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A simple method for determining water content in organic solvents based on cobalt(II) complexes
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Chinese Chemical Letters 22 (2011) 189–192 www.elsevier.com/locate/cclet
A simple method for determining water content in organic solvents based on cobalt(II) complexes Lin Zhou a, Xiao Hua Liu b, Hai Xin Bai b,*, Hong Juan Wang a a
College of Plant Protection, Henan Agricultural University, Zhengzhou 450002, China b College of Sciences, Henan Agricultural University, Zhengzhou 450002, China Received 31 May 2010
Abstract A method to determine water content in organic solvents was developed based on the color change of cobalt(II) nitrate in different solvents. The color-change mechanism and optimal conditions for determining the water content were investigated. The results showed that there was a good linear relationships between the absorbance of cobalt(II) complexes in organic solvents and water contents with g in 0.99890.9994. This method has the advantages of low cost, good reproducibility, good sensitivity, simple in operation, fast in detection, friendly to the environment and no limitation on linear range for determining water content. It was used to determine water in samples with a satisfactory recovery in 97.81%101.24%. # 2010 Hai Xin Bai. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Determination; Water content; Cobalt(II) nitrate; Organic solvent
Water content often has a great importance on organic reactions, and even decides the reaction products, yield or selectivity. The Karl Fischer method [1], Oven-drying method and azeotropic distillation [2,3] are often employed in determining water content. But the methods have disadvantages such as low reaction velocity, being easily disturbed, bad precision, low sensitivity, time consuming, high cost or toxicity [4,5]. Water content is also determined by gas chromatography [6], liquid chromatography [7], fluorescent method [8] and infrared spectroscopic analysis [9], but these methods requires expensive or complicated apparatus [10]. In this work, a method for determining water content has been developed based on the relationship between water content and absorbance of cobalt(II) complexes in organic solvents. It has good reproducibility, simple in orperation, fast in detection, friendly to environment, harmless to people’s health and other virtues. 1. Experimental Cobalt(II) nitrate, ethanol, isopropanol, n-butanol and acetone were purchased from Beijing Chemical Plant of China. After being dehydrated for 3 h at 328 K, Co(NO3)2 was dissolved in organic solvent. The experimental water was secondary distilled water. All the other reagents were of analytical reagent grade. * Corresponding author. E-mail address: [email protected] (H.X. Bai). 1001-8417/$ – see front matter # 2010 Hai Xin Bai. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2010.09.015
[()TD$FIG] 190
L. Zhou et al. / Chinese Chemical Letters 22 (2011) 189–192 1.6
Absorbance
2.5
Absorbance
2.0 1.5
1.2 0.8 0.4 0.0
400
1.0
500
600
Wavelength(nm)
0.5 0.0 200
300
400
500
600
Wavelength(nm) Fig. 1. Absorption spectra of Co(NO3)2 in ethanol solution as a function of water content. The volume and concentration of Co(NO3)2 in ethanol solution were 3.00 mL and 2.587 102 molL1 respectively, the water content in ethanol solution of Co(NO3)2 varied from 0 to 0.01667 mol L1 as the arrow shown in the Figure.
