PRINCIPLES OF EXTRACTION AND THE EXTRACTION OF ...

1 CHAPTER. 2. PRINCIPLES OF EXTRACTION AND. THE EXTRACTION OF SEMIVOLATILE ORGANICS. FROM LIQUIDS. MARTHA J. M. WELLS. Center for the Management, Utilization and Protection of Water Resources and Department of Chemistry, Tennessee Technological University, Cookeville, Tennessee PRINCIPLES OF EXTRACTION . This chapter focuses on three widely used techniques for EXTRACTION of semi- volatile organics from liquids: liquid liquid EXTRACTION (LLE), solid-phase EXTRACTION (SPE), and solid-phase microextraction (SPME). Other tech- niques may be useful in selected circumstances, but these three techniques have become the EXTRACTION methods of choice for research and commercial analytical laboratories. A fourth, recently introduced technique, stir bar sorp- tive EXTRACTION (SBSE), is also discussed.

2 To understand any EXTRACTION technique it is rst necessary to discuss some underlying PRINCIPLES that govern all EXTRACTION procedures. The chemi- cal properties of the analyte are important to an EXTRACTION , as are the properties of the liquid medium in which it is dissolved and the gaseous, liquid, supercritical uid, or solid extractant used to e ect a separation. Of all the relevant solute properties, ve chemical properties are fundamental to understanding EXTRACTION theory: vapor pressure, solubility, molecular weight, hydrophobicity, and acid dissociation. These essential properties determine the transport of chemicals in the human body, the transport of chemicals in the air water soil environmental compartments, and the transport between immiscible phases during analytical EXTRACTION .

3 EXTRACTION or separation of dissolved chemical component X from liquid phase A is accomplished by bringing the liquid solution of X into contact with a second phase, B, given that phases A and B are immiscible. Phase B. may be a solid, liquid, gas, or supercritical uid. A distribution of the com- Sample Preparation Techniques in Analytical Chemistry, Edited by Somenath Mitra ISBN 0-471-32845-6 Copyright 6 2003 John Wiley & Sons, Inc. 37. 38 PRINCIPLES of EXTRACTION ponent between the immiscible phases occurs. After the analyte is distributed between the two phases, the extracted analyte is released and/or recovered from phase B for subsequent EXTRACTION procedures or for instrumental analysis. The theory of chemical equilibrium leads us to describe the reversible dis- tribution reaction as XA XB 2:1.

4 And the equilibrium constant expression, referred to as the Nernst distribu- tion law [1], is X B. KD 2:2 . X A. where the brackets denote the concentration of X in each phase at constant temperature (or the activity of X for nonideal solutions). By convention, the concentration extracted into phase B appears in the numerator of equa- tion ( ). The equilibrium constant is independent of the rate at which it is achieved. The analyst's function is to optimize extracting conditions so that the distribution of solute between phases lies far to the right in equation ( ). and the resulting value of KD is large, indicating a high degree of EXTRACTION from phase A into phase B. Conversely, if KD is small, less chemical X is transferred from phase A into phase B. If KD is equal to 1, equivalent con- centrations exist in each phase.

5 Volatilization Volatilization of a chemical from the surface of a liquid is a partitioning process by which the chemical distributes itself between the liquid phase and the gas above it. Organic chemicals said to be volatile exhibit the greatest tendency to cross the liquid gas interface. When compounds volatilize, the concentration of the organic analyte in the solution is reduced. Semivolatile and nonvolatile (or involatile) describe chemicals having, respectively, less of a tendency to escape the liquid they are dissolved in and pass into the atmosphere above the liquid. As discussed in this book, certain sample preparation techniques are clearly more appropriate for volatile compounds than for semivolatile and nonvolatile compounds. In this chapter we concentrate on EXTRACTION methods for semivolatile organics from liquids.

