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Selective determination of orthophosphate and total inorganic phosphates in detergents by flow injection photometric method
Talanta 52 (2000) 211 – 216 www.elsevier.com/locate/talanta
Selective determination of orthophosphate and total inorganic phosphates in detergents by flow injection photometric method Liu Jing-fu, Jiang Gui-bin * Research Center for Eco-En6ironmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, People’s Republic of China Received 27 July 1999; received in revised form 24 January 2000; accepted 3 February 2000
Abstract A flow injection photometric system was developed for the determination of orthophosphate and total inorganic phosphates in detergents. While orthophosphate was directly determined in the presence of other phosphates by utilizing the kinetic discrimination of flow injection analysis, total inorganic phosphates was analyzed after on-line hydrolysis of polyphosphates in 2.5 mol l − 1 sulfuric acid for 50 s under 145°C. Sodium dodecyl sulfate (SDS) was added to mask the interference of non-ionic surfactants. The detection limits and the sampling rates were 2.5 mg l − 1 P2O5 and 40 h − 1 for total inorganic phosphates, and 1.0 mg l − 1 P2O5 and 80 h − 1 for orthophosphate determination. The proposed method was applied to analyze orthophosphate and total inorganic phosphates in washing powders. The experimental results are in good agreement with those obtained by the Chinese national standard methods. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Flow injection; Spectrophotometry; On-line hydrolysis; Orthophosphate; Total inorganic phosphates; Condensed phosphates; Washing powders
1. Introduction Phosphates had been widely used as assistant detergents. Though it is considered to be related to the eutrophication of waters and was forbidden or limited to use in detergents in many places, phosphates are still used in detergents especially in developing countries. The determination of * Corresponding author. Fax: +86-10-62923563. E-mail address: [email protected] (J. Gui-bin)
phosphorus in detergents is of great importance in product control and environmental protection. Conventional methods for determining orthophosphate and total inorganic phosphates in detergents are very tedious and time-consuming. In these methods, orthophosphate was determined by spectrophotometric method after constitutes separating by ion-exchange column chromatography [1], and total inorganic phosphates was determined by spectrophotometric [2] or gravimetric [3] methods after hydrolyzing the polyphosphates of
0039-9140/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 0 0 ) 0 0 3 3 7 - 4
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detergents into orthophosphates by heating in acid solution. Continuous flow analysis (CFA) has been successfully applied to determine orthophosphate and total inorganic phosphates in detergents. Lundgren [4] developed a CFA method to determine orthophosphate and total phosphate with a sampling rate of 20 h − 1, but 10 or 20 min elapse before the results of the orthophosphates or total inorganic phosphates of the first sample appears. Later, Tsuda et al. [5] proposed a CFA method to determine total phosphate in detergent with a sampling rate of 40 h − 1, but samples must be fused with sodium carbonate for 30 min before determination. Due to the relatively complex system and expensive instrumentation, the usage of these methods is limited. Flow injection (FI) determination of orthophosphate has been well developed since its first reporting by Ruzicka et al. [6], it is the on-line conversion of condensed phosphate into orthophosphate that is difficult. Though FI systems that can on-line hydrolyse condensed phosphate were adopted as post-column detector for HPLC analysis of inorganic phosphate [7–9], and FI methods for the determination of total phosphorus in natural waters [10] or waste waters [11] were also reported, the study on FI determination of phosphate in detergents is very limited. Fogg et al. [12] reported a FI-voltammetric procedure for the determination of total phosphate in washing powders, but the samples should be heated for 1 h to hydrolyze the polyphosphates into orthophosphate. Alexander et al. [13] developed a FI method for the determination of phosphate species in detergents with a calcium ion-selective electrode, but the different phosphate species interfere each other and it is difficult to be applied to implement routine analysis. In this study, a low-cost and effective FI-photometric system was developed to determine total inorganic phosphates and orthophosphate in detergents. Total inorganic phosphates was determined by the molybdenum procedure after it was on-line hydrolyzed to orthophosphate. While orthophosphate in detergents was directly determined utilizing the kinetic discrimination of FIA, i.e. orthophosphate was determined by the molyb-
denum blue method in only about 30 s before coexisting condensed phosphates were hydrolyzed to orthophosphate under acid condition.
