136 METHODS OF ANALYSTS. 



rnometer, graduated to 0.10 C., so that it can be used as a stirrer, and stir 

 the mass slowly until the mercury remains stationary for thirty seconds. 

 Then allow the thermometer to hang quietly, with the bulb in the center of 

 the mass, and observe the rise of the mercury. The highest point to which it 

 rises is recorded as the titer of the fatty acids. 

 Test the fatty acids for complete saponification as follows : 

 Place 3 cc in a test tube and add 15 cc of alcohol (95 per cent by volume). 

 Bring the mixture to a boil and add an equal volume of ammonium hydroxid 

 (0.96 sp. gr.). A clear solution should result, turbidity indicating unsaponified 

 fat. The titer must be made at about 20 C. for all fats having a titer above 

 30 C. and at 10 C. below the titer for all other fats. 



6. lodin Absorption Number. Official, 

 (a) PREPARATION OF REAGENTS. 



(1) Hubl's iodin solution. Dissolve 26 grams of pure iodin in 500 cc of 95 per 

 cent alcohol. Dissolve 30 grams of mercuric chlorid in 500 cc of 95 per cent 

 alcohol. Filter the latter solution, if necessary, and mix the two solutions. Let 

 the mixed solution stand twelve hours before using. 



(2) Hanus iodin solution. Dissolve 13.2 grams of iodin in 1,000 cc of glacial 

 acetic acid (99.5 per cent) showing no reduction with bichromate and sul- 

 phuric acid; add enough bromin to double the halogen content determined by 

 titration 3 cc of bromin is about the proper amount. The iodin may be dis- 

 solved by the aid of heat, but the solution should be cold when bromin is added. 



(5) Decinormal sockum thiosulphate solution. Dissolve 24.8 grams of chem- 

 ically pure sodium thiosulphate, freshly pulverized as finely as possible and dried 

 between filter or blotting paper, and dilute with water to 1 liter at the tem- 

 perature at which the titrations are to be made. 



(//) Starch paste. Boil 1 gram of starch in 200 cc of distilled water for ten 

 minutes and cool to room temperature. 



(5) Solution of potassium iodid. Dissolve 150 grams of potassium iodid in 

 water and make up 1 liter. 



(6) Decinormal potassium bichromate. Dissolve 4.9083 grams of chemically 

 pure potassium bichromate in distilled water and make the volume up to 1 

 liter at the temperature at which the titrations are to be made. The bichromate 

 solution should be checked against pure iron. 



(b) DETERMINATION. 



(1) Standardizing the sodium fltiosulphate solution. Place 20 cc of the 

 potassium bichromate 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 hydro- 

 chloric acid. Allow the solution 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 const :mt shaking continue to add the sodium 

 thiosulphate solution until the blue color just disappears. 



(2) Weighing the sample. Weigh about one-half gram of fat or 0.250 gram 

 of oil o on a small watch crystal or in some other suitable way. Melt the fat, 



Use from 0.100 to 0.200 gram in the case of drying oils which have a very high 

 absorbent power. 



