PH Buffer

[AT]

Yea simple stuff i've been using these calculations for years

[Jim]

Simple as that, eh?

[Aleman]

Yea simple stuff i've been using these calculations for years

I've not used them for a log while, however my wife uses them regularly at work. I'm trying to persuade her that she needs to get her 6th form students to calculate the ratios for Sodium hydrogen carbonate and sodium carbonate, as it has a much more practical purpose then the usual stuff.

[Garth]

Using the three pK's of phosphoric acid and the pH, the ratio's of each of the phosphoric pairs is calculated. For example, the first ionization is given by:
H3PO4 --> H+ + H2PO4-
K1 = [H+][H2PO4-] / [H3PO4], or
[H2PO4-] / [H3PO4] = K1 / [H+]
Normalizing the amount of each of the four moieties, their relative amounts are calculated. The molar amounts of monosodium phosphate and disodium phosphate are derived from the following two equations.
Monosodium Phosphate = [H3PO4] + [H2PO4-]
Disodium Phosphate = [PO43-] + [HPO42-]

duh, obviously, that's just like so.....yesterday

like AT, I've been doing this all my brewing life......

[prolix]

Well I was happy with most of it but then.....

four moieties

please! such exact chemistry and such glib use of language, well if you can pick a single hole in the argument might as well throw the whole lot out.

I bet he calls them nonagons aswell the Philistine!

[Redbloke]

... could some kind soul explain 'Buffer Strength' to me please?

Cheers

[Redbloke]

... thanks DaaB ... I presume that answer was given after a heavy session!

... so, just to clarify; there is no need to change the buffer strenght value (1000) when entering the ph value?

[WallyBrew]

The term strength applies to the concentration of the two phosphates used in making the buffer. The summation of the two concentrations is the figure given in the cell marked buffer strength. This is given in millimoles. One millimole is one thousandth of a mole.

If you look at the bottom line it suggests that a 10millimolar buffer will probably suffice.

For calibration of a pH meter one would normally work in the range 25millimolar.

So no, the term strength does not necessarily imply that the buffer will be any good at the chosen pH. And yes you may want to alter the 1000 figure

I'm sure this is badly explained

[Redbloke]

... thanks for the answer WallyBrew, but I get the feeling I'm trying to run before I can walk I think I'll go the route DaaB is suggesting. It's gonna be PH 5.2 for me, that's if I can get my hands on some.

Cheers