The use of urea (NH2CONH2) to remove nitric oxide (NO) from exhaust streams was investigated using a laboratory laminar-flow reactor. The experiments used a number of gas compositions to simulate different combustion exhaust gases. The urea was injected into the gases as a urea-water solution. The decomposition processes of the urea-water solutions and urea powder were examined. For both the nitric oxide removal and the urea decomposition experiments, a Fourier transform infrared (FTIR) spectrometer was used to determine the concentrations of the product species. The products from the decomposition were examined every 50 K from 500 K to 800 K. The dominant products were ammonia (NH3), isocyanuric acid (HNCO) and carbon dioxide (CO2). In case of urea-water solution decomposition, for gas temperatures between 550 and 650 K, the highest concentrations were for NH3 and HNCO. On the other hand, the concentrations of CO2 were highest for gas temperatures of about 500 - 550 K. For temperatures above about 650 K, the amount of these three dominant prod-ucts slightly decreased as temperature increased. ivFor the nitric oxide removal (SNCR) experiments, the gas mixture was heated to temperatures between 800 K and 1350 K. Depending on the temperature, gas composition, residence time, and urea feed rate, removal levels of up to 95% were obtained. Other by-products such as N2O were detected and quantified. The effects of the urea/NO (beta) ratio were determined by varying the urea concentration for a constant NO con-centration of 330 ppm. The effects of the levels of oxygen (O2) in the exhaust gases and the residence time also were investigated. Increasing the urea/NO ratio and residence time resulted in higher NO removal and increased the temperature window of the nitric oxide removal.

The addition of urea clearance monitoring to the care regimen of renal failure patients provides a dramatic decrease in complications due to improper or inadequate dialysis. Present methods of monitoring urea clearance are computationally complex and expensive to perform, resulting in poor rates of clinical acceptance of this measurement. Dialysate-side urea levels have been shown to relate to traditional measures of dialysis adequacy without the need for complex calculations. The requirements for photometric reagents or electrodes make determination of the urea level expensive and time consuming. This research is focused on the development of an optical measurement system to determine the sample urea level without the need for reagents. An algorithm was developed to predict the urea concentration of the sample from a set of optical transmission parameters recorded from the sample using a specially developed instrument. This instrument records the difference in sample transmission at two different wavelengths. Energy at the first wavelength is absorbed by urea, and the second wavelength is selected such that the matrix of the sample absorbs energy at both wavelengths equally. This effectively nulls out the absorbance of the background matrix, significantly improving the urea detection sensitivity. The algorithm was developed from an analysis of the instrument data and factors causing variations in the data. Calibration, bench study, and clinical protocols were designed, and performed using these protocols. Using a partial least squares approach, the algorithm was fit to a set of training data. The resulting algorithm was used to predict the urea content of patient hemodialysis samples. Compared to a reference standard (Beckman CX7, standard error /dl), the standard error of prediction for this algorithm was 0.47 mg/dl (N = 34 patients). The algorithm was able to predict dialysate urea at clinically relevant levels in samples collected from hemodialysis patients. Qualitative relationships were developed between the sample urea level and the data recorded from the sample. This system has the potential to provide a method that clinicians can use to efficiently and effectively monitor urea removal over the course of a dialysis session.

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A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science (Chemistry) May 1993 / One of the greatest environmental drives in the synthetic resins field has been to decrease the formaldehyde emission from cured urea formaldehyde (UF) resins, without adversely affecting their excellent technical performance. [Abbreviated Abstract. Open document to view full version] / MT2017

