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throline or associates of cationic surfactants with sul- ... Abstract—The results of studying the sorption of various ion associates on polyurethane foams were ...
Journal of Analytical Chemistry, Vol. 57, No. 10, 2002, pp. 875–881. Translated from Zhurnal Analiticheskoi Khimii, Vol. 57, No. 10, 2002, pp. 1036–1042. Original Russian Text Copyright © 2002 by Dmitrienko, Pyatkova, Zolotov.

Sorption of Ion Associates on Polyurethane Foams and Its Application to Sorption–Spectroscopic and Test Methods of Analysis S. G. Dmitrienko, L. N. Pyatkova, and Yu. A. Zolotov Faculty of Chemistry, Moscow State University, Vorob’evy gory, Moscow, GSP-3, 119899 Russia Received January 30, 2002; in final form March 01, 2002

Abstract—The results of studying the sorption of various ion associates on polyurethane foams were generalized. The main sorption-affecting factors were found to be the nature, hydrophobicity, and charge of the associate ion; the nature and concentration of the counter ion; the composition of the polymer unit of the polyurethane foam; and the pH and salt composition of the aqueous phase. Correlation equations were proposed to relate the partition coefficients with the hydration energy of counter ions in the ion associates of cationic dyes and metal complexes of 1,10-phenanthroline and with the number of carbon atoms in the alkyl fragment of cationic alkyltrimethylammonium surfactants. A sorption scheme was proposed and substantiated. Examples were given of the practical use of sorption for determining anionic and cationic surfactants, phenols, 1-naphthol, Fe(III), and Ru(IV).

Polyurethane foams (foamed heterochain polyether and polyester polymers) are of significant interest for analytical chemistry [1–3]. Almost all analytical applications of polyurethane foams are based on the unique sorption properties of these materials. Sorption–spectroscopic and test methods of analysis are among these applications [4–16]. Polyurethane foams are used as solid matrices because of their high sorption capacity to various compounds, chemical and hydrolytic stability, white color, relative availability, and low price. In addition, the components are retained on polyurethane foams due to both adsorption and absorption, which results in a uniform distribution of colored sorbates in the whole volume of the sorbent. Further development of sorption–spectroscopic and test methods of analysis based on the application of polyurethane foams is related to the study of sorption processes and mechanisms. The knowledge of sorption chemistry on polyurethane foams can facilitate the selection of practically important sorption systems, explain, and, in some cases, predict the sorption conditions in unexplored systems. In this work, the sorption of ion associates on polyurethane foams, which presents interest for several reasons, is considered mainly on the basis of our own studies. The variability of the structures of ion associates allows the systematical variation of the chemical nature, size, hydrophobicity, sign, and charge of ions and to study the effect of these parameters on the sorption. In addition, the practical aspect was also taken into account for the selection of test materials: ion associates include many colored and luminescent compounds, which allow us to use the results obtained for

the development of new sorption–spectroscopic and test methods of analysis. EXPERIMENTAL Experiments were performed using ions we conventionally classified into three types (Table 1). The first type includes associates containing large hydrophobic cations of cationic dyes or cationic metals chelates of 1,10-phenanthroline; the second type includes associates containing large hydrophobic anions of sulfophthalein dyes, tetrabromofluorescein, or 4-nitrophenylazo phenol and 1-naphthol derivatives. Associates of the third type contain both ion types, e.g., associates of anionic surfactants with metal complexes of phenanthroline or associates of cationic surfactants with sulfophthalein dyes or 4-nitrophenylazo phenol derivatives. Commercial dyes were purified by recrystallization and used as aqueous solutions; 4-nitrophenylazo phenol derivatives were prepared by azo coupling with 4nitrophenyldiazonium tetrafluoroborates. Synthesized metal complexes of 1,10-phenanthroline were used, as well as cationic and anionic surfactants. Polyurethane foams (PUFs) based on polyethers (trademarks 140, 5-30, and M-40), polyesters (2200, 35-08, and M3), and their copolymer (VP) (OAO Polimersintez, Vladimir; GPO Radikal, Kiev) were used as sorbents. Experimental procedure included the study of sorption on different polyurethane foams in batch mode depending on the equilibration time, concentrations of compounds to be found, and the total composition of

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Table 1. Ion associates studied Associate type I

