[Trent57]

I am no chemist. I thought I could just look this up in some calculator online.. found nothing but complex stuff.

I have 91% isopropyl alcohol. I have 70% isopropyl alcohol.

I desire 80% isopropyl alcohol.

So what ratio do I use? How much 70% do I add per ounce of 91% to make 80%? Or maybe I should add 91% to 70%?

[sjb]

What do you need it for? It can be complex if you need it with decent accuracy, due to the nature of the solutions (liquids). What sort of percentage do you mean (w/w, v/v, etc) and what is the non alcoholic part of the solution?

[Trent57]

I have bottles of 70%, and bottles of 91%. Bottles are standard consumer retail outlet isopropyl alcohol. 

I desire a mixture of 80% for cleaning. 90% is way too much. 70% is a bit too low. 78% works. 84% works. 90% does not work. 70% does not work well.

I am just curious as to the ratio I'd need to mix these in a spray bottle. Doesn't have to be perfect.

Or maybe I am better off adding distilled water to the 91%? I know nothing of these things, chemistry is not my thing.

[billnotgatez]

This is not special chemistry problem
It is more like algebra you learned in high school
you could make the assumptions
   you have just water and alcohol
   you are using percentage based on volume added to volume
   you have a final volume that you have chosen
you create your equation and then solve


I would after that use some brute force method to estimate if my calculations are correct.

below is brute force calculations
see if you can be more precise with algebra

lnsanity check
doing this by volume

The 91% group
100 ml of 91.0% is 91.0 ml of alcohol and 09.0 ml of water
090 ml of 91.0% is 81.9 ml of alcohol and 08.1 ml of water
080 ml of 91.0% is 72.8 ml of alcohol and 07.2ml of water
070 ml of 91.0% is 63.7 ml of alcohol and 06.3 ml of water
060 ml of 91.0% is 54.6 ml of alcohol and 05.4 ml of water
050 ml of 91.0% is 45.5 ml of alcohol and 04.5 ml of water
040 ml of 91.0% is 36.4 ml of alcohol and 03.6 ml of water
030 ml of 91.0% is 27.3 ml of alcohol and 02.7 ml of water
020 ml of 91.0% is 18.2 ml of alcohol and 01.8 ml of water
010 ml of 91.0% is 09.1 ml of alcohol and 00.9 ml of water

the 70% group
100 ml of 70.0% is 70.0 ml of alcohol and 30.0 ml of water
090 ml of 70.0% is 63.0 ml of alcohol and 27.0 ml of water
080 ml of 70.0% is 56.0 ml of alcohol and 24.0 ml of water
070 ml of 70.0% is 49.0 ml of alcohol and 21.0 ml of water
060 ml of 70.0% is 42.0 ml of alcohol and 18.0 ml of water
050 ml of 70.0% is 35.0 ml of alcohol and 15.0 ml of water
040 ml of 70.0% is 28.0 ml of alcohol and 12.0 ml of water
030 ml of 70.0% is 21.0 ml of alcohol and 09.0 ml of water
020 ml of 70.0% is 14.0 ml of alcohol and 06.0 ml of water
010 ml of 70.0% is 07.0 ml of alcohol and 03.0 ml of water

The 80% goal
100 ml of 80.0% is 80.0 ml of alcohol and 20.0 ml of water  <----

the various possibilities

090 ml of 91.0% is 81.9 ml of alcohol and 08.1 ml of water
010 ml of 70.0% is 07.0 ml of alcohol and 03.0 ml of water
giving
100 ml of 88.9% is 88.9 ml of alcohol and 11.1 ml of water

080 ml of 91.0% is 72.8 ml of alcohol and 07.2ml of water
020 ml of 70.0% is 14.0 ml of alcohol and 06.0 ml of water
giving
100 ml of 86.8% is 86.8 ml of alcohol and 13.2 ml of water

070 ml of 91.0% is 63.7 ml of alcohol and 06.3 ml of water
030 ml of 70.0% is 21.0 ml of alcohol and 09.0 ml of water
giving
100 ml of 84.7% is 84.7 ml of alcohol and 13.2 ml of water

060 ml of 91.0% is 54.6 ml of alcohol and 05.4 ml of water
040 ml of 70.0% is 28.0 ml of alcohol and 12.0 ml of water
giving
100 ml of 82.6% is 82.6 ml of alcohol and 17.4 ml of water

050 ml of 91.0% is 45.5 ml of alcohol and 04.5 ml of water
050 ml of 70.0% is 35.0 ml of alcohol and 15.0 ml of water
giving
100 ml of 80.5% is 80.5 ml of alcohol and 19.5 ml of water

