Dilution of phosphoric acid problem

[RazDuBraz]

Hi,

I had my second attempt to make a more accurate dilution of phosphoric acid 75% down to 3.0% (for acidity/alkalinity curve plotting).
Now I´m not sure if I got 3.0% or 3.1% and if there is a way to really make sure.

Materials:
Phosphoric acid 75%: Brewferm, foodgrade
DI water: Hamron Battery Water
Scale: Jadever JWL-3K (new and factory calibrated)
Thermometer: Thermapen ONE
Hydrometer: AllaFrance -10...+30 Oechsle, calibrated with DI water and 10 Oechsle sugar solution
Ball pipette, 0-5 ml +/- 0,03
Measuring cylinder, glass 100 ml

Density reference: Density of phosphoric acid - Steffen's Chemistry Pages

First of all, I would suppose that the stated 75% of the acid could differ somewhat, not sure how much.
Too high density to control with any hydrometer.
Tried to measure up 40 ml in the measuring cylinder on the scale, 4ml at a time, but got something like 72.5..78%, adding upp each addition. Felt very inaccurate.
Filled the measuring cylinder to the 100 ml mark on the scale, and got very close to 75%. Don´t remember exactly but it was within about +/-0.5%.

Giving up on that, I decided to go ahead.

1 kg of 3% = 40 g (30 / 0.75) of acid + 960 g of DI water
Measured up 40.00 g and added the water to a total of 1000.00 g

Did an hydrometer test which showed 15 Oechsle on the mark.
Comparing to the density chart, that would be about a 3.1 % solution, should have been 14.6 Oechsle.
Too big error for me to accept.
Double checked my hydrometer with DI water, perfect 0.
All temperatures were 20.0 C.

What do you guys think?
Is it more likely I have a 3.0% or a 3.1% solution?
Did I do anything wrong?
Is there a better way to do/test this?

Many thanks!

 

[doug293cz]

Are your concentrations by weight or by volume?

Brew on

 

[RazDuBraz]

Only by weight.

The more I think about it, I think I must give up this idea.
Easy way out, order standard 0.1 N solutions. Expensive thou.

 

[balrog]

Couldn't you measure a 12,3,5,10) of the original 75% acid with DI water, and within the significant digits of the dilution and measurement, know whether the original is ok?

[Edit, ok, it changed "1 : ( 2 , 3 , 5 , 10 )" into a smiley on me]

 

[doug293cz]

I don't think you did anything wrong. I doubt your 75% acid is actually 75.000% (it's probably spec'ed at something like 75% +x% / -0%.) If you don't have a certificate of analysis for the starting acid, it is not going to be good enough for the accuracy you are seeking. It seems you are going to be stuck buying analytical standard solutions to get the accuracy you want.

Brew on

 

[AlexKay]

Nailed by @doug293cz.

Spec sheet says 74-76%. That’s a deviation more or less comparable to what you’re getting.

I’d trust the density measurement more than the spec sheet, given the level of care you seem to be taking.

 

[jerrylotto]

What difference does it make? You're going to adjust to a given pH anyway, so it'll just take a little less.

 

[mac_1103]

What difference does it make? You're going to adjust to a given pH anyway, so it'll just take a little less.

a more accurate dilution of phosphoric acid 75% down to 3.0% (for acidity/alkalinity curve plotting)

Looks like the OP is trying to do some analytical chemistry rather that just make good beer. Maybe his water fluctuates a lot or something and he feels like he needs to determine the alkalinity before he can make adjustments.

 

[jerrylotto]

Looks like the OP is trying to do some analytical chemistry rather that just make good beer. Maybe his water fluctuates a lot or something and he feels like he needs to determine the alkalinity before he can make adjustments.

That is what titration is for...

 

[IslandLizard]

You'd need a volumetric flask of a chosen volume (v), and a precise enough scale. Most of those flasks are calibrated at 20°C.

Weigh the (net) volume of your mixture. With the density correlation table (or formula), you can get a decent precise mol/liter indication of your mixed volume. That also eliminates the +/- error of your original stock solution.

 

[RazDuBraz]

Actually, what I was trying to do, was to make some titration curves for different grains.

I have used Beersmith for some time now, also other online tools, and I have also made a couple of pH prediction tools and applications myself. Sometimes I run into that the pH prediction is quite off thou. Usually when it comes to specialty grains in the mix.

I have long suspected that the titration curves for wheats and oats are quite flat.
One goal is also to make better curve calculations based on grain type and color.

I did a couple of titration curves with my phosphoric acid already. But since I want to have as solid data as possible, I was worrying about the accuracy of the acid.

Now I know, that what should be most accurate, would be to buy professionally made 0.1N solutions of acids and bases. Preferably HCl or H2SO4 and NaOH.

 

[Dland]

If the professionally made solutions are hard to get or expensive, you might just try Weyerman acid malt. That product is readily available, pretty consistent and made for brewing.

Of course you would have to do more tests, calculations and graphing for a different product, but somehow I sense that would not be a problem for you.

 

[AlexKay]

Perhaps I’m missing something (is the phosphoric acid supposed to be a titrant, or a calibration analyte?), but wouldn’t it be vastly simpler to use hydrochloric acid here? There are lots of equilibria to worry about with a triprotic acid. And you’re not drinking it; it doesn’t need to be food grade.

In any event, I’ll stick with what I said before: use your hydrometer to measure the concentration. Assuming you’re careful with temperature and calibration, I don’t see any reason not to trust it for the accuracy you need.

 

[jerrylotto]

Phosphate does not affect beer taste like chloride does and again, all you need to use it for is to hit a given pH. I would not add any acid indiscriminately to my mash - I measure the pH and add acid until I hit the sweet spot.

 

[mac_1103]

Phosphate does not affect beer taste like chloride does and again, all you need to use it for is to hit a given pH. I would not add any acid indiscriminately to my mash - I measure the pH and add acid until I hit the sweet spot.

I don't think anybody said they were going to add HCl to the mash. The OP is going to use it to construct acidification curves for each of his malts so that he can do his own calculations for acid additions instead of relying on someone else's software. I have no idea if that's worth the trouble, but it sounds like pretty much the exact opposite of indiscriminate to me.

How long does it take you to hit the sweet spot? An how much does it matter that your mash starts out at too high a pH?

 

[RazDuBraz]

For what it's worth, I'm not just in it for the "ok, that will be beer too". I am one of those weird guys who get a kick out of "hitting the spot" and finding ways to do it over and over again.

Knowing your process is tuned.

A plus is having a good beer in the end.

 

[passedpawn]

That is what titration is for...

^^ this guy. Titrate with phenolphthalein added. For acids, I use a 1M solution of NaOH. The math is easy and can be found on the webz. (in the image below, I was verifying the strength of the HCL that home depot sells - FYI, they are liars).

 

[Dland]

And then you cleaned the glaze off your patio?

BTW, I do believe you about them being liars....