The concentration of phenol in a water sample can be determined by separating th

Question

The concentration of phenol in a water sample can be determined by separating the phenol from the non-volatile impurities in the water by steam distillation. The resulting distillate is treated with 4-aminoantipyrine and K3Fe(CN)6 at pH = 7.9 to form a coloured dye. A phenol standard with a concentration of 4.00 ppm has an absorbance of 0.424 at 460 nm in a 1.00 cm cell. A water sample is steam distilled (transferring the phenol to the distillate) and a 50.0 mL aliquot of the distillate is transferred to to a 100.0 mL volumetric flask, treated with 4- aminoantipyrine and K3Fe(CN)6 at pH = 7.9 and diluted to volume. Question: If the absorbance of this solution is 0.394 at 460 nm, what is the concentration of phenol in the original solution?

Explanation / Answer

Use Lambert Beer Law    A = ecl, i.e. A is proportional to conc.

A=0.424 for ………………4 ppm phenol (in the measuring cell)

A=0.394 for ……………x ppm phenol (in the measuring cell)

X = 3.715 ppm

The original solution is the distillate/aliquot.

The concentration in the distillate is

3.72 ppm x 100 mL/50 mL = 7.43 ppm phenol

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