Phosphoric Acid dilution calculation

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Well-Known Member
Oct 2, 2023
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At present I'm using Lactic acid for mash PH adjustment, but suspect I taste it in certain beer styles. So planning to switch to Phosphoric acid

The amount needed might be easier to measure if it's diluted, maybe to 50% or 25%. And also safer, in case of any splashes.
With 50% strength, it would take in the order of 11ml, to reduce mash PH by half a point (21 litre mash).

An earlier post Phosphoric Acid dealt with dilution, but the answers didn't agree, or weren't very clear. So hoping someone can confirm I've got this right. then method might be useful to others.

Say for dilution from 75% to 25%:
Due to acids density being different to water, it's not simply going to be 1 part acid : 2 parts distilled water by volume.

One formulae # given in that post was: Vol1 x Density1 x Conc%1 = Vol2 x Density2 x Conc%2
A table of densities (at 20°C) is given at Density of phosphoric acid - Steffen's Chemistry Pages

75% strength - density is 1.579 (g/cm³)
25% strength - density is 1.1462 (g/cm³)

Vol1 * 1.579 * 75 = Vol2 * 1.146 * 25
Vol2 = Vol1 * 4.133
So to make 250ml @25%, you use 60.49ml @75% with 189.51ml of distilled water.

As check: putting figures into brewfather: phosphoric 250ml @25% and 60.49ml @75% both show same change in mash ph. So it seems that formula # is correct.

But measuring small amounts by volume is hard (unless you have pipettes). It's usually easier to measure small amounts by weight on gram scales.

Changing calculation, (for 250ml @25%) to weight:

60.49ml @75% is 60.49ml * 1.579 g/cm³ = 95.51g
The density of water at 20°C is 0.99823 g/cm3
189.51ml water is 189.51 * 0.99823 = 189.17g
total weight =284.68g

Volume of 284.68g Phosphoric acid @25% is 284.68g / 1.1462 (g/cm³) = 248.37ml
(should have been 250ml, not sure why the error. Table figures out?)

Anyway, that's 95.51g acid @75% to 189.17g water
which is 1 part acid @75% to 1.98 parts water by weight, or nigh on 1 part acid : 2 parts distilled water by weight, for a 3:1 dilution from 75% to 25%.
I think the 1.198 should have come out as 2, and guessing there's slight errors in the table (every online table seems slightly different) rather than my calculating to just 2 DP.

Or going back to the start equation:
Vol1 x Density1 x Conc%1 = Vol2 x Density2 x Conc%2
and converting it to weight (weight = volume * density)
Weight1 x Conc%1 = Weight2 x Conc%2
Then much easier, as no density table needed.

Weight of water for dilution = ((Start_Concentration% / Finish_Concentration% ) -1) * Weight of acid

For the example, diluting from 75% to 25%
Weight of water = ((75/25)-1) * weight of acid
= 2 * weight of acid

Where diluted by volume, its best to have both the phosphoric acid and distilled water at as near 20°C as possible. Otherwise it involves checking how density variation with temperature might differ between them. All brew day measurements are done at 20°C, right!
I guess, when diluting by weight the temperature won't matter.

Diluting acids should always be done by adding the acid to the water.
Not the other way round, where an exothermic reaction could be dangerous.

I'm buying Growth Technology pH Down, 81% Phosphoric acid (as it's easier to get).

diluting to 25% from 81%
ratio = ((81/25)-1) = 2.24
so for 87g acid I need 194.88g water
totals about 282g, or 246ml (to fit a 250ml bottle)

diluting to 50% from 81%
ratio = ((81/50)-1) = 0.62
so for 82g acid I need 50.84g water
totals about 132.8g, or 99.5ml (to fit a 100ml bottle)

Brewfather only seems to calculate acid addition given in ml. Not in grams (where I could then use scales).
So I might try a double neck dispensing bottle (with calibrated chamber), rather than fiddle with measuring spoons.
A better use for my 100ml star-san bottle once it's empty, than recycling!
Wow! Or you could just use CRS (same thing as AMS) instead, and use the water treatment calculator on this site. No taste impact from CRS, but it does add some chloride and sulphate.

