A comparison of the properties of various preparations of sodium alginate

Abstract

Fifteen specimens of sodium alginate from various brown algae, including species of Laminaria, Fucus and Ascophyllum, have been prepared in the laboratory or obtained from commercial sources. Tests on these samples included measurements of optical rotation, ionic mobility, rate of sedimentation, viscosity and content of sulphated ash. Most specimens were homogeneous by electrophoresis and in the ultracentrifuge. The content of alginate of the samples varied from 61.7 to 99.6%. Rate of sedimentation was strongly dependent on concentration. Electrophoretic mobilities varied only slightly. Viscosity varied much more widely than other physical properties and was the best criterion of state of polymerization. Fucoidin was detected as impurity in some preparations. From the best preparations examined the following values were obtained: [α]d20° = −133°to −140°; μ = 2.9 to 3.1 × 10−4 cm.2 sec.−1 volt−1 at 25°, pH 5.5, I 0.05; s40 = 1.3 to 1.6 S (c = 0.5%); sulphated ash = 34.0 to 35.9%. Viscosity varied from 2 to 20 centistokes at 25°in 0°1n‐NaCl (c = 0.25% w/w). Values for [η] varied from 3.6 to 19.2 corresponding to a degree of polymerization of 210 to 1,100 and particle weight of 42,000 to 222,000. The specific refractive increment of sodium alginate was found to be 0.160 g.−1 cm.3.