ANALYST, FEBRUARY 1992, VOL. 117 145 Synthesis of a Morin Chelating Resin and Enrichment of Trace Amounts of Molybdenum and Tungsten Prior to Their Determination by Inductively Coupled Plasma Optical Emission Spectrometry Xing-yin Luo,* Zhi-Xing Su,* Wen-yun Gao, Guang-yao Zhan and Xi-jun Chang Department of Chemistry, Lanzhou University, Lanzhou 730000, People's Republic of China A morin chelating resin was synthesized using aminated poly(viny1 chloride) as the starting material, and the optimum conditions for the synthesis were established. The parameters governing the characteristics of the resin for the adsorption of MoVi and Wvi including acidity, flow rate, rate constant, saturated capacity of adsorption, effect of re-use, interfering ions and desorption were investigated. The MoV1 and Wvl concentrations in standard samples were determined by using inductively coupled plasma optical emission spectrometry, with satisfactory results.For concentrations of MoVt and W'" of 0.4 mg 1-1, the relative standard deviation was 2.8% for MeV' and 2.6% for WI. The structure of the chelating resin was deduced by infrared spectrometry, and the mechanism of the enrichment of MoV1 and Wi is discussed. Keywords: Morin chelating resin; synthesis; enrichment; molybdenum and tungsten determination; inductively coupled plasma optical emission spectrometry Morin (2' ,3,4' ,5,7-pentahydroxyflavone) is amongst the most sensitive reagents used for the spectrophotometric and spec- trofluorimetric determination of a number of metal ions,'J particularly M0.3~4 In the present work this reagent was attached to an aminated macroporous poly(viny1 chloride) resin by means of the Mannich reaction to give a chelating resin containing morin as the functional group.This chelating resin is resistant to the action of strong acid or base. The conditions for the synthesis of the resin and its ability to absorb trace amounts of MoV1 and Wvl were studied. The resin was used in the analysis of several standard samples with satisfac- tory results. Experimental Apparatus and Instruments An ICP 6500 inductively coupled plasma optical emission spectrometer (Perkin-Elmer) , a Nicolet 170-sx Fourier trans- form infrared (FTIR) spectrometer, a Model pHs-3A digital pH meter and an electrodynamic oscillator were used. The adsorption column consisted of 0.2 g of chelating resin, which was kept in distilled water for 8-10 h before use, in a glass tube (20 cm long, 0.46 cm i.d., 0.15 cm i.d.at the lower end), in which a small pad of cotton-wool had been placed at the lower end beforehand. Materials and Reagents The aminated macroporous poly(viny1 chloride) resin (N content: 10%; particle diameter: 0.36-0.45 mm) was synthe- sized by the procedure described in ref. 5 . All the reagents used were of analytical-reagent grade and distilled water was used throughout. The stock solution of MoV1 or Wvl, at a concentration of 1.0 g I-*, was prepared by dissolving 0.4610 g of ( N H ~ ) ~ M o ~ O ~ ~ - ~ H ~ O or 0.4486 g of Na2W04.2H20 in dis- tilled water and diluting to 250 ml with distilled water. The mixed standard solution of MoV1 and Wvl, each at a concentra- tion of 20 mg 1-1, was prepared by diluting the stock solution with distilled water.Synthesis of the Morin Chelating Resin A 0.5 g amount of morin was placed in a three-necked 250 ml flask fitted with a condenser and agitator. Then, 100.0 ml of * Authors to whom correspondence should be addressed. 