[ironnica]

Hi guys,

I have 2 questions in this.

Using the Henderson Hasselbach equation, I worked out that for a 0.1M Phosphate pH8 buffer, i needed 2.1764g and 12.2339g of NaH2 and Na2H PO4 respectively in 500ml. (pKa2=7.21)

I highly doubt the maths is wrong, did it a few times to make sure. I made this up to 500ml and measured the pH, it was pH 9.5.

1. What reasons could there be for such a difference in the pHs?

I purged this aqueous solution with CO2. The pH dropped to pH6.8.

2. What property of a buffer allows it to sustain a pH (including enduring Carbonic acid formation)?

Thanks very much for any answers received,
Liam

[Babcock_Hall]

As an aside, I would point out that the pK_a you used pertains to very low ionic strength.  As the ionic strength increases, the pKa falls to below 7.  Borek is a better person to explain this than I am, although I don't think it explains the results you obtained.

[Borek]

Using the Henderson Hasselbach equation, I worked out that for a 0.1M Phosphate pH8 buffer, i needed 2.1764g and 12.2339g of NaH2 and Na2H PO4 respectively in 500ml. (pKa2=7.21)

First of all:

molar mass of NaH₂PO₄ is 119.98 g/mol, 2.1746 g is 0.01814 mol.
molar mass of Na₂HPO₄ is 141.96 g/mol, 12.2339 g is 0.08618 mol.

You have 0.01814+0.08618=0.1043 moles of phosphates in 0.5L of solution, this is not a 0.1M buffer, more like 0.2M - and even that not very accurately. This makes me doubt general accuracy of your calculations.

[ironnica]

Sorry, should have specified. The monobasic was hexahydrate i think. Also mistake also. I'm almost fully sure it was per 1L. I'll submit my calculations later. Thanks

[ironnica]

Ok, it's nah2po4.2h2o with a gmm of 156.01g/mol.

Calcs: x=na2hpo4 y=nah2po4
henderson haselbach: 8 =7.21 + log x/y. I.e. 6.16595y=x
also x+y=0.1
Simultaneous eq.
X-6.16595y=0
-x-y=-0.1

X cancel, -7.16595y=-.1
Y=0.01395M
then x=.1-.01395=.08605M

per litre and with specified weights i got the initially mentioned weights. Any advice on the topic would be happily accepted. Ty.
Ps. Actual weights were 2.1792g and 12.2339g respectively

[Borek]

As Babcock suggested, first problem here is with the high ionic strength - close to 0.3. But I would expect it to make the solution pH lower than 8, not higher.