RO Water using Lactic acid instead of phosphuric

[tschmitt]

Gordon Strong starts with a quarter teaspoon of 10% phosphoric acid in his RO water. I want to try adding it to bring down PH but I can not find phosphoric locally. I can get 88% Lactic acid.

Does anyone know how much Lactic to use to find the phosphoric equivalent?
Or does anyone have the formula to do the conversion?

 

[ajdelange]

I can tell you how to solve the problem which is better than just giving you the answer. A tsp is officially 5 mL so it's easy to figure out how much a quarter tsp. is. Use the density of 10% phosphoric acid which you can find on the internet to determine the mass of 1/4 tsp and then multiply that by 10% to determine the actual mass of phosphoric acid. The only tricky part comes next. Phosphoric acid is a weak triprotic acid but in the region of pH in which you will be using it you can treat it like a strong monoprotic acid so determine the amount of protons released by dividing the mass of phosphoric acid by its molecular weight.

Lactic acid is a weak monoprotic acid but again in the region of pH of interest you can consider it strong. Figure out the mass of lactic acid you need by multiplying the amount of protons you would get from the phosphoric by the molecular weight of lactic acid. Divide this by 0.88 to get the mass of the diluted acid and then divide that by the density of 88% lactic acid. That may be a bit hard to find on the internet so I'll give you that here: 1.206 g/cc. This is the number of mL of 88% lactic acid that provides the same number of protons as 1/4 tsp of 10% phosphoric (approximately). A slightly more accurate answer can be had by doing an appreciably more complex calculation and is not justified.

 

[tschmitt]

That is fantastic. Thank you for taking the time to explain & provide.

 

[tschmitt]

1.206 ml = 1/4 teaspoon

So in the end 1/4 teaspoon of phosphoric at 10% = 1/4 teaspoon of 88% lactic
when it comes to its impact on Ph?

 

[mabrungard]

Blindly adding acid to water can get you into trouble. While Gordon's recommendation is sound for brewing pale beers, it is probably not needed or desirable for brewing beers with more acidic grists. The relative lack of alkalinity in RO water makes the need for acidification apply only to the palest beers. Even sparging water that starts as RO water does not need to be acidified. Its alkalinity is low enough already.

 

[Microphobik]

1.206 ml = 1/4 teaspoon

So in the end 1/4 teaspoon of phosphoric at 10% = 1/4 teaspoon of 88% lactic
when it comes to its impact on Ph?

Sorry to bring up an old post, but I would love to know the answer to this as well. I had a hard time understanding the equation above. Sorry.

The water in my area is very inconsistent and so I have decided to start brewing with RO water and following Gordon Strong's recommendations as well. But I can only get 88% Lactic acid. Are they essentially equal by volume, in terms of the impact on PH, as the thread I quoted suggests?

 

[ajdelange]

Errors here

RE #4:

1.206 is the density of 88% lactic acid, not the number of mL in a quarter tsp. A metric tsp is exactly 5 cc so 1/4 tsp is 1.25 cc.

88% Lactic acid is, WRT pH 5.5, 11.5 N
10% Phosphoric acid is, WRT pH 5.5, 1.09N

Thus the lactic acid is, per unit volume, 10.55 time stronger

RE #5:
One can indeed get into trouble by throwing acid about without thinking but beers that are quite dark can require some - even some stouts.

RE #6:

No, they are not approximately equal in strength. 88% lactic is more than 10 times stronger than 10% phosphoric. Now 85% phosphoric acid is 14.9 N WRT pH 5.5 and thus 14.9/11.5 = 1.3 times stronger than 88% lactic.

 

[Microphobik]

So are you saying that if Gordon Strong calls for .25 tsp 10% Phosphoric acid I should use .19 tsp 88% Lactic acid? Not at all sure I did that right. Thanks.

 

[ajdelange]

No, I said 88% lactic is 10.55 times stronger than 10% phosphoric so if Gordon wants 0.25 tsp 10% phosphoric that is equivalent to 0.25/10.55 = 0.024 tsp lactic, i.e. about one tenth as much.

 

[Brewskii]

yeah, i use the 88% lactic and its always just drops to get where i'm going with a 6 gal batch using RO and building from scratch