A small amount of water or sample containing water was added into a quartz cuvette (1.0 cm pathlength) with a given amount of organic solvent solution of Co(NO3)2. After mixed thorougthly, the absorption spectrum of Co(NO3)2 solution was measured by TU-1800 UV-visible spectrophotometer from Beijing Purkinje General Instrument Co.,Ltd. 2. Results and discussion Similar characteristic absorption spectrum was obtained while Co(NO3)2 was dissolved in ethanol, isopropanol and n-butanol, respectively. Fig. 1 showed the absorption spectra of Co(NO3)2 in ethanol. Results indicated that the absorbance decreased as water content increased. Since acetone has a miminmu observable wavelength (i.e. 330 nm) for absorption spectrum while it was used as solvent, only spectrum with a absroption wavelength more than 330 nm were obtained. In in 400600 nm, the Co(NO3)2 spectra (Insert in Fig. 1) in acetone had similar characteristic to that of Co(NO3)2 in the other organic solvents. Results on qualitative spectroanalysis of different samples were showed in Table 1,which indicated that the spectra in 200350 and 400600 nm were derived from existence of NO3 and cobalt(II), respectively. Co(NO3)2 can form complex ions, Co(NO3)42 (mauve) and Co(H2O)62+ (pink) while it exist in orgnaic solvent, and water separately. After water being added in the organic solvents, the Co(NO3)42 partially dissociated into Co2+ and NO3. Co2+ is an electron-deficient ion (electron acceptor) and can react with electron-rich agent (electron donor) to form chargetransfer complex [11]. Water is an electron-rich compound and can react with chloranilic acid to form a pink complex, Co(H2O)62+. Consequently, the content of complex Co(H2O)62+ increased with the decrease of Co(NO3)42, and the solution color changed from mauve to pink, which agreed well with the observed experimental phenomena. The mechanism of color change can be expressed as follows, CoðNO3 Þ4 2 ¼ Co2þ þ 4NO3
(1)
6H2 O : þ Co2þ ¼ CoðH2 OÞ6 2þ
(2)
Table 1 The qualitative analysis for the absorption spectra of different samples. Determined sample 3.00 mL 3.00 mL 3.00 mL 3.00 mL
ethanol + 20 mL H2O ethanol + 20 mL H2O + 50 mL HNO3(0.02 mol/L) ethanol + 20 mL H2O + 50 mL Co(NO3)2 (0.02 mol/L) H2O + 50 mL Co(NO3)2 (0.02 mol/L)
Spectral peak number
Maximum absorption wavelength(nm)
0 2 3 3
– 241 243 239
– 281 291 301
– – 520 511
[()TD$FIG]
L. Zhou et al. / Chinese Chemical Letters 22 (2011) 189–192 2.5
0.56 0.48
Absorbance
2.0
Absorbance
191
1.5
0.40 0.32 0.24 0.16
1.0
0.0
0.4
0.8
450
500
1.2
1.6
Water content(mol/L)
2.0
0.5
0.0 200
250
300
350
400
550
600
Wavelength(nm) Fig. 2. Effect of time on the transformation reaction of cobalt(II) complexes in ethanol containing water. The 3.00 mL ethanol solution of Co(NO3)2 (2.713 102 molL1) and 35 mL water were used. The interval of spectral scan was 3 min. The spectral changes with time and temperature were shown as the arrows.
According to the aforesaid conclusion and results in Table 1, the peaks at 520 and 511 were the characteristic peaks of Co(NO3)42 and Co(H2O)62+. The absorbance decreased as the water content increased, which indicated that Co(H2O)62+ has a smaller molar absorptivity than that of Co(NO3)42. Thus, the absorption spectrum which has a absorption maximum around 520 nm was selected for determining the water content in organic solvents. The spectra were monitored during 0–24 min (Fig. 2). Fig. 2 illustrates that the spectra remained constant as time reached to 9 min, which indicated that the aforementioned complex transformation just need no more than 9 min to reach its chemical equilibrium. Since these resutls were obtained under the laboratory temperatures which could fluctuated with the changes of laboratory conditions. So the results proved that the temperature fluctuation had no effect on the spectra. Therefore, the determination of water content in organic solvents can not be affected by the fluctuation of room temperature. Under selected conditions, the absorbance at the maximum absorption wavelength of Co(NO3)2 (2.612 102 molL1) in the organic solvents was measured in the presence of different water contents. Take ethanol for example, the results