Biochemical Buffers Confusion

[dae]

Biochemical Buffers Confusion

• Quote

Post by dae » Sat Oct 03, 2009 7:22 am

Is it correct to to think that a buffer will always shift a solution toward the buffer's pKa? For instance, if I have an aqueous solution of pH 3 and I add acetic acid (pKa = 4.76), the pH of the solution should rise, right? But I sure I'm missing something because by that logic adding neural water to neutral water should raise the pH since the pKa of water is 15.74.

Part two of my confusion: In my lab we made a solution of 0.25% sodium bicarbonate (NaHCO3-, pKa2 = 10.33). The diH20 started at pH 5.x and became at 8.12. If I didn't measure the final pH, how could I calculate it? Starting with the same diH20, how much sodium bicarbonate would I need to add to get a solution of pH = 10?

Thanks!

[JackBean]

• Quote

Post by JackBean » Sat Oct 03, 2009 12:23 pm

Are you adding the buffer to solution or solution to the buffer? Anyway, whatever two solutions are you mixing, the pH will be something in between the original pH of the original solutions (let's say, you will mix solution of pH 3 and solution of pH 7, so you will get something between 3 and 7, depending on amount, buffering capacity etc. of the original solutions;)

Well, you are only adding salt of weak acid, so you should always get solution of pH in between of pKa1 and pKa2, unless you won't add some alkaline (in this case add NaOH to make bicarbonate buffer)

[dae]

Re:

• Quote

Post by dae » Sat Oct 03, 2009 2:26 pm

Are you adding the buffer to solution or solution to the buffer?

I'm adding sodium bicarbonate as a solid to diH20. Thanks, I should have specified that.

[mith]

• Quote

Post by mith » Mon Oct 05, 2009 6:25 pm

first calculate how much negative ions you'd need or hydroxide ions for pH 10, then calculate amount of solute you need based on the dissociation constant.