HUNGARIAN JOURNAL OF INDUSTRIAL CHEMISTRY VESZPREM Vol. 30. pp. 27-31 (2002) A CHELATE SORBENT PREPARED BY THE MODIFICATION OF LICHROPREP RP-8 WITH TITAN YELLOW AND ITS APPLICATION I. Sow A, R. KOCJAN and R. SWIEBODA (Department of Inorganic and Analytical Chemistry, Medical School, 6 Staszica Str., 20-081 Lublin, POLAND) Received: AprilS, 2001 The new chelating sorbent for metal ions was prepared by impregnation of chemically modified silica LiChroprep RP-8 with ion pairs composed of cation of Aliquat 336 and anion of Titan Yellow. The hypothetical molecular mechanism of binding this ion pair by the surface of the applied carrier was presented. The sorbent was compared with analogous sorbent with plain silica carrier containing the same ion pairs. Higher stability of the new sorbent in comparison to that of the plain silica chelating sorbent was demonstrated. The sorbent obtained was applied for chromatographic separations of some chosen mixtures of metal ions and for additional purification of aqueous solutions of alkali metals from trace amounts of heavy metals. Keywords: Titan Yellow, chelating sorbent, LiChroprep RP-8, extraction chromatography, purification of alkali metal salts Introduction In the trace analysis of metal ions an important role is played by the chelating sorbents [1]. Besides commercial chelating resins used for these purposes, other chelating sorbents have recently found increasing popularity [2], and especially interesting are silica gels modified with various chelating reagents [3]. In analytical chemistry, including trace analysis of metal ions, chemically modified silicas have also found wide applications [4-6]. The chelating sorbents can be obtained in a very simple manner by the impregnation of common silica with ion pairs formed by an alkylammonium cation of a liquid anion exchanger Aliquat 336 (methyltrioctylammonium chloride) and the anion of sulfonated chelating reagent [7]. In ref. [8] the application to trace analysis of some metals of a chelating sorbent obtained by impregnation of silica with a mixture of Aliquat 336 and Titan Yell ow. Our earlier data indicate the high stability of sorbents formed by impregnation of chemically modified silica- RP-8 with mixtures of Aliquat 336 and sulfonated chelating reagents [9,10]. Therefore we decided to prepare the sorbent by modification of RP-8 with Titan Yellow (Fig.l) and compare this to common unsilanized silica gel modified with Titan Yellow. The present paper relates the results of the performed investi gatioris. Experimental All experiments were performed at room temperature (20±1°C). Reagents and solutions LiChroprep RP-8 (0.040-0.063 mm) (Merck, Germany) was used as support for the stationary phase. Aliquat 336 - methyltrioctylammonium chloride (Merck, Germany) was purified by shaking a O.lmol r1 solution in chloroform with equal volume of 0.1 mol r 1 hydrochloric acid and subsequently five times with distilled water and then, by filtering the organic phase through a cellulose filter. Titan Yellow (Merck, Germany), was purified as described by Nyons [II]. Aqueous metal salt solutions were prepared by dilution of Titrisol standard metal salt solutions (Merck). Working solutions were freshly prepared from standard metal salt solution by dilution with doubly distilled water {previously adjusted to an appropriate pH value with 1 mol r 1 hydrochloric acid. acetate buffer, or tetraborate buffer). Freshly distilled chloroform of analytical grade was used as diluent of AHquat 336. 28 Fig.l Titan Yellow (Thiazol Yellow G) Solutions of HC104 and HCl (Suprapur, Merck) were used as eluents. Apparatus A Pye Unicam (Cambridge, UK) single-beam atomic absorption spectrometer was used for the determination of the metals. All pH measurements were performed with a Mera- Elwro N517 (Wroclaw, Poland) direct-reading pH meter, a glass-silver/silver chloride combination ele-ctrode. A voltammetric analyser UPE-2a (Radius, Gdansk, Poland) was used in the determination of trace amounts of Ph, Cd, Cu and Zn in KCl by anodic stripping voltammetry. A graphite electrode, impregnated with epoxy resin and coated with a mercury film in situ, was used as a working electrode having a working area of 12.5 mm2• Procedure The impregnating solution was prepared by shaking an appropriate volume of 0,01mol r 1 solution of Aliquat 336 in freshly distilled chloroform with 5 volumes of O.,OOlmol r 1 aqueous solution of Titan Yellow. After separating the phases, the organic phase was passed through a cellulose filter to remove the residual aqueous phase. LiChroprep RP-8 was impregnated with a chloroform solution of a mixture of Aliquat 336 and Titan Yellow in the following manner: Amount of 10 g of the sorbent was prepared by mixing a 200 ml portion of the organic solution containing l mmol of Titan Yellow (0.651 g) and 2 mmol of Aliquat 336 (0.884 g) with 8~465 g of LiChroprep RP-8. The diluent (chloroform) was then evaporated with the use of a vacuum evaporator on a water bath. l g amount of the chelating sorbent contained 0.1 mmol of Titan Yellow and 0.2 mmol of Aliquat 336. Elution of Titan Yellow from the sorbent with aqueous solutions of mineral acids was performed in the foUowing manner. A 0.1 g amount of the obtained sorbent was shaken for 10 min. with 5 ml of an