# Why does buffer capacity of tartrate buffer generally decrease as the buffer pH increases?

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.

• – MaxW Mar 30 '19 at 17:01
• @MaxW I have seen that source, but for some reason the equation they give for their monoprotic buffer capacity is very different from what is given here (chembuddy.com/?left=pH-calculation&right=pH-buffer-capacity) and the equation for their diprotic buffer capacity gives extremely high values as [H+]/Kw, which is a part of the equation, gives, for example, 10^10, when pH = 4.0. I'm not sure why there's such a big difference from other sources, seeing that the unit are both mol/L/pH. Or am I mistaken about the units being the same? – YeRyeon Seo Mar 30 '19 at 17:20

Generally, the maximum buffer capacity is at $$\ce{pK_a}$$ . The tartaric acid is somewhat special for 2 reasons:
1. It is a diprotic acid with both $$\ce{pK_a}$$ very close, with the $$\ce{pK_{a1}}$$ rather low, being affected by the reason 2. :
$$\ce{pK_{a1}}=2.89,\ce{pK_{a2}}= 4.40 (L+)$$
2. The solution buffer capacity (not limited to presence of specific buffer substances) generally increases toward $$\ce{pH}=0$$. It means, the buffer capacity is not given by concentration of of conjugated acid and base, but also by concentration of $$\ce{H+}$$ itself. That in large extent masks the $$\ce{pK_a}$$ maximum. It means, at a slope, a local peak must be big enough to be a peak.
• For a diprotic acid see aqion.de/site/184 You'll need to plug $a_1$ and $a_2$ from formulas (2.3b) and (2.3c) into formula (5.1a) – MaxW Mar 30 '19 at 19:04