If you prefer sums to a calculator, 1ml of CRS neutralises 180 mg of CaCO3. Or if you like a really rough rule of thumb, use about 1/3 the vol of CRS that you would use of Lactic (at 80%).

I realise that this doesn't answer your question, just an alternative that you may like to consider. Cheers!
Had thought about AMS, but couldn't fathom how to enable the brewfather setting to use it.
So ordered Phosphoric. Next day, found brewfather AMS/CRS worked (in auto) if I also enabled the AUTO setting!

I'd previously just enabled the MASH setting, hoping to set the amount myself, without success.

I've never worked out why people would prefer using AMS/CRS for for lowering PH, when (as OldFart said) it also adds chloride and sulphate. It seems that using acid, leaves you better control of the other individual elements.

Is AMS/CRS best for hard water areas?
My tap water's very low in minerals, and doesn't need much acid anyway.
I don't know about best, but it doesn't add flavour and the salt additions are OK for my water. It is just a mix of hydrochloric and sulphuric. (6.4% Hydrochloric and 8.6% Sulphuric apparently)
CRS/AMS are challenging for some water sources. E.g. for the volume needed for London water, you'd end up with too much sulphate for a Neipa. The phosphate in phosphoric acid does taste (as in cola), but malt already has tons of sulphate in anyway so you don't notice any difference with a little more.
With being in a very soft water area, the big thing I've actually got to to watch out for, is not going too acidic on dark beers.
You are quite happy to use 80% lactic acid, yet you want to dilute the phosphoric acid down so it's safer.
I had a little look for someone on the US forum and brewfather said that 5.5ml of 50% phosphoric acid was the same reduction as 6ml of 80% lactic acid.
Protect your eyes from splashes, gloves if you want, I just syringe it out of the container and put it in the mash water.
Acid burns are far less serious than alkali burns, washing removes it well from the skin. Contact duration is also a factor in the severity of burns so wash promptly.
I tend to save the lactic acid for dry irish stout. I haven't managed to work out the pH drop associated with the ascorbic acid I add to my mash or sparge water, but suspect it has little effect.
Be aware the the phosphate ions will preciptitate calcium ions which are a very important factor in the mashing phase of the process, that is why CRS or AMS is regarded as being better for alkalinity reduction, the sulphate and chloride ions also are in a good balance.
Be aware the the phosphate ions will preciptitate calcium ions which are a very important factor in the mashing phase of the process, that is why CRS or AMS is regarded as being better for alkalinity reduction, the sulphate and chloride ions also are in a good balance.
Starting to get confused now.
Need some guide for making choice. Or could just press auto in brewfather, and hope.
Starting to get confused now.
Need some guide for making choice. Or could just press auto in brewfather, and hope.
If you start from first principles, then the main negative ions (anions) that brewers are interested in adding to beer are chloride and sulphate, and so if you need to reduce the pH by adding acid, it makes sense to add acids with those anions - ie hydrochloric acid and sulphuric acid. And that's exactly what British brewers do commercially, either individually or as proprietary blends such as AMS/CRS, and British homebrewers have tended to follow suit.

However, it is harder to get hold of such acids in the land of the free, and so USians tend to reduce pH with acids containing "other" anions - either phosphoric or lactic. They have a fondness for lactic despite its low taste threshold, because of the influence of German brewing on the US. The Germans have this touching belief that lactic acid from a bottle is completely unacceptable whereas lactic made by bacteria on mouldy grain is completely OK because it's "natural". So the Germans rely on acid malt for pH reduction, and USians often use lactic as a result, even though it would not be anyone's first choice due to its low taste threshold.

All these things are a complicated mixture of chemistry and cultural history, but if you can get hold of them then sulphuric and hydrochloric (or a blend thereof like AMS/CRS) are first choice. Phosphoric's OK if you can't.
More to to with economic and food source protection history than making beer. I can only speculate the people knew far less about the chemistry involved in beer making and ad that synthesis of inorganic acids was not so well known back then. As said both hydrochloric and sulphuric acids are by far better to check high pH with than lactic acid. German beer is best because of a law introduced in 1516 that dictates what can be used 🤣 Don't make me laugh when I can buy Belgian and English beer that tastes better and is not hampered by a law prohibiting the use of perfectly good beer making ingedients!