254 METHODS OF ANALYSIS [Chap. 



(f) Silver carbonate. — Dissolve 0.1 gram of silver sulphate in about 50 cc. of water, 

 add an excess of sodium carbonate solution, allow the precipitate to settle and 

 wash with water several times by decantation until the washings are practically- 

 neutral. This reagent must be freshly prepared immediately before use. 



6 DETERMINATION. 



All evaporations should be made on a ivaler bath, the temperature of which is main- 

 tained at 85°-90°C. 



Evaporate 100 cc. of the vinegar to 5 cc, add 20 cc. of water and agarn evaporate 

 to 5 cc. to expel acetic acid. Treat the residue with about 5 grams of fine sand and 

 15 cc. of the milk of lime and evaporate almost to dryness, with frequent stirring, 

 avoiding the formation of a dry crust or evaporation to complete dryness. Treat 

 the moist residue with 5 cc. of water, rub into a homogeneous paste, then add 

 slowly 45 cc. of absolute alcohol, washing down the sides of the dish to remove 

 adhering paste, and stir thoroughly. Heat the mixture on a water bath, with 

 constant stirring, to incipient boiling, transfer to a suitable vessel and centrifu- 

 galize. Decant the clear liquid into a porcelain dish and wash the residue with 

 several small portions of hot 90% alcohol by volume by aid of the centrifuge. 

 (If a centrifuge is not available, decant the liquid through a fluted filter into a 

 porcelain dish. Wash the residue repeatedly with small portions of hot 90% alco- 

 hol, twice by decantation, and then by transferring all the material to the filter. 

 Continue the washing until the filtrate amounts to 150 cc.) Evaporate to a sirupy 

 consistency, add 10 cc. of absolute alcohol to dissolve this residue and transfer to 

 a 50 cc. glass-stoppered cylinder, washing the dish with successive small portions of 

 absolute alcohol until the volume of the solution amounts to 20 cc. Then add 

 3 portions of 10 cc. each of anhydrous ether, shaking thoroughly after each addi- 

 tion. Let stand until clear, then pour off through a filter, and wash the cylinder 

 and filter with a mixture of 2 volumes of absolute alcohol and 3 of anhydrous ether. 

 If a heavy precipitate has formed in the cylinder, centrifugalize at low speed, de- 

 cant the clear liquid and wash 3 times with 20 cc. portions of the alcohol-ether 

 mixture, shaking the mixture thoroughly each time and separating the precipitate 

 by means of the centrifuge. Wash the paper with the alcohol-ether mixture, and 

 evaporate the filtrate and washings on the water bath to about 5 cc, add 20 cc. 

 of water and again evaporate to 5 cc. ; again add 20 cc of water and evaporate to 5 

 CO.; finally add 10 cc. of water and evaporate to 5 cc. 



These evaporations are necessary to remove all the ether and alcohol, and, when 

 conducted at 85°-90°C., result in no loss of glycerol if the concentration of the latter 

 is less than 50%. 



Transfer the residue with hot water to a 50 cc. graduated flask, cool, add the 

 silver carbonate, prepared from 0.1 gram of silver sulphate, shake and allow to stand 

 10 minutes; then add 0.5 cc. of basic lead acetate solution [VIII, 13 (a)], shake 

 occasionally and allow to stand 10 minutes; make up to the mark, shake well, filter, 

 rejecting the first portion of the filtrate, and pipette 25 cc. of the clear filtrate into 

 a 250 cc. volumetric flask. 



Add 1 cc. of concentrated sulphuric acid to precipitate the excess of lead and 

 then 30 cc. of the strong potassium dichromate solution. Add carefully 24 cc. of 

 concentrated sulphuric acid, rotating the flask gently to mix the contents and avoid 

 violent ebullition, and then place in a boiling water bath for exactly 20 minutes. 

 Remove the flask from the bath, dilute, cool and make up to the mark at room tem- 

 perature. The amount of strong dichromate solution used must be sufficient to 



