[theIceMan_au]

Is there a formula for calculating the required amount of acid (or base) needed to adjust the pH of water by a specific amount?

Specifically, I have a 550 000 litre pool that is at a pH of 8.3, and I need to reduce it to a pH of 7.4. How do I calculate the required amount of acid (preferably hydrochloric acid) needed to do this?

So far I have tried converting both pH's to the hydronium ion concentration, then using the difference to find how many more moles of hydrogen ions are need to increase the hydronium concentration, but this gives me values that seem way to small than expected (only 0.02 mol of 1M HCl needed for the 550000 L pool!).

Thanks - Ice

[Hunter2]

I didnt calculated that, but it could be right, because you are on the alcaline side close to neutral. There is not much H⁺.

[Borek]

This is not trivial. You can't calculate required amount of acid not knowing the solution composition. Most ions present are either weak acids or weak bases, so they will produce some buffering effect.

Best approach is to check the required amount experimentally - say, take a 100 mL sample and titrate it using diluted acid to 7.4, then just scale up.

[theIceMan_au]

The question is theoretical so I don't have any physical sample of water (although I could fabricate a sample similar to the given information).

The water has sodium bicarbonate buffer at 165 ppm. Is there a way to calculate the change in pH with this information?

[Borek]

The water has sodium bicarbonate buffer at 165 ppm. Is there a way to calculate the change in pH with this information?

That's much better. You can treat it as a buffer problem and use Henderson-Hasselbalch equation, compare http://www.chembuddy.com/?left=buffers&right=pH-change (and perhaps http://www.chembuddy.com/?left=buffers&right=composition-calculation for an introduction).

Or, use pH calculator. My approach would be: start with recipe for bicarbonate buffer at 165 ppm, select "New pH calculator from buffer" from the file menu, add HCl solution, see what volume has to be added to modify pH to the required value. That requires guessing, but as you have an instant answer and the pH change is proportional to the volume, it can be done pretty fast.

No matter whether you will calculate pH change manually or using software, in the end you will have to check the final pH with pH meter to be sure it worked the way you wanted it to work.

[theIceMan_au]

Cheers, I'll check that out.