Associate characterization

Compounds studied

R+X–, where R+ is a large hydrophobic Rhodamines: 3B (R3B), 6G (R6G), and butylrhodamine (ButR); cation, and X– is a counter ion (Cl–, Acridine Yellow (AY); cationic chelates of metals with 1,10-phenanthroline: – – [Cu(Phen)2]2+, [Fe(Phen)3]2+, [Ru(Phen)3]2+, and [Cu(Phen)2]+ Br–, I–, SCN–, N O , or Cl O ) 3

4

II

M+An–, where An– is a large hydrophobic anion, and M+ is a counter ion (Li+, Na+, K+, or Rb+)

III

R+An–, where R+ and An– are large hydrophobic cation and anion, respectively

Sulfophthalein dyes (SPDs): thymol blue (TB), bromocresol purple (BCP), bromothymol blue (BTB), bromophenol blue (BPB), and bromocresol green (BCG); tetrabromofluorescein (TBF); 4-nitrophenylazo phenol (AP) and 1-naphthol (AN) derivatives Associates of cationic chelates of metals with 1,10-phenanthroline and anionic surfactants; associates of cationic surfactants with 4-nitrophenylazo phenol derivatives; associates of cationic surfactants with sulfophthalein dyes

the solution, as well as the measurement of diffuse reflection or luminescence spectra of sorbates. Diffuse reflectance was measured with a Spectroton colorimeter (NPO Khimavtomatika, Chirchik). Absorption spectra of solutions were recorded with Hitachi-124 and SF-16 spectrophotometers; absorbance was measured with FEK-56 and KFK-2 photoelectrocolorimeters. Spectra of fluorescence excitation and the fluorescence of solutions and sorbates were measured with a Hitachi MPF-2A spectrofluorimeter. The pH of solutions was controlled using an EV-74 potentiometer with an ion-selective electrode. Sorbents were used as pellets (height, 5–10 mm; diameter, 16 mm; mass, 0.03–0.06 g, depending on the mark of polyurethane foam). The degree of sorption was assessed from partition coefficients and the recovery of compounds. The recovery of compounds (R, %) and partition coefficient (D) were calculated from the equations c0 – c - × 100%, R, % = -----------c0 Table 2. Partition coefficients (log D), maximum sorption (am), and sorption constants (log K) of the ion associates of rhodamines 6G and 3B, butylrhodamine, Acridine Yellow, and singly charged forms of sulfophthalein dyes on PUF 140 (V = 25 mL; mPUF ~ 0.04 g) Dye R6GCl R3BCl ButRCl AYCl TB BCP BTB BCG

log D (c = 1 × 10–6 M)

am × 106 M/g

log K (P = 0.95, n = 5)

2.45 2.86 2.97 3.34 3.15 3.28 3.75 4.08

6.8 11.0 18.0 4.0 2.6 40.0 50.0 55.0

4.84 ± 0.07 4.91 ± 0.05 5.08 ± 0.06 5.08 ± 0.10 5.52 ± 0.05 6.41 ± 0.02 6.46 ± 0.02 6.55 ± 0.04

where c0 is the analyte concentration in the aqueous solution before sorption, and c is its concentration after sorption; and R, % V D = ------------------------------- ---- , ( 100 – R, % ) m where V is the volume of the test solution, mL, and m is the mass of the polyurethane foam pellet, g. The reproducibility of determining partition coefficients was characterized by RSD ≤ 5% (n = 5–7). In most cases, the maximum sorption (am) was calculated from sorption isotherms. RESULTS AND DISCUSSION The sorption of different ion associates on polyurethane foams with different structures of polymer units was regularly studied. The main objective was to establish relationships between the sorption parameters and the physicochemical properties of the adsorbed (absorbed) associates and sorbents. It was found that the sorption of ion associates on polyurethane foams is affected by many factors, namely, the hydrophobicity and charge of associate ions, the nature and concentration of the counter ion, the structure of the polymer units of polyurethane foams, and the acid and salt composition of the aqueous phase. The effect of ion hydrophobicity. To reveal the relationship between the structure of a dye and its sorption capacity, sorption isotherms were obtained for rhodamines, Acridine Yellow (pH 5) [17, 18], and singly charged forms of sulfophthalein dyes (pH 3.5) [19]. Sorption isotherms of dye associates belong to Langmuir isotherms. An increase in the values of partition coefficient, maximum sorption, and sorption constant was observed in the orders R6G < R3B < ButR and TB < BCP < BTB < BCG with the increase in hydrophobicity of the dye molecule, which attested to a significant role of hydrophobic sorbent–sorbate interactions (Table 2). It was noted earlier that the formation of hydrogen bonds between the protonated primary

