[jameskin3e05]

i want to know what pH changes would appear when i add water to dilute a buffer solution~ i think it would not change,right?
But in my experiment,the pH of the buffer solution is increased from 4.95 to 5.23 after dilution~why is that?

i have seen that there were lots of reason~e.g. activity coefficients , ionic strength
but i dont know how these factor affect the pH of the buffer solution

Also,i want to ask if acidic buffer solution resist acid more better than alkali?
                            alkali buffer solution resist alkali more better than acid?

[Borek]

See http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch and other lectures on the ChemBuddy site.

[jameskin3e05]

could u reply my question clearly~?!
i have already seen the website that u post
but that's only a large paragraph,and very difficult!!!

Thanks

[Borek]

There is a table that shows how pH changes with dilution (sorry for the edit of my post in the meantime). That's as close to the answer as possible.

What buffer and what dilution factor did you use? What was the ionic strength of the solution before and after dilution?

[jameskin3e05]

buffer solution made by sodium hydroxide and ethanoic acid with  pH 4.95

5cm of this buffer solution is mixed with 45cm water
the pH value increased from 4.95 to 5.73

but form your graph,it should be unchanged

in fact,i dont know what mean of ionic strength is ,i just copy from your post only

furthermore,i want to know that why adding water would affect the pH of buffer solution~?
pH=pKa+log(salt/acid)
ADDing of water would not change the ratio of salt/acid,isnt it?

[AWK]

Adding of water can change pH of buffer solution, since you, in fact,  do not measure a concentration of H+ during measurements of pH but activity of H+. Activity of H+ and other ions change with concentration of ionic species in solution.

[Borek]

buffer solution made by sodium hydroxide and ethanoic acid with  pH 4.95

What was concentration of the buffer before dilution?

furthermore,i want to know that why adding water would affect the pH of buffer solution~?
pH=pKa+log(salt/acid)
ADDing of water would not change the ratio of salt/acid,isnt it?

Ratio changes because acid dissociates. The more diluted, the more dissociated acid is, which means there is less acidic form and more cojugated base in the solution.