showed that the absorbance of the cobalt(II) complexes at 520 nm(A520) nitrate decreased with the increment of water content (cw) in the organic solvent, and a good linear relationships (Insert in Fig. 2) between A520 and water content was obtained with g = 0.9994. While the other solvents were used, similar linear relationships were obtained and showed in Table 2. Results also proved that The linear range of the every regression equation could be enlarged by increasing the initial concentration of Co(NO3)2 in organic solvent. For the samples with water content beyond the linear range, the water content also could be determined by standard addition method addition method only if the water amount in the added sample do not exceeded the linear range. Therefore, the method could determine water content in organic solvent without limitation of linear range. The data of 12 parallel determinaton with absolute ethanol as sample were processed by statistical methods. The results showed that the method is highly sensitive and well reproducible based on 0.128 mol L1 as detection limit and 1.9% as the relative standard deviation (RSD). The water content in 95%(V/V) ethanol was analyzed based on the method developed in this study with the reagent blank as reference. The average value of water content for 8 parallel Table 2 The linear regression equations corresponding to different organic solvents. Organic solvent
Linear regression equation
related coefficient (g)
Ethanol Isopropanol n-Butanol Acetone
A520 = 0.5391–0.1954 cwater A525 = 1.186–0.5786 cwater A520 = 0.1377–0.2731 cwater A530 = 1.394–0.5096 cwater
0.9994 0.9993 0.9989 0.9992
192
L. Zhou et al. / Chinese Chemical Letters 22 (2011) 189–192
analytical results was 4.93% (V/V) with a 1.20% of RSD. Using similar pretreatment method, the water content in flour was determinted by the method developed in this work. The moisture of flour was determined with National Standard method of China [12] simultaneously. The moisture content in flour was 11.08%, RSD = 1.85%, Er was 1.73%. The recoveries were 97.81%101.24%. The results proved that the determined water content agreed well with the true value and this method was reliable. During the analysis, the concentration of Co(NO3)2 in the organic solvents was 3.33 102 molL1. 3. Conclusions This paper described a simple, rapid and steady-going method for determining water content in organic solvents based on the linear relationships between the absorbance at maximum absorption wavelength of cobalt(II) complexes in the organic solvents (i.e. ethanol, isopropanol, n-butanol and acetone) and water content. This method could determine water content in organic solvent without the limitations of its linear range. The results on determining water content in samples indicated that this method was satisfactory with a good sensitivity and good reproducibility. The development of this method has great importance in laboratory study and industrial production. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 20735003 and No. SKLEAC2010011) of State Key Laboratory of Electroanalytical Chemistry in China, the Projects (Nos. 30700348 and 30700349) Sponsored by the Scientific Research Foundation for the Doctors in Henan Agricultural University of China, as well as the support the Scientific and technological key project (Nos. 082102350006 and 102102310335) of Henan Province in China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
K. Fischer, Angew. Chem. 48 (1935) 394. S. Edward, D. Monica, T.F. Holden, J. Dairy Sci. 51 (1968) 40. M.K. Tian, X.L. Feng, F.L. Zhao, Chin. J. Anal. Lab. 26 (2007) 105. G.Z. Li, Y.M. Liu, Spect. Anal. 24 (2004) 122. X.L. Feng, N. Li, F.L. Zhao, Chin. J. Anal. Chem. 31 (2003) 981. R. Iguclfi, K. Yamaguchi, T. Shibamoto, J. Chromatogr. Sci. 26 (1988) 588. J. Chen, J. Fritz, J Chmmatogr 482 (1989) 279. W. Liu, P. He, Chin. J. Yantai University 12 (1999) 176. M.M. Tripathi, E.B.M. Hassan, F.Y. Yueh, Sensors and Actuators B: Chemical 136 (2009) 20. H.X. Bai, X.R. Yang, J. Chin. Chem. Soc. 54 (2007) 619. W.B. Zhou, H.K. Li, G.Z. Zhao, et al. Chin. J. Anal. Chem. 28 (2000) 783. National Standard of China (GB5009.3-85), 1985.
