Saturday, August 11, 2018

acid base - Why does buffer capacity of tartrate buffer generally decrease as the buffer pH increases?


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From what I see, the buffer capacity of tartrate buffer is generally decreasing as buffer pH increases. Why is this? Also, does this mean that tartrate buffer doesn't have a maximum buffer capacity at pH=pKa?


Or is there a more reliable source where I can get the buffer capacity vs. buffer pH graph for buffers of tartaric acid? This image is from a blog, and I can't find the real source of this image, even with Google image search.



Answer



Generally, the maximum buffer capacity is at pKXa . The tartaric acid is somewhat special for 2 reasons:




  1. It is a diprotic acid with both pKXa very close, with the pKXa1 rather low, being affected by the reason 2. :



    pKXa1=2.89,pKXa2=4.40(L+)



  2. The solution buffer capacity (not limited to presence of specific buffer substances) generally increases toward pH=0. It means, the buffer capacity is not given by concentration of of conjugated acid and base, but also by concentration of HX+ itself. That in large extent masks the pKXa maximum. It means, at a slope, a local peak must be big enough to be a peak.


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