[Sonic Hedgehog]

Hi all,
This is a pretty basic question, but it's been a while since University and I'm a bit rusty with buffers (so please bear with me!)

So I'm trying to prepare a range of buffers for LCMS.  I want to prepare buffers at pH 4, 5, 6, 7, 8.  Buffer concentration is 1M.  I'll need to use volatile buffers such as Ammonium acetate and formate buffers.  pKa of Formic is 3.75, Acetic is 4.76, so I know these buffers won't be too effective at or above pH6, but I'm hoping the high concentration (1M) will be enough to buffer a urine sample for example.
For 100mL buffer I've worked out the mass of NH4.acetate and volume of Acetic acid I need with HH eqn.  When I use an online buffer calculator (Chembuddy) I get different results so I'm not sure what I'm doing wrong.

Columns are:
pH of buffer   mass (NH4.acetate).g     Volume (Acetic).  uL   100mL buffers

   4   1.141   4877   1M NH4.Oac buffer (pH4)
   5   4.893   2091   1M NH4.Oac buffer (pH5)
   6   7.289   311   1M NH4.Oac buffer (pH6)
   7   7.664   33   1M NH4.Oac buffer (pH7)

Buffer calculator

pH of buffer   mass (NH4.acetate).g     Volume (Acetic).  uL

   4   1.78   4.4mL   1M NH4.Oac buffer (pH4)   
   5   6.52   879.7uL   1M NH4.Oac buffer (pH5)   
   6   7.59   90.09uL   1M NH4.Oac buffer (pH6)   
   7   7.71   0.1476uL    1M NH4.Oac buffer (pH7)   

When I plug in the values from the buffer calculator (above) into the HH equation I don't get the target pH.  I'm not sure what I'm doing wrong.  Any ideas?

Thanks for your help

[Borek]

When I use an online buffer calculator (Chembuddy)

No such thing (no online version), unless someone reused the name.

1M in what? You use ammonia and acetic acid, they can't be both 1M at the same time.

1M solutions have quite high ionic strength, so calculations are rather problematic. Also, HH equation is of no use far outside the buffer working range.

I am not convinced the idea of using high concentration far from the buffering range will work.

[Sonic Hedgehog]

Hi Borek,
Sorry I meant that I downloaded it online.

Regarding buffer concentration 1M is the total in this case (final molarity of NH4.acetate plus final molarity of acetic acid), so [A] + [HA].  Is that not convention?

I am not convinced the idea of using high concentration far from the buffering range will work.

I'm not convinced either.  I couldn't think of a volatile buffer that operated at ~pH6-7 however.

Thanks for your help, I'd appreciate any suggestions about buffer selection/concentration you might have. 

Cheers

[Borek]

Regarding buffer concentration 1M is the total in this case (final molarity of NH4.acetate plus final molarity of acetic acid), so [A] + [HA].  Is that not convention?

There are two possible buffers in the solution - acetic acid/acetate and ammonium/ammonia. While yes, convention says buffer concentration is a sum of concentrations of the acid and conjugate base, there was ambiguity as to which pair you mean.

No idea about the volatile part. If you have installed Buffer Maker it calculates the buffer capacity, so you can easily compare how good each buffer is at given pH.