6 Techniques for EXTRACTION of volatile organics from solids and liquids are discussed in Chapter 4. PRINCIPLES of EXTRACTION 39. Henry's Law Constant If the particular extracting technique applied to a solution depends on the volatility of the solute between air and water, a parameter to predict this behavior is needed to avoid trial and error in the laboratory. The volatiliza- tion or escaping tendency (fugacity) of solute chemical X can be estimated by determining the gaseous, G, to liquid, L, distribution ratio, KD , also called the nondimensional, or dimensionless, Henry's law constant, H 0 . X G. H 0 KD 2:3 . X L. The larger the magnitude of the Henry's law constant, the greater the ten- dency for volatilization from the liquid solvent into the gaseous phase [2 4]. According to equation ( ), the Henry's law constant can be estimated by measuring the concentration of X in the gaseous phase and in the liquid phase at equilibrium.

7 In practice, however, the concentration is more often measured in one phase while concentration in the second phase is deter- mined by mass balance. For dilute neutral compounds, the Henry's law constant can be estimated from the ratio of vapor pressure, Pvp , and solu- bility, S, taking the molecular weight into consideration by expressing the molar concentration: Pvp H 2:4 . S. where Pvp is in atm and S is in mol/m 3 , so H is in atm m 3 /mol. Vapor Pressure The vapor pressure, Pvp , of a liquid or solid is the pressure of the com- pound's vapor (gas) in equilibrium with the pure, condensed liquid or solid phase of the compound at a given temperature [5 9]. Vapor pressure, which is temperature dependent, increases with temperature. The vapor pressure of chemicals varies widely according to the degree of intermolecular attractions between like molecules: The stronger the intermolecular attraction, the lower the magnitude of the vapor pressure.

8 Vapor pressure and the Henry's law constant should not be confused. Vapor pressure refers to the volatility from the pure substance into the atmosphere; the Henry's law constant refers to the volatility of the compound from liquid solution into the air. Vapor pressure is used to estimate the Henry's law constant [equation ( )]. 40 PRINCIPLES of EXTRACTION Solubility Solubility is also used to estimate the Henry's law constant [equation ( )]. Solubility is the maximum amount of a chemical that can be dissolved into another at a given temperature. Solubility can be determined experimentally or estimated from molecular structure [6,10 12]. The Henry's law constant, H, calculated from the ratio of vapor pressure and solubility [equation ( )] can be converted to the dimensionless Henry's law constant, H 0 , [equation ( )] by the expression Pvp MW.

9 H0 2:5 . 0:062ST. where Pvp is the vapor pressure in mmHg, MW the molecular weight, S the water solubility in mg/L, T the temperature in Kelvin, and is the appropriate universal gas constant [9]. For the analyst's purposes, it is usually su cient to categorize the escap- ing tendency of the organic compound from a liquid to a gas as high, medium, or low. According to Henry's law expressed as equation ( ), esti- mating the volatilization tendency requires consideration of both the vapor pressure and the solubility of the organic solute. Ney [13] ranks vapor pres- sures as Low: 1 10 6 mmHg Medium: between 1 10 6 and 1 10 2 mmHg High: greater than 1 10 2 mmHg while ranking water solubilities as Low: less than 10 ppm Medium: between 10 and 1000 ppm High: greater than 1000 ppm However, note that in Ney's approach, concentration expressed in parts per million (ppm) does not incorporate molecular weight.

10 Therefore, it does not consider the identity or molecular character of the chemical. Rearranging equation ( ) produces Pvp HS 2:6 . In this linear form, a plot (Figure ) of vapor pressure (y-axis) versus solu- bility (x-axis) yields a slope representing the Henry's law constant at values PRINCIPLES of EXTRACTION 41. Figure Henry's law constant at values of constant H conceptually represented by diagonal (dotted) lines on a plot of vapor pressure (Pvp ) versus solubility, S. of constant H. From this gure it can be deduced that low volatility from liquid solution is observed for organic chemicals with low vapor pressure and high solubility, whereas high volatility from liquid solution is exhibited by compounds with high vapor pressure and low solubility. Intermediate levels of volatility result from all other vapor pressure and solubility combi- nations.