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Home Brew Forums > Home Brewing Beer > Brew Science > Phosphoric Acid
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Old 12-23-2012, 03:17 AM   #1
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Default Phosphoric Acid

I have (had) been using Lactic acid to lower the pH on my mash but was getting some of the lactic tang flavor associated with it since I have to use a ton to lower pH.

I have a bottle of Phosphoric Acid 25% (v/v) (1 + 3) Aqueous Solution, 250 mL of Phosphoric Acid, ACS

I use the Bru'n Water spreadsheet for my calculations and was wondering what strength the acid I have is. The Lactic was 88%, but is the Phosphoric Acid 25%?

Thanks in advance

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Old 12-23-2012, 01:57 PM   #2
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I've seen commercial packages of 10%, 25%, 75%, and 85% phosphoric acid. Assuming the seller is reputable, it is quite possible that the indicated strength is correct.

If you have an aquarium test kit for alkalinity, you should be able to figure out if the acid strength is accurate. For instance, use Bru'n Water's calculator to figure out how much of that acid to add in order to bring the alkalinity to zero (pH = 4.4). Add half that amount to the water and check alkalinity and then add the remainder and again check alkalinity. Also test what the tap water alkalinity is at. That first alkalinity test should report 1/2 the tap water alkalinity value and the zero alkalinity test should read...zero.

How high is your alkalinity or bicarbonate content in your tap water?

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Old 12-23-2012, 05:04 PM   #3
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The easiest way to determine the strength of phosphoric acid solution is to use your hydrometer. Phosphoric acid is heavy so a weak solution has a pretty healthy SG. The graph below shows the relationship. You can proceed in one of two ways. If the solution is less than about 30% in strength then just put some in a clean hydrometer jar and use your hydrometer with the chart. If the solution is strong you will know this because the acid is syrupy and when you pick up the bottle you sense that it is heavier that seems normal for its size. A hydrometer is still an excellent way to determine the SG but as most brewing hydrometers only go up to 1.170 you are limited to less than about 30% w/w. There are, of course, wider range hydrometers (hydrometers used to check automobile battery acid will take you to 1.300 typically and thus 45% w/w, for example) but you may not have or wish to buy such and instrument.

In this case place a small container (about 100 mL) on your hops scale and tare it. Then add about 100 mL of water and weigh. Now pour off the water, dry the container and refill to as close to the same level as you can possibly get (obviously a volumetric flask is ideal for this) with the acid and weigh again. Take the ratio of the weighings. This is the SG. Use that with the chart.

If you have access to a laboratory balance and a pipeter put a weigh boat on the balance, tare it and pipet 1 cc of the acid. The weight, in grams, is the specific gravity.

You can do this with a reloading scale if you happen to have one of those. Convert the tared reading to grams (15.432 grains per gram).

You can do titrations (as suggested by Martin) to determine the strength of polyprotic acids (such as phosphoric) but the math gets a little tricky (because they are polyprotic). In industry the strength of such acids is obtained by measuring the density.

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Old 12-23-2012, 07:13 PM   #4
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thanks for the replies. I use Martin's worksheet and plugged the % in as 88% and added the required acid amounts. It undershot the pH a bit so I added more in increments until I hit 5.35 pH and called it good. I am 5.3 into the boil kettle.

When I used a hydro it lifted the hydro right out of the tube. I will call the wine supply place and ask what the concentration is for future use.

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Old 12-23-2012, 08:37 PM   #5
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Quote:
Originally Posted by Inhiding View Post
When I used a hydro it lifted the hydro right out of the tube. I will call the wine supply place and ask what the concentration is for future use.
That means it is concentrated. Is it syrupy?

You can still assay it by diluting but as things are by weight you must dilute by weight. If you weigh out 3 oz of the acid and add 9 oz of water you will dilute by approximately 4:1. Note these are not fluid oz but oz measured on a scale. 88% acid would fall, after dilution by this much, into the range where it can be measured by a hydrometer that only goes up to 1.170. The concentration of the original (undiluted) acid is

conc_orig = conc_dil*(1 + acid_wt/added_water_wt)/(1 + conc_dil/100)

Concentrations are in %. Thus if you add 9 oz of water to 3 oz of acid and measure SG = 1.100 corresponding to about 17.5% for the dilution the original acid is

17.5*(1 + 3)/(1 - .175) = 84.8%


It is also possible to use volume dilution which is much easier to do but you would need a spreadsheet to solve for the original acid strength. For example if you took 50 cc of the stock acid and added 100 cc of water and found that mix to have SG = 1.140 the strength of the stock is 88%. The spreadsheet is very simple (12 cells) and I can make it available to anyone interested.
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Old 12-25-2012, 08:15 PM   #6
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Using Phosphoric acid also means more material to build healthy membranes for the cells. Although this will also cause some yield loss, probably insignificant though. (The cells will use the sugars to produce more cells as opposed to more alcohol.) Many types of fermentations use phosphates to balance the concentration of cells and a specific target yield. With all that being said, use caution when adding extra nutrient sources as it will change the fermentation characteristics.

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Old 12-25-2012, 08:25 PM   #7
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I don't think the addition of phosphoric acid will make a difference for the yeast. there is easy more phosphate coming from the malt. A.J mentioned that malt is about 1 % phosphate if I remember correctly. That means 40g for a standard 5 gal batch. Compared to the 2-4 g phosphoric acid that you may need.

Phosphate is also not a limiting nutrient in brewers wort.

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