Urea concentrations

For each mol of urea produced in a total-recycle urea process, one mol of water is formed. It is usually discharged from the urea concentration and evaporation section of the plant. For example, a 1200 t/d plant discharges a minimum of 360 t/d of wastewater. With a barometric condenser in the vacuum section of the evaporation unit, the amount of wastewater is even higher. Small amounts of urea are usually found in wastewaters because of entrainment carry-over. 

A 3.5 h treatment of a 70 kg patient (V = 40.6 liters) with a urea clearance of 200 ml,/min should result in a 64% reduction in urea concentration or a value of 0.36 for the ratio d (f this parameter almost always falls between 0.30 and 0.45. The increase in urea concentration between hemodialysis treatments is obtained from equation 13, again assuming a constant V, where (f is the urea concentration in the patient s blood at the end of the hemodialysis, and d the concentration at time t during the intradialytic interval. 

A plot of the reciprocal reaction rate versus the reciprocal urea concentration should give a straight line with an intercept and... 

MaePherson, L. M. D., and Dawes, C. (1991). Urea concentration in minor mucus gland secretions and the effect of salivary film velocity on urea metabolism by streptococcus vestibularis in an artificial plaque./Periodont. Res. 26, 395- 01. 

The sensor is an ammonium ion-selective electrode surrounded by a gel impregnated with the enzyme mease (Figme 6-11) (22). The generated ammonium ions are detected after 30-60 s to reach a steady-state potential. Alternately, the changes in the proton concentration can be probed with glass pH or other pH-sensitive electrodes. As expected for potentiometric probes, the potential is a linear function of the logarithm of the urea concentration in the sample solution. 

The activity of enzymes in the film was estimated in the following way In order to test the activity of urease, we utilized a calorimetric assay based on urea hydrolysis the enzymatic reaction was followed at 590 nm, the suitable wavelength for bromcresol purple (Chandler 1982). Urea concentration was 1.67 ts 10 M. 

By dynamic light scattering it was found that, in surfactant stabilized dispersions of nonaqueous polar solvents (glycerol, ethylene glycol, formamide) in iso-octane, the interactions between reversed micelles are more attractive than the ones observed in w/o microemulsions, Evidence of intermicellar clusters was obtained in all of these systems [262], Attractive intermicellar interactions become larger by increasing the urea concentration in water/AOT/ -hexane microemulsions at/ = 10 [263],... 

The immunoglobulin fraction from each bleed was purified by use of a recirculating isoelectric focusing (RIEF) technique (Bier et al. 1979). The whole serum was diluted 1 3 with urea, to yield a final urea concentration of 3 M, and then desalted by electrodialysis. The urea was added to prevent precipitation under hypotonic conditions. Ampholine (1 percent w/v, pH 3.5 to 10, LKB... 

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. 

Denitrification can be affected by free ammonia, but this inhibition does not appear up to 300 to 400 mg/L NH3.46 This high concentration can justify that no inhibition of the denitrification process has been reported for this kind of wastewater.3-4 Eiroa and colleagues37 observed that nitrate was eliminated much faster at higher initial urea concentrations. However, they also found an increase of nitrite accumulation, which was later removed, due to high urea concentrations. 

Renal Effects. Ingestion of drinking water containing lead was found to be associated with evidence of renal insufficiency in humans (Campbell et al. 1977). Lead concentrations in drinking water were compared to PbB concentrations in 283 residents who ingested this water for a mean of 21.5 years. A highly significant correlation was found for these two parameters. In addition, elevated PbB concentrations were associated with renal insufficiency, reflected as raised serum urea concentrations and hyperuricemia. No renal biopsies were performed. 

The changes in structure of denatured nuclease as a function of urea concentration (Fig. 3) suggest that, as hydrophobic interactions are weakened and the backbone becomes more highly solvated, the chain expands gradually. The data presented by Millet et al. in this volume suggest that this expansion does not continue asymptotically as predicted by simple polymer physical chemistry. This is the behavior expected for a polypeptide chain trapped in a small region of conformation space. Most, perhaps all, of the conformations accessible in the expanded denatured state may have a native-like topology. 

Fig. 7. Dependence of uncorrected (A) diffusion coefficient (D) and (B) number of particles in the observation volume (N) of Alexa488-coupled IFABP with urea concentration. The data shown here are not corrected for the effect of viscosity and refractive indices of the urea solutions. Experimental condition is the same as in Figure 6. 

Fig. 8. Dependence of (A) corrected diffusion coefficient (D), (B) steady-state fluorescence intensity, and (C) corrected number of particles in the observation volume (N) of Alexa488-coupled IFABP with urea concentration. The diffusion coefficient and number of particles data shown here are corrected for the effect of viscosity and refractive indices of the urea solutions as described in text. For steady-state fluorescence data the protein was excited at 488 nm using a PTI Alphascan fluorometer (Photon Technology International, South Brunswick, New Jersey). Emission spectra at different urea concentrations were recorded between 500 and 600 nm. A baseline control containing only buffer was subtracted from each spectrum. The area of the corrected spectrum was then plotted against denaturant concentrations to obtain the unfolding transition of the protein. Urea data monitored by steady-state fluorescence were fitted to a simple two-state model. Other experimental conditions are the same as in Figure 6.

The dependence of the CD of unfolded barstar on temperature and urea concentration was reported by Nolting et al. (1997). In concentrated urea, the CD at 222 nm shows a linear temperature dependence,... 

Fig. 29. Equilbrium unfolding of C40A/C82A/P27A (pseudo-wild-type) barstar monitored by A R, mean residue circular dichroism. Conditions for near-UV CD were 50 /xM protein in 50 mM Tris-HCl buffer, pH 8, 0.1 M KC1, path length 1 cm. (A) Urea-induced unfolding at 25°C at urea concentrations as indicated. (B) Cold-induced unfolding in...

This success does, however, have a drawback the urea concentrations required could not possibly have existed in the primeval ocean. Thus, as in the case of other condensation reactions, one has to assume that there were ponds or lagoons, in which the necessary reagent concentrations could build up via evaporation of water. 

Uremia results in increased permeability of the blood-brain barrier to sucrose and insulin K+ transport is enhanced whereas Na+ transport is impaired. There is an increase in brain osmolarity in acute renal failure due to the increase in urea concentrations. However, in contrast to acute renal failure, the increase in osmolarity in chronic renal failure results from the presence of idiogenic osmoles in addition to urea. CBF is increased in uremic patients but CMR02 and CMR are decreased. In the brains of rats with acute renal failure, ATP, phosphocreatine and glucose are increased whereas AMP, ADP and lactate are decreased, most probably as a result of decreased energy demands. 

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