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Continuous-flow molecular emission cavity analysis for sulphide
Analytica Chimica Acta, 173 (1985) 311-314 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Short Communication
CONTINUOUS-FLOW FOR SULPHIDE
MOLECULAR
EMISSION CAVITY ANALYSIS
N. GREKAS and A. C. CALOKERINOS* Laboratory of Analytical 106 80 Athens (Greece)
Chemistry,
Depa@ment
of Chemistry,
University
of Athens,
(Received 18th January 1985)
Summary. Sulphide (l-10 rg ml-‘) is determined by mixing the sample with an excess of orthophosphoric acid in a segmented continuous-flow system. The hydrogen sulphide evolved is swept into the cavity for generation of S, emission. The analysis is completely automated, requires no sample pretreatment and samples can be analyzed at 24 h-l.
Sulphide can be determined by conventional molecular emission cavity analysis (m.e.c.a.) in aqueous solution [l] or in binary and some ternary mixtures [2]. Sulphide has also been determined in mixtures with sulphate, thiocyanate and thiosulphate after sequential selective removal of ions and oxidation of the remainder by hydrogen peroxide to sulphate [3] . The m.e.c.a. technique also allows the determination of sulphide in the presence of other sulphur compounds in solids [4, 51. Alternatively, sulphide can be determined in a vaporization system in which the sample solution is acidified and the hydrogen sulphide evolved is continuously purged by nitrogen to the m.e.c.a. cavity for generation of SZ emission [6]. Furthermore, the anion can be determined by flow injection introduction of the solution into the cavity [ 71. Recently, a continuous-flow molecular emission cavity analyzer was described [ 81. The sample solution is mixed with aqueous orthophosphoric acid solution and is carried by a segmented continuous-flow stream through a heated reaction coil into a debubbler where the gas evolved is swept by nitrogen into the cavity. This system was successfully used for the determination of sulphite in various samples and of sulphur dioxide in atmospheric air after fixation as disulphitomercurate(I1) [ 81. This communication describes the advantages of the system for determining l-10 c(g ml-’ sulphide. Expertmen tal Apparatus. The photometer,
m.e.c.a. cavity and continuous-flow system were as described previously [ 8 1. Reagents. All reagents were of analytical grade, and deionizeddistilled water was used throughout. A sulphide stock solution (500 pg ml-‘) was 0003-2670/85/$03.30
0 1985 Elsevier Science Publishers B.V.
312
prepared by dissolving 1.87 g of Na,S - 9Hz0 (Ferak Berlin, p.a.) in 500 ml of 0.1 M sodium hydroxide/O.01 M EDTA. The exact sulphide concentration was established iodimetrically. More dilute solutions were prepared in 0.02 M sodium hydroxide/2 X 10” M EDTA by the fewest dilution steps possible. All sulphide solutions were prepared daily. Procedure. All experimental parameters were used as optimized for sulphite [8]. The cavity was conditioned by allowing 0.5-1.0 ml of stock sulphide solution to enter into the continuous flow system and generate intense Sz emission within the cavity. After re-establishment of the baseline, the determination was done as for sulphite [ 81. Results and discussion Analytical parameters. Figure 1 shows a typical recording for a series of sulphide standards by the proposed procedure. The calibration graph, emission intensity (I, mV) vs. pg ml” sulphide (C), was sigmoidal [9] and the log I vs. log C calibration graph was linear over the range l-10 pg ml-’ sulphide (log I = -0.420 + 1.72 log C, r = 0.9996). The relative standard deviation for the slope of the log-log graph for six working days was 2.2%. The limit of detection (signal/noise = 2) was 0.5 pg ml-’ sulphide and the relative standard deviations for 2 and 4 ,ug ml-’ sulphide were 2% and 0.6% (n = 8), respectively. When aqueous solutions of sulphide (l-10 pg ml-‘) were processed, the average error was +1.5%. Under the same experimental conditions, determination of sulphite (log I = -1.10 + 1.31 log C) [8] was less sensitive than that of sulphide. Interferences. Interferences from anions were investigated by determining 10 pg ml-’ sulphide in the presence of equal and fivefold (by wt.) amounts of other anion. The responses were compared with those obtained from an uncontaminated sulphide solution. No effect was observed from thiosulphate,
Fig. 1. Typical recording for a series of sulphide standards.
