Friday, April 3, 2020

How to make hand sanitizer with 91% Isopropyl alcohol if you don't know any algebra

Note: This post has been substantially written, for clarity. The information in the earlier version is correct, but difficult to absorb.
     We tried it. It's runny and it stinks— but only for a few seconds; after which the odor completely disappears and your hands feel fine: smooth, not too dried out from alcohol and no hand lotion stickiness. Aloe rocks, but some of the gel tends to sink in the alcohol so shake before each use!
     You can make this stuff and you may have to if you live in Omaha, where hand sanitizer is almost impossible to find as of this writing, weeks into the American coronavirus epidemic.
     We found some isopropyl alcohol at Walmart Neighborhood Market and snagged the last two bottles of a very nice Walgreens-branded "After Sun" aloe gel (on sale if you buy two!)
Omaha is the home of Marianna, a world-wide supplier of hair products, which has 145,000 square feet of FDA approved production space in Omaha but isn't making any hand sanitizer, even though other cosmetic companies (like L'OrealUSA) here and in Europe have stepped up to the plate. GOP Mayor Stothert praised an Omaha brewery making hand sanitizer (for first responders) but apparently hasn't bothered to ask Marianna, which could really crank out the stuff, to do something for the wider local community. Neither evidently, has GOP Gov. Pete Ricketts.
Pencils ready, class?

How to make hand sanitizer with 91% isopropyl alcohol

     The ratio is simple: Mix aloe gel with 100% alcohol, which must constitute no less than 60% of the total mixture. (Most commercial hand sanitizers are 70% alcohol.)
     That's straightforward: 60-40 alcohol/aloe or 3 parts alcohol and two of aloe.
     But most drug store alcohol (DSA) is 91%, not 100%, so what's the correct ratio, to get to say 70%?
     Here's the math, set off with rules, which you can skip, if you wish:

My 8 oz. bottle of hand sanitizer was nearly empty, so I emptied it into the 3 oz. travel bottle in the truck. Then I washed it out to be refilled.
     I needed to make 6 oz. of mixed hand sanitizer in the 8 oz. bottle (room to shake).
     70% of 6 oz. occupied by 100% alcohol is 4.2 oz, but you bought Drug Store Alcohol (DSA), which was only 91% alcohol.
     So how much more DSA do you need to get to 4.2 oz of pure alcohol?

     The answer is about 1/2 an ounce more, or 4.6 oz.

Here's the equation (involving some very simple algebra.)

.91 x DSA = 4.2 oz.
(4.2 oz. is the 70% of 6 oz. that must be 100% alcohol.)

You solve the equation for DSA, the volume of Drug Store Alcohol you need, by dividing both sides of the equation by .91

.91 x DSA            4.2
----------     =     ------------
    .91                    .91

which simplifies to:

DSA   =  4.6 oz, the same amount of alcohol as 4.2 oz. of 100% alcohol

4.5 is close enough, since you're aiming for 70%, more than you need.

Now all you need do is:
  • Pour exactly 4.5 ounces of water into the empty 8 oz. bottle, and mark it.
  • Then pour exactly 1.5 more oz. of water, to make 6 oz., and mark it again.
  • Pour out the water.
  • Pour in the 91% DSA up to the first mark.
  • Add aloe gel up to the second mark (no higher!)
  • Close the bottle, and shake.

You're done and you're welcome.

No-algebra cheat for using 91% Isopropyl alcohol to make hand sanitizer:

3 parts 91% isopropyl alcohol
1 part aloe gel
Three-to-one (75/25) makes hand sanitizer with an alcohol content of 68.25%, comfortably above what you need and close to the 70% in commercial preparations.


...And don't drink isopropyl alcohol (yuck, have you smelled the stuff?) or leave it unsecured around desperate alcoholics or senseless teenagers who will do anything on a dare.