"Crash Lander" wrote in
:

Not at all a scientific type person, so this may be a dumb question, but
is there any way to evaporate the water out of the isopropyl alcohol
without burning off the alcohol, or is the evaporation point of the
alcohol lower than that of the water? If so, could you evaporate the
alcohol out of the water, and collect the pure alcohol that way?
Crash Lander

Having just read up on isopropyl alcohol the other day for other
reasons, I can help answer this....

[warning! I may have gone overboard on the info]

From http://en.wikipedia.org/wiki/Isopropyl_alcohol

Isopropyl alcohol (also isopropanol)....

Isopropanol is a major ingredient in "dry-gas" fuel additive.
In significant quantities, water is a problem in fuel tanks as
it separates from the gasoline. If the engine tried to combust
the water instead of gasoline serious engine problems could
result. The isopropanol does not remove the water from the
gasoline. Rather, the isopropanol solubilizes the water in
the gasoline. Once soluble, the water does not pose the same
risk as insoluble water.

and...

Isopropyl alcohol forms an azeotrope* with water at 87.4% alcohol.
It is impossible to dehydrate isopropanol further using standard
distillation methods. For this reason, more expensive means, such
as using a drying agent, are necessary for production of 100%
isopropyl alcohol.

*
From http://en.wikipedia.org/wiki/Azeotrope

An azeotrope is a mixture of two or more compounds (molecules)
which at a specific composition (ratio of compounds) maintains
vapor and liquid phases which are in equilibrium and identical
in composition. Due to the uniformity of liquid and vapor,
chemical composition of the azeotrope cannot be changed by
simple boiling (distillation).


So that explains why isopropanol is difficult to purify. Also,
as others have questioned, I'm not sure isopropanol is a good
substitute for ethanol for this test.

From http://en.wikipedia.org/wiki/Ethanol

The product of either ethylene hydration or brewing is an
ethanol-water mixture. For most industrial and fuel uses,
the ethanol must be purified. Fractional distillation can
concentrate ethanol to 96% volume; the mixture of 96% ethanol
and 4% water is an azeotrope with a boiling point of 78.2 °C,
and cannot be further purified by distillation. Therefore,
95% ethanol in water is a fairly common solvent.

So both alcohols will contain some water. Although you may
be able to obtain pure isopropanol, obtaining pure ethanol
is practically impossible.

From the article on ethanol,

In most jurisdictions, the sale of ethanol, as a pure substance
or in the form of alcoholic beverages, is heavily taxed. In order
to relieve non-beverage industries of this tax burden, governments
specify formulations for denatured alcohol, which consists of
ethanol blended with various additives to render it unfit for human
consumption. These additives, called denaturants, are generally
either toxic (such as methanol) or have unpleasant tastes or odors
(such as denatonium benzoate).

Some info on Alka-Seltzer...
From http://en.wikipedia.org/wiki/Alka_Seltzer

Alka-Seltzer is a combination of aspirin (acetylsalicylic acid),
baking soda (sodium bicarbonate) and citric acid...

...which are dissolved...in a glass of water. As the tablets
dissolve, the acid and bicarbonate react vigorously producing
carbon dioxide gas

From http://en.wikipedia.org/wiki/Sodium_Bicarbonate

Sodium bicarbonate, when exposed to an acid, releases carbon
dioxide and water


Based on all the above, I would surmise that Alka-Seltzer not
fizzing in gas is simply due to it not dissolving, thus not allowing
the bicarb and citric acid to mix and thus react. The presence of
ethanol or water in the fuel is what permits the dissolving of the
tablets and thus the fizzing.

In the long run, I'd say the only way to be sure is to test several
samples of fuel with known quantities of ethanol and/or water to
determine at which levels the Alka-Seltzer would react. I'd also
try several samples of known 'good' fuel as a control. You should
also verify the samples with other known good testing methods.

Oh, I'd also try crushing the tablets and trying known 'good' fuel
to see if that allows the reaction to proceed.

IHTH

Brian
--
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