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Author Topic: Phosphate Buffer -- Ionic Strength  (Read 19439 times)

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DUDE778

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Phosphate Buffer -- Ionic Strength
« on: March 02, 2008, 08:33:26 PM »

Given that the stepwise dissociation constants for phosphoric acid are:
Ka1 = 7.5×10-3; Ka2 = 6.2×10-8; Ka3 = 4.8×10-13

To prepare 1.10 L of a buffer solution having an ionic strength of 0.130 and a pH of 7.55 would require:

1. (mass) of Na2HPO4(anhydrous)
2. (mass) of NaH2PO4(anhydrous)

I have no clue what this question is asking or even saying

how would i start
Thank you
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Borek

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Re: Phosphate Buffer -- Ionic Strength
« Reply #1 on: March 02, 2008, 08:55:21 PM »

What is ionic strength, how do you calculate pH of buffer?
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AWK

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Re: Phosphate Buffer -- Ionic Strength
« Reply #2 on: March 02, 2008, 09:43:09 PM »

You should specify a method of calculation of activity coefficients.
The Debye-Huckel or EHT(correction - should be EDH = extended Debye-Hückel equation)  methods are insufficient for a such ionic strength.

Yor acidity constants of H3PO4 are probably for a null ionic strength
« Last Edit: March 05, 2008, 04:02:12 AM by AWK »
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coquim

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Re: Phosphate Buffer -- Ionic Strength
« Reply #3 on: March 02, 2008, 11:20:27 PM »

http://www.columbia.edu/~scb2001/tools/phosphate/phosphate.html

this link may could help you...this little program works good!
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Borek

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Re: Phosphate Buffer -- Ionic Strength
« Reply #4 on: March 03, 2008, 12:52:21 AM »

It completely ignores ionic strength, so it is of no use here.
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Re: Phosphate Buffer -- Ionic Strength
« Reply #5 on: March 03, 2008, 12:59:53 AM »

Given that the stepwise dissociation constants for phosphoric acid are:
Ka1 = 7.5×10-3; Ka2 = 6.2×10-8; Ka3 = 4.8×10-13

To prepare 1.10 L of a buffer solution having an ionic strength of 0.130 and a pH of 7.55 would require:

1. (mass) of Na2HPO4(anhydrous)
2. (mass) of NaH2PO4(anhydrous)

I have no clue what this question is asking or even saying

how would i start
Thank you

Which Ka you should use for a mixture of NaH2HP4 and NaH2PO4 for pH calculation without using ionic strength?
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DUDE778

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Re: Phosphate Buffer -- Ionic Strength
« Reply #6 on: March 11, 2008, 09:14:53 AM »

For this question, assume activity coeffienct of all unity. 

Again, i still feel alittle lost  ???
thank you
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Borek

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Re: Phosphate Buffer -- Ionic Strength
« Reply #7 on: March 11, 2008, 10:02:32 AM »

What do you know about buffer pH calculation?
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DUDE778

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Re: Phosphate Buffer -- Ionic Strength
« Reply #8 on: March 11, 2008, 01:38:11 PM »

Um.. i do know that

HA(aq) + H2O(l) --> H3O+(aq) + A-(aq)
Ka = [H3O+][A-] / [HA]

[H3O+] = Ka[HA] / [A-]
pH = -log[H3O+]

Thats as far as i can think of.... i just can figure out how to start this question though  ???
I think i must be stupied if i cant do a simple pH problem
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Borek

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Re: Phosphate Buffer -- Ionic Strength
« Reply #10 on: March 11, 2008, 07:56:39 PM »

Quote
[H3O+] = Ka[HA] / [A-]
pH = -log[H3O+]
This above is sufficient for pH calculation of your buffer without activities when you know concentrations.
You can approximately calculate ionic strenght for mixture of salt (ignoring dissociation od H2PO4-) as:
I =  c(NaH2PO4) + 3 c(NaH2PO4)

Can you now prepare your buffer neglecting activity coefficients?
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