040 ml of 91.0% is 36.4 ml of alcohol and 03.6 ml of water
060 ml of 70.0% is 42.0 ml of alcohol and 18.0 ml of water
giving
100 ml of 74.8% is 78.4 ml of alcohol and 21.6 ml of water

030 ml of 91.0% is 27.3 ml of alcohol and 02.7 ml of water
070 ml of 70.0% is 49.0 ml of alcohol and 21.0 ml of water
giving
100 ml of 76.3% is 76.3 ml of alcohol and 23.7 ml of water

020 ml of 91.0% is 18.2 ml of alcohol and 01.8 ml of water
080 ml of 70.0% is 56.0 ml of alcohol and 24.0 ml of water
giving
100 ml of 74.2% is 74.2 ml of alcohol and 25.8 ml of water

010 ml of 91.0% is 09.1 ml of alcohol and 00.9 ml of water
090 ml of 70.0% is 63.0 ml of alcohol and 27.0 ml of water
giving
100 ml of 72.1% is 72.1 ml of alcohol and 27.9 ml of water

[sjb]

I desire a mixture of 80% for cleaning. 90% is way too much. 70% is a bit too low. 78% works. 84% works. 90% does not work. 70% does not work well.

Based on that, I imagine simply a 1:1 mix would be fine (so for instance 100 ml of 91% and 100 ml of 70%). This may not give exactly 200 ml of mix, though.

[AWK]

This can be calculated quite accurately using a dedicated calculator converting percentages by volume to percent by weight.
https://handymath.com/cgi-bin/isopropanolwghtvoltble5.cgi

[billnotgatez]

@AWK

When I clicked on your link I got

Internal Server Error
The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator, and inform them of the time the error occurred, and anything you might have done that may have caused the error.
More information about this error may be available in the server error log.

I found this link which may be what you meant

https://handymath.com/cgi-bin/isopropanolwghtvoltble5.cgi?submit=Entry

Thank you for your research

[AWK]

This is the link content (directly from the forum)

Comparing both links, I can see that this part is missing:
? submit = Entry
Apparently, my computer still remembered the entire link and automatically completed it.

[AWK]

Result for some mixture

[Trent57]

Oh wow thanks much - very handy calculators. Now I can experiment a bit.

[billnotgatez]

@Trent57
let us know what you come up with to achieve the 80% goal

[Trent57]

From the calculator, looks like equal parts 70% and 91% will work almost perfectly to get to 80.64068%. 70% will work; just not very well. 91% will not work. 83%+ is just almost too much (evaporates too quickly), too it seems.

I do have another, really weird question... nothing to do with alcohol. Y'all seem very knowledgeable, and this kinda worries me. I got an old house in 2018, estate deal, guy died in 2016 (vacant 2016-2018) - guy had lived here since 1950. Was a clean fanatic, although not remodeled since 1974, very neat and clean. So a short while ago there's a toilet issue, leak of clean water, not sewage. Still, some rust, mildew, so I figure I'll just toss some bleach on it and wipe it up. Ooops, spilled some bleach. No biggie. No issues. Wipe it up. Then I go into the other room egads, nasty gas smell. Some of the bleach went... somewhere... under the cement floor... perhaps around a pipe... and encountered something it reacted with. Go outside good lord... gag you out there... was a strong reaction. Kinda concerned about what it reacted with. I do not think it was the "normal wrong" bleach and cleaner mixing that people do, as this was worse smell yet did not seem to be as noxious. Blew my nose twice, bit of blood. Fine next day. I was around someone that mixed bleach and cleaner once, the smell wasn't as bad but the effect was worse on me. The nasty smell took a long while to dissipate. And it was worse outside (I have no idea why yet).

I do suspect the old man had cleaned in the bathroom with some kind of dry crystal stuff, as there was some behind the toilet I cleaned up - and that would make sense, years of those crystals seeping around the pipe next to the toilet and piling up under the floor. Dunno something just seemed to worry me about it. I do not yet have access to that area under the floor - something I just noticed - so I have to figure that out, too. 

[AWK]

You have a tool that allows you to calculate your problem, admittedly by trial and error, but much more accurately. Using this calculator you can check that the volume contraction problem is not relevant in this concentration range (less than 0.20%). In this case, you can use the simple method for percentages by weight and without a calculator giving a better result of 80.14% (V/V) than the ratio you are pushing for 100: 100 with the result of 80.64.