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Author Topic: ammonium acetate buffer!  (Read 21333 times)

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Berettagtz

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ammonium acetate buffer!
« on: June 21, 2007, 10:22:32 PM »

I know this is probably a simple problem, but I am in the lab and I am trying to make this buffer (around 4.5 pH).  The issue I am having is that I only have liquid acetic acid (glacial) and liquid ammonium hydroxide available.  I need about 500 mL of ammonium acetate buffer so can anyone tell me how to do calculations for this. 
here is how I would do it if I had solid form reagents:
Ammonium acetate buffer - Prepare a 0.1 M ammonium acetate solution by dissolving 7.7 g ammonium acetate in a 1000 mL water. Adjust 1 L of this solution to pH 4.5 by adding acetic acid (about 8 mL) and 5 mL of 1 M p-TSA (equivalent to 5 mM p-TSA).

Thanks in advance.
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AWK

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Re: ammonium acetate buffer!
« Reply #1 on: June 21, 2007, 10:32:35 PM »

Explain an abreviation: p-TSA
You need at leats Ka of acetic acid
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Berettagtz

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Re: ammonium acetate buffer!
« Reply #2 on: June 21, 2007, 10:41:58 PM »

p-Toluene sulfonic acid (p-TSA), and the pKa for acetic acid I believe, is 4.76. 
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AWK

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Re: ammonium acetate buffer!
« Reply #3 on: June 22, 2007, 01:39:04 AM »

Then this is a more complex problem your mixture contains 0.005M ammonium p-toluenesulfonate (PTS-OH is much stronger acid than CH3COOH), ammonium acetate and acetic acid. But for crude calculation you can neglect PTS salt and calculate concentrations of  ammonia and acetic acids for 0.1 M ammonium acetate buffer. Since concentration and moles in 1L are the same numbers you can diminsh acetate by 0.005 mole, then scale moles to 0.5 L.
Finally, knowing density of glacial acetic acid (or its molarity) you can measure a needed its volume).
You need also molar concentration of ammonia (or % concentration and density)

After mixing all reagents add water enough to volume 0.5 L.
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Berettagtz

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Re: ammonium acetate buffer!
« Reply #4 on: June 22, 2007, 02:36:30 AM »

well, the addition of p-toluene sulfonic acid is only to get the pH to about 4.5.  In essence, if the pH is already 4.5 without p-toluene sulfonic acid, then I would not need to add any p-toluene sulfonic acid. 
So, to reframe my first post, I only needed to know how to make ammonium acetate buffer of pH about 4.5 with liquid ammonium hydroxide and liquid acetic acid. 
I think I confused you with p-TSA.
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AWK

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Re: ammonium acetate buffer!
« Reply #5 on: June 22, 2007, 03:17:45 AM »

Now, what does mean 0.1 M buffer solution?

It contains 0.1 mole of acetic acid and acetate in 1 liter of solution. You need o.5 L of buffer prepared from acetic acid, hence you need 0.05 mole of acetic acid (=3 grams)
{H3O+] = Ka x c_acid / c_salt
c_acid + c_salt = 0.1 (these concentrations concerns equilibrium)
You should solve a system of 2 equations with 2 variables and find
c_salts= concentration of ammonia in 1 L. Depending on your ammnia concentration you should calculate volume of ammonia, after mixing add water to 0.5 L.
« Last Edit: June 22, 2007, 04:39:08 AM by AWK »
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Berettagtz

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Re: ammonium acetate buffer!
« Reply #6 on: June 22, 2007, 06:54:43 PM »

OK, so here is what I have so far:
I rearranged the first equation to get:

[acetic acid]=[H3O+][ammonium hydroxide]/Ka

and then I plugged this into the second equation:

[H3O+][ammonium hydroxide]/Ka  + [ammonium hydroxide] = 0.1

so the H3O+ concentration can be calculated from the desired pH, and also Ka value is known.  So then I calculate the [ammonium hydroxide] and plug it into one of the equations to get [acetic acid].  So once I have both unknown concentrations, I take the original concentrations (obtained from the bottle) and it's just a dilution problem from there. 

am I on the right track here?

ps.
for acetic acid I did not find molarity on the bottle, but it says that it contains "more than 80%" acid by mass.  Will this be a problem in my further calculations?
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AWK

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Re: ammonium acetate buffer!
« Reply #7 on: June 22, 2007, 08:25:15 PM »

OK
For a density of 80 % acetic acid see Borek tables
http://www.chembuddy.com/?left=CASC&right=density_tables

For calculations of molarity see Borek lectures
http://www.chembuddy.com/?left=concentration&right=toc
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Berettagtz

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Re: ammonium acetate buffer!
« Reply #8 on: June 22, 2007, 09:37:51 PM »

OK, I calculated the molarity of stock acetic acid and I got 14.24.  The molarity of stock ammonium hydroxide is 14.8.
Then I calculated the desired molarities for both the salt and the acid.  
[ammonium hydroxide]=0.0355 M
[acetic acid]=0.0645 M

 
so how would I calculate the volume I need of each reagent to give me .5 L of buffer?
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