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amino group of Acridine Yellow and the polyether or polyester oxygen atoms of polyurethane foams is most probable [18]. For rhodamine dyes bearing a secondary amino group, such interactions are less pronounced. The sorption of 4-nitrophenylazo phenol derivatives increased with the increase in hydrophobicity of the azo component in the order phenol < 2-cresol < 1-naphthol. It was shown for the associates of cationic alkyltrimethylammonium surfactants with the BTB anion exchanger and 4-nitrophenylazo phenol and 1-naphthol derivatives (type III associates) that the sorption increased with the hydrophobicity of the alkyl fragment of cationic surfactants in the order C10 < C12 < C14 < C16 < C18 [7]. In the presence of cationic surfactants, the log D of the ionic associates of 4-nitrophenylazophenol and 4-nitrophenylazo-1-naphthol as a function of the number of carbon atoms in the alkyl fragment of cationic surfactants can be described by the equations: log D = 0.209nc + 0.547 (r = 0.983) and log D = 0.025nc + 3.313 (r = 0.970), respectively. Effect of the concentration and nature of the counter ion. The concentration and nature of the counter ion are important factors affecting the sorption of associates. Curves with saturation are typical for the recovery of associates as a function of the counter ion concentration. The sorption of type I associates increased with the increase in size and hydrophobicity of the anion in the – – order Cl–, Br– < NO 3 < I– < SCN– < Cl O 4 . A linear dependence was observed between the values of log D and the anion hydration energies (∆H, kJ/mol). The corresponding correlation equations are given below: [Cu(Phen)2]X2, PUF 2200 : log D = 9.20 + 0.028∆H; r = 0.993, [Fe(Phen)3]X2, PUF 2200 : log D = 10.22 + 0.030∆H; r = 0.992, PUF 140 : log D = 11.09 + 0.031∆H; r = 0.989, [Ru(Phen)3]X2, PUF 2200 : log D = 8.10 + 0.029∆H; r = 0.970, [Cu(Phen)2]X, PUF 2200 : log D = 5.48 + 0.009∆H; r = 0.986, R6GX, PUF 140 : log D = 5.75 + 0.010∆H; r = 0.997, PUF 2200 : log D = 5.37 + 0.009∆H; r = 0.967.

interference of cations with the sorption of anionic complexes of metals, mainly of thiocyanate ones, and indicate that the polyether units of polyurethane foams selectively bind alkaline metal ions, especially the potassium ion [21, 22]. The resulting positively charged sorbent fragments favor the extraction of anions. Effect of the polymer fragment of polyurethane foams. The polymer matrix of polyurethane foams exerts a significant effect on the sorption of type I associates. The data in Table 3 indicate that PUFs 35-08, 2200, and 140 possess increasing selectivity to the ion associates of type I. In the whole, the sorption of type I associates, except for the associate bearing the doubly charged AY cation, changes in the order 35-80 > 2200 > 140 > 5-30, MZ, M-40, VP. In most cases, the sorption of type II associates by polyether polyurethane foams is more efficient than by polyester ones. In polyether polyurethane foams, the recovery increases when going from propylene-based PUF M-40 to ethylene and propylene oxide-based PUF 5-30 and farther to ethylene oxide-based PUF 140 (Table 3). This difference in the sorption behavior of polyurethane foams similar in hydrophobicity [20] indicates that, along with hydrophobic interactions, the sorption of ion associates also involves specific (electrostatic) interactions resulting in the formation of hydrogen bonds. In distinction to associates of types I and II, the sorption of type III associates is practically independent on the structure of the polymer unit of polyurethane foams (Table 3). Ion charge. Sorption depends on the charge of associate ions. Other conditions being equal, doubly charged ions of types I–III associates are sorbed by all polyurethane foams worse than singly charged ions (Table 3). The composition of the aqueous phase and the acidity of the solution affect the sorption of ion associates for at least two reasons. First, the compounds studied can occur in different forms (e.g., as singly or doubly charged ions or neutral molecules), depending on pH [18, 19]. Second, when the solution acidity and salt composition change, the sorption properties of polyurethane foams can vary because of their modification by hydroxonium and alkaline metal ions. Scenario of sorption of ion associates. To describe the sorption behavior of associates on polyurethane foams, the following system of equilibria was proposed (considered for type I associates as an example):

