Thu Oct 27, 2016 10:00 am

I came across this thread while looking for an answer to a question that I had. I know it's an old question, but maybe I can shed some light on this for anyone else wondering.

Short answer: 5.35*10^21 molecules, each contributing one (H+) and one anion (CH3CHOHCOO-).

Long answer:

First off, it's simply a lot easier to use the same units for everything. Using non-standard units causes most of the head scratching for everyone. Water reports are in ppm, acid in % by dry weight per volume, equivalents, etc. It's easier for the agencies, manufacturers, or whoever to use what they use, so that's why we are always scratching our heads.

As was stated in a previous reply, acid percentage is grams of dry acid per 100mL of solvent, in this case water.

For an 80% solution of lactic acid, you have 80g lactic acid powder added to 100mL water. Using the molar mass of lactic acid, 90.08 g/mole, you can divide into your 80g starting weight and find your solution to contain 0.888 moles. Multiply this by Avogadro's number, 6.022*10^26, and you will find that you have approximately 5.35*10^23 molecules per 100 mL of solution. I say approximately because adding the acid to the solution increases the volume. If you really wanted to know exactly what you had, you could measure the density and use that to calculate the exact amount.

So, for 1mL, you have 5.35*10^23 / 100 = ~5.35*10^21 molecules. The formula for lactic acid is CH3CHOHCOOH, and it dissociates into CH3CHOHCOO- and H+, so you get one ion of each contributed to your solution for each molecule that you add. In a highly concentrated solution, you do not get complete dissociation because it is a weak acid. However, as you are adding it to a 5 gallon batch, you should see complete dissociation.

Keep in mind that 1 ppm of liquid is not 1 molecule of chemical X per million molecules of solvent Y. It is one milligram per liter of solvent, usually water. So, for 400 ppm, you would have 0.400g of X per liter of water. You still need to multiply by total volume and Avogadro's number, then divide by the molar mass of X to find the number of molecules or ions present.

If you were feeling froggy, you could even calculate the expected pH of your strike water. And if you knew what the statistical contribution was from the components of your mash, you could calculate your pH mid mash.

Hope this helps.