Topic
Urea
About: Urea is a research topic. Over the lifetime, 21394 publications have been published within this topic receiving 382444 citations. The topic is also known as: carbamide & carbonic acid diamide.
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TL;DR: In this article, KOH activation of a nitrogen-containing polymer obtained by heating sucrose and urea was used to obtain a high CO2 adsorption capacity of 0.70mmol/g from dry air containing 400ppm of CO2.
82 citations
TL;DR: In this paper, the quantitative course of electrochemical oxidation of uric acid and xanthine at the pyrolytic graphite electrode has been examined between pH 1-7, the most commonly encountered pH range in biological systems.
Abstract: The quantitative course of electrochemical oxidation of uric acid and xanthine at the pyrolytic graphite electrode has been examined between pH 1–7, the most commonly encountered pH range in biological systems. Basically, uric acid is oxidized in a primary oxidation of the bond to give a readily reducible and highly reactive bis‐imine. The bis‐imine can be detected as a cathodic peak by fast sweep cyclic voltammetry. Complete hydration of the bis‐imine gives rise to uric acid‐4,5‐diol that breaks down to alloxan and urea at pH 1, allantoin and a small amount of urea at pH 7, and at intermediate pH values mixtures of alloxan, allantoin, urea, and occasionally traces of parabanic acid are formed. Xanthine undergoes a oxidation that proceeds by two stages. The first involves oxidation of the bond to give uric acid which, being more easily oxidized than xanthine, is immediately further oxidized to the uric acid bisimine. This then hydrates and fragments in essentially the same way as described for uric acid, the same products being observed. Mechanisms are developed which explain the observed electrochemistry and over‐all formation of products from this reaction. The nature and yields of products formed electrochemically and the effect of pH on the reaction parallels the biological (enzymatic) oxidation of these compounds.
82 citations
82 citations
TL;DR: The tilapia fish Oreochromis alcalicus grahami from Kenya has adapted to living in waters at pH 10.5 by excreting the end product of nitrogen metabolism as urea rather than as ammonia directly across the gills as occurs in most fish.
82 citations
TL;DR: In this paper, a mechanistic model is presented, which combines the process of ammonia volatilization with the simultaneous transformation and movement of urea and its products in soil, for predicting the concentration profiles of Urea, ammoniacal-nitrogen and soil pH, and ammonia losses.
Abstract: SUMMARY
Urea application to soil raises the pH and ammonium concentration, thus providing ideal conditions for ammonia volatilization. A mechanistic model is presented, which combines the process of ammonia volatilization with the simultaneous transformation and movement of urea and its products in soil, for predicting the concentration profiles of urea, ammoniacal-nitrogen and soil pH, and ammonia losses, following application of urea.
The model consists of continuity equations describing the diffusion and reaction of urea, ammoniacal-nitrogen and soil base; it takes into account the volatilization of ammonia and the concurrent acidification of the soil surface; and considers a variable PCo2 profile due to soil respiration and urea hydrolysis. The derivation of the continuity equations and their boundary conditions, calculations of ammonia volatilization, and appropriate methods for numerical solutions are described.
82 citations