56 TESTING MISCELLANEOUS SUPPLIES. 



2. Specific Gravity. 

 Determine with a pyknometer at 15.5 C. 



3. Refractive Index. 



. 



Determine with a Zeiss direct reading refractometer at 15 or 17.5 C. 

 With a pure article the per cent of glycerin may be obtained from either the 

 specific gravity or the refractive index. 



4. Chemical Method of Determining- Glycerol. 



In impure glycerin, such as soap lye, a determination by physical methods is, 

 of course, not reliable. For some mixtures, containing other reducing bodies, 

 there is no really good method. When a chemical method is necessary, and 

 sugar and other organic compounds are absent, Hehner's bichromate method is 

 probably as satisfactory as any available. Place the crude glycerin (about 1.5 

 grams), or a corresponding amount of soap lye, from which all fatty acids have 

 been removed, in a graduated 100 cc flask, dilute slightly, add some oxid of 

 silver, allow to stand ten minutes, and add a slight excess of basic lead acetate. 

 Make up to the mark, filter through a dry paper, and place 25 cc of the filtrate 

 in a perfectly clean beaker. Add first an accurately measured amount (40 to 

 50 cc) of a solution of bichromate (made by dissolving 74.56 grams of potassium 

 bichromate in water and diluting to 1,000 cc, the exact strength being then 

 determined against pure iron or ferrous ammonium sulphate), then 15 cc of 

 strong sulphuric acid. Cover the beaker and heat for two hours in boiling 

 water ; then cool. Add an excess of standard ferrous ammonium sulphate 

 solution and titrate back with a standard bichromate solution containing 7.456 

 grams per liter. (The ferrous ammonium sulphate solution should contain 

 about 240 grams per 1,000 cc.) One cubic centimeter of the strong bichromate 

 solution corresponds to 0.01 gram of glycerol. 



INKING PADS. 



The material used in these pads is printers' roller composition, being com- 

 posed of glue and glycerin generally with a small amount of borax added. 



1. Sampling 1 . 



Weigh the whole pads, remove the felt and weigh again. Cut the pad in 

 quarters and when only one pad is furnished cut up two opposite quarters into 

 vi TV small .-Hid uniform pieces. When several pads are in one sample, take 

 smaller segments from each pad. It is very important that the samples be all 

 cut to a uniform size cubes about 2 mm on each edge. 



2. Moisture. 



Cover the bottom of a small, flat-bottomed aluminum dish (2.5 inches in 

 diameter, and five-eighths of an inch high) with freshly ignited sand, weigh, 

 distribute over the sand from 1.5 to 2 grams of the fine pieces of pad, weiirh 

 :i < 'irately, and heat for two hours in a water oven, cool in a desiccator, and 

 weigh. Duplicates should agree within 0.3 per cent. 



Lewkowitsch, Oils, Fats, and Waxes, 1909, 8:327. 



