Re: 2465b possibly really stupid question (noob)

Richard Knoppow

CVS in Los Angeles has both 70% and 91%. Sold for medical use. Does not say USP on the label. I am not sure why USP registration is important for a cleaning agent. Note that the USP is a private institution, not a government agency.
   The percentage of Isopropyl alcohol is important when used as a cleaner. It is really a measure of how dry it is. Because IPA absorbs moisture from the air truly anhydrous IPA is nearly impossible to produce or to package. The driest form is 99+%. When opened it will absorb moisture from the air until it reaches 91%, which is its equilibrium point. That's why 91% is the driest one can get off the shelf at a drugstore. 99+% is especially packaged, it is available in bottles and in spray cans at places that sell computer supplies but the bottled kind will become 91% if not kept capped and sealed. Presumably spray cans do better but I have no proof.
   The driest kind is important where residual moisture is consequential. For instance, now that trichlor is no longer easily available Kodak recommends 99+ IPA for cleaning the emulsion side of film. Even a little moisture will be taken up by the emulsion causing streaking which may not dry out. Note that IPA should NOT be used on the support side of film because it can attack some types of support.
   Before trichlor was ruled an environmental hazard the standard film cleaner was carbon tetrachloride. However, while an effective cleaner it is highly toxic and should be avoided. Its also hard to obtain.
  Where a small amount of moisture can be tolerated 91% IPA is satisfactory but probably 70% should never be used because it leaves too much moisture behind and it may take much longer to evaporate than you would think. 70% is used as rubbing alcohol because it is less irritating to the skin than stronger forms but still has reasonable germicidal properties.
    Probably all this stuff is on the web somewhere.

On 5/13/2020 11:09 PM, Roy Thistle wrote:
On Wed, May 13, 2020 at 05:37 AM, Chuck Harris wrote:

I won't say you *can't* buy USP labeled IPA in drug stores,
or grocery stores, in the USA, but, I have never found any in
all the years I have been buying it.
Sunmark Isopropyl Rubbing Alcohol, 70% USP, 16 Fl. Oz., at Walmart.
Cheers all.

Richard Knoppow

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