Math puzzle

[William Seger]

I have a bottle of 91% isopropyl alcohol, but scientists recommend 70% for killing coronaviruses. How much water should be added to one ounce of a 91% solution to make it a 70% solution?

(I think I finally got the right answer, but it was tougher than I thought -- but I probably did it the hardest possible way. )

[LunaSea]

1. Simple.

Just use the 91%.
The cooties will be 21% deader.

[William Seger]

2. Apparently not

The water helps the alcohol penetrate farther into the fatty protein shell.

Anyway, I'm slapping my forehead over how complicated I made a simple problem.

[soothsayer]

3. Supposed to mix in aloe Vera gel

Cuz skin irritant otherwise.

[William Seger]

4. I'm not using it as hand sanitizer

I'm using it to sanitize some N95 masks for reuse. The fibers in the disposable ones (also the disposable surgical masks) are not degraded by alcohol and can be reused many times -- just soak them and let them dry.

[sl8]

9. Regarding treatment of masks with alcohol

There is some evidence that treatment with alcohol will reduce the efficacy of N95 masks:

From https://elautoclave.files.wordpress.com/2020/03/stanford-2020.pdf

[...]
In the previous Stanford Medicine COVID-19 report compiled by Amy Price, DPhil (Oxon) and
Larry Chu, MD, we have shared our results based on a one-time disinfection treatment (Table 1).
We concluded that 75% alcohol solution and chlorine-based solutions were detrimental to
the static charge in our meltblown fabric, and resulted in reduced efficiency.
[...]

Also, much more information on decontaminating masks,:
https://news.stanford.edu/2020/04/01/researchers-show-how-to-decontaminate-reuse-n95-masks/

New website explains how hospitals can decontaminate and reuse scarce N95 masks to fight COVID-19

Researchers from Stanford and other universities scoured the scientific literature to create N95decon.org, a web portal that medical professionals can access for trustworthy information on how to decontaminate used N95 masks.

[RockRaven]

5. 0.3 units of water per unit of 91% to get 70% result, basically...

The easiest way to do the math, for me, was to set it up like this:
0.91*1ounce=0.7*X and in this set up, X=final volume of 70%
Divide both sides by 0.7 and X=1.3, meaning 0.3 must be added to the original ounce of 91% stuff.

As a side note, this is just how the math works. In reality, there is another consideration. When you dissolve one thing in another, they occupy a smaller volume than the sum of the original two volumes, and this holds true for these two liquids. It doesn't matter on the scale of mixing your own house cleaning agents, but it could matter, sometimes, in a laboratory setting.

[William Seger]

6. Yep, I did it the hard way

... setting up simultaneous equations where the amount of alcohol, the original amount of water, and the added water were all unknowns. That's unnecessary because as stated, they aren't unknowns: one ounce of 91% solution contains 0.91 oz alcohol, and that doesn't change, so the simple version of the question is what does the total volume need to be such that 0.91 oz is 70% of that total.

[ProfessorGAC]

7. Don't Stress Too Much WS

The efficacy difference between 65 & 75% is negligible.
Talking 10.5 seconds, instead of 10.
One correction to your earlier reply above:
The added water does not really help that much with viruses. The water greatly improves the efficacy on bacteria, because the lack of water causes them to slow respiration, allowing some to survive the evaporation time. The water also changes the vapor pressure above liquid, so it takes longer to evaporate, and we get less cooling by evaporation.
The lipid sheath on a virus is insoluble in water. Extra water would change the solvolytic partitions, actually slowing the dissolution of the lipid into IPA.
The 91% is more effective on viruses, by a bit.
But, since the 70% is much better on bacteria, microbiologists use 70% as the standard. It's effective on any microbe except some spores.

[William Seger]

8. I went back and read the article

... where I read that, and it said the extra water in the 70% solution delayed the evaporation of the alcohol, allowing it to penetrate farther into the covid-19's shell. I've now looked at a couple more articles that say 70% is better for killing bacteria but 91% is fine for killing viruses. Nonetheless, for some reason, the prevailing recommendation still seems to be to use 70% for covid-19 sanitization. Maybe I'll compromise and just add a little water.

[ProfessorGAC]

10. Safe To Kill All Microbes At Once

I worked with several microbiologists. Mostly because for a while I was doing work optimizing conditions to improve purity & quality of the active ingredients in sanitizing cleaners.
Also, several high end surfactant products are so biodegradable, they are susceptible to microbial contamination. So, methods to sanitize and create engineering barriers to entry were done commonly and the micro folks had to challenge test the effectiveness.
I saw lots of tests run for microbial content & challenge testing for antimicrobial agents.
Otherwise I wouldn't know the details I know.