% solution = ??? concentration

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% solution = ??? concentration

Postby bredmakr » Wed Oct 23, 2002 5:39 am

Ok who is the chemistry guru out there? Let's say that I have 1 gallon of 10% solution phosphoric acid (HPO4 correct?). What is the concentration of phosphoric acid in ppm=mg/L in the one gallon container? How do I calculate the concentration in mg/L?
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Molarity?

Postby jayhawk » Thu Oct 24, 2002 9:25 am

I am taking a first year chem course right now, so I am far from an expert but maybe I can help a little. I think you need to know the molarity of the 10% solution you have. That means you have to find out how many moles per litre of HPO4 are in the solution. If you know that figure, then it is fairly straightforward to determine the actual mass(mg/L)of HPO4 in the solution. Perhaps you could contact the manufacturer. They may be able to tell you all you need to know.
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felt good

Postby dartedplus » Thu Oct 24, 2002 12:58 pm

it felt good to say all that junk...didnt it
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geeks R us

Postby Gravity Thrills » Thu Oct 24, 2002 1:40 pm

Every once in a while I have to try to prove to myself that the piece of paper on the wall (somewhere in a closet actually) was really worth the price of admission :-)
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Acid Jazz

Postby Gravity Thrills » Thu Oct 24, 2002 1:41 pm

I'm not a chem guru either, but I do have to retain enough of the stuff to get by in my day job (sadly not brewing) as a biologist and college teacher.

First off, phosphoric acid is H3PO4, not HPO4. This means it has the potential to donate 3 hydrogen ions and drive the pH of a solution down (pH stands for "pondus hydrogenii" or "the power of hydrogen"). In truth, only the first hydrogen is readily given up, which is why phosphoric acid is classified as a weak acid as opposed to something like sulfuric acid.

Now, as far as your question, I think you're thinking in the wrong units. Jayhawk is right, you should be thinking in terms of moles rather than ppm. To dredge up dim memories, 1M (molar) phosphoric acid is 6.02 X 10^23 molecules of (too bad the forum doesn't support scientific notation) H3PO4 dissolved in a liter of water. Recall that this is good old Avagadro's number from high school chem class. These numbers get a little unwieldy, which is why we use the pH scale instead. pH is the negative log(10) of the hydrogen ion concentration -- pH = -log10[H+]. Pure water has a pH of 7.0, which means it has a H+ ion concentration of 1.0*10-7 M. remember the scale is logarithmic, which means that a solution with a pH of 6 is actually 10x more acidic than a solution of pH 7.

The practical answer to your question is that you just need to measure the pH of your gallon of 10% phosphoric acid. If is has a pH of 4.5, then that means it has an H+ concentration of 10^ -4.5 M.

Unfortunately, how acidic your mash or wort will become on addition of x ml of your phosphoric acid solution is not a simple question to answer. That is, of course, because different solutions have different buffering capacities. wort or mash buffering capacity will vary based on things like the dissolved salts in your water, the starting pH of your water, and the amount of dark grain in your grist. Basically (and this is the crux of the whole biscuit), you need to experiment with small additions, mix and check the result. Your results will be repeatable when you brew something similar, so once you figure things out for yourself once, you should be able to easily apply your knowledge to future brews.

A roundabout non-answer to your question, I know, but I hope it helps.

Cheers,
Jim
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Um...

Postby Monkey Man » Thu Oct 24, 2002 7:57 pm

I'm no chemistry guru, but in about a year I will have a piece of paper that I will need to justify the admission price of... Why not get started now... maybe I missed something, but if the solution is 10% H3PO4 and you have a gallon of it then wouldn't you have .1 gallons of H3PO4??
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Maybe so, but...

Postby jayhawk » Thu Oct 24, 2002 8:04 pm

Yes, 0.1 gallons of H3PO4 in 1 gallon of solvent is 10%, but that still doesn't get us to how many mg/L there are in the solution. We still have to determine the mass (g, mg)of that amount H3PO4 to determine the overall concentration of the H3PO4 in solution in mg/L.
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100% of what?

Postby Gravity Thrills » Fri Oct 25, 2002 3:22 am

That's the problem calling a hydrated phosphoric acid solution 100% phosphoric acid. Pure, 100% anhydrous H3PO4 is a crystalline solid. Once you dissolve it in water, what does it really mean to call it 100%? Usually this means it is a 100% SATURATED solution, meaning adding more anhydrous acid won't increase the strength because no more material can be dissolved in the water. The liquid should be referred to as 100% saturated solution, and yes a 10x dilution of that would give a 10% saturated solution. It would be better if strength in molarity were also included in the packaging but I have yet to see that. So as to the original question of lbs/gallon or ppm... still no definitive answer. You can probably go to Argent or any of the chemical supply websites and look up how much solid chemical goes into their 100% solution.

A similar situation I am familiar with is with teh preservative formaldehyde. "100% formaldehyde" is really an aqueous solution that is only 37% formaldehyde by weight and the rest is water. This is always a source of confusion to biology students because when a protocol cdalls for 10% formalin, many students think you should dilute the near 40% stock solution by 1:3. Actually you need to dilute by 1:9, because the protocol is calling for 10% saturated solution.
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Postby JSGilbert » Mon Aug 02, 2004 2:49 pm

Here is how I would approach this problem (eventhough it's been quite a while since the post was started):

If you had 1 million parts, then 10% would be 100,00 parts. So 100,000 parts per million (mg/L).

Here's a more classical approach that arrives at the same answer:

If we have a solution of phophoric acid, let's assume that 1 mL = 1 g. Reasonable.

If we have a 100 mL sample (100 g), then 10% or 10 mL (10g) would be phosphoric acid (generally speaking, of course-some of the phosphoric acid molecules will deprotonate (lose H+ ions)). If we have 1 L, we have 1000 mL; therefore, we have 100 g of phosphoric acid in one liter (g/L) along with 900 g of HOH. Now if we convert from grams to milligrams by multiplying by 1000, we get 100,000 mg or 1 x10(to the fifth power) mg in one liter. So your answer is:

100,000 mg/L !!

This would be a 1 M solution of H3PO4.

Hope that helps!

BTW the pH of a 1 M solution of a strong acid (HNO3, HCl) would be -1!
Phosphoric acid is a weak acid and therefore would have a much higher pH by comparison.
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