Changing the percentage solution of a chemical.

I assumed this to be a simple calculation but I can only find weird formulas online which I cannot follow.

I have 1L of 100% Acetic Acid and want to make 1L of 28% solution. Is it simply 280ml of the 100% solution mixed with 720ml water?

Yes, that will be close enough. With two remarks:
1: NEVER add water to acid, but do it the other way around. So start with eg 500ml of water and then add the desired amount of acid.
2: top off to 1000ml; you may need marginally more than 720ml of water (in total, the initial 500ml + whatever you need to add to make 1000ml).

The first remark is crucial for your safety!!!

I'm most definitely not one of the whimpy types when it comes to handling chemicals, but glacial acetic acid is one of those things I'd rather not have around the house. Nothing wrong with it per se, but the vapors are rather nasty and it's one of those things you really don't want to splash on your clothes, let alone body or God forbid in your eyes. Hence the first remark, as adding water to concentrated acetic acid (or any other highly concentrated or pure acid) can create an instant boil with acid splattering around uncontrollably.

koraks said:

Yes, that will be close enough. With two remarks:
1: NEVER add water to acid, but do it the other way around. So start with eg 500ml of water and then add the desired ampint of acid.
2: top off to 1000ml; you may need marginally more than 720ml of water (in total, the initial 500ml + whatever you need to add to make 1000ml).

Click to expand...

Yes, this, on both counts. Adding water to acid is potentially dangerous, as the water will get hot as the acid dissolves, and with strong/concentrated acids, may get hot enough to boil, spattering the concentrated acid. The same is true for strong akali like sodium hydroxide, BTW. And many liquid/liquid systems won't add volume precisely. The classic one is alcohol in water -- there's a significant loss of volume for the mixture compared to the volumes of the two components when diluting high concentration alcohols. Acetic acid is chemically related to ethanol, and thus may lose volume when being diluted, but it's a general rule when mixing chemicals to start with less water than the arithmetic says, get everything mixed and fully dissolved, then top up to final volume with water.

Also note that a the end, you'll be adding your top-up water to the acid solution, but at that point your acid will be (based on the example above) below about 50% strength and it's safe to add the water to that strength acid. If it were sulfuric acid, I'd start with a bit more water (say, 650 ml instead of 500 ml) to give the acid all the dilution that's practical before the final top-up.

Thanks guys, that's perfect.

"Do as you oughter, add the acid to the water." That's how I remember...

When diluting glacial acetic acid, you're making a vol/vol solution (e.g., x ml acetic acid in x ml total solution). For a 28% solution, you want 100ml total to contain 28ml acetic acid (or multiply by 10 and get 280/1000ml).

The arithmetic for this is intuitive, but for other dilutions can be less obvious. I use the cross-multiply proportion method. Here it is for future reference.

I've been making limoncino lately (a good lock-down activity ), so I'll use that as an example:

Say I have 750ml of 40% lemon-infused vodka that I want to dilute down to 30%. Here's how I find how much total solution I need:

First we make a proportion: Q1/Q2 :: C2/C1 (these should be formatted like fractions: Q1 = quantity 1, Q2 = Quantity 2, C1 = Concentration 1 (%), C2 = Concentration 2). The "x" unknown can go anywhere, but we have to know the other three values. So, for my limoncino recipe, we plug in the three known values:

750ml/x ml :: 30%/40% (750ml is Q1, x is Q2, the quantity we're trying to find. 30% is C2, the desired dilution, 40% is C1, the current dilution).

We multiply numerator of fraction one (750) by the denominator of the second (40) to get 30,000

Then multiply the denominator of the first fraction (x) by the numerator of the second (30) to get 30x.

Now we make the equation: 30,000 = 30x. Let's divide both sides by 30 to get x by itself and end up with: 30,000/30 = x

[If you like, you can just skip the proportion steps and set up the equation: Q1 x C1 = Q2 x C2, in my example: 750 x 40 = 30x,
which is 30,000 = 30x...]

We do the division and get: x = 1000ml. So to make a 30% solution of limoncino, I have to make my total volume 1000ml, i.e., I need to add 250ml water (and then sugar to taste, store in the freezer for a while and enjoy...).

Hope this helps,

Doremus