Phosphoric Acid

I recently bought some 75% phosphoric acid for reducing the alkalinity of my tap water after reading of others on here using similar.

Now I have the bottle and I'm reading the warnings all over it, my question is this:

Can I use a plastic syringe to measure and dose it? I have both 1ml and 5ml plastic syringes.

If not, how are you other guys measuring and dosing?

Google can't seem to answer this one for me...

Yes, you can. I also do it like this. It is not that they will dissolve while having them in your hands.

However, will you use a spreadsheet like Bru'n'water to obtain the correct amount of acid to reduce your alkalinity? A recent experiment I did showed that you really do not need very much acid, and that you are probably better off to first make a bottle of solution which is less concentrated. This makes it simpler to measure the correct amounts.

chthon said:

However, will you use a spreadsheet like Bru'n'water to obtain the correct amount of acid to reduce your alkalinity?

Click to expand...

I've actually put my own spreadsheet together because Bru'n'Water didn't handle dilution of liquor in the manner that I wanted.

I have experiments planned with 2-5L of water to check that my spreadsheet is working and to dial it in if necessary.

Good to hear I'm ok to use the plastic syringes, that will make life nice and easy. I've thought about diluting the 75% conc down to say 50% or 25% but I'm not sure I've got a suitable bottle to store it in or the chemistry knowledge to get it right!

Experimented this afternoon.

4L of water
Alkalinity at the start 153ppm CACO3

Target alkalinity 22ppm CACO3. Spreadsheet says 0.6ml 75% conc. Phosphoric Acid

Alkalinity after adding the acid measured at 19.7ppm CAC03

That'll do for me!

braziliain said:

Good to hear I'm ok to use the plastic syringes, that will make life nice and easy. I've thought about diluting the 75% conc down to say 50% or 25% but I'm not sure I've got a suitable bottle to store it in or the chemistry knowledge to get it right!

Click to expand...

Glass bottles are good to go. I use apothecary bottles of 250 mL and 100 mL to store my diluted solutions. It's phosphoric acid, not fluoridic acid which can be used to etch glass.

chthon said:

I use apothecary bottles of 250 mL and 100 mL to store my diluted solutions.

Click to expand...

Do you just dilute it say 10:1 then add 10 times as much? Or is the chemistry more complex than that?

braziliain said:

I've actually put my own spreadsheet together because Bru'n'Water didn't handle dilution of liquor in the manner that I wanted.

I have experiments planned with 2-5L of water to check that my spreadsheet is working and to dial it in if necessary.

Good to hear I'm ok to use the plastic syringes, that will make life nice and easy. I've thought about diluting the 75% conc down to say 50% or 25% but I'm not sure I've got a suitable bottle to store it in or the chemistry knowledge to get it right!

Click to expand...

Are you using the free version of Bru N Water? If so, it's worth bunging Martin $10 for the paid for version (which you get free updates for too). It has quite a bit more functionality and I wouldn't be surprised if the dilution method that you want isn't covered by it. I can't imagine the number of hours of work that has gone into that spreadsheet, so I wouldn't want to try to replicate it myself.

braziliain said:

Do you just dilute it say 10:1 then add 10 times as much? Or is the chemistry more complex than that?

Click to expand...

The basic formula for dilution is Conc1 X Vol1 = Conc2 X Vol2
The volumes are what you obtain, not what you need to add.

Say that you start from 75% and want to obtain 250 mL of 25%. Then the equation becomes:

75% X vol1 = 25% X 250

Solving this gives: vol1 = 25% X 250 / 75% = 83 mL of the 75% solution.

Add this in the bottle, then add (250 - 83) mL distilled/RO/demineralised water, 167 mL. Then you have 250 mL of a 25% phosphoric acid solution.

Phosphoric acid is polyprotic, but from what I remember, just using the first ionization constant is a good approximation. Keep in mind the pH scale is logarithmic, which means pH 2 is 10x more acidic than pH 3, and 100x more acidic than pH 4. The point being, pH in the twos is really low.

chthon said:

The basic formula for dilution is Conc1 X Vol1 = Conc2 X Vol2
The volumes are what you obtain, not what you need to add.

