70% isopropyl alcohol -- and 30% what?

[Mark Spahn]

Here's a naive chemistry question...

A plastic bottle I have before me is labeled
"70% ISOPROPYL ALCOHOL
First Aid Antiseptic
For Rubbing and Massaging"

Okay, if what's in the bottle is 70% isopropyl
alcohol, what is the other 30%?
Could it be water? (I don't even know whether
water and alcohol mix. Oh wait, they must,
else you couldn't make a mizuwari --
not to be confused with a mizuwani,
which is a sand shark).

Second question: If you leave this 70%
isopropyl alcohol exposed to the air,
which component evaporates away faster,
the alcohol, or the non-alcohol?
Upon evaporation, will the solution become
more alcoholic, or less alcoholic?
-- Mark Spahn (West Seneca, NY)

[Matthew Schlecht]

     The chemistry jinn alerted me to this burning question.

On Mon, Jul 7, 2014 at 8:43 PM, Mark Spahn <mark...@verizon.net> wrote:

Here's a naive chemistry question...

A plastic bottle I have before me is labeled "70% ISOPROPYL ALCOHOL
First Aid Antiseptic For Rubbing and Massaging"

Okay, if what's in the bottle is 70% isopropyl alcohol, what is the other 30%?
Could it be water?

     Yes, the other 30% is water, not accounting for any impurities.

 

(I don't even know whether water and alcohol mix.  Oh wait, they must, else you couldn't make a mizuwari --
not to be confused with a mizuwani, which is a sand shark).

Second question: If you leave this 70% isopropyl alcohol exposed to the air, which component evaporates away faster,
the alcohol, or the non-alcohol?
Upon evaporation, will the solution become more alcoholic, or less alcoholic?

     Ya know, I had always assumed that the 70:30 ratio came from the azeotrope, but...no.  This mixture is made to contain 30% water because...

Disinfecting pads typically contain a 60–70% solution of isopropyl alcohol in water. Water is required to open up membrane pores of bacteria, which acts as a gateway inside for isopropyl.

[ http://en.wikipedia.org/wiki/Isopropyl_alcohol ]

     Pure water boils (100% vapor pressure) at 100 ˚C, pure isopropanol boils at 82.6 °C, and an 87.9 : 12.1 azeotropic mixture of isopropanol/water boils at 80.4 °C.  This azeotrope would boil off first, which would tend to decrease the amount of isopropanol left behind compared to the water fraction, until pure water would be left.

[ http://en.wikipedia.org/wiki/Azeotrope_(data) ]

     Actually, these projections might not be precisely true for evaporation at 室温, but the alcohol would still have a higher vapor pressure even at sub-boiling temperatures, and so the mixture would become enriched in water over time.

Matthew Schlecht, PhD
Word Alchemy
Newark, DE, USA
wordalchemytranslation.com

[Mark Spahn]

Aha, so this knowledge provides a CSI-type

crime-solving technique. 

The blind Mr. Schroedinger, who also has a

defective sense of smell, left out a dish of

water for his finicky cat.  But in reaching for

the cat's plastic bottle of Evian, he instead

picked up the cat's plastic bottle of massage

alcohol (70% isopropyl alcohol). 

The time of death of Schroediner's cat from

acute isopropyl alcohol poisoning can be

determined, carbon-14-like, from the alcohol/water

ratio of the cat's dish of gourmet drinking "water".

-- Mark Sp.

 

 

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