The partition coefficients of type II associates on polyether polyurethane foams in the presence of alkaline metals increase in the order Li+ < Na+ < Rb+ < K+, with a pronounced maximum for potassium [7, 19]. The obtained results agree with the data reported for the JOURNAL OF ANALYTICAL CHEMISTRY

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R+ + X–

RX,

[ RX ] Kac, RX = ---------------------, + – [R ][X ]

(1)

RX

RXf ,

[ RX ] KD, RX = --------------f , [ RX ]

(2)

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Table 3. Partition coefficients (logD) of ion associates on different polyurethane foams Polyurethane foam Ion associate 140

5-30

AYHCl AYH2Cl2 R3BCl [Cu(Phen)2]Cl [Ru(Phen)3]Cl2

3.34 2.34 2.53 3.01 2.63

2.56 0.97 2.11 2.04 2.54

TBP Na TBP2 Na2 AP Na AN Na

4.8 2.0 2.81 3.75

3.04 1.30 2.14 3.35

AP CTMA AN CTMA [Cu(Phen)2] DDS [Cu(Phen)2] (DDS)2 [Fe(Phen)3] (DDS)2 [Ru(Phen)3] (DDS)2

4.0 3.80 3.08 2.46 2.26 4.10

3.97 3.73 3.03 2.38 1.76 3.50

+



Rf + Xf

+

X–

+ Rf

+

Type I 2.54 1.01 2.04 1.67 2.38 Type II 1.95 1.30 1.82 3.48 Type III 3.30 3.84 2.89 2.24 1.57 3.53

[ RX ] f f - , (3) K ac, RX = -------------------------+ – [R ] f [X ] f

(RX)f ,

[R ] f [X ] f -, KRX = -------------------------+ – [R ][X ] +

R+

M-40

– Xf

,



(4)

where [RX]f , [R+]f , [X–]f , [RX], [R+], and [X–] denote the concentrations of the ion associate and ions in the polyurethane foam phase ( f ) and water; Kac, RX and f

K ac, RX are the constants of RX association in water and the polyurethane foam phase; KD, RX is the coefficient of RX partition between the phases; and KRX is the sorption constant. The partition coefficient (D) of associate RX, after the corresponding transformations with account for Eqs. (1) and (3) and the condition of electric neutrality [R+]f = [X–]f, is as follows: – 1/2

K D, RX K ac, RX [ X ] [ RX ] f + [ R ] f - = ---------------------------------------------D = -------------------------------+ f 1/2 + 1/2 [ RX ] + [ R ] K ac, RX K RX [ R ] +

+ 1/2

– 1/2

( 1 + K ac, RX K RX [ R ] [ X ] ) -. × -----------------------------------------------------------------------– ( 1 + K ac, RX [ X ] ) f

1/2

(5)

The equation for the partition coefficient is simplified in the case when the ion associate is completely

VP

2200

35-08

MZ

2.53 1.52 2.08 2.34 2.54

2.54 1.95 2.80 2.39 3.38

3.76 1.48 4.56 – 3.46

2.79 1.01 2.11 – 2.65

3.18

3.23

2.04 3.42

4.39 3.23 Nonsorbable 1.42 1.10 3.29 3.42

3.26 3.43 3.10 2.24 2.19 4.08

2.76 3.56 2.90 2.23 1.72 3.80

– – – – – 3.41

– –

2.81 3.60 2.85 2.41 – 3.83

dissociated in both the aqueous solution and the polyurethane foam phase: – 1/2

+ – 1/2

D = K RX [ X ] [ R ] 1/2

(6)

.

In the case when associate RX is completely dissociated in water but not dissociated in the polyurethane foam phase, we have D = K RX K ac, RX [ X ]. f



(7)

An analysis of Eqs. (6) and (7) shows that the partition coefficient at the constant concentration of cation R+ should increase with the concentration of counter ion X–, and the slope of logD as a function of log[X–] will be 0.5 in case of the complete dissociation of RX in the polyurethane foam phase and 1 if not. The partition coefficient of cation R+ in the form of associate RX does not depend on its concentration in the absence of dissociation in the polyurethane foam phase and decreases in its presence when the R+ concentration increases. At the constant concentration of X–, which is maintained by the addition of any nonsorbed salt MX, the slope of logD as a function of log[R+] will be –0.5 at the complete dissociation of the ion associate in the polyurethane foam phase. It was found that the partition coefficient of rhodamine dye associates at a constant concentration of the counter ion X– depends on their concentration, and the slope of logD as a function of log[R+] will be close

