[MightyMan]

Hey guys,
i need some help with this question:

If a person drank rubbing alcohol (isopropanol), a breathalyser analysis (in units of mg/100 mL blood) would be in error.  Its reading would be:
1. Too high by a factor of 1.3
2. Too low by a factor of 1.3
3. Too high by a factor in 2.6
4. Too low by a factor of 2.6

my professor skimmed over how ethanol is detected in a breathalyser from a colour change of a cromate compound, but i'm not too sure how this relates to isopropanol.
I also know that in the reaction of the cromate colour change, ethanol is oxidized into acetic acid, however isopropanol would oxidized into a ketone, but im not sure how to connect it to the question.

Thanks in advanced.

[Dan]

OK, a breathalyser works by the chromate oxidation of the alcohol which gives a colour change.

Fist of all, what would the oxidation products of ethanol and isopropanol be?

[MightyMan]

The oxidation products for ethanol and isopropanol would be acetic acid and propanone, respectively.

I thought of finding a ratio between the molecular weights of ethanol and isopropanol, and that got me to 1.3, but i dont know if its too high or too low by that factor.
Plus i'd like to understand why...

[sjb]

That's a start, now can you write balanced equations for each of the oxidations?

S

[MightyMan]

2 K₂Cr₂O₇+ 3 C₂H₅OH + 8 H₂SO₄ -> 2 K₂SO₄+ 2 Cr₂(SO₄)₃+ 3 CH₃COOH + 11 H₂O
would be the ethanol (this was provided for me in my lecture notes)

using the equation and replacing ethanol for isopropanol and acetic acid for acetone, i got:
K₂Cr₂O₇ + 3 C₃H₇OH + 4 H₂SO₄ -> K₂SO₄ + Cr₂(SO₄)₃ + 3 C₃H₆O + 7 H₂O

therefore since Cr₂(SO₄)₃ is the colour change and there is 1 more mole of it in the ethanol reaction, would isopropanol would give an error in reading 1.3 (ratio of molecular weights) lower

Does that sound right?

[sjb]

The amount of dichromate is fixed in the breathalyser, so taking this into account, you effectively have the equations

2 K₂Cr₂O₇+ 3 C₂H₅OH + 8 H₂SO₄  2 K₂SO₄ + 2 Cr₂(SO₄)₃+ 3 CH₃COOH + 11 H₂O for ethanol.

and

2 K₂Cr₂O₇ + 6 C₃H₇OH + 8 H₂SO₄  2 K₂SO₄ + 2 Cr₂(SO₄)₃ + 6 C₃H₆O + 7 H₂O for IPA.

so what is the ratio by weight of EtOH : 2  IPA ?

S