LARD AND LARD ADULTERATIONS. 463 



solution in contact with the oils under examination. The effect there- 

 fore of this change will hot be felt upon the number which expresses 

 the percentage of iodine absorbed. 



As an illustration of the progressive change in the strength of the 

 iodine solution the following examples are given. In each case the 

 strength was determined by titration with a deci-normal thiosulphate 

 of soda solution : 



A solution of iodine, .made np as indicated, showed the following 

 strength on the dates indicated : 



The iodine solution was now allowed to stand in a thick green glass 

 bottle until the 10th of November, 1888. On that date it was found that 

 lOcc of the solution of iodine required only 7.3cc of the solution of 

 thiosulphate soda to neutralize it. It is thus seen that during that time 

 two-thirds of the iodine had disappeared. 



Deci-normal solution of thiosulphate of soda (hyposulphite of soda). 

 Ecduce to a fine powder about 30 grams of the purest recrystallized 

 thiosulphate of soda ; spread this salt in a thin layer over a clean white 

 blotting pad; cover with another pad and subject to pressure. After 

 two or three minutes remove the pad, pour the powdered salt into a 

 dish, and repeat the drying operation. It is better to put the salt all 

 into a dish and respread therefrom on the blotting-pad than to stir the 

 salt on the pad with a spatula. By this latter method some fibers of 

 paper may be mixed with the salt. Weigh exactly 24.8 grams of the 

 dried salt and make up to one liter, at the temperature at which the 

 titratious are made, with recently-boiled distilled water. Since the pure 

 salt is used the solution will be exactly deci-normal. Its strength, 

 however, may be set by weighed portions of resubli mated iodine, of 

 which about 1 gram, weighed from a weighing bottle, should be taken 

 for each determination. The solution of hyposulphite of sodium may 

 also be set in the following way : Dissolve 3.874 grams chemically pure 

 bichromate of potassium in distilled water and make the volume up to 

 one liter. Place 20cc of this solution in a glass-stoppered flask to 

 which has been added lOcc of a 10 per cent, iodide of polassium solu- 

 tion and 5cc of strong hydrochloric acid. Allow the thiosulphate of so- 

 dium solution to flow into the flask from a burette until the yellow 

 color of the liquid has almost disappeared. Add a few drops of starch 

 paste, and with constant shaking continue to add the thiosulphate so- 

 lution until the blue color just disappears. The number of centimeters 



