Chemical Instrumentation
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S. Z. LEWIN, New York University, Washington Square, New York 3, N. Y.
T h i s series of articles presents a survey of the basic principles, characteristics, a n d limitations of those instruments which .find important applications in rhemical work. The emphasis i s on commercially available q u i p m a t , a n d approximate prices are quoted to shou, the order qi magnifude uf cost of the various types 3f design and constrriction.
feature equilibrium const,antsapply. Typical dis tribution isotherms, show-ing the relation ship between the concentrntions of snlut, in the two phases as a fnnrt,ion of t,h, solute concentration, nrr ahown in Figur, 2.
14. Chromatographic Equipment Although the term "ehro~nstorrl-nl,ll\." originally referred t o a spwiiir teilmique far the resolution of mixtures of colored substances based on capillary action, usage has lcd t o such a broadening of t,he application of this term thnt the original meaning has long been relegated to the limbo of history. T o t h e rsbent t h a t there is an underlying, unifying principle hehind all t h e techniques thnt are currently referred to as ehrom.ztographir in nature, i t is bhis: there is a selecliz,e dwtribution of componals between two phases, coupled zuilh a f o m prodwing relatim motion qf the phases. This simple principlc is now exploited in such x broad array of practical trrhniques that chromatography has ckveloped in about two dersdrs from :I sparingly used, esoteric research tool to the most popular and active field in analytical rhemist,ry. The forms taken by chromatographic equipment are determined by thc fundnmental merh~nisrnsbeing utilimd for thr separation of the components of thr samplcs, and the proper choice and use of such equipment must be predicated npon :L thorough grasp of basic prinriplrs.
rll'ldp, will hc negatiw. Henrr, p,/p? will l,r greater than r l / r 2and , the vapor is relatively richer in component 1 than is the liquid with which i t is in eq~dihriunn. This difference in rompasition of liquid and vapor a t the same tempernturr is illustrated in Figure 1 far an iclval solut,ion.
:J. O . .
1.0-
.
.
,
. . . ..x . . -x
P U R E 2 -MOLE
. ' . . 1 0 , . . 0 FRACTION-
PURE I
Figure 1 . The vopor in equilibrium with o twoComponent liquid is richer in the more volotile component. ITB. da and x ~ T nore the boiling points of pvre component 1, pvre 2, and roiution of comporition X L respectively. The upper dashed curve shows the comporition of the vopor phore, the lower curve that of the liquid phase, which can exist in equilibrium with each other at any given temperature.
Figure 2. Each curve shows the equilibriun concentrotionr, CI and Cn, of o given solute in hv, liquid phorer, 1 and 2 respectively, ms o functial of concentrotion. I . A straight line (i.e., conrton partition rotiol is characteristic of an equilibrivn in which the solute has the some moieculor forn in both phorer. 11. Positive deviation from con. rtont partition rotio occurs, e.g., when the salute 2. 111 disrocides in phase 1, but not in Negative deviation occurs, e.g., when the rolvtt osrociotcr in phore 2, but not in phase 1 .
The dissolving of a gas in a liquid i: also a partition equilibrium, for the liquic can bc thought of as a n ertrnrtnnt for thc gas, and tho latter distributes itself be. tween the two phases. Henry's Law Selective Distribution c, = Kp, where c, is the concentration o dissolved gas, and p is the gm phasc p a r Selectivity in the distribution of r o n tial pressure, is equivelent to the distrihn Cansidcrations similar t o the :diovc ponents hetwocn two phases mny 1w tion lnw given above. apply t o the selective rlistribut,ion of the achieved in the following ways. 3. Interfacial Adsorption Epuilibrwm I . Vapor-1,iquid Epuilibriwn. T ~ P ramponcnts of a gas mixture hetween t,hc At the interface between n gas and a solid gns phase and a non-volntilc liquid p h ~ ~ c vapor that is in equilihrium with n tmon liquid and a solid, two liquids, pte. in whirh they can dissolve. component liquid is enriched in the mow an adsorption-desorption equilibrium car 2. Lipaid-I,iprid Erlmction Epu
