Diluting Down 99% IPA to the desired concentration: A Lesson in Math & Chemistry

https://onsenlabs.com/blogs/blog/isopropyl-alcohol-ipa-aka-i...

Also includes a dilution table for 91%.

Their table seems reasonable, but the first example they work through is wrong.

>Example 1:

>Suppose a scientist has a bottle of 99% IPA (C1) that they want to dilute down to 70% (C2). To end with 50mL of 70% IPA (V2), how much volume of 99% IPA should they use and dilute with distilled water to create the final solution?

>99% x V1 = 70% x 50mL = 35.35mL

>Thus, adding 35.35mL of distilled water to 14.65mL of 99% IPA creates a 50mL solution of 70% IPA.

(They reversed the water and IPA quantities.)

The table's garbage. It's showing the total volume as exactly equal to water volume plus alcohol volume, out to four digits. That's not how it works. A mixture of water and alcohol (whether ethanol or isopropyl) gets more compact because the different molecules can pack together better, by up to a few percent.

Is it inaccurate enough that it cannot be used to safely create disinfectant-grade 70% dilutions from 99% or 91%?

If you're measuring things by slopping them into a measuring cup instead of weighing them out to the milligram, you'll never know the difference.

It won't matter for "eh somewhere between 55% and 75% is fine", but if you have a good measuring cup you could easily see the difference in volume.

I don't know why the chart assumes you're measuring down to thirds of milligrams, though...

Good catch!

That didn’t answer my question about what kind of water to use.

Tap water will be fine. The alcohol you're diluting will disinfect it.

If you want to be ultra-sure, then use distilled. But be aware it's also bought by people who use it for their CPAP machines, so don't go nuts.

It depends on where you are; my tap water has extremely high mineral (>40 gpg) content, and I could definitely see that interfering with its disinfectant properties (not to mention leaving a residue behind).

It repeatedly says "distilled water."

"… we find that they would need to add 3.31 ounces of (distilled) water with 8 ounces of 99% IPA …"

Frankly I don't think it matters at all. Consider if you wipe down a surface with 70% IPA it'll dissolve all sorts of crud.

It’s preferable to used sterile distilled water or water that has been boiled according to the WHO.

https://www.who.int/gpsc/5may/Guide_to_Local_Production.pdf

Thank you!

Yes. Just dilute it in the correct ratio to bring it to the required concentration. The purity of the water is not particularly important. (If it's drinkable, it's perfectly OK to use. Go ahead and use tap water if you want.)

The formula is simple: 'Concentration Required' divided by 'Concentration Supplied' multiplied by 'Final quantity Required'. That will give you the amount of 91% isopropyl alcohol you need to measure out, then dilute to the 'Final quantity required' with your tap water. (Or better still, use Distilled Water or De-Ionized Water.)

So: You want 500 mls of isopropyl alcohol 70% and you have isopropyl alcohol 91% on hand -

      Concentration required = 70%

      Concentration Supplied  = 91%

      Final quantity required = 500 mls

Amount of isopropyl alcohol 91% needed = 70 / 91 x 500 = 384.6 mls

Dilute that 384.6 mls to 500 mls with water and there's your 500 mls of isopropyl alcohol 70%.

Of course, you don't need to be quite so finicky when it comes to biological measurements. You'd round off the 384.6 mls to 385 mls (or even 380 mls or 390 mls) without any appreciable loss of effectiveness.

If you mix cheap booze (e.g. 40% vodka) with 91% isopropyl, you can make a larger quantity of 70% sanitizer than you could with pure water.

Mixing on a scale can be pretty confusing, though, because the percentages are (I think) by volume, and water and alcohol have different densities.

It can also be confusing if you check the volumes after mixing, since the water-alcohol interaction decreases the molecular spacing and the total volume. V_(water + alcohol) < V_water + V_alcohol.

https://en.wikipedia.org/wiki/Alcohol_by_volume#Volume_chang...

Any booze you buy has a huge tax on it, so you would be better of, with cleaning alcohol.

Normally, yes. Today, I’m not even able to find isopropyl at many stores. I’m actually thinking about ordering flux cleaner for my soldering, for the first time in my life.

You can get methanol pure and cheap from auto stores (in regions that get cold, anyway). Just try not to get it on your skin or breath the fumes too much. It's quite a bit more toxic. Definitely don't use it to make hand sanitizer.

And also ethanol vs isopropanal, might add a bit of complexity

But then don’t you lose the qualities that make water important in disinfecting?

What do you think the other 60% of vodka is?