1418 FOODS AND FOOD ADULTERANTS. 



Solution of potassium dicliromate. Dissolve 3.87i grams of chem- 

 ically pure potassium dichromate in distilled water and make the vol- 

 ume up to 1 liter at the temperature at which the titrations are to be 

 made and standardized against pure iron wire. 



DETERMINATION. 

 STANDARDIZING THE SODIUM THIOSULPHATE SOLUTION. 



Place 20 cc of the potassium dichromate solution, to which has been 

 added 10 cc of the solution of potassium iodid, in a glass-stoppered 

 flask. Add to this 5 cc of strong hydrochloric acid. Allow the solu- 

 tion of sodium thiosulphate to flow slowly into the flask until the yellow 

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

 starch paste, and with constant shaking continue to add the sodium 

 thiosulphate solution until the blue olor just disappears. The num- 

 ber of cubic centimeters of thiosulphate sulution used multiplied by 5 

 is equivalent to 1 gram of iodin. 



Example : Twenty cubic centimeters K 2 Cr 2 O 7 solution required 16. 2 cc 

 sodium thiosulphate; then 16.2 X 5 = 81 = number cubic centimeters 

 of thiosulphate solution equivalent to 1 gram of iodin. Then 1 cc 

 thiosulphate solution 0.012-1 gram of iodin. Theory for decinormal 

 solution of sodium thiosulphate, 1 cc =0.0127 gram of iodin. 



WEIGHING TIIK SA.MPLK. 



Weigh about 1 gram of fat or 0.500 gram of oil a on a small watch 

 crystal or any light weighing glass. The fat is first melted, mixed 

 thoroughly, poured onto the crystal, and allowed to cool. 



Introduce the watch ciystal into a wide-mouth 16-ounce bottle with 

 ground-glass stopper. 



ABSORPTION OF IODIN. 



The fat or oil in the bottle is dissolved in 10 cc of chloroform. 

 After complete solution has taken place, 30 cc of the iodin solution 

 are added in the case of fats, or from 40 to 50 cc b in the case of oils. 

 Place the bottle in a dark place and allow to stand, with occasional 

 shaking for three hours. There must be a large excess of iodin or 

 the results will not be satisfactory. 



a With drying oils which have a very high absorbent power 0.100 to 0.200 gram 

 should be taken. 



b F. Ulzer, Jour. Soc. Chem. Ind., 1898, 17, 276, says iodin should be in excess 

 about twice the amount that is absorbed. The solution loses strength with age, but 

 can be used so long as 35 cc of decinormal thiosulphate neutralize 25 cc iodin 

 solution. R. Henriques, Ztsch. Anal. Chem., 1901, 40, 429, says iodin should be in 

 excess at least 60 per cent of amount added. 



c The time allowed does not give the complete iodin absorption power of an oil or 

 fat and can not be compared with determinations where six to twelve hours have 

 been used. It gives very satisfactory comparative results, but the time factor must 

 be very closely adhered to. 



