Hydrolysis of urea

Two general methods are used for homogeneous precipitation. If the precipitate s solubility is pH-dependent, then the analyte and precipitant can be mixed under conditions in which precipitation does not occur. The pH is then raised or lowered as needed by chemically generating OH or H3O+. For example, the hydrolysis of urea can be used as a source of OH . 

The hydrolysis of urea is strongly temperature-dependent, with the rate being negligible at room temperature. The rate of hydrolysis, and thus the rate of precipitate formation, can be controlled by adjusting the solution s temperature. Precipitates of BaCr04, for example, have been produced in this manner. 

One example of an enzyme electrode is the urea electrode, which is based on the catalytic hydrolysis of urea by urease... 

The enzyme urease catalyzes the hydrolysis of urea. The rate of this reaction was determined for a series of solutions in which the concentration of urea was changed while maintaining a fixed urease concentration of 5.0 pM. The following data were obtained. 

Until the discovery in 1975 of nickel in jack bean urease (which, 50 years previously, had been the first enzyme to be isolated in crystalline form and was thought to be metal-free) no biological role for nickel was known. Ureases occur in a wide variety of bacteria and plants, catalyzing the hydrolysis of urea,... 

Hydroxides and basic salts. The necessity for careful control of the pH has long been recognised. This is accomplished by making use of the hydrolysis of urea, which decomposes into ammonia and carbon dioxide as follows ... 

The experiments were carried out with two enzymes, urease and glutathione-5-trans-ferase (GST). Urease catalyses the hydrolysis of urea (Avrameas 1978), while GST is an... 

Hydrolase enzymes catalyze the hydrolysis of a substrate and are most commonly coupled with potentiometericela trodes The pioneering work in this field focussed on de loping an enzyme electrode for the determination of urea. Urease catalyzes the hydrolysis of urea to ammonium and bicarbonate ions according to the reaction detailed below. 

Adjustment of pH. Some organic reagents hydrolyze in aqueous solution at elevated temperature and release ammonia. Examples are urotropin and urea. The hydrolysis of urea is shown in Figure 8-2. Urotropin yields hydrolysis formaldehyde and ammonia. In this way, the pH is increased. The chemical reactions necessary for the formation of a gel with other components of the mixture can then take place. 

The hydrolysis of urea releases ammonia to the liquid bulk, which can cause inhibition of the methanogenic sludge.21 Nevertheless, these authors found a toxic effect at 1300mg/L N-NH3, this value being higher than the nitrogen concentration of these wastewaters and no inhibitory effects could be expected. 

A wide variety of aerobic and anaerobic microorganisms are able to express the enzyme urease (urea amidohydrolase), which catalyses the hydrolysis of urea to ammonia and carbon dioxide.27 So far,... 

Different effects of formaldehyde on the hydrolysis of urea are reported. On the one hand, Garrido and colleagues,3 applying anoxic conditions, observed that an inhibitory effect started at 50 mg/L formaldehyde and the levels of inhibition were 50% and 90% for concentrations of formaldehyde of 100 mg/L and 300 mg/L, respectively. Similar effects were found by Campos and colleagues,33 working with an anoxic USB, who observed that formaldehyde concentrations in the reactor of 250 to 300 mg/L caused an inhibition of around 53%. This inhibition on the ureolytic activity was also reported by Walker.36 On the other hand, Eiroa and colleagues37 carried out batch assays at different initial urea concentrations from 90 to 370mg/L N-urea in the presence of 430 mg/L formaldehyde. They observed that a complete hydrolysis was achieved and initial urea hydrolysis rates remained constant. 

Chakrabarti, T. and Subrahmanyam, P.V.R., Biological hydrolysis of urea in a continuous flow stirred tank reactor under laboratory conditions—a bench scale study, Proc. 36th Industrial Waste Conference, Purdue University, pp. 477, 1981. 

Latkar, M. and Chakrabarti, T., Performance of upflow anaerobic sludge blanket reactor carrying out biological hydrolysis of urea, Water Environ. Res., 66, 12-15, 1994. 

