536 



ILL I STB A TI IE EXPERIMENTS 



(6) Protective action of e))udsoids (p. 93). Two equal portions (9 c.c.) 

 of neutral gold sol are treated (1) Avitli 1 c.c. of a 0-1 per cent, gelatin sol 

 and (2) with 1 c.c. of distilled water. To lioth are added 1 c.c. of N/lNaCl 

 solution. Examine by pure transmitted light, i.e. by looking through the 

 tubes at a uniformly illuminated screen of white ])aper. 



(c) Six tubes are prepared as follows and examined as in (6) above : 



38. Adsorption. 



(a) Adsorption of colloid to a surface. Pour into a series of Erlenmeyer 

 flasks faintly coloured suspensions of various colloidal dyes. Add a gram 

 of blood- or bone-charcoal to each flask. Shake several times. Filter 

 through ordinary pleated papers. Note the practically colourless filtrate 

 obtained. Return the charcoal to the cleaned flasks and shake with water. 

 Is any colour given of? ? Now add some substance w^hich lowers the surface 

 tension of water, e.g. methylated spirit, tributyrin, etc. The colour appears 

 in the fluid. 



Dyes to try. Congo red, 0-05 per cent. ; Night blue, 0-01 per cent. ; Prus- 

 sian blue, 0-01 per cent. ; Berlin blue, 0-01 per cent. 



(b) Adsorption of colloid to colloid. Capillary analysis. Cut a number 

 of strips 2 X 15 cm. from a good filter paper. (Do not take the slip from 

 too near the edge of the sheet.) Hang two or three of these strips so that 

 each one dips its edge into a narrow-necked vessel (Erlenmeyer flask) con- 

 taining a fluid to be tested, taking care that the papers are immersed to the 

 same and to a sufficient depth (about 2 cm.), and that glass and paper do 

 not come in contact. Filter paper becomes negatively charged in contact 

 with water, and, therefore, positively charged colloids will become " fixed " 

 electrostatically at the licjuid-paper interface, while negatively charged 

 colloids will ascend with their dispersion media. (i.) Flask i. Water, 

 ii. Aqueous night blue or Prussian blue. iii. Aqueous alkali blue. In 10-15 

 minutes examine the height of w^ater and each dye on the strips. 



(ii.) Flask iv. Mixture of 20 c.c. 2 per cent, aqueous alizarin red and 

 0-5 c.c. sat. aqueous picric acid. Leave paper hanging in this mixture for 

 20-30 minutes. Remove and examine. Hold over strong ammonia for 

 a moment to make alkaline (i.e. to redden the alizarin). How do you account 

 for the extremely dark band at the junction of the stains of picric acid and 

 picric-alizarin mixture. 



(c) Adsorption of Salts to Colloids. Cut a series of discs 3-4 mm. thick 

 from a fairly concentrated gelatin gel and place them in a Petri dish con- 

 taining a 2 per cent. a(]ueous solution of commercial aluminium sulphate 

 (contains iron) and leave for some days. In three days or so the gelatin 

 becomes tinged reddish brown (ferric salts). Now test the original solution, 

 the solution after standing with gelatin, and the gelatin itself for iron 

 by adding a few drops of ammonium thiocyanate to each. Note the depth 

 of colour. 



