Objective

To determine the extinction coefficient of vanillin in methanol UV spectroscopy.

Procedure

Two 100mL equimolar stock solutions of vanillin in methanol was prepared. Five dilute solutions were prepared from each stock solution. UV absorbance of each dilute solution was measured using Perkin Elmer Lambda 35 UV spectrometer. Molar extinction coefficient (molar absorptivity) ε for the solutions was then calculated from Beer's law A = εcl.

Results

UV absorbances of dilute solutions recorded in the spreadsheet
Beers law Plots
Solution Series A
Solution Series B
Mol. Wt of Vanillin - 152.14g/mole


Beers_Law_Plot_A.JPGBeers_Law_Plot_B.JPG

[Show the spectra so we can clearly see the shape of the curves JCB]- Runs for this experiment were performed only as absorbance measurements, therefore spectral files are not available.


214A
Wt of empty 100mL volumetric flask (A) -------------------------------- 065.3582g
Wt of 100mL methanol solution of vanillin (152.3mg) + flask -----144.3760g
Wt of (214A) 10mM solution of vanillin ----------------------------------- 079.0178g



214B

Wt of empty 100mL volumetric flask (B) ---------------------------------64.1059g
Wt of 100mL methanol solution of vanillin (152.3mg) + flask-----143.1527g
Wt of (214B) 10mM solution of vanillin ---------------------------------- 079.0468g

214A Dilutions:
  1. 214A-1: 100uL --- 10mL -----0.100mM
  2. 214A-2: 200uL --- 10mL -----0.200mM
  3. 214A-3: 075uL --- 10mL -----0.075mM
  4. 214A-4: 125uL --- 10mL -----0.125mM
  5. 214A-5: 150uL --- 10mL -----0.150mM

Weight measurements for 214A dilutions
214A-1: Wt of empty 10mL volumetric flask ----------------------- 08.4100g
Wt of 10mL vol.flask + methanol solution + 100uL 214A ---------16.2900g
Wt of 10mL vanillin / methanol solution (0.1mM) ------------------- 07.8800g


214A-2: Wt of empty 10mL volumetric flask ------------------------09.6389g
Wt of 10mL vol.flask + methanol solution + 200uL 214A --------17.5191g
Wt of 10mL vanillin / methanol solution (0.2mM) ------------------07.8802g

214A-3: Wt of empty 10mL volumetric flask -----------------------09.6203g
Wt of 10mL vol.flask + methanol solution + 75uL 214A ----------17.4471g
Wt of 10mL vanillin / methanol solution (0.075mM) ---------------07.8268g


214A-4: Wt of empty 10mL volumetric flask ------------------------ 09.6340g
Wt of 10mL vol.flask + methanol solution + 1250uL 214A --------17.5020g
Wt of 10mL vanillin / methanol solution (0.125mM) --------------- 07.8680g



214A-5: Wt of empty 10mL volumetric flask --------------------------09.7935g
Wt of 10mL vol.flask + methanol solution + 150uL 214A ----------17.6429g
Wt of 10mL vanillin / methanol solution (0.150mM) ---------------- 07.8494g


214B Dilutions:
  1. 214B-1: 100uL --- 10mL -----0.100mM
  2. 214B-2: 200uL --- 10mL -----0.200mM
  3. 214B-3: 075uL --- 10mL -----0.750mM
  4. 214B-4: 125uL --- 10mL -----0.125mM
  5. 214B-5: 150uL --- 10mL -----0.150mM


Weight measurements for 214B dilutions
214B-1: Wt of empty 10mL volumetric flask -----------------------9.1104g
Wt of 10mL vol.flask + methanol solution + 100uL 214 --------16.9582g
Wt of 10mL vanillin / methanol solution (0.1mM) ------------------7.8478g


214B-2: Wt of empty 10mL volumetric flask ------------------------9.6337g
Wt of 10mL vol.flask + methanol solution + 200uL 214B -------17.4815g
Wt of 10mL vanillin / methanol solution (0.2mM) ------------------ 7.8478g


214B-3: Wt of empty 10mL volumetric flask ---------------------- 8.4193g
Wt of 10mL vol.flask + methanol solution + 75uL 214B --------16.2840g
Wt of 10mL vanillin / methanol solution (0.075mM) ---------------7.8647g


214B-4: Wt of empty 10mL volumetric flask ------------------------9.6181g
Wt of 10mL vol.flask + methanol solution + 125uL 214B -------17.4474g
Wt of 10mL vanillin / methanol solution (0.125mM) ---------------7.8293g



214B-5: Wt of empty 10mL volumetric flask -------------------------- 9.8172g
Wt of 10mL vol.flask + methanol solution + 150uL 214B ----------17.6427g
Wt of 10mL vanillin / methanol solution (0.150mM) ------------------ 7.8255g

Discussion

Absorbance maxima of vanillin in methanol has been reported [1] to be 230nm, this is in very close to the observed absorbance maxima 231nm, molar extinction coefficient, Log ε is reported to be 4.19 (at 230nm), the obtained Log ε at 231nm is 4.15 (series 214A) ie.14230cm-1M-1. However for series 214B solutions the value was substantially different 13658cm-1M-1; this may be because absorbances for solution 214B-3 (0.075mM) was higher and did not fit well in the Beers law plot, indicating an error in solution making.
The molar absorbtivity at 230nm absorbance was 14066cm-1M-1.
In sheet 4 ε calculations for both solution series A and B were performed, by restricting absorbance range between 0.5-2 absorbance units, this was done in order to eliminate any instrumental inaccuracy. By doing so, average ε for solutions series A and B were calculated as tabluated below
Solution Series
Log ε at 230nm
Log ε at 277nm
Log ε at 308nm
Series A
4.22
4.06
4.05
Series B
4.24
4.07
4.06
NIST
4.13
4.0
3.95



Conclusion

Molar extincition coefficient for vanillin methanol was successfully determined to be 14230cm-1M-1 at 231nm.

Log

2008-12-15

13:00 Made-up stock solutions 214A and 214B
15:00 Made-up dilute solutions; 214A-1, 214A-2, 214A-3, 214A-4, 214A-5; 214B-1, 214B-2, 214B-3, 214B-4 & 214B-5
16:00 Obtained UV absorbances.

References

1.Roy D. Hartley and Annie S. Keene; Phytochemistry Vol 23, Iss 6, 1984, p1305-1307