182 METHODS OF ANALYSIS [Chap. 



hours. Filter with suction and thoroughly wash the precipitate in the flask and 

 on the paper with 95% alcohol by volume. Transfer the portion on the filter to 

 the original flask, rinsing the paper with a jet of hot water. Digest the pre- 

 cipitate with hot water, containing 2 grams of sodium sulphate in solution, un- 

 til the reaction is complete, and boil until the barium sulphate precipitate settles 

 readily. Concentrate by evaporation, if necessary, and transfer to a 100 cc. vol- 

 umetric flask with a little hot water, cool, make up to volume with water and 

 filter. 



27 DETERMINATION. 



Transfer 25 cc. of the filtrate, obtained in 26, to a flask graduated at 25 and 

 27.5 cc, add about 2.5 grams of pulverized uranyl acetate and shake vigorously at 

 frequent intervals for 3 hours, keeping the solution well protected from light. If all 

 the uranyl acetate dissolves, add more so that a small amount remains undissolved 

 at the end of 3 hours. Dilute the solution to the 27.5 cc. mark with saturated uranyl 

 acetate solution, mix well, filter if necessary, and polarize in a 200 mm. tube, using 

 the same precautions as described in 25. Multiply the reading, calculated to the 

 basis of a 200 mm. tube, by 1.1 to correct for the dilution. 



Polarize another portion of the filtrate, obtained in 26, which has not been treated 

 with uranyl acetate. Multiply the algebraic difference in degrees Ventzke between 

 the 2 readings, calculated to the basis of a 200 mm. tube, by the factor 0.036 to obtain 

 the weight of malic acid in grams per 100 cc. in the solution as obtained in 26. 



Method III. — Tentative. 

 (Approximate determination for products containing tartaric acid.) 



28 PREPARATION OF SOLUTION^. 



Prepare the sample as directed under 26 up to the point of filtration and washing 

 of the barium malate precipitate, then dry the precipitate thoroughly and transfer 

 the portion on the filter to the original flask, rinsing the paper with a jet of hot 

 water. Digest the precipitate with hot water, transfer to a 100 cc. volumetric 

 flask with a little hot water, cool, make up to volume with water and filter to re- 

 move insoluble barium tartrate. This amount of water is sufficient to dissolve 

 barium malate up to amounts as large as approximately 0.9 gram in 100 cc. More 

 than 100 cc. of water must be used when more than 0.9 gram of barium malate 

 is present. The amount of barium tartrate dissolved by hot water is so small as to 

 affect only slightly the polarization after treatment with uranyl acetate. 



29 DETERMINATION. 



Proceed as directed in 27, using the solution prepared as directed in 28. 



CITRIC ACID^— TENTATIVE. 

 CApplicable in the presence of sugar and malic and tartaric acids.) 



30 REAGENTS. 



(a) Barium hydroxid solulion. — Approximately N/4. 



(b) 50% barium acetate solution. 



(C) Sulphuric acid (1 to 1) and (/ to 5). 



(d) Potassium or sodium broynid solution. — Dissolve 15 grams of potassium bromid 

 in 40 cc. of water or 16 grams of sodium bromid in 50 cc. of water. 