2. Experimental
2.1. Reagents All chemicals were analytical grade and distilled water was used throughout. Standard stock solution (2000 mg l − 1 P2O5) was prepared by dissolving potassium dihydrogen orthophosphate (KH2PO4) in water. Reagents for total inorganic phosphates determination: working standards, containing 2.5 mol l − 1 H2SO4, were obtained by appropriated dilutions of the stock solution and 10 mol l − 1 H2SO4 with water. Carrier solution (C) was consisted of 1.5% (m/v) ammonium molybdate ((NH4)6Mo7O24 · 4H2O) and 2% (m/v) potassium tartrate (C6H4O6K2 · 1/2H2O) in 0.2 mol l − 1 H2SO4. Reagent (R) was prepared daily, it contains 1.2% (m/v) ascorbic acid and 0.1% (m/v) sodium dodecyl sulfate (SDS). Reagents for orthophosphate determination: working standards were obtained by appropriated dilutions of the stock solution with water. Carrier solution (C) was consisted of 1.5% (m/v) ammonium molybdate ((NH4)6Mo7O24 · 4H2O), 2% (m/ v) potassium tartrate (C6H4O6K2 · 1/2H2O) and 0.1% (m/v) antimonyl potassium tartrate (K(SbO)C4H4O6 · 1/2H2O) in 0.2 mol l − 1 H2SO4. Reagent (R) was prepared daily, it contains 1.2% (m/v) ascorbic acid and 0.1% (m/v) SDS.
2.2. Apparatus The FI system, shown in Fig. 1, consists of a peristaltic pump, a sample injection valve, a thermostat, a self-made photometer, a computer and a laboratory-made high temperature reactor. Selfdeveloped software was installed in the computer to administer the whole system from introducing samples into the system to calculation of the results. When total inorganic phosphates was determined, standard or sample solutions, containing 2.5 mol l − 1 sulfuric acid, were introduced into
L. Jing-fu, J. Gui-bin / Talanta 52 (2000) 211–216
the high temperature reaction coil (HTRC) in which condensed phosphates were hydrolyzed in a 450 cm long and 0.8 mm i.d. polytetrafluoroethylene (PTFE) coil heated to 145°C. This coil is wrapped around a 45 mm o.d. metal tube in which an electrical heating rod is inserted. The temperature is controlled at 1459 3°C with a OMRON E5CS-R1PX temperature controller. The hydrolyzed sample solutions were cooled to 40°C after passing through a 40 cm long and 0.8 mm i.d. PTFE cooling coil (CC), and were debubbled with a T-shaped three way connector before entering into the sample injection valve. Samples were injected into ammonium molybdate solution (C) and confused with ascorbic acid (R) to take chromogenic reaction in an 80 cm long and 0.8 mm i.d. color reaction coil (CRC). The cooling coil (CC) and the color reaction coil (CRC) were placed in 40 90.1°C recycled water supplied by a thermostat. Although absorbance at 880–890 or 710 nm is generally used in the molybdenum method, 620 nm wavelength was adopted in this study as it is the longest wavelength the self-developed photometer provided. On the other hand, the method sensitivity under this wavelength is suitable for route analysis of total inorganic phosphates in washing powders. For the determination of orthophosphate, the hydrolysis system (including the 450 cm long heating coil, the 40 cm long cooling coil and the T-shaped debubbler) was omitted and samples were directly injected into the color reaction system. The constitutes of the carrier solution was also different from those for the determination of total inorganic phosphates.
Fig. 1. Manifold optimized for the determination of orthophosphate and total inorganic phosphates. P, peristaltic pump; S, sample solution; C, carrier solution; R, reagent; HTRC, high temperature reaction coil (450 cm × 0.8 mm i.d., 145°C); CC, cooling coil (40 cm × 0.8 mm i.d., 40°C); DB, debubbler; CRC, color reaction coil (80 cm × 0.8 mm i.d., 40°C); D, photometric detector (620 nm); W, waste.
213
2.3. Sample preparation Sample solution was prepared by dissolving 1.000 g of washing powders in 500 ml of water. This solution was used to directly determine orthophosphate or after filtering if necessary. For the determination of total inorganic phosphates, appropriate amount of sample solution and 10 mol l − 1 H2SO4 were diluted to containing 10–120 mg l − 1 P2O5 and 2.5 mol l − 1 H2SO4.