Chinese Chemical Letters 22 (2011) 189–192 www.elsevier.com/locate/cclet
A simple method for determining water content in organic solvents based on cobalt(II) complexes Lin Zhou a, Xiao Hua Liu b, Hai Xin Bai b,*, Hong Juan Wang a a
College of Plant Protection, Henan Agricultural University, Zhengzhou 450002, China b College of Sciences, Henan Agricultural University, Zhengzhou 450002, China Received 31 May 2010
Abstract A method to determine water content in organic solvents was developed based on the color change of cobalt(II) nitrate in different solvents. The color-change mechanism and optimal conditions for determining the water content were investigated. The results showed that there was a good linear relationships between the absorbance of cobalt(II) complexes in organic solvents and water contents with g in 0.99890.9994. This method has the advantages of low cost, good reproducibility, good sensitivity, simple in operation, fast in detection, friendly to the environment and no limitation on linear range for determining water content. It was used to determine water in samples with a satisfactory recovery in 97.81%101.24%. # 2010 Hai Xin Bai. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. Keywords: Determination; Water content; Cobalt(II) nitrate; Organic solvent
Water content often has a great importance on organic reactions, and even decides the reaction products, yield or selectivity. The Karl Fischer method [1], Oven-drying method and azeotropic distillation [2,3] are often employed in determining water content. But the methods have disadvantages such as low reaction velocity, being easily disturbed, bad precision, low sensitivity, time consuming, high cost or toxicity [4,5]. Water content is also determined by gas chromatography [6], liquid chromatography [7], fluorescent method [8] and infrared spectroscopic analysis [9], but these methods requires expensive or complicated apparatus [10]. In this work, a method for determining water content has been developed based on the relationship between water content and absorbance of cobalt(II) complexes in organic solvents. It has good reproducibility, simple in orperation, fast in detection, friendly to environment, harmless to people’s health and other virtues. 1. Experimental Cobalt(II) nitrate, ethanol, isopropanol, n-butanol and acetone were purchased from Beijing Chemical Plant of China. After being dehydrated for 3 h at 328 K, Co(NO3)2 was dissolved in organic solvent. The experimental water was secondary distilled water. All the other reagents were of analytical reagent grade. * Corresponding author. E-mail address: [email protected] (H.X. Bai). 1001-8417/$ – see front matter # 2010 Hai Xin Bai. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved. doi:10.1016/j.cclet.2010.09.015
[()TD$FIG] 190
L. Zhou et al. / Chinese Chemical Letters 22 (2011) 189–192 1.6
Absorbance
2.5
Absorbance
2.0 1.5
1.2 0.8 0.4 0.0
400
1.0
500
600
Wavelength(nm)
0.5 0.0 200
300
400
500
600
Wavelength(nm) Fig. 1. Absorption spectra of Co(NO3)2 in ethanol solution as a function of water content. The volume and concentration of Co(NO3)2 in ethanol solution were 3.00 mL and 2.587 102 molL1 respectively, the water content in ethanol solution of Co(NO3)2 varied from 0 to 0.01667 mol L1 as the arrow shown in the Figure.