appropriate acid solution in small tubes. The solution was subsequently centrifuged and the dye was then determined by spectrophotometry at 337 nm with reference to a calibration graph. Small polypropylene columns (55 mm x 5 mm) (J. T. Baker. Phillipsburg, NJ~ USA) were packed with 0.1 g of dry sorbent and used to measure the relative capacity of the sorbent towards different metal ions. A 5 ml portion of each solution (previously adjusted to an appropriate pH), containing 0.3 mg of the metal to be determined, was passed through each column for 5 min. Each percolate was analysed by AAS to determine the residual metal ion concentrations. Voltammetric determination of the metals was performed in the following manner. Volumes of 10 ml of 0.5 mol r 1 KCl (analytical grade or Suprapur grade) to which mercuric nitrate was subsequently added [the concentration of Hg(N03)z in the resulting solution was 10-4 mol r 1] were passed through the columns packed with 1 g of the sorbent. Volumes of 2 Jll of standard solutions of cif'+, Cd2+, Pb2+ and Zn2+ ions of 100 ppm concentration were introduced into 10 ml of 0.5 mol r 1 KCl (analytical-reagent grade) (the concentration of each metal ion added in the solution was then 20 ppb). Oxygen was removed from the analysed solutions with a stream of argon of special purity. The solutions were electrolysed for 3 min. at -1.3 V. Anodic oxidation of the metals was subsequently performed at the following conditions: amplitude 1.2 V; potential change of 10 mvs-1• Results and Discussion As discussed in the introduction, chelating sorbents can be obtained in a simple manner by impregnation of silica by ion pairs composed of methyltrioctylammonium cation and sulfonated chelating reagent anion obtained by static extraction process [7). During the preparation of such sorbents, silica gel was impregnated with Aliquat 336 and chelating reagents from chloroform solutions in such proportions that the molar concentration of Aliquat 336 was twice as large as the concentration of the reagent containing a single sulfo group (or four times for reagents with two sulfo groups, e.g. Nitroso-R-Salt or Titan Yellow). It was experimentally found many times that the use of excess amounts of Aliquat 336 improved the quality of sorbents so that they became more resistant to elution of the chelating reagent or the whole ion pairs by the aqueous acid solutions. The problem arises how the ion pairs are bound by the silica surface, what is the role of the double excess of Aliquat 336 and what is the cause of the increased stability of the sorbents prepared with this excess. Hansen et al. [12] in their paper on dynamically modified silica have demonstrated that the Br- anion from the tetraalkylammonium bromide binds the hydrogen ions from the surface silanol groups of silica; the W ions are substituted by the tetraalkylammonium cation. It can be presented those cations of Aliquat 336 are bound to silica surface by a similar mechanism (Fig.2). The mode of binding of the ion pairs: tetraalkylammonium cation - sulfonated chelating :eagent anion, with double molar excess of AJiquat 336 1s probably the following: half amount of Aliquat 336 is bound to the silica surface according to Fig.2; the \)( H. \1.V ·-0 \JI I t I I I I I I I I ;1;/i1}/;;/}/jll/7J;}i2;1t/?;J/7J/J//; Fig.2 Silica impregnated with Aliquat 336 Fig.3 Silica impregnated with Aliquat 336 and modified witch ion pairs composed of cation of Aliquat 336 and anion of Titan Yellow 1 adsorbent surface is covered by a monomolecular layer of octyl groups from Aliquat 336 so that the surface is hydrophobized and it bounds by hydrophobic (and dispersive) interactions the second layer of Aliquat cations forming the chelating ion pairs with the sulfonated reagent. In the hydrophobic interactions the octyl groups from both tetraalkylammonium cations are involved. In effect, the carrier surface is isolated from the aqueous bulk phase and the chelating groups are outside the< double layer, capable of binding metal ions in the bulk aqueous solution. The structure of the sorbent containing LiChroprep RP-8 can thus be represented as in Fig.3. During elution of metal ions bound by sorbent of this type with aqueous solutions of mineral acids, the weaker or stronger accompanying elution of chelating reagents (or even whole ion pairs) is observed so that the properties of the sorbent are changed and its ion exchange capacity is decreased. The anion of chelating reagents are displaced from the ion pairs by the anion of acids used (especially by CI04~ anions which have greatest affinity to the tetraalkylammonium cations). On the other hand, the hydrogen ions from the acids applied in the eluent elute not only metal ions bound by the sorbent (which is advantageous), but also, at higher· concentrations, can displace the whole hydrophobic layer in a process reverse to Fig.2 and then the chelating sorbent is decomposed. It happens especially in cases when to elute some metal ions bound by the sorbent aqueous acid solutions of high concentrations (0.5- 1 mol r1) are to be used. Therefore, we were 29 Fig.4 LiChroprep RP-8 modified with ion pairs composed of cation of Aliquat 336 and anion of Titan Yell ow 50 45 :a 40 2 35 ~ -e-smcagel -fr- Lichroprep RP-8 g 30 HCI ~ 25 ;>< = 20 ~ 15 '