Author Topic: Phosphortic Acid question  (Read 270 times)

Offline Mr_Beer

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Phosphortic Acid question
« on: July 05, 2020, 03:24:12 PM »
I purchased a quart of 85% Phosphoric Acid by mistake.  I should have purchased 10% instead.

At the levels that Beersmith recommends for acid addition during the mash step for my size batches (5 to 7 gallon) with my well water the amount required is .04 teaspoon – obviously impossible to measure.

So I want to dilute the 85% down to a more standard solution of 10% with distilled water that would be easier to handle and measure.

Chemistry and dilution formulas are a long time in my past.  So I thought I would solicit a view on the dilution process.

A strict percentage calculation would yield a .118 conversion factor -- --
   5 ounces of 85% PA to 42 ounces of distilled water

And then I got confused – are there contributing ions and other factors to this dilution? 

Any help would be appreciated.

Offline Silver_Is_Money

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Re: Phosphortic Acid question
« Reply #1 on: July 05, 2020, 04:04:49 PM »
The differing densities of 10% and 85% must also be factored in.


Offline Silver_Is_Money

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Re: Phosphortic Acid question
« Reply #2 on: July 05, 2020, 04:10:17 PM »
Vol1 x Density1 x Conc%1 = Vol2 x Density2 x Conc%2

The Density of 85% Phosphoric Acid is 1.689 g/CC
The Density of 75% Phosphoric Acid is 1.579 g/CC
The Density of 30% Phosphoric Acid is 1.181 g/CC
The Density of 10% Phosphoric Acid is 1.053 g/CC

Offline Silver_Is_Money

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Re: Phosphortic Acid question
« Reply #3 on: July 05, 2020, 04:14:31 PM »
Another method:

(Molarity x Volume) = (Molarity x Volume)

The Molarity of 85% Phosphoric Acid is 14.6502
The Molarity of 75% Phosphoric Acid is 12.0848
The Molarity of 30% Phosphoric Acid is 3.6140
The Molarity of 10% Phosphoric Acid is 1.0745

Offline chezteth

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Re: Phosphortic Acid question
« Reply #4 on: July 05, 2020, 07:07:35 PM »
Aside from diluting it down you could use a syringe for children's cough syrup. You should be able to get some at a local pharmacy, possibly for free. I also have 85% phosphoric acid and the syringe works well. You just need to know the volume needed in milliliters.

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Offline Mr_Beer

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Re: Phosphortic Acid question
« Reply #5 on: July 05, 2020, 09:47:05 PM »
Thank you "Silver_Is_Money" for a precise approach.  I turned it into a small spreadsheet and now am good to go for dilution. 

How you determined the density of the acid at various concentration is PFM to me but the data is much appreciated. 

To others who answered, I appreciate the suggestions but I do not want additional equipment and procedures to complicate my brew day. 
Beersmith has standard computations for 10% Phosphoric Acid and that seems good to me. 

Offline Silver_Is_Money

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Re: Phosphortic Acid question
« Reply #6 on: July 05, 2020, 10:48:35 PM »

Offline Silver_Is_Money

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Re: Phosphortic Acid question
« Reply #7 on: July 05, 2020, 10:51:53 PM »
When diluting always add acid to water.  Never add water to acid.