313
thiocyanate, cyanide, iodide, bromide or carbonate. Nitrite seriously decreased the intensity (Table 1). Reduction of nitrite to nitrogen by sulphamic acid was investigated for eliminating the interference. Aqueous solutions of sulphamic acid introduced into the analyzer did not produce any molecular emission within the cavity and left the emission intensity from pure sulphide solutions unaffected. A 0.02% (w/v) solution of sulphamic acid completely eliminated the interference from equal concentrations of nitrite on 1.0 and 5.0 pg ml-’ sulphide. The effect of an equal concentration of nitrite on 10 Mg ml-’ of sulphide was eliminated by 0.04% (w/v) sulphamic acid. The interference of a fivefold concentration of nitrite on sulphide was only partially eliminated by sulphamic acid. These results are shown in Table 1. Sulphite interferes severely with the determination of sulphide. The emission intensities from 5.0 pg ml-’ sulphide and 10 pg ml” sulphite were 6.0 mV and 1.6 mV, respectively, while the emission intensity from the mixture was 9.2 mV, as would be expected from the sigmoidal nature of the calibration graph. Various attempts were made to eliminate the interference of sulphite on the measurement of sulphide. Tannic acid completely removes the emission of sulphite [8] but 2 X 10J and 5 X lo”% (w/v) tannic acid also decreased the intensity from 8 gg ml” sulphide by 43% and 86%, respectively. Likewise, 0.01% and 0.02% (w/v) glyoxal decreased the intensity from 8 E.cgml-’ sulphite by 80% and lOO%, respectively, because of the formation of the addition compound. The same concentrations of glyoxal had no effect on the intensity from aqueous sulphide solutions. Nevertheless, when 0.02% (w/v) glyoxal was added to a mixture of sulphide and sulphite, the emission intensity was unaffected. This is probably due to disproportionation of glyoxal in the alkaline solution required for stabilization of sulphide. TABLE
1
Effect of nitrite on the emission intensity from sulphide, and elimination of the interference by addition of sulphamic acid Concentration Sulphide
(fig ml-‘) Nitrite
Emission intensity* Sulphamic added acid (%) (w/v) 0.00
10.0 10.0 10.0 5.0 5.0 5.0 1.0 1.0 1.0
0 10 50 0 5.0 25 0 1.0 5.0
100 35.3 2.9 100 35.0 0 100 66.7
0.02
88.2 73.5
0.04
100 88.2b
100 80.0b 100 80.0b
aIntensities from pure sulphide solutions arbitrarily taken as 100. bNo further decrease of interference was observed at higher sulphamic acid concentrations.
314 TABLE 2 Alkaline degradation of organic sulphur compounds Compound
Cont. (mg ml-‘)
Cysteamine * HCl N-Acetyl-L-cysteine Thiodiacetic acid Thioglycollic Cephalexin
acid
1.86 2.55 0.64 13.3 0.10 0.25
to sulphide in 1 M sodium hydroxide
Sulphide sulphur (rg ml-‘) Theoretical
524.5 500.4 137.0 4620 9.21 23.02
Found after hydrolysis? at: 30°C
100” c
0(75) O(60) 4.32(45) 4.91(95) 3.58(60) O(30) O(30)
1.51(75) 0.96( 60) 5.72(60) 6.21( 100) 14.88(60) 5.99(30) 14.93(30)
aThe times of hydrolysis (min) are given in parentheses.
