Abstract. Oxidation of ethanol is carried out by the oxidizing agent, potassium dichromate. The oxidizing agent undergoes a color change followed by the completion of the reaction. This change of color is utilized to measure the absorbance of the solution. The concentration of the unknown ethanol sample is calculated by plotting an absorbance vs concentration graph of the standard solutions.
Module and group
Lab class completed
The aim of the experiment is to determine the concentration of ethanol by calorimetric determination method. The concentration of ethanol is measured following its oxidation with potassium dichromate. Potassium dichromate (K2Cr2O7) undergoes reduction, which is followed by a color change of the solution.
Oxidation is the loss of electrons (OIL), while reduction is gain of electrons (RIG). It can be remembered as (OIL RIG). The oxidation of alcohol (ethanol) results in formation of aldehyde (acetaldehyde). The acetaldehyde is further oxidized to acetic acid. The oxidation state of chromium changes from (+7) to (+3), which occurs due to reduction or gain of electrons. The reduction of chromium is associated with change of color from orange-yellow to bright green (Pavia, 2010).
Cr2O72-+ 3C2H5OH 3 CH3COOH + 11 H2O + 4 Cr3+
(Balanced reaction for oxidation-reduction)
Williams and Reese provide this equation
2K2Cr2O7+ 3C2H5OH +8H2SO4 2 K2SO4+3 CH3COOH + 2Cr2(SO4)+ 11 H2O
The instrument used in this experiment is the UV visible spectrophotometer. The intensity of light is measured before and after it passes through a sample. Transmittance is the ratio of the light before and after it passes through the sample (Clugston & Flemming, 2000).
T = I/Io
The absorbance for a given sample A= –log (%T/100%)
Beer Lamberts’ law defines the relationship between the absorbance of light by a molecule and its concentration. The concentration is proportional to absorbance
A= ε* b* c. A is absorbance, ε is the molar absorptivity, b is the path length and c is the concentration of the solution. (Clugston & Flemming, 2000).
When UV light is transmitted on a sample, concentration of the solution determines the amount of light absorbed. The path length remains constant throughout the experiment.
The absorbance of the known solutions (standard) is determined in the experiment. This is then used to measure the concentration of the unknown ethanol solution.
- Experimental method
The method involves the pipetting of 10, 8, 6, 4 and 2 ml. of the 1M ethanol (standard) to 100 ml. volumetric flasks. The volume was made up by addition of deionized water followed by adequate mixing. 4 ml. of the ethanol sample (with unknown concentration) was aliquoted to the flask. Each of the flasks were labeled. The concentration of the known ethanol solutions was calculated based on the equation.
Concentration in mol dm3- = ml. of ethanol used/100
The concentrations of the standards were calculated as 0.1, 0.08, 0.06, 0.04 and 0.02 mol dm3-respectively. 1 ml. of each of these standard solutions and the unknown sample were pipetted to
8 ml. of dichromate solution. The reaction mixture was warmed to 50.C for 30 minutes.
The last reaction mixture was prepared by adding 1 ml. deionized water to the 8 ml. dichromate solution. The tubes were arranged from left to right on the rack in order of decreasing concentrations at the end of 30 min. The absorbance for each of the tubes was measured. The tube containing the most concentrated solution has an orange yellow color while the lowest concentration was observed to be sky blue. The unknown had a color similar to 10 ml ethanol containing solution. (based on naked eye evaluation)
- Results and discussion. The table shows the absorbance of solution and concentration of ethanol. A solution containing 10 ml ethanol has greatest absorbance while solution with 0ml ethanol has the lowest absorbance
The equation y=mx+c can be used to determine y based on the x or the absorbance of the unknown solution. According to the data equation for the slope is y = 29.082x-0.315
Based on the absorbance of the unknown which was 3.5462
Rearranging the equation, conc. of unknown is 0.132 mol/dm3-
A .The mass of ethanol can be calculated based on the RMM and density.
Density of ethanol is 0.789 gm cm3
Moles =gm/RMM =m* v/ 1000
1 dm3 = 1000 cm3=1000ml.
The equation can be rearranged
g = (m*v/1000)* 46.07
= (0.132* 1000/1000)* 46.07
B. The mass that would be present in 100 cm3
= A/10 = 0.611 %
C. volume = mass/density
Mass =0.611, density is 0.789
=0.7743 (for 4 ml)
So for 100 ml
0.7743 * 25 = 19.36
This is %ABV
- Conclusion. This experiment utilized the principle of visual colorimetry for the measurement of concentration of unknown solution of ethanol based on the oxidation of ethanol by potassium dichromate solution (solution in sulfuric acid).
Williams, M. B. and Reese, Darwin (1950) Colorimetric determination of ethyl alcohol, Anal.Chem., 22, 1556.
Clugston Michael & Flemming Rosalind (2000). Advanced Chemistry. Oxford.
Pavia, Donald L (2010). Introduction to Organic Laboratory Techniques: A Small Scale Approach. Thomson. 9th Ed.