In another set of experiments Graham placed the solution in a shallow bell-jar, the bottom of which was closed by a piece of parchment paper or bladder (i.e., a solid colloid). This membrane separated the solution from pure water in which the apparatus, called a dialyser,
 | Fig: Grahams dialyser |
was placed. Crystalloids passed readily through the colloidal septum, whereas colloids were either arrested or diffused exceedingly slowly.
By means of the dialyser a solution of a colloid may be freed from crystalloidal impurities (e.g., salts). A convenient dialyser consists of a parchment paper tube bent into a U-shape, filled with the solution, and placed in a jar through which passes a slow stream of water.
 | Fig: Tubular dialyser |
Small "thimbles" of parchment paper, slipped over the end of a glass tube and fixed by a short length of rubber tubing, may also be used. Collodion films are still more efficient.
Expt. 2. - Pour a solution of potassium iodide and starch into a dialyser, consisting of a piece of parchment paper tied tightly over the mouth of a bell-jar. Suspend the bell-jar with the parchment paper dipping into distilled water in a dish. After half an hour add chlorine water to the water in the dish. A yellow colour, due to liberated iodine, shows that the iodide has diffused through the parchment paper, but the starch is retained, since this would have given a blue colour with the iodine, as may be seen by adding chlorine water to the liquid in the bell-jar.
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