[justcurious]

I'm trying to buffer and neutralize a very acidic solution by adding the acidic solution into a buffer. However, using 1 M PO4 buffer is not strong enough and requires an enormous volume. Calculating 10 M sodium phosphate buffer requires crazy amounts of solids to be weighed out--I'm not sure if I'm calculating it right. So my questions are:

1. Is there a different buffer I can use that can buffer and neutralize a very strongly acidic solution, while keeping pH 7-8 at all times?

2. I'm not sure if I'm calculating it correctly to make 10 M phosphate buffer. Using HH equation, I need 1.695 mol NaH2PO4.H2O (which is 234 g for 500 mL solution!) and 3.3 mol Na2HPO4 (469 g!!!).

Any advice would be great! Thanks.

[Borek]

10M buffer can be well above solubilities of substances involved. Not to mention the fact activity coefficients in such soup are impossible to calculate, so your buffer will be off by definition.

No idea what you are trying to achieve, but sounds to me like you are using a wrong approach.

[dipesh747]

2. I'm not sure if I'm calculating it correctly to make 10 M phosphate buffer. Using HH equation, I need 1.695 mol NaH2PO4.H2O (which is 234 g for 500 mL solution!) and 3.3 mol Na2HPO4 (469 g!!!).

Why on earth would you need to make that much of it? for every 2.34 g you need 5 mL of solution, the concentration ratio is still the same.

[justcurious]

Thanks for the responses. I knew it didn't sound right.  Essentially, I'm trying to neutralize an acidic solution so I can extract it into organic solvents but I need a pH between 7-8 for maximal extractability. Too high of a pH and it is too soluble in water; too low and it precipitates out and is difficult to get back in.

What I have been doing is calculating equivalents of NaOH to neutralize all the acid. It works but it hasn't been that exact and I always have to fine-tune the final pH with HCl. So it would be nice to have some buffering window (for minimal number of operations). Any advice on how this would be doable?

[Borek]

Perhaps neutralize it to around 7 first (using pH meter, or just a pH strips), THEN add the buffer.

[Honclbrif]

Slowly add 10% NaOH until you've neutralized your solution. If you over-shoot the neutral point, just add a little bit of acid until you hit that sweet spot.

For a fun bonus, if you know how many moles of acid you're starting with, you can add an equal number of moles of base to get you in the right neighborhood without lots of testing and back-titration.