3. Results and discussion
3.1. Hydrolysis of condensed phosphates The key procedure for quick determination of total inorganic phosphates is to hydrolyze condensed phosphates quantitatively and quickly. Experiments show that the hydrolysis of tripolyphosphate, the major phosphate in detergent, is significantly influenced by the temperature and acid concentration. Keeping the temperature at 90°C, the hydrolysis rate of tripolyphosphate increase with the increasing of H2SO4 concentration until 10 mol l − 1. Higher concentration of H2SO4 was not tried as its high viscosity and its harmful effect to the pump tube (Tygon). Though tripolyphosphate can be hydrolyzed completely in about 25 s under 90°C and 9 mol l − 1 H2SO4, 2.5 mol l − 1 H2SO4 was adopted as the high viscosity of higher concentration of sulfuric acid results in serious containment between samples and the requirement of long washing time. To investigate the hydrolysis efficiency of the proposed FI online hydrolysis system, two solutions of a typical heavy duty washing powder were determined under different temperature. Both of them contain the same washing powder concentration and 2.5 mol l − 1 H2SO4, but one was determined after boiling 40 min in 0.5 mol l − 1 H2SO4 to quantitatively hydrolyze the condensed phosphates, while another was determined immediately after mixing the sample and the sulfuric acid solutions. Results shown in Fig. 2 demonstrate that the hydrolysis rates of condensed phosphates in detergents increase with the increasing of temperature in the range of 50–130°C but keeps unchanging until
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Fig. 2. Effect of temperature on the hydrolysis of polyphosphates in washing powders. Using the manifold for total inorganic phosphates determination as shown in Fig. 1. The sulfuric acid concentration in samples were 2.5 mol l − 1. 2, manual pre-hydrolyzed; , on-line hydrolyzed.
160°C. Fig. 2 also indicates that condensed phosphates in detergents can be hydrolyzed quantitatively at above 130°C as equal signals were obtained for these two solutions. The hydrolysis of potassium pyrophosphate and sodium tripolyphosphate in the proposed on-line hydrolysis system under 145°C was studied in a similar way, results show that the signal ratios of without and with pre-hydrolysis for the pyrophosphate and tripolyphosphate were 1.0590.02 and 1.029 0.02, respectively. Introducing a color solution in the on-line hydrolysis system show that it takes about 50 s for the color solution to pass through the on-line hydrolysis system. Therefore, it can be concluded that the condensed phosphates in detergents could be hydrolyzed quantitatively in 50 s under 145°C and 2.5 mol l − 1 H2SO4.
3.2. Chromogenic reaction of orthophosphate Two commonly used spectrophotometric methods for the orthophosphate determination, the method based on the blue color of reduced phosphomolybdate solution and the method based on the yellow color given with a vanadomolybdate reagent, were tried in this study. While the former one is seriously interfered by silicate, the later one
is seriously interfered by non-ionic surfactants. The molybdenum blue method is adopted in this study as its higher sensitivity and the interference of silicate can be easily eliminated by adding tartrate in the molybdate solution (C). Ascorbic acid was used as reduce reagent for both the total inorganic phosphates determination and the orthophosphate determination, but antimonyl potassium tartrate was used as catalyst in the orthophosphate determination because the orthophosphate concentration in detergents is relatively low. Potassium tartrate and SDS were added in C and R, respectively, to eliminate the interference of silicate and non-ionic surfactants. The concentration of ammonium molybdate, potassium tartrate, ascorbic acid, antimonyl potassium tartrate, SDS and sulfuric acid were optimized utilizing the univariant method and results were given in reagents preparation described above. As the molybdenum blue method is temperature dependent, the color reaction was controlled at 409 0.1°C with a thermostat. Under the optimized conditions, the interference of some major coexisting substance in washing powders was studied. Results showed that for the determination of 20 mg l − 1 P2O5, at least 100 mg l − 1 sodium dodecylbenzenesulfonate and SiO2, 400 mg l − 1 Triton X-100, 1000 mg l − 1 sodium sulfate, sodium carbonate, sodium chloride and sodium peroxocarbonate did not interfere the determination.
