Citric acid buffer solution for prac

[antony22]

Hi Guys,

Citric acid has three different values for pKa because it is a polyprotic acid. I am aiming to make a buffer solution with sodium citrate but I am not sure which pKa value to use in the Henderson-Hasselbalch equation. Does anyone have any ideas?

 

[Mike H]

Questions to ask yourself -

1.) What pH do you want your buffer to be at in the end?

2.) Under what conditions is a buffer going to work most effectively in relation to pKa?

3.) Have you ever made a phosphate buffer? If so, what's the purpose of chemical companies selling monobasic, dibasic, and tribasic potassium/sodium phosphate?

 

[Borek]

Citric buffer can't be easily compared with phosphoric buffer - way too low separation of pKa values.

 

[Mike H]

Citric buffer can't be easily compared with phosphoric buffer - way too low separation of pKa values.

I was mostly thinking that one might use judiciously chosen amounts of citric acid and a citrate salt to prepare their desired buffer, much like one would use properly chosen amounts of monobasic & dibasic potassium phosphate (or whatever is appropriate) for their desired phosphate buffer.

Although I didn't remember that about citric acid. Good to know for future use!

 

[Fra]

Hi Guys,

Citric acid has three different values for pKa because it is a polyprotic acid. I am aiming to make a buffer solution with sodium citrate but I am not sure which pKa value to use in the Henderson-Hasselbalch equation. Does anyone have any ideas?

These simple equations are useless except for the simplest cases.

You need to write down the full coupled equilibrium for all the coupled protolysis, and in the general case you can't solve it analytically but it's easy to solve by any numerical method like Newton raphson.

It can be done with excel macros. Not only is this a nice to have tool for the future, it's also a really good exercise!

/Fredrik

 

[Borek]

You need to write down the full coupled equilibrium for all the coupled protolysis, and in the general case you can't solve it analytically but it's easy to solve by any numerical method like Newton raphson.

It can be done with excel macros. Not only is this a nice to have tool for the future, it's also a really good exercise!

Oh well... now that you called for numerical methods and software:

Buffer Maker - the ultimate buffer calculator

It does everything you mentioned, plus some. And you can get support at PF

 

[Fra]

That looks nice :)

I also migrated some old excel macro tools I made for brewing chemistry and fermentation calculations into simple interface VB applications. That's sometimes quicker than excel. The nice thing is if you have some excel macros one can pretty much copy and paste the code into a VB app. So for these kind of not so cpu intense things basic is neat since it's easily compatible with excel.

So are you the software author? :)

Edit: Hey, I didn't pay attention to your signature before! ;-)

/Fredrik