COLLOIDS 527 



Add ahout 20 drops of neutral rod (O-Of) per cent, in alcoliol) to each. Add 

 N/IO NaOH to the water till it has approximately the same pR as the 

 bicarbonate. Titrate liotli with N'nOHCl to the same end jxiiiit. 



26. Dialysis (see pp. 557-560). 



(1) In using any dialvser it is advisabh' to look to the following points : — 



1. Test for leaks. 



2. See that neither the preservative nor tlie fluid to be dialysed act on 

 the substance of the membrane, e.g. bile ])igment increases the permea- 

 bility of collodion. (Bile may be dialysed through a double dialyser. ?.e. a 

 collodion tube suspended in a larger tube of the same material. Blood 

 pigment occasionally presents the same difficulty.) 



3. II the dialysate is wanted as well as the dialysed fluid, dialysis must 

 be carried out by changing the external fluid from time to time. 



4. If it is not necessary to keep the external fluid for examination, rapid 

 dialysis may be obtained by keeping up a continuous flow of water in the 

 outer vessel. This is most conveniently done by placing the dialyser or a 

 series of dialysers in a sink, the level of water in which may be adjusted 

 by means of a wide glass tube running through the waste plug. The water 

 supply is led to the bottom of the sink. 



(2) [a) Dialyse {a) Egg albumin + Sodium Chloride. (/)) Starch + Iodine + 

 HCl. {(') Starch + Gkicose. Test dialysate for both constituents. 



{b) Using a glass dialyser with a collodion membrane dialyse a mixture 

 of either congo red, litmus or alizarin and hydrochloric acid. 



(c) Dialyse some blood serum. What is the precipitate ? How do you 

 explain this ? 



[d] An acid Perfusate from an Alkaline Solution Prepare some colloidal 

 ferric hydrate either by dialysing 5 ])er cent, ferric chloride or by gradually 

 adding 1 c.c. of 30 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 K4FeCy.; solution. When 

 the iron content becomes very small (after 48 hours) add some HCl to the 

 colloid. Note the increase in the dif?usibility of the Fe. How can you 

 explain this ? Why does an acid perfusate come from an electro-negative sol ? 



27. Faraday-Tyndall Phenomenon (p. 79). 



Arrangement of apparatus. The fluid to be examined is placed in a prismatic 

 cell (small flat-sided specimen jar, say 5 X 5 X 2| in.). One large face of the 

 cell is covered with a black velvet curtain. Light from any optical projector 

 is passed through the cell in a plane parallel to the long side. A lens is 

 interposed so that the focus falls about the middle of the cell forming a cone. 

 A darkened room is essential for any but individual demonstrations. 



A. (1) Fill the cell with water and show that the beam is hardly shown. 

 (With conductivity water the beam is not seen, see preparation of special 

 water, p. 557.) 



(2) Dust lycopodium or puff smoke into the beam outside the cell to 

 show how small particles in air af?ect the visibility of the light. 



(3) Add about 1 c.c. of an egg albumin solution to the water in the cell. 

 Why is the cone bluish in tinge ? 



(4) Replace the solution with another containing a red gold sol. ^^ hy 

 is the cone green ? 



(5) Try also 1 per cent, starch, 1 per cent, mastic, a saturated solution of 

 cane sugar or milk whey. 