Urease is an exceptional metallohydrolase that contains two proximite Ni11 ions within its active site.24,25 It efficiently catalyzes the hydrolysis of urea to finally give carbon dioxide and two moles... 

Both complexes (867) and (868) promote the hydrolysis of urea in a two-step process.2080 Heating of (867) or (868) in acetonitrile solution produced ammonia with kinetic first-order dependence on complex concentration and an observed rate constant of (7.7 0.5) x 10-4 h-1 to yield a cyanate complex as the reaction product. When the reaction was carried out in 50% aqueous acetonitrile solution, ammonia was produced at the same rate but without buildup of the cyanate-containing product, suggesting that the latter is hydrolyzed in the presence of water. The hydrolysis rate was also first order in water, indicating that it occurred by attack of an external water on the coordinated cyanate.2080... 

Capsules with high enzyme loading and activity prepared by templating BMS spheres can also be used as biomimetic reactors [89]. For example, PGA/ PLL capsules with pre-loaded urease (prepared via a BMS sacrificial template) are capable of catalyzing the hydrolysis of urea and have been shown to induce the exclusive formation of CaC03 particles inside the capsules [89]. 

Fig. 13.3 Scanning electron microscopic images of LDH particles with various size (A) 100, (B) 200, (C) 1500, and (D) 4500 nm. LDH particles (A) and (B) were synthesized under hydrothermal conditions and (C) and (D) were prepared using hydrolysis of urea (see Table 13.1).

It is worth noting that the partide sizes of samples prepared by the urea hydrolysis method are larger than other LDHs. Such a finding can be rationalized by considering the slow hydrolysis of urea [61], since it can be classified as a very weak Bronsted base (pfCb= 13.8). Its hydrolysis mechanism consists of the formation of ammonium cyanate as the rate-determining step, followed by fast hydrolysis of the cyanate to ammonium carbonate ... 

Enzymes are usually named in terms of the reaction that is catalyzed, commonly adding the suffix -ase to the name of the stoichiometrically converted reactant or substrate. For instance, an enzyme that catalyses the hydrolysis of urea is urease. Enzyme names can only be applied to single enzymes, especially those with termination -ase. For systems that involve the action of two or more enzymes the use of the term should be avoided and the word system should be included. 

The oxidation of urea was not complete. The incomplete step in the photolysis of urea is much more likely to be hydrolysis of urea to produce ammonia, since the oxidation of ammonia is known to occur under the conditions of the experiment. 

Historically the earliest Ni-containing enzyme to be described was urease from jack bean meal, which was crystallized by James Sumner in 19261. However, analytical techniques did not allow urease to be recognized as a Ni-containing enzyme until 50 years later. Urease catalyses the hydrolysis of urea to ammonia and carbamate, which spontaneously hydrolyses to give carbonic acid and a second molecule of ammonia. 

Figure 3.15 Precipitation of calcite in three soils (A, B, C) as pH increases during hydrolysis of urea, and simultaneous changes in pH, P and dissolved organic C (DOC) in the soil solution. Moist soil was incubated with CaCl2 and urea at constant CO2 pressure. Numbers with symbols are initial concentrations of urea (M). Data of Huang (1990). Reproduced by permission...

The removal of fertilizer N in the crop as NH4+ does not lead to acidification. Hydrolysis of urea fertilizer—by far the main form of N fertilizer used in wetland rice, together with ammonium bicarbonate in some countries—consumes 1 mol of H+ per mol of NH4+ formed (Table 7.1, Process 1). So although absorption of N as NH4+ leads to a net export of H+ from the roots to balance the resulting excess intake of cations over anions (Table 7.1, Process 5), this acidity is matched by the H+ consumed in urea hydrolysis. Likewise there is no net generation of acidity as a result of NH3 volatilization, although 1 mol of H+ is left behind per mol of NH4+ converted to NH3 (Table 7.1, Process 3). 

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