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.

Volatilização de amônia do solo após a aplicação de ureia convencional ou com inibidor de urease

Abstract: Ammonia volatilization is an important process of N loss which decreases the use efficiency of N by plants when urea is applied on the soil surface. To overcome this problem, some chemical compounds were mixed with urea to inhibit the urease action. The purpose of this study was to compare applications of an alternative fertilizer with urease inhibitor to reduce ammonia volatilization with conventional urea, under certain environmental and soil conditions. Four experiments were carried out in 2007 and 2008, under laboratory conditions, with samples of a Humic Haplumbrept. The treatments varied according to each experiment in terms of soil conditions, such as pH (4.0, 5.5, 6.3 and 6.8), soil water content (5, 10 or 20 % moisture), temperature (18 or 35 oC), aside from the fertilizer physical state (solid or liquid) and application method (over the surface or soil-incorporated). The experimental units consisted of plastic trays into which 12 kg of soil (dry basis) were filled in a 15 cm layer. Ammonia gas traps were installed across the soil surface. Frequent measurements were performed during the first 28 days of soil-fertilizer incubation. The peak of ammonia volatilization from the soil occurred in the first week after the application of traditional urea, and two or three days later for urea with urease inhibitor. Ammonia loss was not always higher from conventional than from treated urea, nor from solid than from liquid fertilizers. Ammonia volatilization increased with increases in soil pH, temperature and N rate and was lower at the lowest (5 %) and highest (20 %) soil moisture content. For surface-applied fertilizers, the maximum daily N loss rate was 14 kg ha-1 and the total cumulative loss ranged from 2 to 50 % of the applied N, depending mainly on the physical state of the fertilizer, temperature and on soil moisture. Soil incorporation of urea fertilizers was the best option to minimize ammonia volatilization in all treatments.

An NMR Investigation of the Effect of Hydrogen Bonding on the Rates of Rotation about the C−N Bonds in Urea and Thiourea

Abstract: The interaction between urea and tetrabutylammonium acetate was investigated in dimethylformamide/dimethyl sulfoxide solutions using ^1H and ^(15)N NMR. The chemical-shift behavior of the urea protons is consistent with a urea-acetate hydrogen-bonded complex involving both carboxylate oxygens and the urea hydrogens trans to the carbonyl oxygen with K_(assoc) = 120 ± 10. Line shape analysis of the temperature-dependent ^1H NMR spectra show that ΔG^⧧ for rotation about the C−N bond of urea changes only slightly from 11.0 ± 0.1 to 11.2 ± 0.1 kcal/mol on 1:1 molar addition of tetrabutylammonium acetate to a dilute solution of urea. A parallel investigation of the interaction of thiourea with tetrabutylammonium acetate gave a binding constant, K_(assoc) = 90 ± 10. The ΔG^⧧ for rotation about the C−N bond of thiourea was found to increase from 13.5 ± 0.1 to 14.0 ± 0.1 kcal/mol on 1:1 addition of tetrabutylammonium acetate to a dilute solution of thiourea in dimethylformamide/dimethyl sulfoxide. Measurements were also made of the self-association of several ureas and of ΔG⧧ for rotation about both C(O)−N bonds of 1,1-dimethylurea.

[47] Ion-exchange chromatography in the presence of urea

Abstract: Publisher Summary The separation of mixed oligonucleotides by ion-exchange chromatography on diethylaminoethanol (DEAE)-cellulose and DEAE-Sephadex is a process, which depends not only on ionic interaction between the oligonucleotides and the exchanger, but also on secondary binding forces such as hydrophobic, and hydrogen bonds. By incorporation of 7 M urea into the eluting system, these secondary forces are largely eliminated. Thus, nuclease digests of nucleic acids can be separated primarily based on their net charge. The urea prevents aggregation of guanine-rich oligomers. The 7 M urea does not eliminate all secondary binding forces, particularly with DEAE-cellulose. The oligomers, which are fractionated according to size, can be subfractionated by rechromatography on Dowex 1 resin or on DEAE-Sephadex or DEAE-cellulose in the presence of 7 M urea at pH values where the bases are partially or fully protonated. To remove urea from the column, the column is washed well with water and by 0.02 M ammonium bicarbonate until no chloride is detected in the washings (acidic silver nitrate test). Mononucleotides are best freed of urea by an analogous procedure using Dowex 1 bicarbonate resin. Larger oligonucleotides can be freed of urea and salts by dialysis but mono, di- and certain tri- and larger oligonucleotides pass through dialysis membranes.