236 



CUVETTE COLORIMETRY 



in the bottom of the flask, and before being used for the first time 

 the solution is boiled in the flask to drive off water and gases. A 

 small glass ball (Fig. 79) is used to close the flask when not in use. 

 The flasks are heated conveniently in a copper block supplied with 

 an electric coil. Holes in the block permit the flasks to be inserted to 

 a depth of about 14 mm.; the temperature is kept constant at 295°. 

 At this temperature the acid is clear; it fumes but does not boil. 



; u 



/l2 mm.\ 



) C 



) 



> (. ;/A 



) 



14-5 mm. 



Fig. 78. Arrangement for 

 digestion. 



Fig. 79. Glass Fig. 80. Removal 



stopper for diges- of tubes from diges- 

 tion flask. tion flask. 



From Brilel et al. (1946) 



To insert the digestion tubes the flask is removed from the block, 

 allowed to cool for 30 sec, and then the tubes are inserted by means 

 of forceps and arranged in a row around the circumference of the 

 flask (Fig. 78). After replacement of the flasks in the block the 

 digestion is watched for a few minutes to see whether liquid seals 

 form across the lumen of the tubes and rise upward. If they rise 

 higher than the middle of a tube the flask is removed from the block 

 for a minute or two and the seals collapse. After the initial stage the 

 digestion is allowed to proceed unattended for 5-6 hr. The tubes are 

 removed from the slightly cooled flasks by means of a conical glass 

 rod as indicated in Figure 80. They are rinsed on the outside with 

 distilled water, dried with a clean towel, and stored in a desiccator 

 over phosphorus pentoxide until ready for the determination of 

 ammonia. 