Alkaline degradation of organic sulphur compounds. The simplicity and sensitivity of the method are useful for the determination of sulphide produced by alkaline degradation of various organic sulphur-containing compounds in 1 M sodium hydroxide (Table 2). Cephalexin was found to degrade completely after 30 min at 100°C and the sulphide yields for 0.10 and 0.25 mg ml-’ were 65.0% and 64.9%, respectively, which are in good agreement with results obtained by Fogg et al. [lo, 111. The method was also used to determine the relatively small amounts of sulphide produced by decomposition of the other compounds. REFERENCES 1 R. Belcher, S. L. Bogdanski, D. J. Knowles and A. Townshend, Anal. Chim. Acta, 77 (1975) 53. 2 M. Q. Al-Abachi, R. Belcher, S. L. Bogdanski and A. Townshend, Anal. Chim. Acta, 86 (1976) 139. 3T. S. Al-Ghabsha, Ph.D. Thesis, Birmingham University, 1979; A. C. Calokerinos and A. Tow&rend, Prog. Anal. At. Spectrosc., 5 (1982) 63. 4 S. A, Schubert, J. W. Clayton and Q. Fernando, Anal. Chem., 52 (1980) 963. 5 J. H. Tzeng and Q. Fernando, Anal. Chem., 54 (1982) 971. 6 E. Henden, Ph.D. Thesis, Birmingham University, 1976; E. Henden, N. Pourreza and A. Townshend, Prog. Anal. At. Spectrosc., 2 (1979) 355. 7 J. L. Burguera and M. Burguera, Anal. Chim. Acta, 157 (1984) 177. 8 N. Grekas and A. C. Calokerinos, Analyst (London), 110 (1985) 335. 9 A. C. Calokerinos and T. P. Hadjiioannou, Anal. Chim. Acta, 148 (1983) 277. 10 M. A. Abdalla, A. G. Fogg and C. Burgess, Analyst (London), 107 (1982) 213. 11 A. G. Fogg, M. A. Abdalla and H. P. Henriques, Analyst (London), 107 (1982) 449.
Short Communication
CONTINUOUS-FLOW FOR SULPHIDE
MOLECULAR
EMISSION CAVITY ANALYSIS
N. GREKAS and A. C. CALOKERINOS* Laboratory of Analytical 106 80 Athens (Greece)
Chemistry,
Depa@ment
of Chemistry,
University
of Athens,
(Received 18th January 1985)
Summary. Sulphide (l-10 rg ml-‘) is determined by mixing the sample with an excess of orthophosphoric acid in a segmented continuous-flow system. The hydrogen sulphide evolved is swept into the cavity for generation of S, emission. The analysis is completely automated, requires no sample pretreatment and samples can be analyzed at 24 h-l.
Sulphide can be determined by conventional molecular emission cavity analysis (m.e.c.a.) in aqueous solution [l] or in binary and some ternary mixtures [2]. Sulphide has also been determined in mixtures with sulphate, thiocyanate and thiosulphate after sequential selective removal of ions and oxidation of the remainder by hydrogen peroxide to sulphate [3] . The m.e.c.a. technique also allows the determination of sulphide in the presence of other sulphur compounds in solids [4, 51. Alternatively, sulphide can be determined in a vaporization system in which the sample solution is acidified and the hydrogen sulphide evolved is continuously purged by nitrogen to the m.e.c.a. cavity for generation of SZ emission [6]. Furthermore, the anion can be determined by flow injection introduction of the solution into the cavity [ 71. Recently, a continuous-flow molecular emission cavity analyzer was described [ 81. The sample solution is mixed with aqueous orthophosphoric acid solution and is carried by a segmented continuous-flow stream through a heated reaction coil into a debubbler where the gas evolved is swept by nitrogen into the cavity. This system was successfully used for the determination of sulphite in various samples and of sulphur dioxide in atmospheric air after fixation as disulphitomercurate(I1) [ 81. This communication describes the advantages of the system for determining l-10 c(g ml-’ sulphide. Expertmen tal Apparatus. The photometer,
m.e.c.a. cavity and continuous-flow system were as described previously [ 8 1. Reagents. All reagents were of analytical grade, and deionizeddistilled water was used throughout. A sulphide stock solution (500 pg ml-‘) was 0003-2670/85/$03.30
0 1985 Elsevier Science Publishers B.V.