A small amount of water or sample containing water was added into a quartz cuvette (1.0 cm pathlength) with a given amount of organic solvent solution of Co(NO3)2. After mixed thorougthly, the absorption spectrum of Co(NO3)2 solution was measured by TU-1800 UV-visible spectrophotometer from Beijing Purkinje General Instrument Co.,Ltd. 2. Results and discussion Similar characteristic absorption spectrum was obtained while Co(NO3)2 was dissolved in ethanol, isopropanol and n-butanol, respectively. Fig. 1 showed the absorption spectra of Co(NO3)2 in ethanol. Results indicated that the absorbance decreased as water content increased. Since acetone has a miminmu observable wavelength (i.e. 330 nm) for absorption spectrum while it was used as solvent, only spectrum with a absroption wavelength more than 330 nm were obtained. In in 400600 nm, the Co(NO3)2 spectra (Insert in Fig. 1) in acetone had similar characteristic to that of Co(NO3)2 in the other organic solvents. Results on qualitative spectroanalysis of different samples were showed in Table 1,which indicated that the spectra in 200350 and 400600 nm were derived from existence of NO3 and cobalt(II), respectively. Co(NO3)2 can form complex ions, Co(NO3)42 (mauve) and Co(H2O)62+ (pink) while it exist in orgnaic solvent, and water separately. After water being added in the organic solvents, the Co(NO3)42 partially dissociated into Co2+ and NO3. Co2+ is an electron-deficient ion (electron acceptor) and can react with electron-rich agent (electron donor) to form chargetransfer complex [11]. Water is an electron-rich compound and can react with chloranilic acid to form a pink complex, Co(H2O)62+. Consequently, the content of complex Co(H2O)62+ increased with the decrease of Co(NO3)42, and the solution color changed from mauve to pink, which agreed well with the observed experimental phenomena. The mechanism of color change can be expressed as follows, CoðNO3 Þ4 2 ¼ Co2þ þ 4NO3
(1)
6H2 O : þ Co2þ ¼ CoðH2 OÞ6 2þ
(2)
Table 1 The qualitative analysis for the absorption spectra of different samples. Determined sample 3.00 mL 3.00 mL 3.00 mL 3.00 mL
ethanol + 20 mL H2O ethanol + 20 mL H2O + 50 mL HNO3(0.02 mol/L) ethanol + 20 mL H2O + 50 mL Co(NO3)2 (0.02 mol/L) H2O + 50 mL Co(NO3)2 (0.02 mol/L)
Spectral peak number
Maximum absorption wavelength(nm)
0 2 3 3
– 241 243 239
– 281 291 301
– – 520 511
[()TD$FIG]
L. Zhou et al. / Chinese Chemical Letters 22 (2011) 189–192 2.5
0.56 0.48
Absorbance
2.0
Absorbance
191
1.5
0.40 0.32 0.24 0.16
1.0
0.0
0.4
0.8
450
500
1.2
1.6
Water content(mol/L)
2.0
0.5
0.0 200
250
300
350
400
550
600
Wavelength(nm) Fig. 2. Effect of time on the transformation reaction of cobalt(II) complexes in ethanol containing water. The 3.00 mL ethanol solution of Co(NO3)2 (2.713 102 molL1) and 35 mL water were used. The interval of spectral scan was 3 min. The spectral changes with time and temperature were shown as the arrows.
According to the aforesaid conclusion and results in Table 1, the peaks at 520 and 511 were the characteristic peaks of Co(NO3)42 and Co(H2O)62+. The absorbance decreased as the water content increased, which indicated that Co(H2O)62+ has a smaller molar absorptivity than that of Co(NO3)42. Thus, the absorption spectrum which has a absorption maximum around 520 nm was selected for determining the water content in organic solvents. The spectra were monitored during 0–24 min (Fig. 2). Fig. 2 illustrates that the spectra remained constant as time reached to 9 min, which indicated that the aforementioned complex transformation just need no more than 9 min to reach its chemical equilibrium. Since these resutls were obtained under the laboratory temperatures which could fluctuated with the changes of laboratory conditions. So the results proved that the temperature fluctuation had no effect on the spectra. Therefore, the determination of water content in organic solvents can not be affected by the fluctuation of room temperature. Under selected conditions, the absorbance at the maximum absorption wavelength of Co(NO3)2 (2.612 102 molL1) in the organic solvents was measured in the presence of different water contents. Take ethanol for example, the results showed that the