calculate pH of phosphoric acid

 1/4 teaspoon of 10% phosphoric acid  for 19 litres. calculated from bate: pH 4.16

For 1 drop = 0.05ml, it means roughly 25 drops

Step 1. Convert mass ( or volume) and concentration ( % w/w) to mole per litre ( mol/L)

Step 2. Use BATE or other app to calculate pH

If aiming for pH 5.6, conc will be 2.5 e-6, or 1/100 tsp , 0.044 ml, roughly 1 drop

From homebrewtalk, AHA, summer rye, hefe Gordon Strong suggested to use 1/4 teaspoon of 10% phosphoric acid, target pH of 5.5. From BF 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, with~ 5% minerals. Anyways, make your own pH measurements .....

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

my excel

For 2L, 

for pH4.16, then it means ~2.6 drop

for pH5.6, then it means ~0.1 drop

Better still, 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 

Chem help YouTube 

Brunwater

Online pH app