Determination of Pb(II) in natural water with dibromo-p-methyl-sulfonazo by 1light-absorption ratio variation approach2Jie Zhang a*3a College of Environmental Science and Engineering, Tongji University, Shanghai 4200092, China567Abstract8Pb ions can react sensitively with dibromo-p-methylsulfonazo (DMSA) to form a blue 9complex, which absorption is maximal at 630 nm. By means of the light-absorption 10ratio variation approach (LARV A), the direct determination of Pb was developed in 110.3 M nitric acid and plots ΔA r-1 vs. c Pb-1 is linear where ΔA r is the light-absorption 12ratio variation and c Pb Pb(II) concentration between 0 and 5.0 mg/L. The limit of 13detection (3σ) of Pb( II ) is only 0.02 mg/L. This method was applied to direct 14analysis of natural water with satisfactory results.1516Keywords: Light-absorption ratio variation, Spectrophotometry, 17Dibromo-p-methylsulfonazo, Determination of Pb (II), Natural water1819Introduction2021Pb(II), one of the most topically toxic metals, is widely distributed in the water and 22soil with recognized accumulative and persistent characteristics.1 When accumulated 23in the human body, Pb(II) will cause damages to organ and systems (especially in 24young children).2-4In recent years, water pollution causes the children blood lead 25accidents occurred frequently in China. Although many sophisticated techniques for 26the determinations of trace Pb(II) have been developed including MS, ICP, AAS, 27chromatography and chemiluminescence,5-9factors such as the low cost of the 28instrument, easy handling, lack of requirement for consumables, and almost no 29maintenance have caused spectrophotometry to remain a popular technique, 30particularly in laboratories of developing countries with limited budgets.10Various 31spectrophotometric reagents such as sodium diethyldithiocarbamate, 11dibromo- 32methyl-carboxysulfonazo, 122-(5-bromo-2-pyridylazo)-5-diethylaminophenol, 13Dib 33romo-p-chloro-arsenazo14have also been reported for the spectrophotometric 34determination of lead in various complex materials and samples. Meanwhile, the 35further development in analytically significant chromophoric system and methodology 36offer new possibility for a variety of analytical applications. As a sensitivity 37increasing spectrophotometric method, the light-absorption ratio variation approach 38(LARV A) was established by Gao's group,15 which is based on the sensitive variation 39of the light-absorption ratio of a complexation system. The analytical sensitivity and 40accuracy are often greatly improved.16,17For example, Tang et al used 41bromopyrogallol red to determine iron ions at pH 6.23 by LARV A.18Zhao et al 42synthesized the MSCPA chromophore to react with copper ions at trace level.19 As an 43asymmetric bisazo derivative of chromotropic acid with one o-sulfonic functional 44group (Fig. 1), dibromo-p-methylsulfonazo (DMSA) was ever reported as a good 45sensitive and selective chromogenic reagent to barium, strontium and lead 20,21.46However, the excessive DMSA co-existing in the reaction solution often interfered the 47accurate determination of the DMSA-Pb(II) complex. In this work, the LARV A was 48applied to the direct determination of trace amounts of Pb(II) in water using the 49DMSA-Pb(II) complexation so as to raise the analytical accuracy and sensitivity. The 50parameters influencing the determination were evaluated. The applicability is in the 51linear range from 0 to 5 mg/L Pb(II) and the detection limit only 0.02 mg/L. The 52method was successfully applied to determinate the trace amounts of Pb(II) from 53natural water samples and certified reference material.54Experimental section5556Apparatus and reagents57A Model Lambda - 25 spectrometer (Perkin-Elmer Instruments, USA) was used58to record the absorption spectra. A Model BS110S electronic balance (Sartorius 59Instruments, Beijing) was used to accurately weight the standard substances and 60DMSA. A stock standard Pb(II) (1000 mg/L) solution was purchased from the 61National Research Center for Certified Reference Materials (Beijing, China). Both 620.100 and 1.00 mg/L Pb(II) standard use solutions