95% ethanol, 20.0 ml of formaldehyde solution (40%), 2.5 ml of concentrated HCI and 1.0 g of aminated poly(viny1 chloride) resin were added. After heating the mixture at 72 "C for 10 h, the chelating resin was washed with distilled water until the washings were of neutral pH and dried under IR radiation. The dried resin was then placed in an extractor and extracted with 95% ethanol in order to wash off the free morin.The resin was then dried for later use. The procedure described in ref. 6 was used to determine whether there was any morin in the extracting agent. The synthesis procedure can be described as follows: 72 "C, HCI + CH20 + morin- 95% C,H,OH -CH=CH-C H 2-C H- I YH 7% CH2 -NH-CH2-mOrin Procedure A 0.2 g amount of the morin chelating resin, which had been stored in distilled water for 8-10 h, was loaded onto the adsorption column. Then, 2.0 ml of the mixed stock standard MoV1 and Wvl solution were transferred by pipette into beakers and diluted with distilled water to 100.0 ml. The solutions were adjusted to pH 2.0 and then passed through the adsorption column at a flow rate of 1.0 ml min-1. Molyb- denum(v1) and Wv' were desorbed quantitatively with 25 ml of 0.1 moll-1 NaOH, and the desorption solutions were evapor- ated to about 5 ml, then transferred into 10 ml flasks and diluted to the mark with distilled water.Molybdenum(v1) and Wvl were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) using the following instru- ment settings: forward power, 1100 W; viewing height, 14 mm; plasma gas (argon) flow rate, 14 1 min-1; auxiliary gas (argon) flow rate, 0.6 1 min-1; nebulizer gas (argon) flow rate, 1.0 1 min-1; and wavelengths, 204.598 nm for MoV1 and 209.475 nm for Wvl. Results Effect of Acidity on Adsorption Equal concentrations of mixed MoV1 and Wvl standards were diluted to equal volumes, and the solutions concentrated using146 1.0 0.9 0.8 0" 0.7 0.6 0.5 0.4 \ ANALYST, FEBRUARY 1992, VOL.117 - - - - - - - 100.0 g 97.5 - c 0 .- c 95.0 2 92.5 90.0 100.0 - 97.5 s - C .- c 95.0 P a U 92.5 90.0 2 1 0.5 cHcl/mol I-' I 1 I I " J 1 2 3 4 5 6 7 PH - - - - - I 1 I I I Fig. 1 Effect of acidity on adsorption for: A, Mo; and B, W r 1 the adsorption column. The acidity was maintained in the range from 2.0 moll-' HCl to pH 7.0. The results (Fig. 1) show that MoV1 is adsorbed quantitatively at all the acidities, whereas Wvl is not adsorbed completely in 2.0 moll-1 HCl. As Wvl forms W03-H20 at pH 61.0,' the adsorption of MoV1 and Wvl at pH 2.0 is considerable. Adsorption Rate By using the procedure described above, the flow rates of the MoV1 and Wvl solutions through the column were varied from 0.5 to 2.5 ml min-1. The results, given in Fig. 2, show that MoV1 and Wvl are adsorbed quantitatively at a flow rate of 0.5-1.0mlmin-1.Hence a flow rate of 1.0ml min-1 was selected for the adsorption of MoV1 and Wvl. Dynamic Saturated Capacity of Adsorption A 0.05 g amount of chelating resin was weighed accurately and, after storing in distilled water for 8-10 h, it was loaded onto the adsorption column. Then, a 100 mg 1-1 MoV1 solution (pH2.0) was passed through the column at a flow rate of 1.0 ml min-1 and the eluate was collected in 10 ml fractions in order to determine the concentration of MoV1, until c = co, where co is the initial concentration of MoV1 in the solution and c is the concentration of MoV1 in the eluate. The results, given in Fig. 3, show that the dynamic saturated capacity of adsorption of the resin for MoV1 was 4.17 mmol per gram of dry resin.A 0.1OOO g amount of resin was weighed accurately and, using a similar method, the dynamic saturated capacity of adsorption of the resin for W1 was determined. The value obtained was 0.762 mmol per gram of dry resin; the results are given in Fig. 3. 