3.3. Characteristics of the methods Using the optimized manifold, some characteristics of these two methods such as linear range, correlation coefficient, detection limit, R.S.D. and sample throughputs were determined and the results were shown in Table 1. Correlation coefficients were obtained by determining five orthophosphate standards covering the linear ranges. R.S.D. were measured by repeated injection of a sample containing 50 mg l − 1 P2O5 (for total inorganic phosphates determination) or 5 mg l − 1 P2O5 (for orthophosphate determination). Detection limits were calculated as 3s above the
L. Jing-fu, J. Gui-bin / Talanta 52 (2000) 211–216 Table 1 Analytical characteristics of the proposed flow injection (FI) methods Analyte
Orthophosphate
Linear range (mg l−1 P2O5) Correlation coefficient Detection limit (mg l−1 P2O5) R.S.D. (n = 11) (%) Sample throughput (h−1)
Total inorganic phosphates
0–20
0–120
0.9999
0.9993
1.0
2.5
1.2 80
2.0 40
blank value (where s is the S.D. (n = 11) for a blank solution).
3.4. Sample analysis Orthophosphate and total inorganic phosphates in five samples of heavy duty washing powders were determined using the proposed methods and the Chinese national standard method. Results were shown as P2O5% for consistency with traditional methods [3 – 5]. As results shown in Table 2, the errors between these two methods are relatively high, but the orthophosphate results were acceptable according to the Chinese national standard method [2]. Two reasons might be responsible for the large errors of the total inorganic phosphates results. One is that the reproduction of the manual ion exchange column chromatography method is not very good and the total inor-
215
Table 3 Comparison of total inorganic phosphates concentrations in washing powders obtained by the proposed flow injection (FI) and the standard methoda Sample
1 2 3 4 5 6 7 8 9 10 11 12 13 14 a
Total inorganic phosphates (P2O5%) FI
Standard
Error (%)
9.73 9.48 9.63 9.63 8.34 9.24 8.30 9.36 9.98 13.77 17.85 26.80 19.86 25.16
9.59 9.50 9.37 9.46 8.25 9.11 8.36 9.51 10.08 13.84 17.79 27.20 19.33 25.46
1.4 −0.3 2.8 1.8 1.0 1.4 −0.7 −1.5 −1.0 −0.5 −0.4 −1.4 2.7 −1.2
Manual quinoline phosphomolybdate gravimetric method
[3].
ganic phosphates content was obtained from the sum of different forms of phosphate. Another one is that higher results were obtained when condensed phosphates were on-line hydrolyzed in the proposed FI procedure according to the above study. Therefore, the total inorganic phosphates in another 14 samples of washing powders were analyzed by the proposed FI method and the conventional gravimetric method. Table 3 demonstrates that the results obtained by these two methods agree very well. The regression equation
Table 2 Comparison of orthophosphate and total inorganic phosphates concentrations in washing powders obtained by the proposed flow injection (FI) and the standard methoda Sample
1 2 3 4 5 a
Orthophosphate (P2O5%)
Total inorganic phosphates (P2O5%)
FI
Standard
Error (%)
FI
Standard
Error (%)
0.66 0.24 0.77 0.72 0.63
0.63 0.25 0.82 0.70 0.58
4.8 −4.0 −6.1 2.8 8.6
11.54 7.36 12.38 9.16 10.90
11.21 7.04 11.92 9.04 10.49
2.9 4.5 3.8 1.3 3.9
Manual ion exchange column chromatography method [2].
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between these two sets of results was C (total inorganic phosphates by FI, P2O5%) = 0.9859C (total inorganic phosphates by gravimetric, P2O5%)+ 0.2080, and the correlation coefficient was g= 0.9994. Paired t-test shows that t= 0.70B t0.05(13) =2.16, which means that there were no significant difference between two methods.
Acknowledgements This work was jointly supported by the Chinese Academy of Sciences under Contract No.KZ951B1-209 and the National Natural Science Foundation of China under Contract No.29825114.
References [1] JIS K3362-1978, Test methods for detergents.
.
[2] Chinese National Standard GB/T 13173.4-91, Separation and determination of different forms of phosphate in detergents-Ion exchange column chromatography. [3] Chinese National Standard GB 9984.2-88, Sodium tripolyphosphate for industrial use — determination of total phosphorus pentoxide content — quinoline phosphomolybdate gravimetric method. [4] D.P. Lundgren, Anal. Chem. 32 (7) (1960) 824. [5] T. Tsuda, S. Takano, K. Furuya, K. Kunnihiro, J. Jpn. Oil Chem. Soc. 25 (3) (1976) 156. [6] J. Ruzicka, E.H. Hansen, Anal. Chim. Acta 78 (1975) 145. [7] Y. Hirai, N. Yoza, S. Ohashi, Anal. Chim. Acta 115 (1980) 269. [8] Y. Hirai, N. Yoza, S. Ohashi, J. Chromatogr. 206 (1981) 501. [9] R.S. Brazell, R.W. Holmberg, J.H. Moneyhun, J. Chromatogr. 290 (1984) 163. [10] M. Aoyagi, Y. Yasumasa, A. Nishida, Anal. Chim. Acta 214 (1988) 29. [11] R.L. Benson, I.D. Mckelvie, B.T. Hart, Anal. Chim. Acta 291 (1994) 233. [12] A.G. Fogg, G.C. Cripps, B.J. Birch, Analyst 108 (1983) 1485. [13] P.W. Alexander, J. Koopetngarm, Anal. Chim. Acta 197 (1987) 353.
