Preservation of urine for ammonia determination with a direct method - Gips, C.H.; Reitsema, A.; Wibbens-Alberts, M. (Elsevier)
Determination of the urinary ammonia concentration can be done by applying Berthelot's indophenol reaction directly on diluted urine. When non-infected urine is stored in stoppered sterile bottles at room temperature, the ammonia content is unaltered during the first 48 h, while the ammonia content (and pH) of deep-frozen urine does not change for at least 8 weeks. The method is easy, rapid and requires no special apparatus.
http://linkinghub.elsevier.com/retrieve/pii/0009-8981(70)90021-5
Keywords: WW-YZ