were prepared by diluting the 63above solution. A 0.6407 mM DMSA was prepared by dissolving 50 mg of 64dibromo-p-methylsulfonazo (content >99%, Jameskin Reagents Institute of Shanghai, 65China) in 100 ml of deionized water. It was used to complex Pb(II).An Optima 2100 66DV inductively coupled plasma optical emission spectrometer (ICP-OES) 67(PerkinElmer, Franklin, MA, USA) was used to verify the accuracy of the method.68The wavelength of Pb (II) was 220.23 nm, which recommended by the manufacturer.69The standard reference material(GBW 08571) was pretreated according to the 70reference22.7172Recommended procedures73A solution containing less than 50 μg of Pb(II) was transferred into a 10-ml74calibrated flask. 3 ml of 1 M HNO3 and 0.60 ml of 0.6407 mM DMSA was added.75The solution was diluted to 10 ml with deionized water and mixed well. With the 76same method, a reagent blank without Pb(II) was prepared. After 10 min, the 77absorbances (Aλ1 and Aλ2) of the sample solution and those (Aλ10 and Aλ20) of the blank 78were measured at 525 and 630 nm with a 1-cm cell against water reference. The ΔA r 79was calculated byΔA r =Aλ2/Aλ1 - Aλ20/Aλ10. According to the standard equation: ΔA r-1 = 80pc Pb-1 + q, the concentration(c Pb, mg/L) of Pb may be calculated, where both p and q 81are the regression constants. Because the sensitivity factor p-1 is the inverse ratio to 82the concentration of DMSA added,15 the less DMSA added may bring out the higher 83analytical sensitivity. Nevertheless, too low DMSA will result in an obvious error, e.g.84increasing the fraction of instrument noise signal. It is appropriate for the addition of 85chromophore to produce a peak absorbance between 0.05 and 0.2.8687Results and discussion8889Absorption spectra90The complex reaction between DMSA and Pb(II) occurred in the acid medium 91because the reagent will form a 1:2 blue complex with lead ions under the acid 92condition.23The absorption spectra of the Pb(II)–DMSA solutions in various acid 93concentrations are shown in Fig. 2. All of the curves exhibited that the absorption 94peaks of the solutions are located at 630 nm and the valleys at 525 nm. These two 95wavelengths were used in the determination of Pb(II) by LARV A. Curve 2 indicated 96that the absorbance difference between the peak and the valley approaches maximum 97in 0.3 M HNO3.98Effect of the reaction time on the absorption spectra of the Pb(II)–DMSA 99complex were determined. The absorbance difference between peak and the valley 100indicated that the reaction is complete in 5 min. Moreover, the light absorption of the 101complex solution is stable for at least 180 min.102103Effect of DMSA, calibration graphs and LOD of Pb(II)104From variation of ∆A r of the solutions with the initial constant molar ratio of 105Pb(II) to DMSA at 10 μg/μmol (Fig. 3), ∆A r approaches a maximal constant when 106DMSA is more than 0.050 mM. The sensitivity ∆A r/c Pb becomes higher with decrease 107of DMSA concentration. However, the noise of spectrometer would become serious if 108the DMSA concentration is too low. In order to optimize the addition of the DMSA 109solution, the determination of replicated reagent blanks i.e. without Pb(II) were 110carried out.111Four series of standard Pb(II) solutions between 0 and 3.0, 0 and 2.0, 0 and 5.0，0 112and 8.0 mg/L were prepared and 0.40, 0.50, 0.60 and 0.80 ml of 0.6407 mM DSMA 113were added, respectively. The absorbances of each solution were measured at525 and 114630 nm against water reference and ΔA r was calculated according to the 115recommended procedures. Their regression equations are given in Table 1. 