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to –0.5, which is indicative of an almost complete dissociation of the R3B and R6G associates in the PUF 2200 phase. The slope of logD as a function of log[X–], – where X– is Br–, I–, or Cl O 4 , is close to 0.4. The sorption of the dye is no longer dependent on the counter ion concentration when the RX ion pair is its single form in the aqueous solution. The dissociation of rhodamine dye associates in the sorbent phase is confirmed spectroscopically by the complete stability of the amplitude parameters of rhodamine dye sorbates on all of the polyurethane foams studied in the presence of Cl–, Br–, and I– ions. By contrast, a significant quenching of fluorescence upon the increase in the atomic number of halogenide ions (the known heavy atom effect) is observed during the extraction of the ion associates of rhodamine dye halogenides by benzene (where they are not dissociated). On the basis of the experimental results obtained, a scenario of sorption of ion associates on polyurethane foams was proposed (Fig. 1). This scheme includes the formation of an associate with a counter ion in the aqueous solution according to Eq. (1) and its distribution between phases followed by dissolution in hydrophobic polymer films composing the polyurethane foam skeleton in accordance with Eq. (2) (Fig. 1a). A partial or complete dissociation of the associates according to Eq. (3) is possible in the polyurethane foam phase. Any impact on the system that displaces equilibria (1) and (2) to the right, e.g., an increase in the counter ion concentration, association constant, or associate size and hydrophobicity, increases the sorption. The observed parameters of the sorption of ion associates on polyurethane foams agree with the extraction mechanism of sorption on polyurethane foams [23–25]. At the same time, we believe that the real sorption mechanism is much more complicated and that the main processes can be accompanied by other processes, especially at a sufficiently high concentration of counter ions (usually cations or anions of alkaline metal salts) and in acid solutions. At the contact of polyurethane foams with alkaline metal salts or mineral acids, the sorbent is modified through the fixation of alkaline metal ions by their polyether or polyester units and the protonation of basic groups, mainly of terminal amino groups (Fig. 1b). The polyurethane foams modified by alkaline metal salts and mineral acids actually represent ion exchangers with positive charges localized on the surface. The sorption of negatively charged particles, e.g., of type II associate anions, on these sorbents apparently follows the ordinary regularities of ion exchange. A large hydrophobic anion exchanges with an anion from the layer of counter ions in the polyurethane foam phase and remains there mainly because of electrostatic interactions (scheme III). Hydrophobic interactions made an additional contribution to the energy of sorbate–sorbent interaction. JOURNAL OF ANALYTICAL CHEMISTRY

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(a) Aqueous phase

R+ + X–

879

RX (I)

Polyurethane foam (f)

(b) Aqueous phase

M+ + X–

R+ + An–

MX

RAn

(VI)

(II)

Polyurethane foam (f)

(R+)f + (X–)f

(RX)f

(MX)f

(IV)

(III) (M+)f + (X–)f

(V)

(RAn)f

Fig. 1. Sorption of ion associates on polyurethane foams.

A polyurethane foam modified by alkaline metal salts and mineral acids, provided the dissociation of these electrolytes in the sorbent phase, can be considered as a polymer ampholyte capable to act as both anion and cation exchanger. One of possible sorption schemes includes the exchange of dye or cationic surfactant cation for a cation from the primary adsorption layer in the polyurethane foam phase (scheme IV). The sorption of associates containing surfactant ions follows scheme V. Hydrophobic interactions are the main sorbate–sorbent interactions in this case. In addition, the formation of surfactant associates can also be due to the interaction between a surfactant cation or anion presorbed on polyurethane foam and counter ions (scheme VI). Analytical applications. The results presented above and the conclusions about the sorption of colored or luminescent ion associates on polyurethane foams made on their basis, indicate that the sorbents in the study are suitable for the quantitative extraction of many ionized compounds and their subsequent direct determination in the sorbent matrix by diffuse reflection spectroscopy, luminescence, or visual observation. Determination is performed after the sorption of the component analyte (which was initially present in the test sample in the ionized form) as an ion associate with a colored or fluorescent counter ion. At this basis, we developed highly sensitive methods for the determination of cationic [8] and anionic [9] surfactants (Table 4). To decrease the blank value, we used doubly charged counter ions: cationic complex [Fe(Phen)3]2+ for anionic surfactants and doubly charged forms of sulfophthalein dyes for cationic surfactants. In some cases, analytes (organic compounds or metal ions) were first converted into colored or luminescent ions and then adsorbed (absorbed) on polyurethane foam as ionic associates with colorless counter ions. This approach was used for the development of sorption–photometric and photometric procedures for determining phenols [7, 10], 1-naphthol [11], Fe(III) [8], and Ru(IV). Phenols and 1-naphthol were converted to negatively charged 4-nitrophenylazo derivatives by the reaction with 4-nitrophenyldiazonium tetrafluoroborate and adsorbed (absorbed) on polyurethane

