Preparation of Electrolyte-free Gelatin

The method described here is a modification of the one employed by Loeb. His method consisted principally in bringing powdered gelatin, by means of acetic acid, to a pH slightly below the isoelectric point and then washing it several times with distilled water. This process brought about a complete removal of various cations. No special care was taken to remove anions except so far as they were replaced by the acetate ion of the acetic acid. In our modified method, of which a brief description was given in a former publication, ~ the gelatin is treated, in addition to acetic acid, also with dilute alkali, and then washed at pH 4.7 with H20, thus bringing about a removal of both cations and anions. The details of the process are as follows. 1000 gin. of Cooper's non-bleached powdered gelatin, the fine dust of which has been removed by screening it through an 80 mesh sieve, are soaked for 1 hour, with constant stirring, in 20 liters of ~/128 acetic acid, at a temperature of 5 ° to 10°C. A glazed 5 gallon earthenware jar immersed in a water bath cooled by means of a coil from a refrigerating machine is employed as a container. The gelatin is washed three times with cold distilled water by allowing the gelatin grains to settle, removing the sypematant liquid as thoroughly as possible, and then refilling the container. The supernatant liquid is conveniently removed by the use of an inverted Btichner funnel, 14 cm. in diameter, connected by means of a long rubber tubing to a 4 liter suction flask and then to a suction pump. A padding made of towel cloth is spread on the funnel in order to filter out the gelatin. The suspension of gelatin is stirred for about 20 minutes in the washing liquid before renewing it.