0.3 ' I I I 1 0.1 0.3 0.5 0.7 [W]/mmol per gram of dry resin 1 I I I I 1 0 1.0 2.0 3.0 4.0 [Mo]/mmol per gram of dry resin Fig. 3 Dynamic saturated capacity of adsorption for: A, Mo; and B, W 5.0 4.0 CI, 3.0 0 E 2.0 1 .o 3.0 F - C 2.0 1 .o n 0 40 80 120 160 200 tlmin Fig. 4 Rate constant of adsorption for: A, Mo; and B, W Determination of Rate Constant A 0.1OOO g portion of resin was placed in each of two 100 ml conical flasks. A 50.0 ml volume of a MoV1 solution of pH 2.0 (Mo = 100 mg 1-1) was added to one of the flasks and 50.0 ml of a Wvl solution of pH 2.0 (W = 100 mg 1-l) were added to the other. Both flasks were attached to the electrodynamic oscillator and shaken at normal speed (100 cycles min-1).The metal uptake was determined at intervals of 20 min until equilibrium was reached (about 160 min). The results are shown in Fig. 4. According to Brykina et aZ.,g the isothermal adsorption equation for a low concentration of ions can be expressed as -ln(l - F) = kt, where F = QJQm, and t is the reaction time, Qt is the adsorption capacity at reaction time t, Qm is the adsorption capacity at equilibrium and k is the rate constant. The values of k obtained from the slope of a linear calibration graph were 4.58 x 10-4 s-1 for MoV1 and 3.88 x 10-4 s-1 for Wvl.The results are given in Fig. 4. Desorption Conditions and Desorption Curves After MoV1 and Wvl had been adsorbed by the resin following the above procedures, the columns were desorbed with 0.01, 0.05,O. 10,0.25 and 0.50 moll-1 NaOH solution, respectively. The results, given in Fig. 5, show that MoV1 and Wvl are desorbed quantitatively when the concentration of NaOH solution is higher than 0.1 moll-'. Hence 0.1 moll-1 NaOH solution was selected for the desorption of MoV1 and Wvl. By using the eluent selected above, the desorption curves of MoV1 and Wvl were obtained. The results, given in Fig. 6, show that 25 ml of eluent were sufficient for desorption.ANALYST, FEBRUARY 1992, VOL. 117 147 100 80 20 0 0.1 0.2 0.3 0.4 0.5 [NaOH]/mol I-' Fig.5 Effect of concentration of NaOH (25 ml) on desorption efficiency of: A, Mo; and B, W I I L 0 5 10 15 20 25 Amount of NaOH added/ml Fig. 6 Desorption curves of: A, Mo; and B, W, using 0.1 moll-1 NaOH as eluent Table 1 Results of the re-use of the resin Adsorption (YO) No. of times resin used Mo"' W V ' 1 99.8 100.0 2 100.0 100.5 3 100.5 100.0 4 99.3 99.9 5 99.6 99.3 6 99.5 100.1 Stability and Re-use of the Resin After the resin has been treated with strong acid or base, it can still be used for the adsorption of MoV1 and Wvl, with recoveries in the range 95.4-100%. Experiments were also carried out to determine the number of times the resin could be used. A column containing adsorbed MoV1 and Wvl was eluted with 3 mol 1-1 NaOH or 6mol1-1 HCI and then washed with 20 ml of 0.1 moll-1 HCI and distilled water until the washings were of neutral pH.This enrichment, desorption and neutralization procedure was repeated six times and the absorbing ability of the resin for MoV1 and Wvl was virtually unchanged. The results are given in Table 1. Interfering Ions Various interfering ions