chapter 16 an introduction to analytical separations introduction to analytical chemistry

CHAPTER 16 AN INTRODUCTION TO

ANALYTICAL SEPARATIONS

Introduction toIntroduction toAnalytical ChemistryAnalytical Chemistry

Copyright © 2011 Cengage Learning16-2

An Introduction to Analytical Separations

As shown in Table 16-1, several general methods are used for dealing with interferences in an analysis, including (1) masking, (2) chemical or electrolytic precipitation, (3) distillation, (4) solvent extraction, (5) ion exchange, (6) chromatography, and (7) electrophoresis.


16A Masking

In masking, a reagent is added to the solution of the sample to immobilize, or chemically bind, the interferent as a complex that no longer contributes to or attenuates the signal from the analyte.


16D-1 Principles

The partition of a solute between two immiscible phases is an equilibrium phenomenon that is governed by the distribution law.

(16-1)


16D-1 PrinciplesThe concentration of A remaining in an aqueous

solution after i extractions with an organic solvent ([A]i) is given by the equation

where [A]i is the concentration of A remaining in the aqueous solution after extracting Vaq mL of the solution having an original concentration of [A]0 with i portions of the organic solvent, each with a volume of Vorg.

(16-2)


16D-2 Applying Extraction to Inorganic Separations

Separating Metal Ions as Chelates Many organic chelating agents are weak acids that react

with metal ions to give uncharged complexes that are very soluble in organic solvents, such as ethers, hydrocarbons, ketones, and chlorinated species. Most metal chelates, on the other hand, are nearly insoluble in water.



Separating Metal Ions as Chelates the equilibria that develop when an aqueous solution of a

divalent cation, such as zinc(II), is extracted with an organic solution containing a large excess of 8-hydroxyquinoline.

The equilibrium constant for this reaction is



• Separating Metal Ions as Chelates The equilibrium-constant expression can be simplified to

or



Separating Metal Ions as Chelates Equilibrium constants K vary widely from metal ion to metal

ion, and these differences often make it possible to selectively extract one cation from another by buffering the aqueous solution at a level where one is extracted nearly completely and the second remains largely in the aqueous phase.


16E Separating Ions by Ion Exchange

Ion exchange is a process by which ions held on a porous, essentially insoluble solid are exchanged for ions in a solution that is brought in contact with the solid.


16E-1 Ion-Exchange Resins

Synthetic ion-exchange resins are high-molecular-weight polymers that contain large numbers of an ionic functional group per molecule.


16E-1 Ion-Exchange Resins

Cation exchange is illustrated by the equilibrium

Anion exchanger and an anion Ax– is


16E-2 Ion-Exchange Equilibria

Ion-exchange separations are ordinarily performed under conditions in which one ion predominates in both phases.

In the removal of calcium ions from a dilute and somewhat acidic solution.

(16-3)



K in Equation 16-4 represents the affinity of the resin for calcium ion relative to another ion (here, H⁺).

(16-4)



for a typical sulfonated cation-exchange resin, values of K for univalent ions decrease in the order Ag+ > Cs+ > Rb+ > K+ > NH4

+ > Na+ > H+ > Li+. For divalent cations, the order is Ba+ > Pb+ > Sr2+ > Ca2+ > Ni2+ > Cd2+ >Cu2+ > Co2+ > Zn2+ >Mg2+ > UO2

2+


16F-1 General Description of Chromatography

Components of a mixture are carried through the stationary phase by the flow of the mobile phase, and separations are based on differences in migration rates among the mobilephase components.


16F-2 Classifying Chromatographic Methods

In column chromatography, the stationary phase is held in a narrow tube, and the mobile phase is forced through the tube under pressure or by gravity. In planar chromatography, the stationary phase is supported on a flat plate or in the pores of a paper.


Table 16-2


Figure 16-4

Figure 16-4 (a) Diagram showing the separation of a mixture of components A and B by column elution chromatography. (b) The output of the signal detector at the various stages of elution shown in (a).


Figure 16-5

Figure 16-5 Concentration profiles of solute bands A and B at two different times in their migration down the column in Figure 16-4. The times t1 and t2 are indicated in Figure 16-4.


