Rubbing Alcohol

[ednadolski]

If you leave 99% alcohol out it will absorb water from the air till it becomes 91% ...

Interesting, I did not know that.   But if you leave it out for too long it will all evaporate away anyways.

Ed

[Chris333]

So I just searched and it looks like it will absorb water till 65%. But yeah if it open to absorb water why doesn't it evaporate?

https://electronics.microcare.com/resources/faqs/hygroscopic-mean-important/

[Chris333]

And one place says a 91% bottle will turn to 70% soon as it's opened. Not sure it would happen instantly like that though.

[peteski]

And one place says a 91% bottle will turn to 70% soon as it's opened. Not sure it would happen instantly like that though.

Chris, you know better than to believe everything you see in the Internet.    Verify your sources, and make sure you get the same info from multiple reliable sources.

This entire absorption of moisture (water) from ambient air seems fishy to me.  Of course alcohol will evaporate if left open to ambient air. The evaporation speed will likely be dependent on the surface area of the alcohol exposed to open air.   I would think that its evaporation rate would be faster than any possible moisture absorption.  And what about the relative humidity and temperature of ambient air? You ambient air could have 70% relative humidity or maybe down to 35% RH.  That would likely also affect the absorption rate and amount.  So will the temperature of the air. I really think you have to take this entire moisture absorption of water with a grain of salt (BTW, salt really absorbs moisture from the air). 

[Chris333]

Yeah I was trying to find a source...

Found many different places that say it absorbs water, but cannot find the rate. It is list as why you cannot buy 100% alcohol because just making it introduces it to air and water.

[Tom L]

Smell wise, I don’t know but it’s noticeably less than the stuff I used to buy at the grocery store or wherever for like $3 or $4 for the 91%.   Maybe the Medical grade has something to do with it?

The difference in performance isn’t worth the extra $, but the days of cheap rubbing alcohol are over for a while, I suppose, and I didn’t want to drive around looking for some.

Prior to this I used Denatured alcohol to strip a few shells and it worked ok as well.  It was the stuff from the paint department at the Depot. My casual non-scientific (and probably incorrect) observation was that it was faster than Isopropyl on smooth surfaces but didn’t work as well in areas like fans and louvres.

Anyway, for stripping paint I always use a sealed container and don’t open it for about a day.  I live in a dry place, I can’t imagine it would absorb much moisture from the air, but who knows.........our world is a mysterious place.

I never did well in chemistry, so none of it makes sense to me 

Tom L
Wellington CO

[MK]

Alcohol is hygroscopic in nature.  That's a fact.

Back in my RC nitro days, we run "nitro" for our engines.  "Nitro" is mostly alcohol base with nitromethane and a few other ingredients.  They sell them by the gallon though later on they sell them in quart size (to lessen the moisture absorption and avoid the hazmat charges but that's another story).

Enough moisture absorbed by the nitro and it goes bad.  You can't fire up your engine or if it does fire, it would run rough or be very hard to tune the needle for fuel adjustment.  Moisture doesn't get absorbed by leaving the cap open but for each time you open it to refuel your engine or squirt bottle, you let in a new fresh volume of air into the gallon bottle.  And the more you use the fuel, the more space is left for "new" air each time you pour the nitro out.  Now you close the cap with this "new" air and whatever moisture is in that volume is then absorbed by the alcohol.

Worse scenario is when the racing season ends (e.g., winter) and you have 20% fuel left in that gallon jug.  I can guarantee you that by Spring, when race season starts, most likely that 20% is no good any more.

Some film chemicals (back in the days) are like that but more for the reaction to air rather than moisture.  They have accordion jugs where you can make the jug fit the liquid to avoid any empty volume.  Similar to this.

https://www.bhphotovideo.com/c/product/1557486-REG/cinestill_tcs_1003_cs_accordian_chemical_bottle.html?sts=pi&pim=Y

[peteski]

Mike, but you're talking weeks or days for the glow fuel to absorb enough mosture to be useless, not hours or days.  And I'm sure the absorption amount also depends on the relative humidity.  If you live in a high-humidity area (on the East COast of U.S.) this will be more of a problem than if you lived in the Arizona desert (where air is very dry and it might not contain enough moisture to make the glow fuel to go bad).

[Joetrain59]

70% and 91% scarce here in CT. too. I just got lucky on Friday, and got 2 qts. at Wallyworld. Bottom shelf, all the way in the back. Has to ask a Gent to to get them for me.
 I use it a lot for paint stripping, and in the garage too. Clean tools with it. As they say, "a clean tool is a safe tool".
   Joe D

[mmagliaro]

A few months ago, I bought a gallon of 99% off eBay - the type in 4 1 quart containers.  It was a little expensive - about double the typical cost of alcohol.   You can get a quart for about $14 with free shipping right now.   But... you can cut it with water to make 91% of 70%, which will get the cost down.   We're just using it as-is, both for modeling and for bathroom/antiseptic purposes.   Since a pint used to cost around $3 in the store, $6/quart,  14 is kind of high.  You can get a gallon for about $32, free shipping.  That's actually not bad, $8/quart.  When you consider the shipping, that's a pretty darn good price.

[Chris333]

I remember buying quarts for 88 cents to strip paint.

[MK]

Mike, but you're talking weeks or days for the glow fuel to absorb enough mosture to be useless, not hours or days.  And I'm sure the absorption amount also depends on the relative humidity.  If you live in a high-humidity area (on the East COast of U.S.) this will be more of a problem than if you lived in the Arizona desert (where air is very dry and it might not contain enough moisture to make the glow fuel to go bad).

This is true but I was commenting on your statement questioning whether alcohol absorbs moisture at all, not the rate of absorption.

This entire absorption of moisture (water) from ambient air seems fishy to me...I really think you have to take this entire moisture absorption of water with a grain of salt (BTW, salt really absorbs moisture from the air). 

And Chris reading somewhere that as soon as you open a 91% bottle it turns into 70% bottle is hogwash based on my experience with nitro fuel.  If that were to happen we would only get one race out of that $30 gallon bottle!   

[MK]

I remember buying quarts for 88 cents to strip paint.

I was at my local ShopRite (supermarket) yesterday and they had a whole shelf of 70% rubbing alcohol for $2.99, limit 2.  3x what it normally goes for!  And ShopRite is know of their good prices.

[GonzoCRFan]

Yeah I was trying to find a source...

Found many different places that say it absorbs water, but cannot find the rate. It is list as why you cannot buy 100% alcohol because just making it introduces it to air and water.

Depending on the mixture, one also might not be able to easily purify the alcohol to 100% to begin with. Ethanol and water form what is called an an azeotropic mixture, where the mixture has a lower boiling point than either of its components. This is why our life-affirming friend Everclear is only 95% ethanol: it's cost-prohibitive to "break the azeotrope"  to get a higher yield of ethanol.

[cv_acr]

Well, IPA also damages paints (we use to strip the paint from Kato shells).  It is toxic if you drink it (that is why it is called denatured), so it is not consumable, but it is fine for its intended purposes.

That's not what "denatured" means; denatured means something is specifically added so it's not pure.