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:
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+)
- 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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