Selective determination of orthophosphate and total inorganic phosphates in detergents by flow injection photometric method Liu Jing-fu, Jiang Gui-bin * Research Center for Eco-En6ironmental Sciences, Chinese Academy of Sciences, P.O. Box 2871, Beijing 100085, People’s Republic of China Received 27 July 1999; received in revised form 24 January 2000; accepted 3 February 2000
Abstract A flow injection photometric system was developed for the determination of orthophosphate and total inorganic phosphates in detergents. While orthophosphate was directly determined in the presence of other phosphates by utilizing the kinetic discrimination of flow injection analysis, total inorganic phosphates was analyzed after on-line hydrolysis of polyphosphates in 2.5 mol l − 1 sulfuric acid for 50 s under 145°C. Sodium dodecyl sulfate (SDS) was added to mask the interference of non-ionic surfactants. The detection limits and the sampling rates were 2.5 mg l − 1 P2O5 and 40 h − 1 for total inorganic phosphates, and 1.0 mg l − 1 P2O5 and 80 h − 1 for orthophosphate determination. The proposed method was applied to analyze orthophosphate and total inorganic phosphates in washing powders. The experimental results are in good agreement with those obtained by the Chinese national standard methods. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Flow injection; Spectrophotometry; On-line hydrolysis; Orthophosphate; Total inorganic phosphates; Condensed phosphates; Washing powders
1. Introduction Phosphates had been widely used as assistant detergents. Though it is considered to be related to the eutrophication of waters and was forbidden or limited to use in detergents in many places, phosphates are still used in detergents especially in developing countries. The determination of * Corresponding author. Fax: +86-10-62923563. E-mail address: [email protected] (J. Gui-bin)
phosphorus in detergents is of great importance in product control and environmental protection. Conventional methods for determining orthophosphate and total inorganic phosphates in detergents are very tedious and time-consuming. In these methods, orthophosphate was determined by spectrophotometric method after constitutes separating by ion-exchange column chromatography [1], and total inorganic phosphates was determined by spectrophotometric [2] or gravimetric [3] methods after hydrolyzing the polyphosphates of
0039-9140/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 9 - 9 1 4 0 ( 0 0 ) 0 0 3 3 7 - 4
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L. Jing-fu, J. Gui-bin / Talanta 52 (2000) 211–216
detergents into orthophosphates by heating in acid solution. Continuous flow analysis (CFA) has been successfully applied to determine orthophosphate and total inorganic phosphates in detergents. Lundgren [4] developed a CFA method to determine orthophosphate and total phosphate with a sampling rate of 20 h − 1, but 10 or 20 min elapse before the results of the orthophosphates or total inorganic phosphates of the first sample appears. Later, Tsuda et al. [5] proposed a CFA method to determine total phosphate in detergent with a sampling rate of 40 h − 1, but samples must be fused with sodium carbonate for 30 min before determination. Due to the relatively complex system and expensive instrumentation, the usage of these methods is limited. Flow injection (FI) determination of orthophosphate has been well developed since its first reporting by Ruzicka et al. [6], it is the on-line conversion of condensed phosphate into orthophosphate that is difficult. Though FI systems that can on-line hydrolyse condensed phosphate were adopted as post-column detector for HPLC analysis of inorganic phosphate [7–9], and FI methods for the determination of total phosphorus in natural waters [10] or waste waters [11] were also reported, the study on FI determination of phosphate in detergents is very limited. Fogg et al. [12] reported a FI-voltammetric procedure for the determination of total phosphate in washing powders, but the samples should be heated for 1 h to hydrolyze the polyphosphates into orthophosphate. Alexander et al. [13] developed a FI method for the determination of phosphate species in detergents with a calcium ion-selective electrode, but the different phosphate species interfere each other and it is difficult to be applied to implement routine analysis. In this study, a low-cost and effective FI-photometric system was developed to determine total inorganic phosphates and orthophosphate in detergents. Total inorganic phosphates was determined by the molybdenum procedure after it was on-line hydrolyzed to orthophosphate. While orthophosphate in detergents was directly determined utilizing the kinetic discrimination of FIA, i.e. orthophosphate was determined by the molyb-
denum blue method in only about 30 s before coexisting condensed phosphates were hydrolyzed to orthophosphate under acid condition.
