XIX] VINEGARS 255 



leave an excess of about 12.5 cc. at the end of the oxidation, the amount given above 

 (30 cc.) being sufficient for ordinary vinegar containing about 0.35 gram or less of 

 glycerol per 100 cc. 



Standardize the ferrous ammonium sulphate solution against the dilute potas- 

 sium dichromate solution by introducing from the respective burettes approxi- 

 mately 20 cc. of each of the 2 solutions into a beaker containing 100 cc. of water. 

 Complete the titration using the potassium ferricyanid solution as an outside indi- 

 cator. From this titration calculate the volume {F) of the ferrous ammonium sul- 

 phate solution equivalent to 20 cc. of the dilute and also, therefore, to 1 cc. of the 

 strong dichromate solution. 



In place of the dilute dichromate solution, substitute a burette containing the 

 oxidized glycerol with an excess of the strong dichromate solution and ascertain how 

 many cc. are equivalent to (F) cc. of the ferrous ammonium sulphate solution and 

 also, therefore, to 1 cc. of the strong dichromate solution. Then 250 divided by this 

 last equivalent equals the number of cc. of the strong dichromate solution pres- 

 ent in excess in the 250 cc. flask after oxidation of the glycerol. 



The number of cc. of the strong dichromate solution added, minus the excess 

 found after oxidation, multiplied by 0.02 gives the grams of glycerol per 100 cc. of 

 vinegar. 



7 SOLIDS.-TENTATIVE. 



Measure 10 cc. of the sample into a tared, fiat-bottomed platinum dish of 50 

 mm. bottom diameter, evaporate on a boiling water bath for 30 minutes, and dry 

 for exactly 2k hours in a water oven at the temperature of boiling water. Cool in 

 a desiccator and weigh. It is essential that the size and shape of the dish and the 

 time of drying be followed strictly. 



8 TOTAL REDUCING SUBSTANCES BEFORE INVERSION.— TENTATIVE. 



Proceed as directed under VIII, 25, using 10 cc. of the sample. In the case of 

 malt vinegar, express the results as dextrose; in all other cases as invert sugar. 



9 REDUCING SUGARS BEFORE INVERSION AFTER EVAPORATION.— TENTATIVE. 



Evaporate 50 cc. of the sample on the water bath to a volume of 5 cc. Add 25 

 cc. of water and again evaporate to 5 cc. Transfer to a 100 cc. volumetric flask, 

 make up to the mark, and proceed as directed under 8, using a quantity equivalent 

 to 10 or 20 cc. of the sample. 



1 REDUCING SUGARS AFTER INVERSION.-TENTATIVE. 



Proceed as directed under 9. After the last evaporation to 5 cc. transfer to a 

 100 cc. volumetric flask with 70 cc. of water, and invert as directed under VIII, 

 14. Nearly neutralize with sodium hydroxid solution, make up to the mark and 

 proceed as directed under VIII, 25, using a quantity equivalent to 10 or 20 cc. of 

 the sample. 



11 LEAD PRECIPITATE.-TENTATIVE. 



To 10 CC. of the sample in a test tube, add 2 cc. of 20% lead acetate solution, 

 shake and let stand 30 minutes. Describe the precipitate as turbid, light, normal, 

 heavy or very heavy. 



12 . POLARIZATION.-TENTATIVE. 



If the lead precipitate is normal, add to 50 cc. of the sample 5 cc. of basic lead 

 acetate solution [VIII, 13 (a)], shake, let stand 30 minutes, filter and polarize, 