116Simultaneously, twelve replicated blanks of each series were determined and their 117standard deviations (σ)shown in Table 1. The LOD of Pb(II), defined as 3σwas 118calculated and series 3 has the lowest LOD at 0.02 mg/L Pb. Therefore, 0.60 ml of 1190.6407 mM DSMA added is suitable for analysis of water samples.120121Effect of Foreign Ions122Fourteen foreign ions including 12 kinds of metal ions and PO43- and Cl- were 123added in order to investigate the selectivity of this method in the presence of HNO3 124and Pb(II). Their effects on ∆A r are shown in Table 2. All the foreign ions have not 125affected the direct determination of 2.00 mg/L Pb(II) (error < 10%). The 126recommended method is highly selective and suitable for the direct monitoring of 127natural water.128129Analysis of samples130Three types of surface water were analyzed. They were sampled from Huangpu 131River (Shanghai, China), Nanxi River (Zhejiang, China) and tap water. A known 132volume of a sample was transferred into a 10-ml calibrated flask and then treated 133according to the recommended procedures. The determination results of Pb are listed 134in Table 3. The recovery rates of Pb(II) are between 100 and 110%.135In order to establish the validity of the proposed procedure, the method has been 136applied to the determination of the content of Pb(II) in certified reference material 137GBW08571.The determined values by the LARV A (1.87 ± 0.22 ug g-1) was in good 138agreement with the cer tified value for the analyte(1.96 ± 0.09 ug g-1).(Mean value ±139standard deviation, n = 3)140Comparative information from some studies on spectrophotometric 141determination of Pb(II) by various methods in Table 4.The suggested method 142possesses advantages with respect to sensitivity, selectivity, acidity range and ease of 143operation.144145Conclusion146147The DMSA was selected as a chromophore for determination of trace amounts of 148Pb(II) in nitric acid medium by LARVA.Under the optimal conditions, the 149light-absorption ratio variation is linear in the range of Pb(II) between 0 and 5.0 mg/L 150and LOD of Pb(II) only 0.02 mg/L. 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Lett., 2051991, 24, 1675-1684206207208Figures’ captions209210Figure 1 Structure of dibromo-p-methylsulfonazo (DMSA)211212Figure 2Absorption spectra of the Pb(II)-DMSA solutions in various acid 213concentrations. 1 to 5: 0.2, 0.3, 0.4, 0.5 and 0.6 M HNO3214215Figure 3Variation of ΔA r /c Pb(II)of the solutions with the same ratio of Pb(II) to 216DSMA at 10 μg/μmol.217Tables218219 Table 1 Calibration equations for determination of Pb(II) with DMSA220Series DSMA, mM c Pb, mg/L aStandard equation R b σ c LOD,mg/L1 0.0256 0 - 3.0 ΔA r -1 = 19.4c Pb -1 +5.920.9971 0.0100 0.062 0.0320 0 - 2.0 ΔA r -1 = 35.0c Pb -1 -0.9460.9977 0.0033 0.043 0.0384 0 - 5.0 ΔA r -1 = 15.3c Pb -1 +3.670.9958 0.0034 0.024 0.0512 0 - 8.0 ΔA r -1 = 16.3c Pb -1 +3.610.9901 0.0036 0.02aEvery series contained nine lead concentrations ；b Linear correlation coefficient ；c 221Standard deviation from 12 replicated blanks.222 223224 Table 2 Calibration graphs for determination of Pb(II) with DMSA225Solution No. (i )Ions added Added, mg/L aError, %b1 Pb(II) 2.002 Ca(II) 10.0 8.83 Zn(II) 50.0 -0.74 Fe(III) 50.0 0.45 Co(II) 50.0 -1.36 Mn(II) 50.0 3.27 Ni(II) 50.0 -4.78 Cd(II) 50.0 -3.49 Cr(III) 50.0 4.9 10 Cu(II) 250.0 -1.2 11 Mg(II) 250.0 -1.9 12 K(I) 250.0 -3.9 13 Al(III) 250.0 4.5 14 PO 43- 250.0 0.1 15 Cl - 250.0 0.3a2.00 mg/L Pb(II) added into each solution from No. 2 to 15; b Error =(∆A r i -∆A r 1)/∆A r 1 226 ×100 (i from 2 to 15). 227 228229 Table 3 Determination of Pb(II) in natural water230Sample from Pb(II) added, mg/L Pb(II) found, mg/L a Recovery, % bb(II) foundby ICP, mg/LHuangpu River 0 0.05 0.0480.05 0.10 100 0.112Nanxi River0 0.02 0.026 0.01 0.03 100 0.031Tap water0 0.07 0.078 0.10 0.18 110 0.181a Three replicated determinations;b calculation e.g. 110%=(0.18-0.07)/0.10×100% 231232Table 4Comparison of the present method with other spectrophotometric methods 233for the determination of Pb(II).234Reagent Media max/nm Remarks RefPyridine-2-acetaldehyde-Salicyloylhydrazone. Benzene 3801.Poor sensitive, 2:salting-out agent used,242,5-Dimercapto-1,3,4-thiadeazole HCl 375 1: Sensitive, 2: Fe, Mninterfere25Benzil Amonoxime IsonicotinoylHydrazone Aqueous 4051: Low sensitive, 2: smallacidity range26N-Hydroxy-N,N-diphenylbenzamidineCHCl3+Methanol525 Cu interferes. 272-(2Thiazolylazo)-p-cresolpH9.0–10.0650 Ni,Co,Zn,Fe,Cu interfere. 28Dibromo-p-methylsulfonazo with LARV A Aqueous 630 Rapid and wide acidityrange,selectivityThiswork235237OHOHN N HO 3S SO 3HN N BrBr H 3C CH 3HO 3S238 239241242 243245246。