Figure 16-6

Figure 16-6 Two-component chromatograms illustrating two methods of improving separation: (a) original chromatogram with overlapping peaks, improvement brought about by (b) an increase in band separation, and (c) a decrease in bandwidth.


16F-6 Relative Migration Rates of Solutes

Distribution Constants

(16-6)

(16-5)



Retention TimesThe time required for this peak to reach the detector

after sample injection is called the retention time and is given the symbol tR .



The average linear rate of solute migration, ,

The average linear velocity, u, of the molecules of the mobile phase is

(16-7)

(16-8)



Relating Migration Rates to Distribution Constants

The total number of moles of solute in the mobile phase is equal to the molar concentration, cM, of the solute in that phase multiplied by its volume, VM.



(16-9)



The Retention Factor, k For solute A, the retention factor kA is defined as

where KA is the distribution constant for solute A. Substitution of Equation 16-10 into Equation 16-9 yields

(16-10)



The Retention Factor, k we substitute Equations 16-7 and 16-8 into Equation 16-9:

(16-11)

(16-12)



The Selectivity Factor The selectivity factor α of a column for the two solutes A and

B is defined as

(16-13)

(16-14)

(16-15)


Figure 16-8

Figure 16-8 Illustration of fronting and tailing in chromatographic peaks.


Figure 16-8

In tailing, the tail of the peak, appearing to the right on the chromatogram, is drawn out, and the front is steepened. With fronting, the reverse is the case.


16F-7 Band Broadening and Column Efficiency

Quantitative Measures of Column Efficiency plate height H plate count or number of theoretical plates N.

where L is the length of the column packing.

(16-16)



Quantitative Measures of Column Efficiency the efficiency of a column is reflected in the breadth of

chromatographic peaks, the variance per unit length of column is used by chromatographers as a measure of column efficiency.

(16-17)



Determining the Number of Plates in a Column

(16-18)


Figure 16-10

Figure 16-10 Determining the number of plates

2R= 16 .

tN

W


16F-8 Variables Affecting Column Efficiency

The Effect of Mobile-Phase Flow Rate The extent of band broadening depends on the flow rate of

the mobile phase. Figure 16-11 show a minimum in H at low linear flow rates.



The Effect of Mobile-Phase Flow Rate Plate heights for liquid chromatographic columns are an

order of magnitude or more smaller than those encountered with gas chromatographic columns.

Offsetting this advantage is that it is impractical to employ liquid chromatographic columns longer than about 25 to 50 cm (because of high-pressure drops), whereas gas chromatographic columns may be 50 m or more in length. Consequently, the total number of plates and thus overall column efficiency are usually superior with gas chromatographic columns.



Other Variables That Influence Plate Heights It has been found that plate heights can be decreased, and

thus column efficiency increased, by decreasing the particle size of column packings, by lowering the viscosity of the mobile phase. Increases in temperature also reduce band broadening in most cases.


Figure 16-11

Figure 16-11 Effect of mobile phase flow rate on plate height for (a) liquid chromatography and (b) gas chromatography.

16-39


16F-9 Column Resolution

Effect of Retention Factor and Selectivity Factor on Resolution

(16-19)

(16-20)


16F-9 Column Resolution

Effect of Resolution on Retention Time As mentioned earlier, the goal in chromatography is the

highest possible resolution in the shortest possible elapsed time. Unfortunately, these goals tend to be incompatible.

(16-22)


16G Sample Preparation

Sampling is an important step in analytical chemistry. It determines the quality of an analysis.

The lengthy process of sample preparation accumulates great errors.

Selective sample handling technique can go a step further to restore, or even enrich, the concentration from dilution.



The analyst must care about the sample cleanup to remove interference by fractionation, derivatization or extraction.

The traditional liquid-liquid extraction (LLE) serves a need for sample preparation.

Solid-phase extraction (SPE) comes in two versions: one is the loosely packed tiny column of silica-based bonded phase.



Another version resembling an HPLC guard column is sturdy and easy for automation.

If dilution from sample preparation calls for enrichment, solid phase microextraction (SPME) is a choice.


THE END

chapter 16 an introduction to analytical separations introduction to analytical chemistry

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