II 2 3 Phase Equilibrium

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Physical Chemistry Laboratory Experiment II-2

PHASE RULE: TERNARY LIQUID SYSTEM

References: See relevant sections of text

"Experiments in Physical Chemistry" O.F. Steinback

and C. J. King, American Book Co., N.Y. (1950), pp.

124-129.

Background:

Know the following and their interrelations:

Phase rule: application to a two phase ternary system. Graphical representation and interpretation of a

triangular phase diagram. Conjugate phases (solutions). Tie line. Distribution ratio.

Objectives:

Determination of the solubility limits in a ternarysystem of water and two other liquids, one of which is

completely miscible and the other is partly miscible with

water.

Construction of the solubility curve of the system beingstudied on triangular graph paper.

Determination of the tie lines of several ternaryconjugate solutions of the system being studied.

Determination of the distribution ratio of the misciblecomponent in the two immiscible phases.

Chemicals:

Iso-propyl alcohol, benzene.

Other pairs of liquids suitable for the experiment, may be

suggested by the instructor.

Apparatus:

Stoppered Erlenmeyer flasks (125 ml)

Constant temperature water bath (25.0 + 0.1oC)

Refractometer


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Experiment II-2 Physical Chemistry Laboratory

Procedure:

I. Determination of the solubility limits.1) Prepare eleven mixtures of benzene and anhydrous iso-

propyl alcohol, of a wide range of composition (see

Steinback and King: table 22, p. 127). Deliver the

necessary amounts into clean dry stoppered Erlenmeyer

flasks from burets.

2) Place the flasks and one containing distilled water ina constant temperature water bath (25.0 + 0.1oC) and

allow enough time to establish thermal equilibrium (15

to 20 minutes).

3) Titrate each mixture, carefully and slowly, withdistilled water (at 25.0 + 0.1oC) until the firstpermanent turbidity appears. The composition of this

cloud point is on or near the solubility curve.

It is important to shake the mixture vigorouslyafter each addition with water. Air bubbles

formed by shaking must not be confused with a

cloud point.

To expedite the titration, withhold about 1-2 mLof the alcohol from the initial mixture; titrate

rapidly with distilled water until the cloud

point is reached, then add the remaining alcohol

and continue titration to the cloud point.

4) Measure the refractive index of each titrated mixture. Save the titrated mixtures.

II.Determination of the tie lines.

1)After completion of step I add excess distilled water,(about 2 ml) delivered by a buret, to at least six or

seven of the titrated mixtures. Allow sufficient time,

in a constant temperature water bath, for the two

conjugate phases to separate into two layers (one hourand preferably longer).

NOTE: The volumes of each layer must be large enough

to sample for analysis, thus do not use the mixture

with the smallest benzene amount.

Alternately, Prepare conjugate solutions of a wide


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Physical Chemistry Laboratory Experiment II-2

range OF composition (see Steinback and King, table

23, p. 127).

2) Analyze each layer of each solution by measuring therefractive index.

Samples may be removed from the conjugate solutions for

refractive index measurements as follows:

Upper layer - use a disposable pipet. Don't insert the

dropper tip too close to the interface

between the two layers.

Lower layer - when inserting the pipet through the upper

layer squeeze the bulb gently to prevent

liquid from entering the dropper. Upon

withdrawing likewise squeeze gently sothat a small amount of the sample is lost

into the upper layer.

Transfer the samples into small vials and stopper the

vials immediately.

Treatment of results:

I. Determination of the solubility curve1) Calculate the composition, inweight percent, of each mixture at the cloud point.

2) Tabulate data includingrefractive index, volume and weight percent of each

component in each mixture.

3) Construct the solubility curveon a triangular coordinate graph paper.

5) Interpret the meaning of the areasbounded by the curve and of various selected points

marked clearly on the diagram. This would includepoints in the two regions, on the solubility curve and

at the plait point. Apply the phase rule in your

interpretation.

II. Refractive Index - Composition Calibration Curve1.Fasten a sheet of rectangular coordinate graph paper


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Experiment II-2 Physical Chemistry Laboratory

to the triangular graph paper with the long side

parallel and level with to the triangle base line.

2.Construct a refractive index scale on the y-axis ofthe rectangular paper. Maximize its range, but use

only about half of the length of the vertical axis.

3.Plot the calibration curve points at the intersectionof a vertical projection from each cloud point

composition and its refractive index.

4.To determine the composition of a given solution,locate its refractive index on the calibration curve,

and project downward to the solubility curve.

III. The tie lines1.Use the refractive index calibration curve and the

refractive indices of the conjugate solutions to

determine the intersection of the tie lines with the

solubility curve. Follow the projection procedure in

step II (4) and mark the compositions on the

solubility curve.

2.Connect the two points of each conjugate pair ofsolutions to obtain the corresponding tie line.

3.On a separate graph paper plot the weight percent ofalcohol in the benzene-rich layer as ordinate vs. the

weight percent of alcohol in the water-rich layer as

abscissa, for the conjugate solutions. Draw 45o line

through the origin, it represents equal distribution.

Comment on the distribution ratio in this system.


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