Visible Spectrophotometric Determination of the Partition Coefficient of Methyl Violet A Microscale Extraction Experiment David C. Sonnenberger a n d Edward L. Ferroni Illinois Benedictine College, Lisle, IL 60532
A major topic i n our General Chemistry Laboratory program is methods of chemical separation. One of t h e standard techniques our students a r e introduced t o is solvent extraction. W i t h t h e increased costs involved i n operating a laboratory (chemical purchase, disposal, equipment, etc.), we have started a conversion of our program t o microscale. This experiment is designed t o demonstrate t h e principles of separation by solvent extraction, partition coefficient, a n d t h e construction a n d use of a Beer-Lambert standard curve. T h e urocedure utilizes metbvl violet a s t h e solute. Standard scale extraction experiments utilizing indicators have been described p r e v i o u ~ l y ? . ~ The Experlment Reagents Required Standard methyl violet aqueous solutions (1.5 X 10W M, 3.0 X 10-6 M, 6.0 X lo-", and 1.2 X 10W M) are needed for the construction of the standard curve. The methvl violet indicator, used bv the students to prepare their stock solutibns, is made by dissolving 0.50 g methyl violet in 1L of distilled water. Construction o f Beer's Law Curve Set the wsrelennh of thr spertrophotometer to 580 nm. Measure l the percent tranamittance i%T, of the four standard m ~ t h y violet solurrons. Convert ro'i'to absorbance ( A ) . and record these values. Plot absorbance vs. concentration of methyl violet on a sheet of graph paper. Draw the best straight line through the points that intercepts the origin. This line is your standard (working) curve. Preparation o f Methyl Violet Stock Solution To a clean 50-mL beaker add 20 mL of distilled water and 0.5 mL of methyl \,iolet indicator. Mix thoroughly. This solut~onis your methyl violet stock solurion. Using a vdumecric pip?[, transfer 1.00 mL 0: the methyl violet stock sol&on to a clean 10-mL volumetric flask, and dilute to the mark with distilled water. Measure the %Tof this solution, convert % Tto absorbance, and record these values. Determination of Partition Coefficient of Methyl Violet Toaclean 15-mL centrifuge tuhe, add 3mL ofmethylviolet stock solution and 3 mL of 2-octanol. Cover the tube with parafilm, and
' Arreguin, B.; Padilia, J.; Herran, J. J. Chem. Educ. 1962, 39, 539. Leonard, C. B.. Jr. J. Chem. ~ d u c1967,44.363. .
agitate the ronrenrs for ahout :30r using a Vortex mirror. Allow the layers toseparate. L'aing a Pasteur piper. transfer the aqueousphase from the centrifuge tuhe to a clean, dry test tuhe. If the aqueous layer is cloudy, centrifuge the solution for about 1min before the transfer. Using avolumetric pipet, transfer 1.00mL of the aqueous phase to a clean 10-mL volumetric flask, and dilute to the line with distilled water. Measure the % T of this solution, convert %T to absorbance, and record these values. Repeat the ahove procedure. From the standard curve generated ahove, use the absorbance values for the solutions in the 10-mL volumetric flasks to determine the concentration of methyl violet in the solutions. Using these concentration values, calculate the concentration of methylviolet in the methyl violet stock solution and in the aqueous phase of the 2octanol extraction. Hint: recall how the solutions in the volumetric flasks are . urenared and the notion of dilution. . In the extraction procedure, the volumes of aqueous and organic phases are identical. As a result of this, the following relationship holds: [methyl violet (stock)] = [methyl violet (aq)]
+ [methyl violet (org)] (2)
U~ingeq 2, calculate the concentration of mrthyl violet in the organic phase after the extraction. Determine thr value of thedistrihution coefficient of methyl violet iletween water and the 2.uumnol utilizing the equation [methyl violet (erg)] Kd = [methyl violet (aq)]
Extraction Efficiency To a clean, dry 15-mL centrifuge tube, add 3 mL of methylviolet stock solution and 1.5 mL of 2-octanol. Cover the tuhe with parafilm, and agitate the contents for about 30 s using a Vortex mixer. Allow the layers to separate. Using a Pasteur pipet, remove the organic phase from the centrifuge tuhe. Add 1.5 mL of fresh 2octanal to the aqueous phase and repeat the extraction. After the layers have separated, transfer the aqueous phase from the centrifuge tuhe to a clean, dry test tuhe. If the aqueous phase is cloudy, centrifuge the solution for about 1min before the transfer. Following the procedure outlined ahove, determine the concentration of methyl violet in the aqueous phase after the extractions. Compare the amount of methyl violet extracted by the two smaller portions of 2-octanol with that extracted by the larger portion. Each section of t h e procedure is designed t o illustrate a chemical concept. I n t h e segment o n t h e Beer-Lambert Law
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January 1989
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plot, the student learns that the amount of light absorbed by a substance is directly (linearly) proportional to the amount of the light-absorbing substance present. In the partition coefficient section. the student observes. visuallv as well as through the use of instrumentation, howa solute EandisrribUte itself between two immiscible solvents. The final sccrion of the procedure demonstrates how the efficiency of the separation process increases by performing multiple extractions. This experiment, including all calculations, can be com-
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pleted in about two hours. An average value of 2.1 for Kd, with a 95% confidence interval of 0.2, was obtained by the authors. Attempts to carry out this experiment using higher concentrations of methyl violet as the solute gave rise to irreproducible results. Acknowledgment
The authors would like to thank E. M. Winkler and D. J. Rausch for helpful discussions.
