Current-voltage (CV) characteristics of polyvinyl alcohol (PVA) - glutardialdehyde
hydrogel cylinders were measured in aqueous KCl solutions. To this end a new special
apparatus was constructed where the gel cylinder connects two electrolyte reservoirs.
The measured quantities are the electric current flowing through the gel and the potential
difference between the two reservoirs. Concentration polarization near the gel-liquid
interfaces is decreased considerably by applying an intense mechanical stirring in
both reservoirs. Under these conditions below 1 V concentration polarization is negligible,
and the CV curves are nearly straight lines. It was found that the gel applied here
is a weakly charged anionic hydrogel. Concentration of fixed anions was determined
from the slope of these lines measured in 0.001 and 0.01 molar KCl solutions. Fixed
anion concentration of the same piece of gel was measured also with a different method,
when the gel was used in an acid-base diode. In this case one reservoir contained
0.1 molar HCl, and the other 0.1 molar KOH. From the results of the two measurements,
the concentration (4.45x10(-3) M) and the pK value (4.03) of the fixed acid groups
responsible for the anionic character of the gel was calculated. The pK value is compatible
with fixed carboxylic acid groups contaminating the PVA gel. Furthermore, concentration
polarization phenomena in the boundary layers nearby the gel were studied in 0.001
M KCl solutions, measuring the diodelike CV characteristic of a gel cylinder, when
stirring was applied only at one side of the gel. Boundary layers facing the cathode
or the anode responded in a different way to stirring. The difference cannot be explained
completely with the hypothesis of electroconvection suggested previously.