were added, respectively, to the diluted standard solutions of MoV1 and Wvl (0.4 mg 1-1) at a 100-fold excess over MoV1 and Wvl. The MoV1 and Wvl were then enriched and determined. Table 2 shows that a 100-fold excess of concomitant ions causes little interference and that a 50-fold excess of Fellr interferes seriously. However, if 0.1 g of ascorbate is used to reduce a 250-fold excess of Fell1 to Fe", the Table 2 Effect of interfering ions on MoV1 and Wv' at a concentration of 0.4 mg 1-l Recovery (%) Interfering Concentra- ion tion/mg 1-l MoV1 WV' 40.0 40.0 40.0 20.0 40.0 40.0 40.0 40.0 40.0 40.0 40.0 40.0 250.0 250.0 100.6 100.1 98.8 20.7 98.9 97.8 96.6 94.0 92.7 91.1 97.2 93.3 101.2 98.6* 97.7 97.6 200.5 21.3 91.7 97.7 97.1 96.6 97.7 106.3 105.1 91.4 100.2 96.7* * The recoveries were obtained after 250 mg 1-1 of Fellr had been masked with 5000 mg I - * of ascorbate.Table 3 Lowest limits of adsorption Amount Concentration Recovery (YO ) MoV1 Wvl VolumeA MoVi Wvl MoV1 Wvl 0.010 0.0125 0.50 20.0 25.0 95.7 73.3 0.010 0.025 0.50 20.0 50.0 95.3 91.8 addedmg (PPb) interference from Fell1 can be eliminated and the recoveries of MoV1 and Wvl are 98.1 and 96.3%, respectively. Lowest Limits of the Resin for Adsorption of MoV1 and Wv' Mixed solutions of MoV1 and Wvl of lower concentration (ppb level) were determined as described under Procedure.The results, given in Table 3, show that the morin chelating resin possesses a fairly strong absorbing ability for MoV1 and Wvl: 20ppb of MoV1 and 50 ppb of Wvl can be absorbed quantitatively. Precision and Analysis of Standard Samples Eight portions of a standard solution containing equal concentrations of MoV1 and Wvl were accurately transferred by pipette into beakers and diluted with distilled water to give a concentration of 0.4 mg 1-1. The diluted solutions were analysed as described under Procedure; the average results for eight determinations were 0.393 mg 1-1 for MoV1 and 0.398 mg 1-1 for Wvl and the relative standard deviations were 2.8% for MoV1 and 2.6% for Wvl.The four standard samples were dissolved as follows: a 0.1000 g amount of a standard sample was weighed accurately and dissolved in 10 ml of an acid mixture [HCI-HN03 (3 + l)] with heating (the undis- solved residue was removed by filtration). The solution was then transferred into a 100 ml flask and diluted to the mark with distilled water. A 5.0 ml aliquot of the solution was transferred by pipette into a 100 ml beaker and the contents of Mo and W in the standard sample were determined as described under Procedure. The results are given in Table 4. It can be seen that the contents of Mo and W determined are in agreement with the certified values. Discussion Conditions for the Synthesis of the Morin Chelating Resin The effects of the conditions used for the synthesis on the morin content of the chelating resin were investigated.The148 ANALYST, FEBRUARY 1992, VOL. 117 Table 4 Results of the analysis of standard samples using the WpGsed method Concentration (%) Relative Certified standard value Found deviation Sample Element (YO) Average (%) 1. 20Cr3MoWV (No. 192)* Mo 0.64 0.63 0.61 0.62 0.62 3.0 W 0.38 0.40 0.40 0.38 0.39 2.6 W 0.43 0.41 0.43 0.43 0.42 2.3 2. CrMoWV (No. H34)* Mo 0.46 0.45 0.44 0.45 0.45 2.2 3. 