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Chemistry Forums for Students => Analytical Chemistry Forum => Topic started by: DUDE778 on March 03, 2008, 02:33:26 AM

Title: Phosphate Buffer -- Ionic Strength
Post by: DUDE778 on March 03, 2008, 02:33:26 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

Title: Re: Phosphate Buffer -- Ionic Strength
Post by: Borek on March 03, 2008, 02:55:21 AM

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

Title: Re: Phosphate Buffer -- Ionic Strength
Post by: AWK on March 03, 2008, 03:43:09 AM

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

Title: Re: Phosphate Buffer -- Ionic Strength
Post by: coquim on March 03, 2008, 05:20:27 AM

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

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

Title: Re: Phosphate Buffer -- Ionic Strength
Post by: Borek on March 03, 2008, 06:52:21 AM

It completely ignores ionic strength, so it is of no use here.

Title: Re: Phosphate Buffer -- Ionic Strength
Post by: AWK on March 03, 2008, 06:59:53 AM

Quote from: DUDE778 on March 03, 2008, 02:33:26 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?

Title: Re: Phosphate Buffer -- Ionic Strength
Post by: DUDE778 on March 11, 2008, 03:14:53 PM

For this question, assume activity coeffienct of all unity. 

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

Title: Re: Phosphate Buffer -- Ionic Strength
Post by: Borek on March 11, 2008, 04:02:32 PM

What do you know about buffer pH calculation?

Title: Re: Phosphate Buffer -- Ionic Strength
Post by: DUDE778 on March 11, 2008, 07:38:11 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

Title: Re: Phosphate Buffer -- Ionic Strength
Post by: Borek on March 11, 2008, 07:48:50 PM

http://www.chembuddy.com/?left=pH-calculation&right=pH-buffers-henderson-hasselbalch

http://en.wikipedia.org/wiki/Henderson-Hasselbalch_equation

http://www.chembuddy.com/?left=pH-calculation-questions&right=pH-buffer-q2

Title: Re: Phosphate Buffer -- Ionic Strength
Post by: AWK on March 12, 2008, 01:56:39 AM

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 H₂PO_4-) as:
I =  c(NaH₂PO₄) + 3 c(NaH₂PO₄)

Can you now prepare your buffer neglecting activity coefficients?