560 ILLUSTRATIVE EXPERIMENTS 



from one end. Thoroughly soak and test a piece. Cut it to a convenient 

 length and with a large cork-borer excise a circular piece from both ends 

 about \ in. from the opening. Bend the tube into U-t^hape and place it in 

 a tall cylinder. A glass rod longer than the diameter of the cylinder thrust 

 through the holes at the ends of the dialyser acts as support. The tube 

 may now be filled, by means of a funnel, with the fluid to be dialysed, while 

 the cylinder is filled with water at the same time and at the same rate. This 

 is to prevent undue strain on the tube. 



TYPICAL COLLOIDS 

 (a) SOLS 



80. 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 OT 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. maybe 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 adtl more hydrazine. If it is greenish, too much 

 hydrazine has been added. 



"(2) Gold Sol for Colloidal Gold Test. 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 2-5-3 c.c. N/5 pure KX'Oo. Bring to the boil, stirring 

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

 commercial formalin (1 c.c. 40 per cent, formalin 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^ of Colloidal Gold. Take a series of small test 

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

 tube put 1 c.c. of triply distilled water. To the first tube of one row add 

 1 c.c. of N/50HC1. Mix. Remove 1 c.c. of the mixture and add this to 

 the second tube, and so on down the row. This will give a series of acid 

 solutions as follows : N/100, N/200, etc. The other series of tubes is treated 

 in exactly the same way with N/50NaOH. The final 1 c.c. of each series 

 is discarded. Two drops of alizarin red and 1 c.c. of the gold sol are added 

 to each tube and the contents mixed. The tube showing the neutral alizarin 

 colour is picked out and its value determined. The whole gold sol is now 

 neutralised by the addition of the exact amount of NaOH or HCl as the 

 case may be. 



81. Colloidal Iron (Experiment 26 (d) ). 



Heat 250 c.c. of distilled water in a 500 c.c. Erlenmeyer flask or in a tall 

 beaker flask. When the water is boiling vigorously, place the burner a little 

 out from the centre of the flask so as to produce a rotatory movement of 

 the water. Add to the boiling water, drop by drop, 1 c.c. of a 30 per cent, 

 solution of ferric chloride. The liquid turns a clear reddish brown colour. 



