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.