38CrWVAI (No. 169-2)* W 0.32 0.32 0.33 0.30 0.32 0 4. Standard steel? Mo 0.26 0.25 0.27 0.25 0.26 0 * Certified reference materials supplied by the Central Iron and Steel Research Institute of the Department of Metallurgical Industry of China.f Certified reference material supplied by the Iron and Steel Factory at Chongqin, Sichan province. Table 5 Effect of the conditions of the synthesis on the morin content of the resin Reaction Morin Reaction Morin Morin Morin temperature1 content* time1 contentt HCHOI content$ Morinl contents "C ("/I h (%) ml (% 1 g (Yo) 30 9.1 4.0 21.9 1.0 9.6 0.05 25.6 50 21.9 6.0 28.6 2.0 13.8 0.1 35.5 72 32.6 8.0 31 .O 4.0 35.5 0.2 21.9 10.0 32.6 6.0 25.9 0.3 16.7 * Aminated resin, 0.2 g; HCHO, 2.0 ml; morin, 0.1 g; 95% C2H50H, 20 ml; concentrated HCl, 0.5 ml; and reaction time, 10 h. f Aminated resin, 0.2 g; HCHO, 2.0 ml; morin, 0.1 g; 95% C2H50H, 20 ml; concentrated HCl, 0.5 ml; and reaction temperature, 72 "C. $ Aminated resin, 0.2 g; morin, 0.1 g; 95% C2H50H, 20 ml; reaction time, 10 h; reaction temperature, 72 "C; and concentrated HCI, 0.5 ml.§ Aminated resin, 0.2 g; HCHO, 4.0 ml; 95% C2H50H, 20 ml; reaction time, 10 h; reaction temperature, 72 "C; and concentrated HCI, 0.5 ml. 4000 3333 2666 2000 1666 1333 1000 666 333 Wave nu r n berlcm - Fig. 7 IR spectra of aminated poly(viny1 chloride) resin (spectrum l), morin chelating resin (spectrum 2), morin chelating resin saturated with MoV1 (spectrum 3) and morin chelating resin saturated with Wvl (spectrum 4) results, shown in Table 5, indicate that a higher content of morin can be obtained. However, if the proportion of materials used is incorrect and the reaction time is too long, the mechanical strength of the chelating resin obtained is affected; moreover, the catalytic effect of acid in the Mannich reaction must be taken into account.Hence, the following optimum conditions were selected for the synthesis: aminated poly(viny1 chloride) resin, 0.2 g; formaldehyde, 4.0 ml; morin, 0.1 g; ethanol, 20.0 ml; concentrated HCI, 0.5 ml; reaction time, 10 h; and reaction temperature, 72 "C. Structure of the Morin Chelating Resin and Adsorption Mechanism Fig. 7 shows the IR spectra of the aminated resin (spectrum 1), the morin chelating resin (spectrum 2) and the morin resin saturated with MoV1 (spectrum 3) or Wvr (spectrum 4). The peaks in Fig. 7 can be analysed as follows9~*0 (vmax/cm-*). Spectrum 1: 3340.5 (YN-H); 2908.7 and 2826.8 (6CH2 and YC-H); 1632.4 [vC=C of the highly conjugated system resulting from the dehydrochlorination reaction in the course of the amination of poly(viny1 chloride)5 and 6N-H]; 1571.6 (8N-H); and 1114.9 (pN-H).Spectrum 2: 3345.9 (YN-H); 2928.8 (vC-H); 1627.2 (vC=C + vC=O); 1440.2 (YC=C of the aromatic rings); the two peaks that were present in spectrum 1, at 1571.6 and 1114.9 cm-1, were not visible and this showed that the reaction was essentially complete. The additional peak at 1173.0 cm-1 in spectrum 2 may be due to the YC-0 of phenol and that at 1068.9 cm-1 to pN-H. Hence, according to the principle of the Mannich reaction and the IR spectra, the structure of the resin might be as shown below: -CH=CH-CHZ-CH- I or -CH =CH-CHZ-CH- I NH FH2 HO 0 OHANALYST, FEBRUARY 1992, VOL. 117 149 On comparing spectrum 3 with spectrum 2 in Fig. 7, it can be seen that the peaks at 3345.9 and 2928.8 cm-1 change slightly.At 1525.9 cm-1 a new but weak peak appears; this is due to the reaction of MoV1 with the >C=O function of the resin, which causes the peak for >C=O to be shifted from 1627.2 to 1525.9 cm-1. Two other peaks at 1173.0 and 1068.9 cm-l become weaker and are