286 TITRI METRIC METHODS 



13. Run controls in which the complete analysis is duplicated 

 without the unknown sample. 



UREA 



Urea has been determined by the urease method in a number 

 of laboratories by making use of one or another of the micro 

 procedures for the estimation of ammonia. In the urease method, 

 the enzyme is added to decompose the urea, and the ammonia thus 

 formed is absorbed in acid and measured. 



Procedures employing diffusion cells were described by Conway 

 (1933), Gibbs and Kirk (1934), Borsook (1935), and Kinsey and 

 Robison ( 1946) . The various forms of diffusion cell used by these 

 investigators have been described (page 168). The method of 

 Kinsey and Robison (1946), which can estimate as little as 1 fig. 

 urea in 4 fA. plasma or aqueous humor with a precision of ±5%, 

 will be described. The methods which do not utilize the diffusion cell 

 for the estimation of ammonia can also be applied in the present 

 case. Thus, the method of Linderstr0m-Lang and Holter ( 1933b) 

 (page 230) was developed with the application to the measurement 

 of urea in mind as one of the potentialities. A colorimetric method 

 is given on page 214. 



Kinsey and Robison Method for Urea 



SPECIAL REAGENTS 



Glycerol-Boric Acid. Add 25 ml. pure glycerol to 100 ml. 25% 



glycerol saturated with boric acid. 

 Urease Extract. Dissolve 1 g. powdered double strength urease 



(Squibb) in 100 ml. boiled saturated sodium chloride. 

 Saturated Sodium Metaborate in Saturated Potassium Chloride 



(boiled). 

 0.002 N Hydrochloric Acid. 

 Gramercy Universal Indicator (Fisher Scientific Co.) diluted with 



15 parts distilled water. 

 Mineral Oil. 



PROCEDURE 



1 . Measure the sample into the central container of the diffusion 