312
prepared by dissolving 1.87 g of Na,S - 9Hz0 (Ferak Berlin, p.a.) in 500 ml of 0.1 M sodium hydroxide/O.01 M EDTA. The exact sulphide concentration was established iodimetrically. More dilute solutions were prepared in 0.02 M sodium hydroxide/2 X 10” M EDTA by the fewest dilution steps possible. All sulphide solutions were prepared daily. Procedure. All experimental parameters were used as optimized for sulphite [8]. The cavity was conditioned by allowing 0.5-1.0 ml of stock sulphide solution to enter into the continuous flow system and generate intense Sz emission within the cavity. After re-establishment of the baseline, the determination was done as for sulphite [ 81. Results and discussion Analytical parameters. Figure 1 shows a typical recording for a series of sulphide standards by the proposed procedure. The calibration graph, emission intensity (I, mV) vs. pg ml” sulphide (C), was sigmoidal [9] and the log I vs. log C calibration graph was linear over the range l-10 pg ml-’ sulphide (log I = -0.420 + 1.72 log C, r = 0.9996). The relative standard deviation for the slope of the log-log graph for six working days was 2.2%. The limit of detection (signal/noise = 2) was 0.5 pg ml-’ sulphide and the relative standard deviations for 2 and 4 ,ug ml-’ sulphide were 2% and 0.6% (n = 8), respectively. When aqueous solutions of sulphide (l-10 pg ml-‘) were processed, the average error was +1.5%. Under the same experimental conditions, determination of sulphite (log I = -1.10 + 1.31 log C) [8] was less sensitive than that of sulphide. Interferences. Interferences from anions were investigated by determining 10 pg ml-’ sulphide in the presence of equal and fivefold (by wt.) amounts of other anion. The responses were compared with those obtained from an uncontaminated sulphide solution. No effect was observed from thiosulphate,
Fig. 1. Typical recording for a series of sulphide standards.
313
thiocyanate, cyanide, iodide, bromide or carbonate. Nitrite seriously decreased the intensity (Table 1). Reduction of nitrite to nitrogen by sulphamic acid was investigated for eliminating the interference. Aqueous solutions of sulphamic acid introduced into the analyzer did not produce any molecular emission within the cavity and left the emission intensity from pure sulphide solutions unaffected. A 0.02% (w/v) solution of sulphamic acid completely eliminated the interference from equal concentrations of nitrite on 1.0 and 5.0 pg ml-’ sulphide. The effect of an equal concentration of nitrite on 10 Mg ml-’ of sulphide was eliminated by 0.04% (w/v) sulphamic acid. The interference of a fivefold concentration of nitrite on sulphide was only partially eliminated by sulphamic acid. These results are shown in Table 1. Sulphite interferes severely with the determination of sulphide. The emission intensities from 5.0 pg ml-’ sulphide and 10 pg ml” sulphite were 6.0 mV and 1.6 mV, respectively, while the emission intensity from the mixture was 9.2 mV, as would be expected from the sigmoidal nature of the calibration graph. Various attempts were made to eliminate the interference of sulphite on the measurement of sulphide. Tannic acid completely removes the emission of sulphite [8] but 2 X 10J and 5 X lo”% (w/v) tannic acid also decreased the intensity from 8 gg ml” sulphide by 43% and 86%, respectively. Likewise, 0.01% and 0.02% (w/v) glyoxal decreased the intensity from 8 E.cgml-’ sulphite by 80% and lOO%, respectively, because of the formation of the addition compound. The same concentrations of glyoxal had no effect on the intensity from aqueous sulphide solutions. Nevertheless, when 0.02% (w/v) glyoxal was added to a mixture of sulphide and sulphite, the emission intensity was unaffected. This is probably due to disproportionation of glyoxal in the alkaline solution required for stabilization of sulphide. TABLE
1
Effect of nitrite on the emission intensity from sulphide, and elimination of the interference by addition of sulphamic acid Concentration Sulphide
(fig ml-‘) Nitrite
Emission intensity* Sulphamic added acid (%) (w/v) 0.00
10.0 10.0 10.0 5.0 5.0 5.0 1.0 1.0 1.0
0 10 50 0 5.0 25 0 1.0 5.0
100 35.3 2.9 100 35.0 0 100 66.7
0.02
88.2 73.5
0.04
100 88.2b
100 80.0b 100 80.0b
aIntensities from pure sulphide solutions arbitrarily taken as 100. bNo further decrease of interference was observed at higher sulphamic acid concentrations.
