[prankster1590]

Say I have 20 liters of a solution with a pH of 5,63. It requires 0,015 mol H⁺/liter solution to lower the pH by 1 degree. The wanted pH is 5,4.

So the change in pH = 0,23.

This requires 0,23*0,015 mol H⁺/liter solution = 0,00345 mol H⁺/liter solution. That is for 20 liters solution 20 l*0,00345 mol H⁺/liter solution = 0,069 mol H⁺ or HCl.

But lactic acid is a weak acid. pK_a=3,86

According to the Henderson-Hasselbalch equation about 97,2% of lactid acid is dissociated in A^- = H⁺ at the wanted pH of 5,4.

So a guess of the amount of lactic acid needed is the amount of strong acid needed divided by 0,972.

That would be: 0,069 mol H⁺ or HCl / 0,972 = 0,071 mol lactic acid.

An 88% lactic acid solution is about 11,8 mol lactic acid / l = 11,8 mmol/ml so 0,071 mol = 71 mmol = 6 ml of 88% lactic acid solution.

So to change the pH from 5,63 to 5,4 of a 20 l solution, about 6 ml of a 88% lactic acid solution is added.


So the question is: Is this a valid calculation? I found it on a beer brewers forum.

[Hunter2]

The main question first is, what is in the 20 l solution. pH alone is to less information.  Is it also lactic acid or something  different.
Normaly in practice a small sample will be taken and the pH adjusted by adding of the acid. No calculation.

[prankster1590]

The main question first is, what is in the 20 l solution. pH alone is to less information.  Is it also lactic acid or something  different.
Normaly in practice a small sample will be taken and the pH adjusted by adding of the acid. No calculation.

Its tapwater. So its probably carbonates that cause the buffercapacity. I gave the amount of H⁺ to change the pH with 1 degree. I gave the calculation.

What more info can I give.

And yeah. I know I can test it but it's about the calculation.