shifted to 1190.5 and 1098.1 cm-1, respectively; this shows that MoV1 can also react with the -OH and -NH- groups of the resin. Some obvious changes can be seen below 1000cm-1 and the peaks can be assigned as followsll (vmax/cm-l): 945.9 and 909.0 (vasymMo=O); 845.8 (vaSymMo=O); 711 .O and 664.3 (vasymMo-O-Mo). Therefore, it can be concluded that the mechanism of the adsorption of MoV1 onto the resin consists of two arts: the first is a chelating 3 or 5 to form a chelating structure with a five- or six-membered ring; the second part is an association mechan- ism, i.e., MoV1 reacts with the -NH- group to form an ion associate.On comparing spectrum 4 with spectrum 2 in Fig. 7, it can also be seen that there is only a slight change in the peaks at 3345.9 and 2928.8 cm-1; no peak appears at 1525 cm-1 and the peak at 1173.3 cm-1 is smaller and is shifted to 1178.3 cm-1. This is a small change and might indicate that Wvl reacts weakly with >C=O and -OH to form a chelating structure. That the peak at 1068.9 cm-* becomes weaker and is shifted to 1099.3cm-1 shows that the reaction between Wvl and the -NH- group of the resin is fairly strong. Compared with spectrum 3, some obvious changes can be seen in spectrum 4 below 1100 cm-1 and the peaks can be assigned as follows12 (vmaX/cm-l): 974.9 (vaSymW=O); 887.9 (vaSymW=O); 805.3 (60-W-0).Therefore, it can be concluded that the mechan- ism of the adsorption of Wvl onto the resin also consists of two parts: the main part is an association mechanism, i.e., Wvl reacts with the -NH- group to form an ion associate; the minor part is a chelating mechanism, i e . , Wvl reacts with the >C=O function and the OH group in position 3 or 5 of the resin to form a chelating structure with a five- or six- membered ring. mechanism, i.e., MoV1 reacts with P ,C=O and -OH in position Conclusion The proposed method using a morin chelating resin to adsorb trace amounts of MoV1 and Wvl from sample solutions selectively is satisfactory. The proposed method was also found to be efficient for the determination of trace amounts of MoV1 and Wvl in standard samples. The method is rapid, accurate and convenient. In addition, the chelating resin is stable and its synthesis is also simple and rapid. 1 2 3 4 5 6 7 8 9 10 11 12 References Goppelsroeder, F., Fresenius 2. Anal. Chem., 1868, 7 , 195. Sanz-Medel, A., and Garcia Alonso, J. I., Anal. Chim. Acta, 1984, 165, 159. Almassy, G., and Viyvari, M., Magy. Kem. Foly., 1956,62,332. Murata, A., and Yume Muchi, F., Shizuoka Daigaku Koga- kubu, Kenkyu Hokoku, 1958,9,97. Guo, X . W., Zhang, J. F., Su, Z. X., and Cao, D. R., Guangpuxue Yu Guangpu Fenxi, 1990, 10(3), 48. Spot Tests in Organic Analysis, Publishing House of Fuel Chemistry Industry, Beijing, 1972, pp. 315-316. Cotton, F. A., and Wilkinson, G., Advanced Inorganic Chemistry, Wiley, New York, 3rd edn., 1972, pp. 545-583. Brykina, G. D., Marchak, T. V., Krysina, L. S., and Velyavskaya, T. A., Zh. Anal. Khim., 1980, 35, 2294. Zhong, H. Q., Elementary IR Spectral Methods, Publishing House of Chemistry Industry, Beijing, 1984, pp. 118-132. Ning, Y. C., Structure Identification of Organic Compounds and Organic Spectroscopy, Qin Hua University Press, Beijing, 1989, Cousius, M., and Green, M. L. H., J. Chem. SOC., 1964, 1567. Sengupta, A. K., and Nath, S. K., Indian J . Chem., Secr. A , 1981, 20(2), 203. pp. 329-351. Paper 1104722A Received September 11, 1991 Accepted September 26, I991