Author Topic: Diluting 90% alcohol to 70% alcohol.  (Read 845 times)

230RN

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Diluting 90% alcohol to 70% alcohol.
« on: December 07, 2020, 10:46:15 AM »
Check me on this.  It sounded too simple and I'm somehow uncomfortable with it.

A neighbor acquired a supply of 90% isopropyl.

Wanted to know how to dilute it to 70%

I noodled this one out:

Place a suitable container on an accurate kitchen scale.  Zero the scale by pressing the "Tare" or "Zero" button.

Put some (not too much) 90% isopropyl in the container.  Take its weight.

Now 90% of that weight is isopropyl alcohol, so 0.9 X that weight is the weight of the isopropyl alcohol.

Since you want weight of alcohol divided by total finished (diluted)   weight to equal 70%, (0.7)  then:

Weight of alcohol / total diluted weight  = 0.7

Or, transposing terms,

Weight of alcohol / 0.7 = Total diluted weight.

So while the container is on the scale, add enough water to make the total weight what you just calculated.

I believe (if this is correct) the same method can be used volumetrically by using a straight-sided container and measuring the starting height with a ruler, taking 90% of that, for the "height" of alcohol, then:

"Height" of alcohol / 0.7 = "Height" of diluted alcohol, so add water to make that total height.

Sounds too simple.  Where might I have gone wrong?

Terry
« Last Edit: December 07, 2020, 10:59:10 AM by 230RN »

Nick1911

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Re: Diluting 90% alcohol to 70% alcohol.
« Reply #1 on: December 07, 2020, 10:57:51 AM »
Is alcohol content measured by weight, or by volume?

Think it's usually only measured by weight in 3.2% states...

zahc

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Re: Diluting 90% alcohol to 70% alcohol.
« Reply #2 on: December 07, 2020, 11:06:31 AM »
Alcohol concentration is measured by both weight and volume. Alcohol by volume is more common for beverages but wt% would be more typical. But I think they actually still measure by weight even when mixing to ABV.
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AZRedhawk44

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Re: Diluting 90% alcohol to 70% alcohol.
« Reply #3 on: December 07, 2020, 11:08:38 AM »
Do not use weight.  Water and alcohol have different gravities, or densities.  One liter of alcohol weighs less than one liter of water.

I believe most alcohol given a ratio is done by volume, not weight.

If you have 1 liter of 90% alcohol and want to have 70% alcohol, you need to add water to dilute the alcohol.

In your current 1 liter of solution, you have 900ml of alcohol.  If you want a 70% solution, you need to know the total solution volume a 70% solution would have with 900ml of alcohol in it.

With V as our total volume variable:

0.7*V + 0.3*V = V

When 0.7*V = 900ml, divide both sides by 0.7.  V = 1285ml.  For a 70% solution with 900ml of alcohol, you need a total volume of 1285ml.  That means add 285ml to your 1000ml solution for a total of 385ml water to your 900ml alcohol.
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MillCreek

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Regards,
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Quote from: Angel Eyes on August 09, 2018, 01:56:15 AM
You are one lousy risk manager.

Brad Johnson

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Re: Diluting 90% alcohol to 70% alcohol.
« Reply #5 on: December 07, 2020, 12:04:39 PM »
If you want to keep a written formula around, it's pretty simple. Three steps that flow from one to another with no fancy math involved. It's basically what AZred used above, just from a different angle mathematically speaking.

CLV x CAC = CAV
CAV / TAC = TLV
TLV - CLV = water needed to reach target concentration


CLV = Current Liquid Volume (ml or oz, doesn't matter so long as you use the same unit all the way through)
CAC = Current Alcohol Concentration*
CAV = Current Alcohol Volume
TAC = Target Alcohol Concentration*
TLV = Target Liquid Volume

*All concentrations expressed as a decimal.

Example using 1000ml total current liquid volume. Current concentration is 91%. Target concentration is 70%.

1000 x .91 = 910
910 / .70 = 1300
1300 - 1000 = 300ml water need to reach a target concentration of 70%

____________________________________________

If you want something you can keep as a self-calculating spreadsheet, open a new sheet and paste the following into cell D2.

=*((A2*(B2/100))/(C2/100))-A2
Remove the red asterisk. Had to put it there or the forum software thought I was trying to insert an emoticon.


Your first row will be labels. Label cells thusly (or with whatever text suits your fancy)
Cell A1) Current Volume
Cell B1) Current Alcohol Percent
Cell C1) Target Alcohol Percent
Cell D1) Liquid Needed

Now all you have to do is enter the liquid amount and the percentages as whole numbers (i.e. 91% is entered as a simple 91 with no decimal). The result is given in the same liquid units as the Current Volume entry.

Brad
« Last Edit: December 08, 2020, 09:46:27 AM by Brad Johnson »
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French G.

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Re: Diluting 90% alcohol to 70% alcohol.
« Reply #6 on: December 07, 2020, 12:27:19 PM »
Simple, advocate for keeping it 90%. Way easier to set people on fire with that.
AKA Navy Joe   

I'm so contrarian that I didn't respond to the thread.

230RN

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Re: Diluting 90% alcohol to 70% alcohol.
« Reply #7 on: December 08, 2020, 04:16:36 PM »
Jesus H. Christ.

These solutions mostly seem to follow my logic in OP except for the known problem of volume shrinkage and some methods  take that small difference into account.  But since you're not mixing pure alchol with pure water, I doubt the difference is significant in this operation.

In other words, I don't think precise percentages are necessary.  MillCreek cited a study a while ago which found that 70% alcohol (I believe isopropyl) was more effective in killing some germs than 90%.  Hence the concern about dilution.  However, I doubt the germs could tell the difference between  68.47%  and 72.92% alcohol concentration.

So unless my friend  is embarking on a quantitatively significant project I'm going to recommend the methods outlined in my OP but note that the amount of additional water should be reduced slightly*.  If he desires more precision, I will provide MillCreek's link in Reply #4.

Incidentally, I finally found an old bottle of 91% iso, but the label did not specify whether that percentage was taken volumetrically or gravimetrically.  They do provide a "Questions?" phone number, 1-800-632-6900, and if I ever get curiouser about this, I might call them.  For myself, I think the gravimetric method is more supportable in terms of eliminating that shrinkage factor question, but volumetric is OK as long as you report it that way with details on how the shrinkage factor was dealt with.

I found Nick1911's  "3.2%" remark amusing.  :)

Terry, 230RN

* I'll never forget seeing a recipe for something years ago whose instructions included adding a certain ingredient:  "Enough to fit on the point of a knife." :rofl:
« Last Edit: December 08, 2020, 04:33:33 PM by 230RN »

Jim147

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Re: Diluting 90% alcohol to 70% alcohol.
« Reply #8 on: December 08, 2020, 04:51:59 PM »
Buy some real alcohol and dilute it by waving a sprite cap over the top.
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And sometimes goes on and on and on.

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230RN

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Re: Diluting 90% alcohol to 70% alcohol.
« Reply #9 on: December 08, 2020, 08:59:55 PM »
I thought that was the dry martini method of mixology.  Fill a glass with whiskey and show it to a bottle of vermouth.

Neither shake nor stir nor whisk nor swirl.

« Last Edit: December 08, 2020, 09:34:13 PM by 230RN »