2. Experimental
2.1. Reagents All chemicals were analytical grade and distilled water was used throughout. Standard stock solution (2000 mg l − 1 P2O5) was prepared by dissolving potassium dihydrogen orthophosphate (KH2PO4) in water. Reagents for total inorganic phosphates determination: working standards, containing 2.5 mol l − 1 H2SO4, were obtained by appropriated dilutions of the stock solution and 10 mol l − 1 H2SO4 with water. Carrier solution (C) was consisted of 1.5% (m/v) ammonium molybdate ((NH4)6Mo7O24 · 4H2O) and 2% (m/v) potassium tartrate (C6H4O6K2 · 1/2H2O) in 0.2 mol l − 1 H2SO4. Reagent (R) was prepared daily, it contains 1.2% (m/v) ascorbic acid and 0.1% (m/v) sodium dodecyl sulfate (SDS). Reagents for orthophosphate determination: working standards were obtained by appropriated dilutions of the stock solution with water. Carrier solution (C) was consisted of 1.5% (m/v) ammonium molybdate ((NH4)6Mo7O24 · 4H2O), 2% (m/ v) potassium tartrate (C6H4O6K2 · 1/2H2O) and 0.1% (m/v) antimonyl potassium tartrate (K(SbO)C4H4O6 · 1/2H2O) in 0.2 mol l − 1 H2SO4. Reagent (R) was prepared daily, it contains 1.2% (m/v) ascorbic acid and 0.1% (m/v) SDS.
2.2. Apparatus The FI system, shown in Fig. 1, consists of a peristaltic pump, a sample injection valve, a thermostat, a self-made photometer, a computer and a laboratory-made high temperature reactor. Selfdeveloped software was installed in the computer to administer the whole system from introducing samples into the system to calculation of the results. When total inorganic phosphates was determined, standard or sample solutions, containing 2.5 mol l − 1 sulfuric acid, were introduced into
L. Jing-fu, J. Gui-bin / Talanta 52 (2000) 211–216
the high temperature reaction coil (HTRC) in which condensed phosphates were hydrolyzed in a 450 cm long and 0.8 mm i.d. polytetrafluoroethylene (PTFE) coil heated to 145°C. This coil is wrapped around a 45 mm o.d. metal tube in which an electrical heating rod is inserted. The temperature is controlled at 1459 3°C with a OMRON E5CS-R1PX temperature controller. The hydrolyzed sample solutions were cooled to 40°C after passing through a 40 cm long and 0.8 mm i.d. PTFE cooling coil (CC), and were debubbled with a T-shaped three way connector before entering into the sample injection valve. Samples were injected into ammonium molybdate solution (C) and confused with ascorbic acid (R) to take chromogenic reaction in an 80 cm long and 0.8 mm i.d. color reaction coil (CRC). The cooling coil (CC) and the color reaction coil (CRC) were placed in 40 90.1°C recycled water supplied by a thermostat. Although absorbance at 880–890 or 710 nm is generally used in the molybdenum method, 620 nm wavelength was adopted in this study as it is the longest wavelength the self-developed photometer provided. On the other hand, the method sensitivity under this wavelength is suitable for route analysis of total inorganic phosphates in washing powders. For the determination of orthophosphate, the hydrolysis system (including the 450 cm long heating coil, the 40 cm long cooling coil and the T-shaped debubbler) was omitted and samples were directly injected into the color reaction system. The constitutes of the carrier solution was also different from those for the determination of total inorganic phosphates.
Fig. 1. Manifold optimized for the determination of orthophosphate and total inorganic phosphates. P, peristaltic pump; S, sample solution; C, carrier solution; R, reagent; HTRC, high temperature reaction coil (450 cm × 0.8 mm i.d., 145°C); CC, cooling coil (40 cm × 0.8 mm i.d., 40°C); DB, debubbler; CRC, color reaction coil (80 cm × 0.8 mm i.d., 40°C); D, photometric detector (620 nm); W, waste.
