This analysis is based upon the oxidation of ethanol by potassium dichromate (orange) under acidic conditions to produce acetaldehyde, (see reaction 1). The dichromate then oxidizes the acetaldehyde to acetic acid, (see reaction 2). As the dichromate oxidizes the various reactants, it is in turn reduced to Cr3+ which is green. The green color is the result of a characteristic absorbance of light at a wavelength in the region of 600nm. Reaction 1
3CH3CH2OH + K2Cr2O7 + 4H2SO4 →3CH3CHO + Cr2(SO4)3 + K2SO4 + 7H2O Reaction 2
3CH3CHO + K2Cr2O7 + 4H2SO4 →3CH3CO2H + Cr2(SO4)3 + K2SO4 + 4H2O The absorbance is directly proportional to the concentration of Cr3+ in the solution (The Beer-Lambert law). The concentration of Cr3+ is also proportional to the concentration of ethanol in the original sample. Measurements are made for a series of standard solutions containing a known concentration of ethanol and a graph is plotted of absorption against concentration. This graph, known as a calibration curve, can then be used to calculate the concentration of ethanol in an unknown sample.
|Chemicals |Apparatus | |0.1M Potassium dichromate |250ml volumetric flask (6) | |2 M Sulphuric acid |Conical flask (4) | |0.2M Na2S2O3 solution |Burette and stand (1) | |Solid Potassium Iodide |Wash Bottle | |Starch indicator |25ml volumetric pipette (3) | | |10ml volumetric pipette(1) | | |1ml volumetric pipette (1) | | |Thin glass stirrer rods | | |Pipette fillers | | |10cm3 measuring cylinder | | |Funnel | | |White tile | | |Hot water bath | | |Electronic balance | | |Heater (2) |
To find out the alcohol content in wine.
Part A - Oxidation of alcohol
1) Record the brand of wine to be analyzed and the manufacturer's specification of the alcohol content. 2) Use a pipette to transfer 10.00 ml wine to a 200 ml volumetric flask. Make up the volume to 200.0 ml using distilled water and mix thoroughly. 3) Place 10.00 ml aliquots of the diluted wine into each of three 250 ml conical flasks. 4) Record the concentration of the standard potassium dichromate solution supplied. Place a 20.00 ml aliquot of this solution in each flask. Wearing gloves and using a measuring cylinder, slowly add approximately 10 ml sulphuric acid solution to each flask. ? 5) Stopper each flask loosely and heat for 10 minutes in a water bath at 45 - 50° C. (Do...