CH 470 INSTRUMENTAL ANALYSIS Homework Set 3 -- Chromatography 1. You need to ana

Question

CH 470 INSTRUMENTAL ANALYSIS Homework Set 3 -- Chromatography 1. You need to analyze the fuel oil in a river that runs along a highway. You start by collecting the sample. Fuel oils are best analyzed by gas chromatography. Aqueous sohutions don't work well in a GC. Therefore you need to extract the fuel oil into an organic phase. Fortunately, being an organic molecule fuel oil is more attracted to an organic solvent than to an aqueous one. The distribution coefficient is 8.3. Your lab has been extracting 100 ml aqueous samples with three 10 ml aliquots of petroleum ether. You think that takes too much time so you shortcut the procedure by performing only one extraction with the whole 30 ml of petroleum ether. What is the percent error introduced by your haste?

Explanation / Answer

The equation for liquid - liquid extraction in multiple steps is given as,

(Qaq)n=(Vaq / DVorg+Vaq)n

where,

Qaq = fraction of analyte in aquous phase

Vaq = volume of aquous phase taken

D = distribution coeffiecient

Vorg = volume of organic phase

n = no. of steps taken for extraction

Now,

for 30 ml organic layer in single step,

Qaq = [(100mL) / (8.3*30 + 100)]

= 0.286

Qorg = 1- 0.286 = 0.713 = 71.3% organic compound come in organic layer.

For multiple step extraction,

Qaq = [(100mL) / (8.3*10 + 100)]3

=0.163

Qorg = 1- 0.163 = 0.837 = 83.7% organic compound come in organic layer

Error = 12.4 %

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