213
2.3. Sample preparation Sample solution was prepared by dissolving 1.000 g of washing powders in 500 ml of water. This solution was used to directly determine orthophosphate or after filtering if necessary. For the determination of total inorganic phosphates, appropriate amount of sample solution and 10 mol l − 1 H2SO4 were diluted to containing 10–120 mg l − 1 P2O5 and 2.5 mol l − 1 H2SO4.
3. Results and discussion
3.1. Hydrolysis of condensed phosphates The key procedure for quick determination of total inorganic phosphates is to hydrolyze condensed phosphates quantitatively and quickly. Experiments show that the hydrolysis of tripolyphosphate, the major phosphate in detergent, is significantly influenced by the temperature and acid concentration. Keeping the temperature at 90°C, the hydrolysis rate of tripolyphosphate increase with the increasing of H2SO4 concentration until 10 mol l − 1. Higher concentration of H2SO4 was not tried as its high viscosity and its harmful effect to the pump tube (Tygon). Though tripolyphosphate can be hydrolyzed completely in about 25 s under 90°C and 9 mol l − 1 H2SO4, 2.5 mol l − 1 H2SO4 was adopted as the high viscosity of higher concentration of sulfuric acid results in serious containment between samples and the requirement of long washing time. To investigate the hydrolysis efficiency of the proposed FI online hydrolysis system, two solutions of a typical heavy duty washing powder were determined under different temperature. Both of them contain the same washing powder concentration and 2.5 mol l − 1 H2SO4, but one was determined after boiling 40 min in 0.5 mol l − 1 H2SO4 to quantitatively hydrolyze the condensed phosphates, while another was determined immediately after mixing the sample and the sulfuric acid solutions. Results shown in Fig. 2 demonstrate that the hydrolysis rates of condensed phosphates in detergents increase with the increasing of temperature in the range of 50–130°C but keeps unchanging until
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Fig. 2. Effect of temperature on the hydrolysis of polyphosphates in washing powders. Using the manifold for total inorganic phosphates determination as shown in Fig. 1. The sulfuric acid concentration in samples were 2.5 mol l − 1. 2, manual pre-hydrolyzed; , on-line hydrolyzed.
160°C. Fig. 2 also indicates that condensed phosphates in detergents can be hydrolyzed quantitatively at above 130°C as equal signals were obtained for these two solutions. The hydrolysis of potassium pyrophosphate and sodium tripolyphosphate in the proposed on-line hydrolysis system under 145°C was studied in a similar way, results show that the signal ratios of without and with pre-hydrolysis for the pyrophosphate and tripolyphosphate were 1.0590.02 and 1.029 0.02, respectively. Introducing a color solution in the on-line hydrolysis system show that it takes about 50 s for the color solution to pass through the on-line hydrolysis system. Therefore, it can be concluded that the condensed phosphates in detergents could be hydrolyzed quantitatively in 50 s under 145°C and 2.5 mol l − 1 H2SO4.
3.2. Chromogenic reaction of orthophosphate Two commonly used spectrophotometric methods for the orthophosphate determination, the method based on the blue color of reduced phosphomolybdate solution and the method based on the yellow color given with a vanadomolybdate reagent, were tried in this study. While the former one is seriously interfered by silicate, the later one
is seriously interfered by non-ionic surfactants. The molybdenum blue method is adopted in this study as its higher sensitivity and the interference of silicate can be easily eliminated by adding tartrate in the molybdate solution (C). Ascorbic acid was used as reduce reagent for both the total inorganic phosphates determination and the orthophosphate determination, but antimonyl potassium tartrate was used as catalyst in the orthophosphate determination because the orthophosphate concentration in detergents is relatively low. Potassium tartrate and SDS were added in C and R, respectively, to eliminate the interference of silicate and non-ionic surfactants. The concentration of ammonium molybdate, potassium tartrate, ascorbic acid, antimonyl potassium tartrate, SDS and sulfuric acid were optimized utilizing the univariant method and results were given in reagents preparation described above. As the molybdenum blue method is temperature dependent, the color reaction was controlled at 409 0.1°C with a thermostat. Under the optimized conditions, the interference of some major coexisting substance in washing powders was studied. Results showed that for the determination of 20 mg l − 1 P2O5, at least 100 mg l − 1 sodium dodecylbenzenesulfonate and SiO2, 400 mg l − 1 Triton X-100, 1000 mg l − 1 sodium sulfate, sodium carbonate, sodium chloride and sodium peroxocarbonate did not interfere the determination.
