XIII] FRUITS AND FRUIT PRODUCTS 183 



(e) 6% potassium permanganate solution. 



(f) Ferrous sulphate solution. — Dissolve 20 grams of ferrous sulphate in 100 cc. 

 of water containing 1 cc. of concentrated sulphuric acid. 



(g) Bromin water. — Freshly prepared, saturated solution. 



31 DETERMINATION. 



Proceed as directed in 26 up to "Filter with suction and thoroughly wash the 

 precipitate in the flask and on the paper with 95% alcohol by volume". Transfer the 

 precipitate from the filter to the flask with a jet of hot water, boil until alcohol 

 can no longer be detected by odor, and add enough of the sulphuric acid (1 

 to 5) to precipitate all the barium originally added and to allow 2 cc. in excess. 

 Evaporate by careful boiling to a volume of 60-70 cc, cool and add 5 cc. of freshly 

 prepared saturated bromin water, or enough to show a distinct excess. Transfer 

 with water to a 100 cc. volumetric flask and dilute to the mark at standard 

 temperature. Mix thoroughly, allow the precipitate to settle and filter through 

 a dry paper. The precipitate may be separated by centrifugalizing and the super- 

 natant liquid decanted, if necessary. Pipette an aliquot of the filtrate, containing 

 not more than 250 mg. of citric acid, calculated from the total acidity of the sample, 

 into a 300 cc. Erlenmeyer flask. If possible, the amount of citric acid in the ali- 

 quot should exceed 50 mg. Add 10 cc. of the sulphuric acid (1 to 1) and 5 cc. of the 

 potassium or sodium bromid solution, mix, warm the flask in a water bath to4S°-50°C. 

 and allow it to remain in the bath for 5 minutes. After removing from the bath add 

 rapidly from a pipette, drop by drop with frequent interruptions, 25 cc. of the 5% 

 potassium permanganate solution and shake vigorously; avoiding a temperature 

 during oxidation exceeding 55°C. Set the flask aside until the hydrated peroxid of 

 manganese begins to settle. The supernatant liquid should be dark brown, showing 

 an excess of permanganate; if an excess is not indicated, add more permanganate. 

 Shake, again set aside to settle and repeat this operation until the precipitate 

 assumes a yellow color and most of it has dissolved. Finally, while the solution 

 is still warm, remove the last undissolved portion of hydrated peroxid of manganese 

 precipitate and also the excess of bromin by adding, drop by drop, the clear ferrous 

 sulphate solution. Allow the solution to cool, shaking occasionally. If the opera- 

 tions have been properly conducted, a heavy white precipitate of pentabromacetone 

 is obtained which becomes crystalline on occasional shaking and in this condition 

 is entirely insoluble in water. Allow the mixture to stand overnight, collect it by 

 means of gentle suction on a tared Gooch crucible provided with a thin pad of asbestos, 

 previously dried over sulphuric acid in a vacuum desiccator, wash with water slightly 

 acidified with sulphuric acid and finally wash twice with water. Dry the precipitate 

 to constant weight over sulphuric acid in a vacuum desiccator, protecting the precipi- 

 tate from strong light. The weight of pentabromacetone multiplied by the factor 0.424 

 gives the equivalent weight of anhydrous citric acid (HsCeHjO?). Occasionally the 

 pentabromacetone is first obtained in the form of oily droplets. These become 

 crystalline on standing or on cooling and are usually discolored by negligible traces 

 of manganese or iron. 



The above method may bo applied directly to the sample without previous pre- 

 cipitation of the citric acid as the barium salt when the amount of sugar or other 

 permanganate reducing substances is not excessive. In this case begin the deter- 

 mination with the addition of 2 cc. of sulphuric acid (1 to 5) and the treatment 

 with bromin water. 



32 METALS.-TENTATIVE. 



Proceed as directed under XII. 



