Isopropyl Rubbing Alcohol (IPA) comes in a variety of strength. Most common are 70%, 91% and 99%. There are a variety of strengths because isopropyl alcohol is used for so many things.
The lower the alcohol concentration, the weaker the solution and the slower it evaporates. Conversely, the higher the concentration, the stronger the solution and the faster it evaporates. These qualities are why different concentrations are used for different purposes.
One use of 70% isopropyl rubbing alcohol is a post-work-out rubdown to soothe exerted muscles. This concentration is used because at 70% alcohol and 30% water, it won’t evaporate before being effective but won’t be strong enough to irritate skin or create uncomfortably strong fumes.
91% and 99% IPA are used for cleaning things that shouldn’t be wet, like computer parts and other electronics. At 99% concentration, IPA will evaporate almost immediately. 70% wouldn’t work in such an application since, with 30% water, the solution will take much longer to evaporate.
IPA is also used for getting ink out of clothing. Sometimes 70% IPA will get the job done. If not, increasing to 91% or 99% may work where the weaker solution was unable.
In situations where the solution strength isn’t important, 70% IPA is the most popular since, containing more water and less alcohol, it is cheaper.
Uses of Rubbing Alcohol -- powered by ehow