Lithium ions, and, to a lesser extent, the sodium ions are bound by polyacrylic acid, as evidenced by pH titrations of polyacrylic acid in dilute (0.01 M) solutions of the alkali metal bases and solutions of their corresponding salts. The formation constant for the binding of lithium to polyacrylic acid was found to be 1.9. When dilute solutions of polyacrylic and polymethacrylic acids were titrated with quaternary ammonium bases, the polyacids became progressively weaker with increasing size of the counterion. These phenomena suggest that the counterions are largely held in close proximity to the polyanion chain, with distances of approach comparable to the sum of the ionic radii. Titrations of these acids with a mixture of a large and small cation also served to point up the increased chain potential in the presence of large cations. All of these effects are absent with the analogous low molecular weight acids.

The reduced specific viscosity of polymethacrylic acid increased with counterion size at the same degree of neutralization for the cations: sodium < tetramethylammonium < tetraethylammonium < tetrapropylammonium < tetrabutylammonium. The increase on going from sodium to tetrapropylammonium at 50–70% neutralization was 20–25% for 0.003–0.01 M polymer, about 4% in 0.001 M solutions. These data indicate that a significant fraction of even large counterions are close to the chain. Conductometric titrations showed that the equivalent conductance of the polyanion was about 40 for most of the salts studied and at low values of α. The binding of counterions based upon the assumption that this value was constant over α led to values which were in reasonable agreement with those determined by Wall using transport experiments.

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A diaminobenzidine (DAB) stain for myelin in glutaraldehyde fixed, osmicated, semithin epoxy sections is described. One or 1.5 microm sections, dried onto slides, are first etched with a 1:2 dilution of saturated sodium ethoxide:absolute ethanol, then incubated in 0.05% aqueous DAB with 0.01% hydrogen peroxide. DAB specifically stains osmium fixed myelinated nerve fibers. This permits high resolution light microscopic study of myelinated nerve fibers in semithin sections of tissues that also can be studied by electron microscopy.

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