412 ILLUSTRATIVE EXPERIMENTS 



ii. Aqueous night blue or Prussian blue. iii. Aqueous alkali blue. In 

 10 to 15 minutes examine the height of water 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. 



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

 from a fairly concentrated gelatine gel and place them in a Petri dish 

 containing a 2 per cent, aqueous solution of commercial aluminium sulphate 

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

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

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

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

 of colour. 



28. An Acid Periusate from an Alkaline Solution. Prepare some 

 colloidal ferric hydrate either by dialysing 5 per cent, ferric chloride or 

 by gradually adding 1 c.c/of SO^per cent, ferric chloride to 25 c.c. of boiling 

 water. Put 10 to 15 c.c. of this sol into a dialysing thimble suspended in 

 an Erlenmeyer flask, containing distilled water and some indicator (litmus 

 or methyl red). Estimate the concentration of iron in the wash-water by 

 abstracting, at each change of water, 10 c.c. of the fluid and adding K 4 FeCy 6 

 solution. When the iron content becomes very small (after 48 hours) add 

 some HC1 to the colloid. Note the increase in the diffusibility of the Fe. 

 How can you explain this ? Why does an acid perfusate come from an 

 electro-negative sol ? 



29. Preparation of Colloidal Gold. (1) Protected Solution (Ostwald). To 

 100 c.c. of ordinary distilled water add a few drops of 1 per cent, neutral 

 gold chloride. Mix and add a few drops of 0-1 per cent, tannic acid sol. 

 Heat over a free flame till boiling, shaking it constantly. If the red colour 

 does not appear on boiling add a little more tannic acid and a little more 

 gold chloride alternately. Divide into two parts. 



A. To one part, while still hot, add about an equal volume of water. 



B. Cool the other part before diluting. A is violet in colour while B is 

 cherry red. Blue gold sol. may be prepared from neutral 0-05 gold chloride. 

 Take three portions of the gold chloride solution and add (i) 5-10 drops, 

 (ii) 10-15 drops, and (iii) 15-20 drops respectively of a hydrazine hydro- 

 chloride solution prepared by adding a tiny crystal to about 20 c.c. of water. 

 If (ii) does not turn blue add more hydrazine. If it is greenish, too much 

 hydrazine has been added. 



(2) Gold Sol for Colloidal Gold Test (p. 80). Heat 150 c.c. of triply 

 distilled water (from a resistance glass still). Add 1 c.c. of 1 per cent, gold 

 chloride solution and then 2-5 to 3 c.c. N/5 pure K 2 C0 3 . Bring to the boil, 

 stirring vigorously. Add gradually but not too slowly, 2-3 c.c. of 1 per 

 cent, commercial formaline (1 c.c. 40 per cent, formaline in 99 c.c. of water) 

 and remove the flame. See that the sol (which should be ruby red without 

 any purple tinge) is neutral to alizarin red and that a 5 c.c. sample of it is 

 completely reduced in one hour by adding 1'7 c.c. of 1 per cent. NaCl. 



(3) Determination of the C n of Colloidal Gold. Take a series of small test 

 tubes of equal bore, etc., and range them in two equal rows. Into each 



