[daf44]

So I have a 1 liter solution with pH = 7,5 and I want to decrease the pH to 3,9. I have a 2M HCl solution to do this. How much HCl solution do I need to add?

My calculation:
pH is the negative log of the hydrogen ion concentration in mol/L
pH = -log([H+]) --> [H+] = 10^-(pH)

In the beginning we have pH = 7,5, so the [H+] concentration is 10^-7,5 = 3,16*10^-8 mol/L
The goal is pH = 3,9, so the [H+] concentration is 10^-3,9 = 1,26*10^-4 mol/L
Required [H+] concentration to decrease pH = 1,26*10^-4 - 3,16*10^-8 = 1,26*10^-4 mol/L

HCl --> H+ + Cl-, so since the stoichiometric are 1:1, the required HCl concentration is also 1,26*10^-4 mol/L

I have a 2 mol/L solution, so I need 1,26*10^-4 / 2 = 6,29*10^-5 liter of this 2M solution to decrease the pH from 7,5 to 3,9 of the original solution.
Is this correct?

[Borek]

Yes, but no.

Yes - you are right about final amount of H⁺ needed and about volume of the 2M solution required to add that amount.

No - the real answer depends on why the initial pH is 7.5. Pure water has pH of 7.0, if the pH is not 7.0, that means solution contains some other ions. What if it is a buffer?

[daf44]

The original solution is a mixture of blood plasma and Xanthan (thickening agent). It consist of roughly 5 m% protein.

[Babcock_Hall]

What do you mean by 5 m%?  Does m mean mass, moles, milli or something else?

[Borek]

The original solution is a mixture of blood plasma and Xanthan (thickening agent).

So it definitely has some buffering properties and you can't calculate amount of acid without knowing a bit more about the composition. The safest approach is to titrate against a pH meter.

[daf44]

What do you mean by 5 m%?  Does m mean mass, moles, milli or something else?

Mass %

[daf44]

So it definitely has some buffering properties and you can't calculate amount of acid without knowing a bit more about the composition. The safest approach is to titrate against a pH meter.

Ok, that sounds indeed as the best solution for the moment