40 METHODS OF ANALYSIS. 



3. To prepare pure sugar, further purify the purest commercial sugar in the 

 following manner: Prepare a hot saturated aqueous solution, precipitate the 

 sugar with absolute ethyl alcohol, spin the sugar carefully in a small centrifu- 

 gal machine, and wash in the latter with absolute alcohol. Redissolve the 

 sugar thus obtained in water, again precipitate the saturated solution with 

 alcohol, and wash as above. Dry the second crop of crystals between blotting 

 paper and preserve in glass vessels for use. Determine the moisture still con- 

 tained in the sugar and take this into account when weighing the sugar which 

 is to be used. 



(NOTE. The Bureau of Chemistry is prepared to furnish pure sucrose for | 

 those who may need it for control work. Wherever this arrangement is not 

 feasible quartz plates, the values of which have been determined by means of 

 chemically pure sugar, shall serve for the control of the saccharimeters. This 

 control of quartz plates by means of chemically pure sugar should, as a rule, 

 apply only to the Bureau of Chemistry, which is to test the correctness of sac- 

 charimeters ; for those who execute commercial analyses, the repeated control 

 of the instruments is to be accomplished, now as before, by quartz plates.) 



4. In effecting the polarization of substances containing sugar employ only 

 half-shade or triple field instruments. 



5. During the observation keep the apparatus in a fixed position and so far 

 removed from the source of light that the polarization nicol is not warmed. 



6. Sources of light may be gas, triple burner with metallic cylinder, lens, and 

 reflector; gas lamps with Auer (Welsbach) burner; electric lamp; petroleum 

 duplex lamp ; sodium light. Make several readings and take the mean thereof, 

 but no one reading may be neglected. 



7. In making a polarization use the whole normal weight for 100 cc, or a 

 multiple thereof, for any corresponding volume. 



8. As clarifying and decolorizing agents use either subacetate of lead, alumina 

 cream, or concentrated solution of alum. Boneblack and decolorizing powders 

 are to be excluded. 



9. After bringing the solution exactly to the mark at the proper temperature, 

 and after wiping out the neck of the flask with filter paper, pour all of the well- 

 shaken clarified sugar solution on a rapidly acting filter. Reject the first por- 

 tions of the filtrate and use the rest, which must be perfectly clear for polari- 

 zation. 



(b) PREPARATION OF REAGENTS. PROVISIONAL, 



(1) Lead subacetate solution. Prepare by boiling 430 grams of normal lead 

 acetate, 130 grams of litharge, and 1,000 cc of water for half an hour. Allow 

 the mixture to cool and settle and dilute the supernatant liquid to 1.25 specific 

 gravity with recently boiled water. Solid lead subacetate may be substituted 

 for the normal salt and litharge in the preparation of the solution. 



(2) Alumina cream. Prepare a cold saturated solution of alum in water and 

 divide into two unequal portions. Add a slight excess of ammonium hydroxid 

 to the larger portion and then add by degrees the remaining alum solution until 

 a faintly acid reaction is secured. 



(C) DETERMINATION OF SUCROSE IN THE ABSENCE OF RAFFINOSE. OFFICIAL.* 



Dissolve the normal weight of the substance in water, clarify with lead sub- 

 acetate, and dilute to 100 cc. Filter and polarize the filtrate at 20 C. in a 200 

 mm tube. The reading obtained is the direct reading or polarization before 



In the presence of much levulose, as In honeys and fruit products, the optical method 

 for sucrose gives too high results. 



