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The Acidity of Nafion
From time to time we
receive questions about the acidity and pH value for Nafion®.
Unfortunately, a pH value cannot be assigned to Nafion.
pH is defined as, "A
symbol for the degree of acidity or alkalinity of a solution; originally and
still often expressed as the negative logarithm of the hydrogen ion
concentration in gram equivalents per liter of solution." Mathematically it is
expressed as pH = -log [H+] .
The concept of pH
presumes that the material in question can be dissolved in water (in solution).
Nafion does not dissolve in water. It therefore does not have a pH value. High
purity water (distilled, deionized) has a pH = 7. This corresponds to a neutral
solution, with a pH=1 being very acidic (high in concentration of hydrogen ions)
and a pH=14 being very basic (very low in concentration of hydrogen ions). If
Nafion were immersed in water, in theory, the pH of the water would remain
unchanged. Actually what happens is that the water does become acidic but not
because Nafion is adding H+ to the water. Water is comprised of H20
molecules, H+ ions and -OH groups. The Nafion absorbs the -OH groups
and leaves excess H+ in the water causing the pH to decrease.
The simplistic answer to
the question is therefore that Nafion has a pH of 7. This is an
oversimplification, but hopefully solves the problem of concerns with Nafion. We
reiterate that Nafion is not poisonous, does not cause any burns when in contact
with the body, and in fact was in years past implanted into the human body as
the tip of pacemaker electrodes inserted into the human heart in order to
stabilize the rate of heartbeat.
The confusion is probably
related to the term "acidic." Since Nafion has no pH, it is not acidic per se.
Instead Nafion is a super-acid catalyst. In Lewis acid-base terms, this means
that Nafion can very readily accept a free electron. This makes Nafion
electrically conductive. It also makes Nafion able to stimulate certain types of
chemical reactions that require removal of an electron in order to initiate the
electron.
Nafion is a copolymer of
two monomeric subunits: tetrafluoroethylene (Teflon®) and
perfluoro-3,6-dioxa-4-methyl-7-octene sulfonic acid. The Teflon serves as a
backbone, with the other monomer attached at regular intervals along the length
of the Teflon chain. The other monomer ends in a sulfonic acid group. If this
sulfonic acid were able to be released and dissolve into solution, it would be a
strong acid. However, Nafion is extremely resistant to chemical attack
(corrosion-resistant) and the sulfonic acid cannot be broken from the Nafion,
even by very strong chemical reactions. While firmly bound within the Nafion
matrix, the sulfonic acid is harmless.
This is very similar to
many other plastics. Many plastics contain cyanide or other poisonous chemical
groups. Once polymerized into solid plastic, however, these potentially
hazardous groups are permanently bound, and are no longer harmful. |
