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### Topic: Conc of NH3 in solutions of different pH ?  (Read 3098 times)

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#### greengumbo

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##### Conc of NH3 in solutions of different pH ?
« on: December 13, 2007, 09:16:31 AM »
Hi everyone,

Im new to the forum so big hello !

Also I have not done any serious chemistry until this year when my thesis demands a small amount. Im basically interested in the transport of solutes across membranes and NH3 transport.

I started with a solution (Barth's buffer) consisting of :

23 mM NaCl
2.4 mM NaHCO3
1 mM KCl
0.33 mM Ca(NO3)2 * 4 H2O
0.41 mM CaCl2 * 2 H2O
0.82 mM MgSO4 * 7 H2O
5 mM Tris/HCl

This was adjusted from the starting ~70mOsm to 200mOsm with the addition of
NH4Cl (I dont have the exact mass added to acheive this but can go find it if needed!) and then adjusted to pH 6.5, 7.5 and 8.5.

So the question is : What is the concentration of NH3 (as opposed to NH4+)at each of the pHs ?

#### Borek

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##### Re: Conc of NH3 in solutions of different pH ?
« Reply #1 on: December 13, 2007, 11:15:14 AM »
You have two equations describing the system - dissociation constant definition and mass balance (sum of NH3 and NH4+ is the same as initial NH4+ concentration). Just solve for NH3.
« Last Edit: December 13, 2007, 11:27:07 AM by Borek »
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#### greengumbo

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##### Re: Conc of NH3 in solutions of different pH ?
« Reply #2 on: December 14, 2007, 08:03:11 AM »
Hi Borek,

Thanks for your post. Im sorry for being so ignorant but i still dont understand what I need to do ? Im not really sure how one goes about solving this at all!

Cheers

Ewan

#### Borek

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##### Re: Conc of NH3 in solutions of different pH ?
« Reply #3 on: December 14, 2007, 09:39:34 AM »
You have two equations:

Kb = [NH4+][OH-]/[NH3]

[NH4+] + [NH3] = C

Find C (total concentration) from the mass of amonium chloride added, solve for [NH3].
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