[Jaki_jaki]

Hello,

I am reading a protocol in which they said the follow:

The water layer was acidified to pH 6 with concd. HCl, diluted with 0.25 M phosphate buffer at pH 6.2 (25ml).

I have no idea how to do this buffer! any help?

[Jaki_jaki]

Somebody knows how to do it...

thanks for advance!

[sjb]

Somebody knows how to do it...

thanks for advance!

Give people a chance to answer, we're not all on 25 hours a day, 8 days a week.

Anyway. What pair of salts do you think you should be using?

[Jaki_jaki]

I would say monosodium phosphate and disodium phosphate....

[ATMyller]

Phosphate buffer usually refers to aqueous solution of potassium hydrogen phosphate and sodium dihydrogen phosphate or aquaeous solution of sodium dihydrogen phosphate and sodium hydrogen phosphate, but can sometimes mean McIlvane's citrate-phosphate buffer made of citric acid and sodium phosphate solutions.
You can calculate the required amounts using Henderson-Hasselbach Equation or try to find a buffer table for the required concentration (can usually be found from chemical supplier websites).

[Jaki_jaki]

And in my case? I am not a chemist but I have to prepare that to do a special experiment....

If I have to prepare 0.25 M of this buffer...how many amount I need of each salt...

Sorry and I would understand if u dont answer...

[Jaki_jaki]

in fact the question is what means....HCl diluted in buffer solution...I have to add HCl to the buffer solution? and how much?

[AWK]

in fact the question is what means....HCl diluted in buffer solution...I have to add HCl to the buffer solution? and how much?

Phosphate buffer contains two hydrogen phosphate salts. 0.25 M means that sum of both concenetration is just 0.25 M

http://www.cnr.berkeley.edu/soilmicro/methods/phosphate%20buffer.pdf
http://home.fuse.net/clymer/buffers/phos2.html

[Borek]

Let's not forget about

http://www.chembuddy.com/?left=Buffer-Maker&right=buffer-calculator

[Honclbrif]

I've always found neutralizing a solution of the acidic salt to be the easiest and fastest way to make a buffer. E.g, if you want a 0.1 M phosphate buffer of pH 7.2, you could work out the HH equation, or measure out the mass of dihydrogen phosphate needed, make a solution thats about 90% of the required vol., drop a pH probe in, and titrate with NaOH until the pH is correct. Then dilute to the final vol, check the pH again.

[Babcock_Hall]

The 0.25 M refers to the total concentration of phosphate species.  I would start with the monosodium form and titrate with NaOH.  You won't need much NaOH in moles, relative to NaH2PO4, because the practical pKa of phosphate is around 6.9 or a little less under your conditions.  The thermodynamic pKa (7.2) refers to an ionic strength of zero.  I would weigh out the appropriate amount of salt and put into a beaker with about 80% of the water I needed.  Then I would add NaOH until a pH of 6.2 is reached.  Then I would pour the solution into a graduate cylinder, add water to the appropriate mark, and mix.