Adjusting the pH of Water

[theIceMan_au]

How do I calculate the required acid or base needed to change the pH from one value to another?

At the moment all I can think of is to use the formula:

 

[latex]

[H_3O^+] = 10^{-pH}

[/latex]

 

To find the concentration of hydronium ions at each pH, then using the difference to find the amount of strong acid (or base) required to reach this level. However, this seems to give me results which are much lower than what would seem reasonable.

 

Is there a method of doing this?

 

[hypervalent_iodine]

Your method seems fine to me. Could you give a specific example of question that you've done?

[theIceMan_au]

For example, I have a 550 000 litre pool at a pH of 8.3, and I need to reduce this to a pH of 7.4. I find the difference in pH and multiply this by 550000 to get:

 

[latex]

550000 \times (10^{-7.4} - 10^{-8.3}) = 0.019 \; \text{mol}

[/latex]

 

For a 550000 litre pool, this seems like an almost insignificant amount of acid needed.

Edited May 26, 2014 by theIceMan_au

[John Cuthber]

Knowing the initial pH isn't enough to answer the question.

You need to know the buffering capacity

 

For example, you could fill the pool with a saturated solution of bicarbonate of soda and the pH would be about 8.3.

Adding 0.019 moles of acid would make no measurable difference.

[theIceMan_au]

I know that the pool has a solution of bicarbonate soda at 165 ppm (which is above the desired range), but we have not been taught how to calculate changes in pH involving buffers (I loosely understand how buffers work).

 

Do you know any good pages that explain how to calculate changes in pH with buffers?