Question

A wine collector has purchased a case of vintage wine at auction. To his disappointment, the wine had an unpleasant odour upon opening a bottle for tasting. Fearing for his investment, the collector has asked you to forensically test for contamination of the wine. To perform the required analysis you have decided to follow the standard colorimetric analysis for total phenol concentration on a 750 mL bottle he has given you.

(i) What is the reagent used to detect total phenol content? What is the peak wavelength observed by UVvis spectrophotometry (λmax)? (Graduate Capability - Chemistry knowledge & skills; Graduate Capability - Critical Thinking)

(ii) Performing tests quickly after sampling is often important for total phenol analysis from environmental waterway samples and, in this case, after sampling a sealed bottle of wine. For analysis, the time delay should not be longer than 4 hours. Discuss with reference to phenol chemistry why this is the case. (Graduate Capability - Chemistry knowledge & skills; Graduate Capability - Critical Thinking)

The working range of the total phenol test is 0-5.00 mg/L. You have at your disposal multiple 500 ± 0.2 mL and 100 ± 0.15 mL volumetric flasks for calibration standard preparation, a 2L Winchester of 99.95% phenol, unlimited distilled/deionized water, and standard 5 ± 0.015, 10 ± 0.025, 25 ± 0.05 mL pipettes:

(iii) Propose a methodology to produce a stock solution of ~ 5.00 mg/L (ρ(phenol) = 1.07 g/cm³) and the dilution steps taken to produce 6x calibration standards (~ 0, 1, 2, 3, 4, 5 mg/L pure phenol in solution). Determine the concentration of your standards with errors (

Answer 1

i) The reagent used to detect total phenol content is Folin-ciocalteu reagent or Folin's phenol reagent. It is a mixture of phosphomolybdate and phosphotungstate. The peak wavelength observed by UV vis spectrophotometry is 765nm.

ii) Performing tests immediately after sampling is important and time delay should not be longer than 4 hours because this reagent measures other reducing substances like nitrogen containing compounds, thiols, vitamins, proteins along with phenol if kept for a longer time.