3.3. Characteristics of the methods Using the optimized manifold, some characteristics of these two methods such as linear range, correlation coefficient, detection limit, R.S.D. and sample throughputs were determined and the results were shown in Table 1. Correlation coefficients were obtained by determining five orthophosphate standards covering the linear ranges. R.S.D. were measured by repeated injection of a sample containing 50 mg l − 1 P2O5 (for total inorganic phosphates determination) or 5 mg l − 1 P2O5 (for orthophosphate determination). Detection limits were calculated as 3s above the
L. Jing-fu, J. Gui-bin / Talanta 52 (2000) 211–216 Table 1 Analytical characteristics of the proposed flow injection (FI) methods Analyte
Orthophosphate
Linear range (mg l−1 P2O5) Correlation coefficient Detection limit (mg l−1 P2O5) R.S.D. (n = 11) (%) Sample throughput (h−1)
Total inorganic phosphates
0–20
0–120
0.9999
0.9993
1.0
2.5
1.2 80
2.0 40
blank value (where s is the S.D. (n = 11) for a blank solution).
3.4. Sample analysis Orthophosphate and total inorganic phosphates in five samples of heavy duty washing powders were determined using the proposed methods and the Chinese national standard method. Results were shown as P2O5% for consistency with traditional methods [3 – 5]. As results shown in Table 2, the errors between these two methods are relatively high, but the orthophosphate results were acceptable according to the Chinese national standard method [2]. Two reasons might be responsible for the large errors of the total inorganic phosphates results. One is that the reproduction of the manual ion exchange column chromatography method is not very good and the total inor-
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Table 3 Comparison of total inorganic phosphates concentrations in washing powders obtained by the proposed flow injection (FI) and the standard methoda Sample
1 2 3 4 5 6 7 8 9 10 11 12 13 14 a
Total inorganic phosphates (P2O5%) FI
Standard
Error (%)
9.73 9.48 9.63 9.63 8.34 9.24 8.30 9.36 9.98 13.77 17.85 26.80 19.86 25.16
9.59 9.50 9.37 9.46 8.25 9.11 8.36 9.51 10.08 13.84 17.79 27.20 19.33 25.46
1.4 −0.3 2.8 1.8 1.0 1.4 −0.7 −1.5 −1.0 −0.5 −0.4 −1.4 2.7 −1.2
Manual quinoline phosphomolybdate gravimetric method
[3].
ganic phosphates content was obtained from the sum of different forms of phosphate. Another one is that higher results were obtained when condensed phosphates were on-line hydrolyzed in the proposed FI procedure according to the above study. Therefore, the total inorganic phosphates in another 14 samples of washing powders were analyzed by the proposed FI method and the conventional gravimetric method. Table 3 demonstrates that the results obtained by these two methods agree very well. The regression equation
Table 2 Comparison of orthophosphate and total inorganic phosphates concentrations in washing powders obtained by the proposed flow injection (FI) and the standard methoda Sample
1 2 3 4 5 a
Orthophosphate (P2O5%)
Total inorganic phosphates (P2O5%)
FI
Standard
Error (%)
FI
Standard
Error (%)
0.66 0.24 0.77 0.72 0.63
0.63 0.25 0.82 0.70 0.58
4.8 −4.0 −6.1 2.8 8.6
11.54 7.36 12.38 9.16 10.90
11.21 7.04 11.92 9.04 10.49
2.9 4.5 3.8 1.3 3.9
Manual ion exchange column chromatography method [2].
216
L. Jing-fu, J. Gui-bin / Talanta 52 (2000) 211–216
between these two sets of results was C (total inorganic phosphates by FI, P2O5%) = 0.9859C (total inorganic phosphates by gravimetric, P2O5%)+ 0.2080, and the correlation coefficient was g= 0.9994. Paired t-test shows that t= 0.70B t0.05(13) =2.16, which means that there were no significant difference between two methods.
Acknowledgements This work was jointly supported by the Chinese Academy of Sciences under Contract No.KZ951B1-209 and the National Natural Science Foundation of China under Contract No.29825114.
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.
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