M. Iannone M.U. 6/20/2012

Determination of the Distribution Coefficient of I2 between Water and an Organic Solvent

Iodine is somewhat soluble in both water and organic solvents. In water it associatesstrongly with I- to form I3- ion. In this experiment, you will extract aqueous iodine/iodidesolutions with dichloromethane or another solvent to determine how I2 distributes itself betweenthe two phases. Two equilibria are involved:(1) I3(aq) F I(aq) + I2(aq) K = (1.38 0.03)103 at 250C [1](2) I2(aq) F I2(CH2Cl2). KDThe distribution coefficient is the equilibrium constant of equation 2:

(3) K IID

CH Cl

H O

=[ ][ ]

2

2

2 2

2

.

Procedure: [2]1. Solutions:Prepare 0.200M I solution (A).Prepare 100 mL of solution B by adding 2.010-4 mol of solid I2 to 100 mL of solution A. (I2dissolves very slowlystir or sonicate.) Because of reaction 1, solution B will have [I] 0.198M and [I3] 0.002M.2. Dilute aliquots of solution B with solution A to form 4 solutions with known [I3] rangingfrom 0.001M to 0.0002M.[3] Also, prepare solution X in which solution B is diluted 1:1 withwaterto test if KD depends on [I-]. Measure absorbance of each in a spectrometer orcolorimeter at 475 nm. Use solution A as the blank.3. Extract an aliquot of each solution with an equal volume of dichloromethane (or cyclohexane,toluene, hexane etc.). Important: allow enough time and mixing to reach equilibrium. Measureabsorbance of the aqueous layer in the colorimeter, using solution A as the blank. The liquidmust have a depth of at least 1 in the cuvette.

Calculations and error analysis: Find slope of calibration data by linear regression. Report it with the standard error. Include

graph in report. Note: force zero intercept by entering zero in the Const box of the Linestfunction dialog box.

Use spectrophotometric measurement and calibration data to determine [I3] in the aqueouslayer after extraction for each solution. Propagate error of slope.

Determine KD for each trial. Use the equilibrium constant for reaction (1) to find [I2(aq)]. Findthe weighted average value for KD with uncertainty.

Discussion:Your discussion should relate the nature of the I2 molecule and the two solvents to the valueobtained for the distribution coefficient. Discuss the role of the I2 I3 equilibrium in theaqueous phase. Does KD depend upon [I], (1) in theory and (2) according to the results fromsolution X? Can you think of a better way to measure KD?

1 Average of 16 values given in J. Sol. Chem., 13, 9 (1984)

2 G. Weiss, T. Greco and L. Rickard, Experiments in General Chemistry, Prentice Hall, New York (1997), pages

155-1593 Under the conditions of this experiment (0.2M I), over 99% of the aqueous I2 is converted: I2 + I I3.

M. Iannone M.U. 6/20/2012

Sample calculations of distribution coefficient: forced intercept

Absorbance of I3- solutions at 475 nm before andafter extraction with CH2Cl2

[I3-]/Mbefore extraction

Absorbancebefore extraction

Absorbanceafter extraction

C 1.0010-3 0.653 0.360D 6.0010-4 0.410 0.215

E 4.0010-4 0.280 0.152 F 2.0010-4 0.145 0.080X 1.0010-3 0.645 0.260

Abs = [I3- ] (67019)M1

[I3-] for solution F after extraction

13 M)19670(080.0]I[

=

= (1.19 0.03) 10-4 M (propagation of error formula used)[I2] in organic layer = [I3-]before [I3-]after (if equal volumes solution & CH2Cl2 used)

= (2.0010-4 M) - (1.19 0.03) 10-4 M=(8.10.3)10-5 M

[I2] in aqueous layer after extraction, from equilibrium expression:( )M10)3.02.8(]I[

X] sol.for 0.100 [* ]I[

]I[ ]I[*200.0]I[

]I][I[10)02.038.1(K7

2

3

23

3

23

=

===

399][][

2

2

=

=aqueous

organicD I

IK

Values for KD from all solutions:C 1256 weight = 1/42 = 0.06D 1266 0.06E 1116 0.07F 995 0.08X 1145 0.09

The Beers law plot is not the primary source of imprecision in the data. The 95% confidencelimits reflect the overall reliability of the result for KD, which should be reported as:

KD = 114 16.

Linest results: (do notinclude in body of report)

670.3125 018.738929 #N/A0.9968837 0.02998231279.5731 41.1502563 0.0035957

Weighted average = (data wt)/wts= 114

Uncertainty = 2wts/1 =

Standard deviation of KD values = 1295% conf. limit = 16