Question

Standard solutions of ethanol in water were prepared and the density was measured with the pycnometer and the refractive index with the Abbé refractometer, from the following data indicate and justify Which of the two methods would you choose to determine ethanol in a sample whose concentration approximate is 5000 ppm

% Ethanol, w/w	0,0	0,5	1,0	2,0	4,0
Density. g/ml	0,9982	0,9973	0,9963	0,9945	0,9910
Refractive index	1,3330	1,3333	1,3336	1,3342	1,3354

Answer 1

The plot of the standard curves according to the data given is as shown below. The concentration is assumed to be 0, 0.5, 1.0, 2.0, 4.0% instead of 0,0 0,5 1,0 2,0 4,0 as given in the table.

The concentration given in ppm is converted to percentage since the standard curves are prepared by concentration in %. The conversion is:

ppm is parts per million. 1ppm = 1/1000000 and 1% is 1/100 Hence 1ppm = 0.0001%

Hence 5000 ppm is 0.5%.

To determine the concentration of ethanol in a sample of ~5000 ppm, I would suggest to use the density method since the change in density with concentration is expressed in the third decimal digit in the case of Concentration-Density standard curve shown above. Whereas using the refractive index the changes are more pronounced in the higher concentration range from 1-4%.