absorbance of the cobalt(II) complexes at 520 nm(A520) nitrate decreased with the increment of water content (cw) in the organic solvent, and a good linear relationships (Insert in Fig. 2) between A520 and water content was obtained with g = 0.9994. While the other solvents were used, similar linear relationships were obtained and showed in Table 2. Results also proved that The linear range of the every regression equation could be enlarged by increasing the initial concentration of Co(NO3)2 in organic solvent. For the samples with water content beyond the linear range, the water content also could be determined by standard addition method addition method only if the water amount in the added sample do not exceeded the linear range. Therefore, the method could determine water content in organic solvent without limitation of linear range. The data of 12 parallel determinaton with absolute ethanol as sample were processed by statistical methods. The results showed that the method is highly sensitive and well reproducible based on 0.128 mol L1 as detection limit and 1.9% as the relative standard deviation (RSD). The water content in 95%(V/V) ethanol was analyzed based on the method developed in this study with the reagent blank as reference. The average value of water content for 8 parallel Table 2 The linear regression equations corresponding to different organic solvents. Organic solvent
Linear regression equation
related coefficient (g)
Ethanol Isopropanol n-Butanol Acetone
A520 = 0.5391–0.1954 cwater A525 = 1.186–0.5786 cwater A520 = 0.1377–0.2731 cwater A530 = 1.394–0.5096 cwater
0.9994 0.9993 0.9989 0.9992
192
L. Zhou et al. / Chinese Chemical Letters 22 (2011) 189–192
analytical results was 4.93% (V/V) with a 1.20% of RSD. Using similar pretreatment method, the water content in flour was determinted by the method developed in this work. The moisture of flour was determined with National Standard method of China [12] simultaneously. The moisture content in flour was 11.08%, RSD = 1.85%, Er was 1.73%. The recoveries were 97.81%101.24%. The results proved that the determined water content agreed well with the true value and this method was reliable. During the analysis, the concentration of Co(NO3)2 in the organic solvents was 3.33 102 molL1. 3. Conclusions This paper described a simple, rapid and steady-going method for determining water content in organic solvents based on the linear relationships between the absorbance at maximum absorption wavelength of cobalt(II) complexes in the organic solvents (i.e. ethanol, isopropanol, n-butanol and acetone) and water content. This method could determine water content in organic solvent without the limitations of its linear range. The results on determining water content in samples indicated that this method was satisfactory with a good sensitivity and good reproducibility. The development of this method has great importance in laboratory study and industrial production. Acknowledgments This work is supported by the National Natural Science Foundation of China (No. 20735003 and No. SKLEAC2010011) of State Key Laboratory of Electroanalytical Chemistry in China, the Projects (Nos. 30700348 and 30700349) Sponsored by the Scientific Research Foundation for the Doctors in Henan Agricultural University of China, as well as the support the Scientific and technological key project (Nos. 082102350006 and 102102310335) of Henan Province in China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
K. Fischer, Angew. Chem. 48 (1935) 394. S. Edward, D. Monica, T.F. Holden, J. Dairy Sci. 51 (1968) 40. M.K. Tian, X.L. Feng, F.L. Zhao, Chin. J. Anal. Lab. 26 (2007) 105. G.Z. Li, Y.M. Liu, Spect. Anal. 24 (2004) 122. X.L. Feng, N. Li, F.L. Zhao, Chin. J. Anal. Chem. 31 (2003) 981. R. Iguclfi, K. Yamaguchi, T. Shibamoto, J. Chromatogr. Sci. 26 (1988) 588. J. Chen, J. Fritz, J Chmmatogr 482 (1989) 279. W. Liu, P. He, Chin. J. Yantai University 12 (1999) 176. M.M. Tripathi, E.B.M. Hassan, F.Y. Yueh, Sensors and Actuators B: Chemical 136 (2009) 20. H.X. Bai, X.R. Yang, J. Chin. Chem. Soc. 54 (2007) 619. W.B. Zhou, H.K. Li, G.Z. Zhao, et al. Chin. J. Anal. Chem. 28 (2000) 783. National Standard of China (GB5009.3-85), 1985.