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I want to buffer the pH of my solution but I don’t want to add any mobile solutes. I imagine that large molecular mass buffers could be used such as phosphorylated polysaccharides or proteins. The limit of such buffers would be solid phase materials that can protonate or deprotonate at their surfaces, but aren’t mobile.

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  • $\begingroup$ Solid acids / bases don't affect the pH of a solution, because hydronium / hydroxide ions stay close to their surface. $\endgroup$
    – Mithoron
    Sep 16 '21 at 0:23
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Just like wave-particle duality eventually ends up in meta-physics, you are extending the concept of pH too far where the concept of pH becomes murky. The simple answer to your query is ion-exchange resins, which readily protonate and deprotonate at the surface. You have dozens of strong acid, weak acid, strong base, weak base resins. They work perfectly!

Let us consider a very simple case using a sulfonic acid resin, which consists of a polymer with sulfonic acid groups ($\ce{-SO3H}$. The sulfonate $\ce{-SO3-}$ group is fixed and the associated proton is exchangeable with another cation in a quantitative way. It looks something like this...

acid resin

Suppose this resin is in contact with 0.01 M HCl. The pH of the solution is 2, but what is the local pH of the resin surface? The number of protons at the resin surface are pretty high hence the pH is different. Such difficulties render the extension of pH to solid surface quite difficult. This type of work of done in the 1940s with indicators adsorbed at the resin.

In short, such materials exist but we cannot call them solid phase buffers, because just like pH buffering concept cannot be applied to surfaces. One can ask the question of a pH at a solid surface in contact with an aqueous solution but we cannot ask what is the pH or the buffer capacity of solid surfaces.

EDIT The comment asked about the titration of a resin. Yes, direct titration of ion-exchange resins is doable and we do get typical titration curves. As stated earlier, this work was extensively done in the 1940s-50s. This figure is an example of a sulfonic acid resin (phenol+aldehyde type) with NaOH directly.

enter image description here

Ref: Gregor, H. P., & Bregman, J. I. (1948). Characterization of Ion Exchange Resins. I. Acidity and Number of Constituent Cation Exchange Groups1, 2. Journal of the American Chemical Society, 70(7), 2370-2373.

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    $\begingroup$ The pH/buffering of the resin itself or near the surface may be ill defined, but can't you reasonably talk about the effect of the resin on the aqueous bulk, at least on a phenomenological level? (That is, not the pH of/near the solid surface, but of the separate aqueous phase.) E.g. take your aqueous solution in a flask, dump in the ion exchange resin. Shake and let settle. Measure the pH of the aqueous supernatant. Add a quantity of acid/base. Repeat. Does the resulting curve look anything like a titration of a soluble buffer, or is it like the resin isn't there, or something else? $\endgroup$
    – R.M.
    Sep 16 '21 at 12:13

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