Accuracy in the determination of blood-urea by the urease aeration-titration method

Abstract

Improvements in technique and apparatus to overcome sources of error in the urease aeration-titration method of determining blood-urea are described. Reaction- and acid-tubes of resistance glass 250 mm. X 30 mm., connected by zigzig resistance glass tubes, which effectually arrest alkaline spray, are arranged serially in a rigid, saddle-shaped metal stand, and handled en bloc during incubation, aeration, etc. Isobutyl alcohol and saturated K2CO3 soln. (instead of solid) are added to the reaction-tubes through tap-funnels. Anti-foam liquid is not used in the acid-tubes, giving complete absorption of NH3, and enabling accurate titrations to be done in the tubes. The N/100 acid is measured in an automatic pipette. The effects of CO2 on the titration end-point, the technique of accurate titra-tion and of reading a "blue line" burette accurate to 0.01 ml. are described. Though aeration removes only about 96% of the NH3 present, the modified method gives a high degree of relative accuracy. The av. difference between the members of 90 pairs of detns. of urea contents of 14-600 mgm. per 100 ml. blood corresponded to 0.02 ml. of iV/75 NaOH or 0.4 mgm. of urea per 100 ml. of blood, and was less than the possible error due to measurements.