75% methanol in water using phosphate buffer Ph=3 , 25 mM ?

[WESAM]

Hi , Tom and every one

Is my preparation correct ? :

Buffer preparation : in 1L volumeric flask put

1- 50.00 ml of 0.5 M tetrasodium pyrophosphate

2- 57.75 ml of 1.00 M phosphoric acid .

Fill up to 1L by water .

so now 75% methanol in water with buffer of phosphate at 25 mM can be done as the following in 900 ml flask :

675 ml methanol was put in 900 ml flask + 225 ml fromthe buffer that was prepared in 1L ( because i want to be buffered at ph=3 ).


Any help will much appreciated

[HW Mueller]

Check out
http://www.liv.ac.uk/buffers/buffercalc.html

Why do you use pyrophosphate? How fast does that hydrolyze to phosphate? Normally one would use phosphoric + NaH2PO4 for this buffer.
A quick head calc tells me you have ~ 0.1 M aqu. solution, why that 57.75ml phosphoric?
If you think that you have ~0.025 M solution in the final mixture than you are wrong, as 675mL MeOH + 225mL aqu. solution does not give 900mL.

[WESAM]

Hi

becase i use this calcuator
http://www.chembuddy.com/?left=Buffer-M ... reparation

it gives me this amount that i mentioned , but why 675mL MeOH + 225mL =900 ml !! .

What is the correct way to prepare 75% methanol in water but i need it buffered at ph=3 .

thanks