Intrinsic Viscosity of Gelatin

Abstract

THE intrinsic viscosity ([η]) is commonly used to characterize high polymers; but its determination is often complicated by adsorption of the high-polymer solute to the apparatus¹. It has been widely assumed and has recently been categorically stated² that adsorption effects can be neglected in the determination of the intrinsic viscosity of gelatin. The examination of more than a score of gelatin samples in this laboratory indicates that under normal working conditions this is not so. Considerable errors attend the use of sintered glass filters in the preparation of samples through removal of significant amounts of gelatin by adsorption. The extent of this error depends on the size and porosity of the filter disk. For example, when 50 ml. of a 0.1 per cent solution of alkali-bone gelatin ([η] = 65 ml./gm.) in molar potassium chloride at pH 7 was filtered through a 2-cm. diameter disk, the adsorptions determined by chemical analysis of the filtrate³ were 4, 7 and 13 per cent, for disks of porosity 1, 2 and 3 respectively. It is possible that the influence of this adsorption on the viscosity is greater than indicated by these figures since high-molecular-weight species of polymers are usually more readily adsorbed. When membrane filters (Membranfilter Gesellschaft No. 1) were used for filtering the solution, the adsorption was not detected analytically and may be presumed to be less than 1 per cent.