PREPARATION OF COLLOIDAL SOLS 561 



The sol contains 2-2 millimoles per cent, of hydrochloric acid from the 0-3 

 gm. of FeCl.,. Most of this acid can be removed by dialysis. Caution. Do 

 not push dialysis too far or the sol will undergo coagulation. The un- 

 dialysed sol may be used for the experiments on cataphoresis, diffusion, 

 precipitation and protection. The dialysed sol or B.P. dialysed iron are 

 necessary for the experiment on mutual precipitation. 



82. Sulphur Sol. 



(i.) Ode'n's. Prepare 100 c.c. of 2N (approx.) sulphurous acid by titration 

 against phenolphthalein. Pass clean H2S for an hour or so through it and 

 continue till the smell of SO2 has practically disappeared. Allow the fluid 

 to stand for 24 hours and then decant off the milky yellow supernatant sol. 

 Before use dilute the sol 100 times with distilled water. 



(ii.) A sulphur sol may be prepared by neutralising a very dilute solution 

 of ammonium sulphide with hydrochloric acid. 



83. Purple of Cassius. 



Add a few cubic centimetres of a 0-01 per cent, solution of stannous 

 chloride to a 0-05 per cent, solution of gold chloride. The stannous chloride 

 reduces the gold chloride (cf. tannic acid and formalin), producing colloidal 

 gold, and itself being converted in the process to the stannic form. The 

 colloidal stannic amdioTxned from this is a hydrophilic colloid and " protects " 

 the gold. The purple-red colour is, therefore, due to an adsorption complex 

 of gold and tin. 



84. Gelatin i per cent. Sol. 



(a) Free the gelatin from the bulk of extraneous matter by immersing it in 

 a large bulk (about 100 vols.) of cold distilled water. Change the water 

 several times. 



(b) Boil about 75 c.c. of distilled water for every 100 c.c. of sol required. 

 Remove the flame and add the cleaned and sivollen gelatin as free as possible 

 from wash water. Stir continuously till the gelatin has completely dissolved. 

 Cool. 



(c) When quite cold stir in the amount of distilled water necessary to give 

 100 c.c. or whatever multiple of 100 c.c. is desired. Accuracy may be obtained 

 by first weighing the beaker, then beaker plus cooled contents, and so arriving 

 at the weight of additional water necessary. 



85. Starch i per cent. 



{a) Put the weighed potato starch in a mortar, and, adding small quantities 

 of cold water at a time, grind into a smooth thick cream. 



(6) Gradually stir in about half of the total volume of water required to 

 make a 1 per cent. sol. This water should be fairly warm, but not hot. 



(c) Dilute to about twice its volume with warm water. 



(d) Boil on a sand bath for half an hour to convert dispersoid to a true 

 colloid. 



(e) Cool, and when cold make up to the desired volume with cold water. 

 (/) Filter through a well washed grey paper. 



{g) Cover the surface with a thin film of toluene to prevent rapid bacterial 

 infection. 



86. Gum Mastic. 



One to five grams of gum mastic are placed in a medium-sized mortar and 

 dissolved by trituration with repeated amounts of 96 per cent, alcohol. The 

 solution is made up to 100 c.c. with alcohol and filtered ; 10 c.c. of this 

 solution are placed in a 500 c.c. beaker and 200 c.c. of distilled water added 

 at once in one portion. Filter and allow to stand for about an hour. Use 

 the upper two-thirds of the suspensoid. 



