 20 Ca 2 +, 10 Mg 2 +, 5 Fe 3 +, 2 Fe 2 +, 10 Pb 2 +. 15.0 Si0 2 , 40 c1-, 50 co;-, 10 N03, 40 so~-. 15 F-, 5 cro~-. 50 Ca 2 +, 20 Mg 2 +, 10 Fe 3+, 10 Pb 2 +, 20 Cu 2 +. 20.0 Si0 2 , 2 PO!-, 50 Cl-, 10 co;-, 10 NO;, 10 SO~-. 10 so;-, 20 F-, 20 Br-, 200 Ca 2 +, 100 Mg 2 +, 20 AP+, 5 Co 2 +. 10.0 Si0 2 , 20 CI-, 100 co;-, 20 NO;, 5 NO:!, 50 so~-. 50 I-, 10 Ca 2 +, 50 Mg 2 +, 10 Fe 2+, 10 Hg2+. 15.0 Si0 2 , 200 c1-, 5 co;-, 10 so~-. 20 so;-, 5 Br-, 20 I-, 20 Ca 2 +, 20 Mg 2 +, 10 Fe 3 +, 10 NF+. 20.0 Si0 2 , 5 Po:-. 20 c1-, 20 co;-, 10 NO;, 10 so~-. 20 ca2+, 10 Mg•+, 5 Fe 3+, 5 Fe 2+, 10 Pb 2+, 10 Co 2 +. 20.0 Si0 2 , 5 Po:-. 40 c1-, 10 co;-, 20 No;, 5 NO;, 20 so~-. 10 so;-, 50 Ca 2 +, 100 Mg2+, 20 Hg 2 +, 10 Cu 2 +.
2
3
4 5
6 7
8
or by adding precipitants such as calcium, mercury(II) or silver were unsuccessful. Determination of silicon in synthetic samples. In order to test the applicability of the proposed method, a series of synthetic water samples was prepared, containing various amounts of common cations and anions appropriate to several types of waters. The results were compared with those obtained by applying the standard spectrophotometric method based on the formation of molybdosilicic acid in hydrochloric acid medium and monitoring the absorbance at 410 nm [2]. The composition of the samples is shown in Table 2, and the results obtained are given in Table 3. The results indicate the useful-
266 TABLE 3 Comparison of results obtained with the flow injection system and the standard spectrophotometric procedure [ 2] Sample (Table 2)
Si0 2 added (tJg ml- 1 )
1 2 3 4 5 6 7 8
5.0 20.0 15.0 20.0 10.0 15.0 20.0 20.0
Si0 2 found (tJg ml- 1)
Error(%)
Standard method
F.i.a.
Standard method
5.3 21.3 16.3 19.8 11.2 15.3 23.1 20.3
5.0 19.9 16.2 19.9 10.0 14.9 19.7 20.2
5.4 6.3 8.8 -1.2 12.4 1.9 15.5 1.7
F.i.a.
0.2 -0.5 8.3 -0.6 0.1 -0.7 -1.3 1.1
ness of the f.i.a. method, which has the following improvements over the standard procedure. The standard method requires the use of 50 ml of sample solution, in addition to another 50 ml of solution for the blank, whereas the f.i.a. method requires only. 0. 7 5 ml of test solution, which is diluted with distilled water to 25 ml; blanks are unnecessary. A minimum time of 8 min is needed for complete colour development in the standard method; the flowinjection technique permits determining up to 120 samples per h. Samples containing <2 Jl.g ml-1 Si0 2 cannot be determined by the standard method, whereas the flow procedure allows the determination of ;;:.:30 Jl.g I-1 • Finally, the flow-injection procedure is more accurate (Table 3). REFERENCES 1 W. J. Williams, Handbook of Anion Determination, Ellis Horwood, Chichester, 1977. 2 American Public Health Association, American Water Works Association and Water Pollution Control Federation, Standard Methods for the Examination of Water and Wastewater, 15th edn., Am. Public Health Assoc., New York, 1980, p. 429. 3 H. Bennett, Analyst (London), 102 (1977) 153. 4 L. G. Hargis, Anal. Chem., 42 (1970) 1494. 5 A. G. Fogg, N. K. Bsebsu and B.·J. Birch, Talanta, 28 (1981) 473. 6 K. Krzysztof, Chem. Anal., 14(6) (1969) 1325. 7 G. Ellion and J. A. Radle, Anal. Chem., 33 (1961) 1623. 8 Y. Israel and R. M. Barnes, Anal. Chem., 56 (1984) 1188. 9 A. G. Fogg and N. K. Bsebsu, Analyst (London), 106 (1981) 1288. 10 A. G. Fogg and G. C. Cripps, Analyst (London), 108 (1983) 1485. 11 Y. Hirai, T. Yokoma, N. Yoza, T. Tarutani and S. Ohashi, Bunseki Kagaku, 30 (1981) 350. 12T. Yokoyama, Y. Hirai, N. Yoza, T. Tarutani and S. Ohashi, Bull. Chem. Soc. Jpn., 55 (1982) 3477. 13J. Thomsen, K. S. Johnson and R. L. Petty, Anal. Chem., 55 (1983) 2378. 14J. Holzbecher and D. E. Ryan, Anal. Chim. Acta, 64 (1973) 151.