Solid-phase extraction (SPE) is commonly used for the preconcentration of trace metals in the analysis of various materials. Previously, we studied the SPE of heavy [1] and platinum-group metals as neutral chelates [2] and of radionuclides as coordinatively solvated compounds [3,4] and mixed complexes [5] by alkylated silica gels. Methods based on the formation of ion associates are also successfully used. Methods for the separation of 90 Sr [6], 137 Cs, 144 Ce, 60 Co, 214 Bi, and 210 Po [7] were developed using the dicyclohexano-18-crown-6-sodium dodecylsulfonate (DCH18C6-NaDDS) system.The aim of this work was to study the chemical mechanism of the SPE of barium complexes formed in the DCH18C6-NaDDS and dibenzo-18-crown-6 (DB18C6-NaDDS) systems and to discuss the chemical mechanism of the SPE of various ion-associate complexes formed by charged metal complexes based on a comparison and analysis of the experimental results and published data.
EXPERIMENTALStarting substances. The following reagents were used: DCH18C6, DB18C6, sodium dodecylsulfonate, and methanol. The following reagents were of chemically pure grade: Ba(NO 3 ) 2 , Ca(NO 3 ) 2 · 4H 2 O, KNO 3 , Mg(NO 3 ) 2 · 6H 2 O, NaNO 3 , HNO 3 , HCl, and NaOH. The following stock solutions of the reagents were prepared: aqueous solutions of the above salts of alkali and alkaline earth metals, an aqueous 0.1 M sodium dodecylsulfonate solution, and alcoholic 0.1 M solutions of the macrocyclic compounds.Adsorbents. Alkylated silica gel with C 16 hydrocarbon radicals, a Diasorb C 16 adsorbent (particle size of 40-160 µ m; carbon content of 14.7%), and commercial Diapak C 16 cartridges with the above adsorbent from BioKhimMak (Moscow State University, Russia) were used as adsorbents. The adsorbent volume in a cartridge was 1 mL. According to certified data, the specific surface area of the C 16 adsorbent used in this study was equal to 300 ± 30 m 2 /g, and the density of grafting of alkyl groups was no lower than 3.3 µ mol/g [8]. The adsorbent was prepared for operations in accordance with a published procedure [9]. To prepare a modified adsorbent, 5 mL of ethanol was initially passed through a Diapak ë 16 cartridge using a peristaltic pump. Next, 10 mL of a reagent solution ( c DCH18C6 = c NaDDS = 1 × 10 -2 M) and 5 mL of water (for the removal of an excess of the reagents) were passed. A Diapak-nitrile adsorbent with the terminal nitrile group was also used (particle size of 40-160 µ m; carbon content of 4.55%).Procedure. The extraction of barium was studied under both static and dynamic conditions. In the former case, a solution with a certain metal concentration and reagent solutions with corresponding values of pH were added to a weighed portion of the C 16 adsorbent pretreated with methanol. The solution with the adsorbent was shaken for a time required for reaching a constant value of metal recovery; thereafter, the adsorbent was filtered off. In the study of adsorption under dynamic conditions, the test solution was pumped through a Diapak C 16 cartridge a...