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Effect of Phosphate Dilution on pH

Effect of Phosphate Dilution on pH - Chemistry Forum

Effect of Phosphate Dilution on pH - Chemistry Forum. Discuss chemical reactions, chemistry.


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  #1  
Old 11-18-2003, 12:36 AM
N. Ron
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Default Effect of Phosphate Dilution on pH



I have an interesting problem....I know I learnt about this in Chem classes,
but that was a long time ago and can't remember!

Why does the pH of a dibasic sodium phosphate buffer solution increase upon
dilution? For example, a 10-fold dilution in the buffer increased the pH
from 7.25 to 7.35. If you just look at the Henderson-Hasselbach equation, as
long as the ratio of the base to the conjugate acid stays the same (as with
dilution), the pH should not change.

I presume that the aw (water activity coefficient) increases with dilution
for dibasic phosphate (relatively more than monobasic phosphate) resulting
in the above change. However, I cannot find any hard data on the actual aw
values at various dilutions.

Or am I on the wrong track?

Please help. Google did not return any useful matches.

Niles


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  #2  
Old 11-18-2003, 02:41 AM
Bruce Sinclair
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Default Effect of Phosphate Dilution on pH

In article <bpbpi6$mqp$1@bob.news.rcn.net>, "N. Ron" <nileshron@rcn.nospam.com> wrote:

isn't pH related to the concentration of H ? Isn't concentration
related to dilution ?

Bruce


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  #3  
Old 11-18-2003, 08:29 PM
Marvin Margoshes
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Default Effect of Phosphate Dilution on pH


"N. Ron" <[Only registered users see links. ]> wrote in message
news:bpbpi6$mqp$[Only registered users see links. ].rcn.net...
classes,
upon
as
with

Writing the ionization equations of buffers in terms of concentration is a
simplification. Activities should be used. The simplification is used
because activity coefficents can't be calculated, except for very dilute
solutions.


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  #4  
Old 11-20-2003, 09:35 AM
zwiebelane
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Default Effect of Phosphate Dilution on pH

"N. Ron" <[Only registered users see links. ]> wrote in message news:<bpbpi6$mqp$[Only registered users see links. ].rcn.net>...



I belive TRIS (tris-hydroxymethyl)aminomethane does just the
opposite... and decreases ~0.1 pH unit per 10 fold dilution.
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  #5  
Old 11-20-2003, 05:40 PM
Libor Striz
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Default Effect of Phosphate Dilution on pH

Bruce Sinclair <bruce.sinclair@NOSPAMagresearch.NOTco.NOTnz> Tue, 18 Nov
2003 02:41:36 GMT wrote ...

You are right in both questions, but wrong in buffer context.
The pH is constant for constant ratio of activities
of conjugated acid / base pair for wide range of conc.


But those activities ..... :-(

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  #6  
Old 11-20-2003, 05:40 PM
Libor Striz
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Default Effect of Phosphate Dilution on pH

N. Ron <[Only registered users see links. ]> Mon, 17 Nov 2003 19:36:35 -0500 wrote
....

Activity coefficient for very dilute solutions (0.001 mol/l or so,
higher konc = more complicated expresions ) is AFAI remember cca:

log10(Acoef) = -0.5 x z^2 * sqrt( I )

where "z" is a charge of a ion and "I" is "ion power"

I = 0.5 sum ( z^2 * c ),

c is mol. konc. of a ion.

So for infinite dilution koef. go to 1.
For real concentrations both go down,
but for hydrogenphosphate(z=2) drops much faster then for
dihydrogenphosphate(z=1).

If you are interested, I can search
formulas for higher konc ( maybe up to 0.1M ) in my books.


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