Say that you start from 75% and want to obtain 250 mL of 25%. Then the equation becomes:

75% X vol1 = 25% X 250

Solving this gives: vol1 = 25% X 250 / 75% = 83 mL of the 75% solution.

Add this in the bottle, then add (250 - 83) mL distilled/RO/demineralised water, 167 mL. Then you have 250 mL of a 25% phosphoric acid solution.

Click to expand...

I hate to inform you of this, but your equation is totally invalid. The real dilution equivalence is:

Molarity1 x Vol1 = Molarity2 x Vol2 (or Normality1 x Vol1 = Normality2 x Vol2)

It only takes 60.49 mL of 75% Phosphoric Acid to make 250 mL of 25% Phosphoric Acid.

75% Phosphoric Acid is 12.0848 Molar
25% Phosphoric Acid is 2.9241 Molar

60.49 mL x 12.0848 Molar Concentration at 75% = 250 mL x 2.9241 Molar Concentration at 25% = TRUE or FALSE

731 = 731 = TRUE

Looking at this another way, as the proof test:

Density of 75% Phosphoric Acid = 1.5790 g/CC
Density of 25% Phosphoric Acid = 1.1462 g/CC

60.49 mL x 1.579 g/mL x 75% = 71.6353 grams of pure 100% Phosphoric Acid initial
250 mL x 1.1462 g/mL x 25% = 71.6375 grams of pure 100% Phosphoric Acid final

71.64 grams initial = 71.64 grams final = TRUE

The only slight difference in grams computed is attributable to rounding error.

If you start out with a certain weight in grams of pure phosphoric acid, and dilute it to 250 mL, you had best end up with the same number of grams of pure Phosphoric Acid, no? Your method assuredly fails this test miserably as follows:

83 mL x 1.5790 g/mL x 0.75 = 98.293 grams of pure 100% phosphoric Acid
250 mL x 1.1462 g/mL x 0.25 = 71.6375 grams of pure 100% Phosphoric Acid

98.293 grams initial = 71.64 grams final = FALSE

Your method fails to account for the density changes involved in dilution.

braziliain said:

I recently bought some 75% phosphoric acid for reducing the alkalinity of my tap water after reading of others on here using similar.

Now I have the bottle and I'm reading the warnings all over it, my question is this:

Can I use a plastic syringe to measure and dose it? I have both 1ml and 5ml plastic syringes.

Click to expand...

Yes, a syringe is a good method. But when handling such acids be sure to wear PPE including goggles, a face shield, an apron, and rubber gloves.

When diluting an acid never add water to acid. Mixing the two evolves lots of heat, and if you add water to acid the mixture will potentially boil, splashing acid on you. The rule is to always add acid to water. And assure that you are adding acid to sufficient water.

An alternative dilution formula that skips molarity and normality:

Vol1 x Density1 x Conc%1 = Vol2 x Density2 x Conc%2

I've been giving some thought with regard to a potentially ideal concentration percent target by which to dilute 75% (or 85%) Phosphoric Acid, and I've settled upon 30%. At the 30% concentration level Phosphoric Acid has sufficiently close enough to the same acid strength (within the range of typical mash and wort pH's) as for AMS/CRS, such that pH reduction data for AMS/CRS can be used directly for 30% Phosphoric Acid.

And while I'm assuredly not an expert in this, so thereby this part of my thought process must be taken with due suspicion and a proverbial "grain of salt", my first assumption would be that the relative danger level for 30% Phosphoric Acid must likewise be relatively similar overall to that for AMS/CRS.

Provided that I calculated it correctly, when 1 Liter of 75% Phosphoric Acid is diluted into distilled water to achieve a final volume of 3.35 Liters (or 3,350 mL), this results in a 30% concentration. The precise calculation comes out to 3,344 mL, but the difference is trivial, and who knows, your 75% might analyze at 75.1% or so anyway. It's generally acceptable to ship a product that has a bit more of what it claims to have, as opposed to a bit less (which is likely to be in violation of laws), so my presumption would be that due to this the marketers may well prefer to err to the high concentration side a tad.

https://wissen.science-and-fun.de/chemistry/chemistry/density-tables/density-of-phosphoric-acid/