Exp018

=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// = ε//c////l**.**// =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



//[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.

**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
 * 214A

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 -1 M -1. However for series 214B solutions the value was substantially different 13658cm -1 M -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 -1 M -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 -1 M -1 at 231nm.

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.