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Table 4. Metrological characteristics of sorption–spectroscopic procedures using polyurethane foams (V = 25 mL) Analyte CTMA CP CTMA

DDS DDS Phenol 2-Cresol 3-Cresol 4-Cresol 1-Naphthol Fe(III) Ru(IV)

Reagent, sorbent BPB PUF M-40 BPB PUF 5-30 [Fe(Phen)3]Cl2, PUF 140 [Fe(Phen)3]Cl2, PUF 5-30 4-NPDA, PUF 5-30

1,10-Phen, HA, PUF 140 1,10-Phen, HA, PUF 2200

Determination method

Analytical range, µg/mL

Detection limit, ng/mL

RSDmin, %

Test material

DRS

0.2–3 0.8–8

40 200

5 5

Model solutions, river water

DRS

0.4–12

100

5

DRS

0.1–20

20

5

DRS

1.2–50

300

5

DRS

3 4 4 10 3 0.3

3 2 3 5 6 6

Model solutions, river water, cosmetic cream

DRS

0.01–0.8 0.01–0.8 0.01–0.8 0.03–1 0.01–1 0.001–1.2

LS

0.002–0.1

0.4

7

Model solutions

Model solutions, shampoos

Model solutions

Note: (DRS) diffuse reflection spectroscopy; (LS) luminescence spectroscopy; (4-NDPA) 4-nitrophenyldiazonium tetrafluoroborate; (BPB) bromophenol blue; (CTMA) cetyltrimethylammonium bromide; (CP) cetylpyridinium chloride; (DDS) sodium dodecylsulfate; (1,10-Phen) 1,10-phenanthroline; (HA) hydroxylamine hydrochloride.

foam as ion associates with a cetyltrimethylammonium (CTMA) cation. Metals to be determined were converted into colored or luminescent complexes with 1,10phenanthroline and adsorbed (absorbed) as associates [Fe(Phen)3](ClO4)2 and [Ru(Phen)3](DDS)2 (DDS denotes dodecyl sulfate). Metrological characteristics of the procedures developed are given in Table 4. The study of the sorption of ion associates on polyurethane foams formed the basis for the development of test devices and the corresponding test procedures for the determination of the total concentrations of phenols [10, 16], 1-naphthol [11, 16], and cationic and anionic surfactants [16]. The procedures were certified and included into the state register of measurement means of the Gosstandart of Russian Federation. ACKNOWLEDGMENTS This work was supported by the Russian Foundation for Basic Research, project no. 01-03-33102. REFERENCES 1. Dmitrienko, S.G. and Zolotov, Yu.A., Usp. Khim., 2002, vol. 71, no. 2, p. 180. 2. Braun, T., Navratil, J.D., and Farag, A.B., Polyurethane Foam Sorbents in Separation Science, Florida: Boca Raton, 1985.

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SORPTION OF ION ASSOCIATES ON POLYURETHANE FOAMS 14. Goncharova, L.V., Dmitrienko, S.G., Pyatkova, L.N., Makarova, S.V., and Zolotov, Yu.A., Zavod. Lab., 2000, vol. 66, no. 5, p. 9. 15. Dmitrienko, S.G., Gurariy, E.Ya., Nosov, R.E., and Zolotov, Yu.A., Anal. Lett., 2001, vol. 34, p. 425. 16. Dmitrienko, S.G., Pyatkova, L.N., Sviridova, O.A., and Medvedeva, O.M., Partnery Konkurenty, 2002, no. 1, p. 19. 17. Dmitrienko, S.G., Loginova, E.V., Myshak, E.N., and Runov, V.K., Zh. Fiz. Khim., 1994, vol. 68, no. 7, p. 1295. 18. Dmitrienko, S.G., Loginova, E.V., Myshak, E.N., and Runov, V.K., Zh. Fiz. Khim., 1997, vol. 71, no. 2, p. 317.

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