314 TABLE 2 Alkaline degradation of organic sulphur compounds Compound
Cont. (mg ml-‘)
Cysteamine * HCl N-Acetyl-L-cysteine Thiodiacetic acid Thioglycollic Cephalexin
acid
1.86 2.55 0.64 13.3 0.10 0.25
to sulphide in 1 M sodium hydroxide
Sulphide sulphur (rg ml-‘) Theoretical
524.5 500.4 137.0 4620 9.21 23.02
Found after hydrolysis? at: 30°C
100” c
0(75) O(60) 4.32(45) 4.91(95) 3.58(60) O(30) O(30)
1.51(75) 0.96( 60) 5.72(60) 6.21( 100) 14.88(60) 5.99(30) 14.93(30)
aThe times of hydrolysis (min) are given in parentheses.
Alkaline degradation of organic sulphur compounds. The simplicity and sensitivity of the method are useful for the determination of sulphide produced by alkaline degradation of various organic sulphur-containing compounds in 1 M sodium hydroxide (Table 2). Cephalexin was found to degrade completely after 30 min at 100°C and the sulphide yields for 0.10 and 0.25 mg ml-’ were 65.0% and 64.9%, respectively, which are in good agreement with results obtained by Fogg et al. [lo, 111. The method was also used to determine the relatively small amounts of sulphide produced by decomposition of the other compounds. REFERENCES 1 R. Belcher, S. L. Bogdanski, D. J. Knowles and A. Townshend, Anal. Chim. Acta, 77 (1975) 53. 2 M. Q. Al-Abachi, R. Belcher, S. L. Bogdanski and A. Townshend, Anal. Chim. Acta, 86 (1976) 139. 3T. S. Al-Ghabsha, Ph.D. Thesis, Birmingham University, 1979; A. C. Calokerinos and A. Tow&rend, Prog. Anal. At. Spectrosc., 5 (1982) 63. 4 S. A, Schubert, J. W. Clayton and Q. Fernando, Anal. Chem., 52 (1980) 963. 5 J. H. Tzeng and Q. Fernando, Anal. Chem., 54 (1982) 971. 6 E. Henden, Ph.D. Thesis, Birmingham University, 1976; E. Henden, N. Pourreza and A. Townshend, Prog. Anal. At. Spectrosc., 2 (1979) 355. 7 J. L. Burguera and M. Burguera, Anal. Chim. Acta, 157 (1984) 177. 8 N. Grekas and A. C. Calokerinos, Analyst (London), 110 (1985) 335. 9 A. C. Calokerinos and T. P. Hadjiioannou, Anal. Chim. Acta, 148 (1983) 277. 10 M. A. Abdalla, A. G. Fogg and C. Burgess, Analyst (London), 107 (1982) 213. 11 A. G. Fogg, M. A. Abdalla and H. P. Henriques, Analyst (London), 107 (1982) 449.