calculate pH of phosphoric acid

Question: adding 1/4 teaspoon of 10% phosphoric acid  to 19 litres of water, what will be the pH?

Let's go through the calculations step by step.

Step 1. Convert mass ( or volume) of 10% phosphoric acid to mole per litre ( mol/L)

The above calculation shows that 1/4 tsp of H3PO4 acid is 6.97e-5 mol/L

Step 2. Input this value to BATE (or other app) to calculate pH

The answer: pH = 4.16

If you use pipette instead of teaspoon, 

1 drop = 0.05ml. So 1/4 TSP means roughly 25 drops

If you actually aim for pH 5.6 (which is optimum pH for mashing), conc will be 2.51 e-6, or 1/100 tsp , 0.044 ml, roughly 1 drop

Let's roll back a bit.

From  recipes of  BarleyWine, summer rye, hefe Gordon Strong suggested to use 1/4 teaspoon of 10% phosphoric acid, which is targeting pH of 5.5.

Discussion on homebrewtalk forum.

However using online BF water calc, Sparge pH would be 4.6. but he also added 1 tsp of CaCl2. 2H20. Mash pH change from 5.7 to 5.64. byo hbf uk. YouTube .

Obviously Gordon is NOT using RO water, BUT with~ 5% minerals. Anyways, make your own pH measurements ( ....don't just copy)

He is adding more than required, due to CaCl2 he added same time? TBC.

Here is the calculations , comparing 0.25 tsp vs 0.009 tsp of 10% phosphoric acid.

my excel

For 2L, 

for pH4.16, then it means ~2.6 drop

for pH5.6, then it means ~0.1 drop

Approach 2: using target pH value to find mol/L :

Using the calculated mol/L to find required volume ( or mass) or acid required 

For pH4, or  more diluted acids, molarity=10^(-1*pH)

Molarity= moles/ L

using Henderson-Hasselbalch equation, as explained in BATE pH calculator cheatsheet:

pH 3 to 4, 10% phosphoric acid:

pH 4 to 5, 10% phosphoric acid:

pH 4 to 5, 1% phosphoric acid:

So for 19L volume, 10% phosphoric acid as starting point is practical. But for 2L volume, 1% would be better.

Please note water have to be distilled water, or R.O. water. Tap water has other minerals and will change the final pH.

Again you need to find out alkalinity, or residual alkalinity, beforehand.

Reference :

10% phosphoric acid means there is 10 g of phosphoric acid in 100 g of the acid solution. ie 10% w/w

Chem help YouTube 

Brunwater

Online pH app