Objective

To determine the solubility of vanillin in methanol by UV spectroscopy.

Procedure

A saturated solution of vanillin in methanol was prepared and subjected to dilutions until UV absorbance for them fell with in the range from Exp018 (1.1-3.0 absorbance units) in which extinction coefficient of the aldehyde was obtained. Concentrations of the unknown dilute solutions and there by the saturated solution was then calculated using there UV absorbance and the molar extinction coefficient from a Beers Law plot.

[A plot of dilution vs concentration (see sheet 3 on exptl data spreadsheet) shows a discontinuity in what should be a straight line fit between the two highest and three lowest concentrations. This brings up the question of instrument accuracy -- many spectrophotometers are not acceptably accurate at absorbance values greater than 1.0. Should you be using all of your data points when calculating averages, or defining an absorbance range to work in? --BH]
1- Lack of absorbance measurements for a solution containing 175uL of saturated solution may have caused the discontinuity in the straight line fit between the points.
2.The makers specified photometric accuracy for Perkin Elmer lambda 35 at 2 A using NIST 1930D filter is estimated to be ±0.005 A. In the absence of NIST standard, photometric accuracy test could not be performed. Therefore evaluating the accuracy in the absorbance of the measurements may be difficult. Hence, all the data points were used.

Results

Calibration Report (pdf)
Experimental Data
Spectral overlay [215A-1 to 215A-5]
Empty Quartz cuvette Vs Air
Empty Quartz Cuvette Vs another
Methanol Vs Air
Methanol Vs Methanol

Discussion

Molar absorbtivity used - 14066cm-1M-1 at wavelength - 230nm These values were initially obtained in Exp018 & Exp018-WS1.
Path length - 1cm
Absorbance for methanol was obtained with empty reference , (no sample placed in the reference slot).therefore no subtraction spectrum was applied to methanol spectrum. However for other samples, methanol was used as the background/reference solution.
Solubility measurements using molar absorbtivities at 277nm and 308nm resulted in lower concentrations for the saturated solution. Solubility measured at at three different maxima using molar absortivity obtained from NIST database [what is the solvent used in the NIST spectrum? JCB] is closer to the previously measured values from EXP013, Exp011, UC-EXP207
.Exp019-BeersLawPlot.JPG
Wavelength nm
Molar Absorbtivity cm-1M-1 from Exp018
Solubility (M)
230
14066
4.06
277
11281
3.44
308
11140
3.31
Wavelength
Molar Absorbtivity cm-1M-1 from NIST
Solubility (M)
230
13514.5
4.22
277
9660.5
4.01
308
8778.1
4.20
The concentration of the saturated solution calculated based on ε from Exp018 is 3.60M (averaged for above three data points) with a standard deviation of 0.39.
When ε obtained from NIST spectrum was used, concentration of the saturated solution was calculated to be 4.14M (averaged for above three data points) with a standard deviation of 0.11

ε Source
CONCENTRATION (M)
STDEV
Exp018
3.60
0.39
NIST
4.14
0.11


Conclusion

Concentration of a saturated solution of Vanillin in methanol has been calculated to be 4.14M

Log


2008-12-15

13:00 Made-up a saturated solution of vanillin in 700uL methanol...once it appeared like the solution had become saturated, the solution was vortexed for 13 min, to make sure it was definitely saturated, obtained a picture of the saturated solution.. [show this picture if possible --BH]
15:00 The following dilute solutions were made
  • 215A - 200uL saturated solution in 100mL methanol (measured in a 100mL volumetric flask)
  • 215A-1 - 100uL 215A in 10mL methanol (measured in a 10mL volumetric flask)
  • 215A-2 -125uL 215A in 10mL methanol (measured in a 10mL volumetric flask)
  • 215A-3 -150uL 215A in 10mL methanol (measured in a 10mL volumetric flask)
  • 215A-4 -200uL 215A in 10mL methanol (measured in a 10mL volumetric flask)
  • 215A-5 -225uL 215A in 10mL methanol (measured in a 10mL volumetric flask)

[What was used to dispense the aliquots of 215A and saturated solution? Is there potential for error because of the volatile solvent (see my general notes)? How could you demonstrate experimentally that such error was not significant, or at least consistent between measurements? --BH]
A micropipettor was used to dispense the small volumes. Pipetting errors when using volatile solvents may be reduced by using solvents stored at low temperatures (-20C ). The linear nature of the data set suggests that experimental errors were minimum or consistent.

18:10 performed a calibration on Perkin Elmer UV-Vis lambda 35 --- Passed
19:05 Completed absorbance measurements for the samples 215A-1 to 215A-5