Toxicity, urea

For reasons of safety and toxicity, urea is the preferred selective reducing agent for mobile SCR applications. Under the hydrothermal conditions in the exhaust system, urea decomposes to ammonia which reduces the nitrogen oxides on the surface of the SCR catalyst [18,19], If urea is used instead of ammonia, the DeNO chemistry involves isocyanic acid as an important intermediate which will lead to a complication of the SCR chemistry [20],... 

A a. The urea cycle converts toxic ammonia into less toxic urea for excretion from the body. 

UF resins in their cured state are non-toxic. Urea itself is also harmless. High temperatures and high relative humidity can result in formaldehyde odour problems in a room containing particleboard manufactured with UF resins. However, free formaldehyde and formaldehyde generated by slow hydrolysis of the aminoplastic bond may cause some inflanunation of the mucous membranes of the eyes, nose and mouth. Even a low concentration of formaldehyde vapour can be disagreeable. However, such irritations usually. disappear in a short time without permanent damage. The problem of formaldehyde... 

Due to its high potential for NOx reduction under lean conditions, modification of the SCR process (using non-toxic urea instead of ammonia) is presently under investigation for mobile applications.In this case there are limitations to the catalyst volume that can be placed on-board. The operating conditions are also continuously changing in temperature and flow rate. Finally, the temperature window has to be enlarged with respect to stationary applications, in particular towards the low temperatme region. 

A common basic amino acid is the lower homologue of lysine, L-ornithine (L-2,5-diaminovaleric acid, 2-41), which is an intermediate in the biosynthesis of arginine and an important amino acid of the ornithine (urea) cycle, which has the function of converting toxic ammonia into less toxic urea in mammals. Ornithine formed in dough through the action of yeast Sac-charomyces cerevisiae) is the main precursor of the typical aroma of bread crust, for which 2-acetyl-l-pyrroline together with 6-acetyl-l,2,3,4-tetrahydropyridine and its isomer 6-acetyl-2,3,4,5-tetrahydropyridine are responsible (see Section 8.2.12.4.1). 

Ammonia is one end product of the metabolism of all the amino acids and of most other nitrogenous substances. It is a very toxic substance, relatively low concentrations in the blood causing serious neurological signs. Ammonia is converted to non-toxic urea by the Krebs-Henseleit urea cycle (Figure 6.3),... 

The consumption of urea for urea—formaldehyde resins has decreased in recent years because of the new findings about the toxicity of formaldehyde slowly released by the resin. 

Tolbutamide. Tolbutamide (l-but5l-3-(p-tolylsulfonyl)urea), with mol wt 270.35, is known as Orinase. It is a white to off-white practically ododess crystalline powder having a slightly bitter taste, mp 126—132°C. It is practically insoluble in water, and soluble in alcohol and chloroform. The toxicity of tolbutamide appears to be low, and reactions are similar to those observed with other sulfonylureas. 

Fig. 43. Schematic of a hoUow-fiber artificial kidney dialyser used to remove urea and other toxic metaboUtes from blood. Several million of these devices...

Both urea— and melamine—formaldehyde resins are of low toxicity. In the uncured state, the amino resin contains some free formaldehyde that could be objectionable. However, uncured resins have a very unpleasant taste that would discourage ingestion of more than trace amounts. The molded plastic, or the cured resin on textiles or paper may be considered nontoxic. Combustion or thermal decomposition of the cured resins can evolve toxic gases, such as formaldehyde, hydrogen cyanide, and oxides of nitrogen. 

Phosgene [75-44-5] M 98.9, b 8.2 /756mm. Dried with Linde 4A molecular sieves, degassed and distilled under vacuum. This should be done in a closed system such as a vacuum line. HIGHLY TOXIC, should not be inhaled. If it is inhaled operator should lie still and made to breath ammonia vapour which reacts with phosgene to give urea. 

PHENYL UREA PESTICIDES, FLAMMABLE LIQUID, TOXIC, n.o.s.. 2768 ... 

