GENERAL PROCEDURES FOR RADIOASSAY 105 



flask F, containing a measured amount of standard C()2-free NaOIT 

 (usually 2 mmoles), is also connected to C , the system evacuated through 

 D to about 20 mm, and stopcock D closed. Since maintaining adecjuate 

 reduced pressure is essential, the apparatus is allowed to stand for a few 

 minutes, and the manometer observed for pressure changes. 



Flask A is rotated 180°, bringing the side arm B upright and spilling 

 the oxidant onto the sample. The bulb of flask .1 is now immersed in the 

 160°C bath, preferably by manipulating the entire ring stand to which the 

 assembly is attached. An ice-water bath is then placed so as to cool 

 absorption flask F. After 10 min flask .4 is removed from the hot bath, 

 the apparatus is allowed to stand an additional 5 min, and F remains in 

 the ice bath. If no leakage has occurred and COo absorption is aderjuate, 

 the manometer reading returns to 20 to 30 mm. C02-free air or purified 

 nitrogen is admitted through stopcock D to equalize the pressure. 



Absorption flask F is removed, and its contents titrated with 0.2 A^ 

 HCl to a faint but definite phenolphthalein pink. From the titration 

 figures obtained, the amount of CO2 liberated by combustion is deter- 

 mined, one er[uivalent of base being used per mole of CO2. Care is taken 

 to avoid excess acid, which would cause CO2 loss. After the titration is 

 complete, 1.5 to 2.0 mmoles of excess base is added to ensure CO2 reten- 

 tion and to furnish base for the subsequent use of BaCl2-NH4Cl precipitat- 

 ing reagent. 



The entire alkaline carbonate solution may now be used for BaCOs 

 plating, but usually aliquots of the homogeneous solution are taken, since 

 absorption of CO2 in NaOH instead of Ba(0H)2 permits the use of ali- 

 ((uots for plate preparation. 



Typical procedures for precipitation of the BaCOs from the samples of 

 Na2C03 contained in the XaOH solutions are as follows (27) : The entire 

 sample or aliquots are transferred to a heavy-walled 40-ml centrifuge 

 tube. If the total carbonate present, as determined by titration, is not 

 adequate to give a satisfactory BaCOs plate (approximately 16 mg BaCOs 

 is required), aliquots of a standard Na2C03 (0.5 to 0.75 N) may be added. 

 Phenolphthalein indicator is added, and the tube placed in a 45 to 55°C 

 water bath. About 5 ml of 0.5 A^ BaCU in 0.4 A^ NH4CI is added force- 

 fully to the sample by a syringe fitted with a fine needle. Additional 

 NaOH must be added quickly if the reagent decolorizes the solution. 

 The tubes should be kept covered as much as possible to minimize CO2 

 contamination. The precipitated sample is allowed to remain at 45°C 

 for at least 10 min. The sides of the tube are washed down with a few 

 milliliters of 3 : 1 alcohol-ether, and the solution centrifuged at about 

 2500 rpm. The supernatant liquid is removed by aspiration, the precip- 

 itate suspended in 15 ml of distilled water, and the suspension centrifuged 

 again. The wash water is removed by aspiration, and the BaCOj is 



