214 CAPILLARY TUBE COLORIMETRY 



hydrogen sulfide. Add 20 ml. 85% phosphoric acid and slowly 

 boil for 1 hr. under a reflux condenser. Decolorize with a few drops 

 of bromine, boil off the excess bromine, and cool. To 12 g. lithium 

 carbonate and 25 ml. phosphoric acid, add slowly 150 ml. water. 

 Boil off the carbon dioxide and when solution is complete, cool 

 and mix with the cone, uric acid reagent and dilute to 1 1. Keep 

 in well-stoppered bottles protected from light. 



PROCEDURE 



1. Introduce into a uniform capillary tube (0.35 mm. inner 

 diameter) a 5 mm. column of tungstic acid supernatant (page 198) 

 or other soln. to be analyzed, 5 mm. cyanide soln., and 1 mm. uric 

 acid reagent, keeping the three columns separated by air spaces. Seal 

 off both ends of the portion of the tube containing the liquids. 



2. In a similar manner prepare tubes with the seven standard 

 solns. 



3. Mix the liquids simultaneously in all of the tubes by centri- 

 fugation. 



4. Four min. after the mixing immerse the tubes for 1 min. in 

 boiling water. 



5. Compare the colors. 



UREA 



Walker and Hudson ( 1937) adapted the capillary tube apparatus 

 to the determination of urea by the hypobromite method. This 

 adaptation enables the analysis of 0.3 /xl. liquid containing 2 to 25 

 milligram per cent urea nitrogen with an average deviation from the 

 macro method of ±3.8%. The measurement of known quantities of 

 urea added to dialyzed horse serum could be made with an average 

 error of 2.3%. For other methods see page 286. 



Walker and Hudson Method for Urea 



SPECIAL REAGENTS 



Sodium Hypobromite Solution. Prepare according to Stehle 

 ( 1921) : In a 50 ml. Erlenmeyer flask, mix 2 ml. of a soln. contain- 

 ing 12.5 g. sodium bromide and 12.5 g. bromine/100 ml., with 2 ml. 

 of a soln. containing 28 g. sodium hydroxide/100 ml. Gently 



