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J Dairy Res. 2008 Nov;75(4):466-70. doi: 10.1017/S0022029908003488. Epub 2008 Jul 22.

A method for estimation of urea using ammonia electrode and its applicability to milk samples.

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Department of Dairy Chemistry, National Dairy Research Institute, Karnal-132 001, India.


A method for the estimation of urea in milk using ammonia electrode is described. Urea is first degraded by urease enzyme into ammonium ion and carbon dioxide at neutral pH. The ammonium ion is then converted into ammonia at alkaline pH. A linear inverse relationship was observed between logarithmic concentration of ammonia or urea and electrode response. Repeatability, expressed as a coefficient of variation, was 1.77% at a level of 8.92 mm-urea in milk. The method was validated in milk samples spiked with between 2 x 10-3 and 10 x 10-3 m-urea and recovery of added urea was quantitative. Whereas, preservative sodium azide at 0.5 g/l or 2 g/l level did not affect results, lower values of urea concentration in presence of Bronopol at 0.5 g/l were observed. Urea levels in milk samples estimated by this method were comparable to standard enzymatic method. The method is simple, fast and is not prone to interference from other milk constituents.

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