The use of a conductive polypyrrole urea biosensor for the detection of urea in blood plasma is investigated.Urease was incorporated into a polypyrrole film by galvanostatic polymerisation. The presence of urea was verified, and the activity of the enzyme in the polypyrrole film was confirmed. The inherent electroactive properties of the polypyrrole-urease film has enabled the production of a flow injection amperometric biosensor for the reliable determination of urea. Greater sensitivity and stability was achieved when a pulsed amperometric detection system was implemented. The analysis of urea in human blood plasma by potentiometry, amperometry and pulsed amperometry was achieved with the assistance of an anion exchange separation prior to the electrochemical detection of urea. A polypyrrole-sulfite oxidase film was developed for use in an amperometric biosensor for sulfite determination. The response of the biosensor to sulfate was linear from 0 to 80 mg/L and the minimum detectable amount was 5 mg/L. Useful interferants in sulfite determination such as ascorbic acid, sodium nitrate and sodium sulfate did not respond to the biosensor. The excellent reproducibility of the sulfite response provides the basis for the construction of a disposable or renewable biosensor for sulfite determination. The analysis of sulfite in wine and beer was accomplished and no pretreatment was required. / Doctor of Philosophy (PhD)

Three experiments were conducted to study dietary effects on urea-nitrogen (N) recycling as a strategy to improve the efficiency of N utilization in ruminants. Experiment 1 examined the effects of feeding diets containing two levels of dietary crude protein (CP; 10.8 vs. 14.0%) and ruminally-degradable protein (RDP; 73.4 vs. 76.0% of CP) on urea-N recycling to the gastro-intestinal tract (GIT), N balance, and microbial protein production in beef heifers. Feeding the low CP diet decreased N intake (P < 0.01), ruminal ammonia-N (NH3-N) concentration (P < 0.01) and urinary N excretion (P <0.01). Endogenous urea-N production increased (P = 0.03) with increasing dietary CP level, as did urinary urea-N loss (P = 0.04). However, urea-N transfer to the gastro-intestinal tract (GIT) was similar across diets, with most of this N returned to the ornithine cycle. Microbial N supply was unaffected (P > 0.05) by dietary treatment. Experiment 2 examined the effects of feeding diets containing two levels of ruminally-degradable starch (RDS; 28.6 vs. 69.2% of total starch) and RDP (48.0% vs. 55.0% of CP) on urea-N recycling to the GIT, N balance, duodenal nutrient flow, and ruminal microbial protein production in beef heifers fed low CP (10%) diets. Ruminal NH3-N concentration was greater (P = 0.01) in heifers fed high RDP as compared with those fed low RDP, and it was also greater (P = 0.01) in heifers fed low RDS as compared with those fed high RDS. Microbial N flow to the duodenum increased as RDP level increased on the high RDS diet, but was not affected by RDP level on the low RDS diet (interaction; P = 0.04). Urea-N entry rate and urea-N transfer to the gastro-intestinal tract were similar (P > 0.05) across diets. The amount of recycled urea-N that was incorporated into microbial N increased as RDP level increased on the high RDS diet, but the opposite was observed on the low RDS diet (interaction; P = 0.008). Experiment 3 examined the effects of feeding diets containing two levels of CP (14.9 vs. 17.5%) and RDP (63.0 vs. 69.0% of CP) on urea-N recycling to the GIT, microbial protein production, N balance, omasal nutrient flow, and milk production in lactating dairy cows. Nitrogen intake (P < 0.01) and both urinary N (P < 0.01) and urea-N (P < 0.01) output were greater for cows fed the high compared with those fed the low CP diet. Ruminal NH3-N concentration tended to be greater in cows fed the high than those fed the low CP diet (P = 0.06), and was greater in cows fed high RDP as compared with those fed the low RDP diet (P < 0.01). However, N balance, milk yield, and microbial N supply were unaffected (P > 0.05) by dietary treatment. The proportion of endogenous urea-N that was recycled to the GIT (i.e., GER: UER) was greater (P = 0.02) in cows fed the low CP compared with those fed the high CP diet. In summary, the results of this thesis show that reducing dietary CP level in beef and dairy cattle reduces urinary N excretion whilst maintaining microbial N supply. In addition, judicious combinations of RDP and RDS when feeding low CP diets can potentially enhance the efficiency of microbial N production. These data show that through careful dietary manipulation, overall efficiency of N utilization can be improved leading to a reduction in N excretion into the environment.

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