If you have a fairly broadband light source (e.g., incandescent) you could measure the spectral power distribution of light through a consistent volume/thickness of the solution (you could also do the same with the container empty to get a baseline reading). From the measured spectral power distribution you could calculate chromaticity coordinates. If you have solutions of known concentrations you could use those to develop a calibration curve, and then compare the chromaticity coordinates of an unknown sample to that curve to estimate the concentration. It would be important to make sure the set-up and measurement/light source geometries were consistent for all of your measurements.

There is/was a quantitative enzymic method that uses urease and the acidic shift in the pH due to urea (in soybeans). Otherwise, maybe the Bertholet reaction may work.

Dear Sir. Concerning your issue about the quantitative color test for the presence of urea in a solution . Urea is the major nitrogenous end product of protein metabolism in mammals. Here, we describe a quantitative, sensitive method for urea determination using a modified Jung reagent. This assay is specific for urea and is unaffected by ammonia, a common interferent in tissue and cell cultures. We demonstrate that this convenient colorimetric microplate-based, room temperature assay can be applied to determine urea synthesis in cell culture. I think the following below links may help you in your analysis:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2804236/pdf/nihms162473.pdf

http://www.jbc.org/content/152/2/453.full.pdf

https://collections.nlm.nih.gov/ext/dw/101265798/PDF/101265798.pdf http://amrita.olabs.edu.in/?sub=79&brch=17&sim=206&cnt=2

Thanks