Alkyl uracyls have been known for some time to act as diuretic agents in experimental animals. The toxicity of these agents precluded their use in the clinic. Appropriate modification of the molecule did, however, yield diuretic agents with application in man. Reaction of allylamine with ethyl isocyanate affords the urea, 89 (the same product can of course be obtained from the same reagents with reversed functionality). Condensation with ethyl cyanoacetate affords aminotetradine (90). In... 

Dimethyl carbonate (DMC) is a colorless liquid with a pleasant odor. It is soluble in most organic solvents but insoluble in water. The classical synthesis of DMC is the reaction of methanol with phosgene. Because phosgene is toxic, a non-phosgene-route may be preferred. The new route reacts methanol with urea over a tin catalyst. However, the yield is low. Using electron donor solvents such as trimethylene glycol dimethyl ether and continually distilling off the product increases the yield. ... 

Its solubility characteristics in aqueous systems are such that retention of toxicity to insects by dissolved crystal protein is always suspect, and loss of activity on dissolution owing to denaturation is often observed. The protein is soluble only in relatively strong aqueous alkali. Thus, it has been variously reported to be soluble in 0.01N- to 0.05N sodium hydroxide (1) and alkali at pH 10.5 in the presence of thioglycollate (35) we have also observed its solubility in alkali at pH 9.5 in the presence of urea and potassium boro hydride. One difference between the characteristic proteins produced by various strains of crystalliferous bacilli is observed in the degree of alka-... 

A series of poly(ester-urethane) urea triblock copolymers have been synthesized and characterized by Wagner et al/ using PCL, polyethylene glycol, and 1,4 diisocyanatobutane with either lysine ethyl ester or putrescine, as the chain extender. These materials have shown the elongation at break from 325% to 560% and tensile strengths from 8 to 20 MPa. Degradation products of this kind of materials did not show any toxicity on cells. 

Mechanism of action can be an important factor determining selectivity. In the extreme case, one group of organisms has a site of action that is not present in another group. Thus, most of the insecticides that are neurotoxic have very little phytotoxicity indeed, some of them (e.g., the OPs dimethoate, disyston, and demeton-5 -methyl) are good systemic insecticides. Most herbicides that act upon photosynthesis (e.g., triaz-ines and substituted ureas) have very low toxicity to animals (Table 2.7). The resistance of certain strains of insects to insecticides is due to their possessing a mutant form of the site of action, which is insensitive to the pesticide. Examples include certain strains of housefly with knockdown resistance (mutant form of Na+ channel that is insensitive to DDT and pyrethroids) and strains of several species of insects that are resistant to OPs because they have mutant forms of acetylcholinesterase. These... 

Treatment of a chiral amine with phosgene is the cheapest way to prepare symmetrical ureas [29]. Nevertheless, due to the toxicity and reactivity of that reagent, it can advantageously be replaced by triphosgene [30] or l,l -carbonyldiimidazole [31-34] or other derivatives such as l,l -carbonyldi-2(lH)-pyridinone [35]. This procedure can be extended to thiophosgene (Scheme 1) and its thio-analogues, such as l,l -thiocarbonyldi-2(lH)-pyridinone to produce thioureas [36] chiral diamines can thus be transformed into the corresponding monoureas or monothioureas. 

Emission control from heavy duty diesel engines in vehicles and stationary sources involves the use of ammonium to selectively reduce N O, from the exhaust gas. This NO removal system is called selective catalytic reduction by ammonium (NH3-SGR) and it is additionally used for the catalytic oxidation of GO and HGs.The ammonia primarily reacts in the SGR catalytic converter with NO2 to form nitrogen and water. Excess ammonia is converted to nitrogen and water on reaction with residual oxygen. As ammonia is a toxic substance, the actual reducing agent used in motor vehicle applications is urea. Urea is manufactured commercially and is both ground water compatible and chemically stable under ambient conditions [46]. 

While ammonia, derived mainly from the a-amino nitrogen of amino acids, is highly toxic, tissues convert ammonia to the amide nitrogen of nontoxic glutamine. Subsequent deamination of glutamine in the liver releases ammonia, which is then converted to nontoxic urea. If liver function is compromised, as in cirrhosis or hepatitis, elevated blood ammonia levels generate clinical signs and symptoms. Rare metabolic disorders involve each of the five urea cycle enzymes. 

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