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An experiment to determine the amount of urea in a specimen of urine.


Metabolism produces a number of toxic by-products, particularly the nitrogenous wastes that result from the breakdown of proteins and nucleic acids. Amino (NH2) groups are the result of such metabolic reactions and can be toxic if ammonia (NH3) is formed from them. Ammonia tends to raise the pH of bodily fluids and interfere with membrane transport functions. To avoid this the amino groups are converted into urea, which is less toxic and can be transported and stored to be released by the excretory system. Urea is the result of two amino groups being joined to a carbonyl (C=O) to form CO(NH2)2, the process of which is called the ornithine cycle and takes place in the liver. The ornithine cycle was developed by Hans Krebs in 1932 and is similar to the Krebs cycle through the use of oxaloacetate. One of the steps in the cycle the breakdown of arginine into ornithine and urea, a reaction catalysed by the enzyme arginase. (See below)

(Fig 1.0)

Arginine Orthinine Urea

Urease is the enzyme which catalyses the hydrolysis of urea according to the following equation:

(NH2)2CO(aq) + 3H2O(l)  CO2(g) + 2NH3(g)

The acidic ammonium carbonate is formed because the carbon dioxide dissolves in water to produce carbonic acid (H2CO3), which immediately reacts with ammonia to form the ammonium carbonate. This is shown by the following equation:

2NH3(g) + H2CO3(aq)  (NH4+)2CO3(aq)

The resulting solution can then be titrated against hydrochloric acid with methyl orange as the indicator in order to determine how much urea was present initially.
The point of neutralisation using a methyl orange indicator is determined using the following colour changes.

 Acid  Red.
 Neutral  Yellow.
 Alkali  Orange.

Enzymes are nearly all made up of globular proteins. The structure of enzymes can be divided into three categories:

1. The primary structure, which is the sequence of amino acids. 2. The secondary structure, which is the coiling of the protein into an alpha helix 3. The tertiary structure, which is the 3D shape into which the protein is folded. This shape gives the enzyme its properties and specificity. The shape is held together by ionic bonds, disulphide bridges and the weaker hydrogen bonds.


Six urea solutions were prepared an placed in conical flasks one of which was of unknown concentration. The flasks were sealed to prevent CO2 and NH3 gases from escaping and then placed in a water bath at 35oC for 1 hour. The temperature was kept at 35oC as each enzyme has an optimum temperature at which it works best, so it was important that the temperature remained constant for the duration of the reaction.

After 1 hour all the flasks were removed. A burette was washed first with distilled water to remove any impurities. Then with HCl to prevent the acid from being neutralised by the remaining water, as this would increase the pH of the acid and give a less accurate titre. It will also remove any impurities not dealt with by the water. The burette was then carefully filled to the top with HCl. A 10cm3 pippet was used to place portions of the urea solution into a beaker, into which a few drops of methyl orange were placed to act as an indicator. The beaker was arranged on top of a white tile so that the end-point of the titration could be determined more accurately. At the start of the titration the solution was yellow and at the end-point it turned red. This process was repeated for each solution, and the volume needed to completely neutralise 10cm3. Each time the procedure was repeated 3 times and the average titre would be calculated.


(Fig 2.0)
Concentration of urea (g/100cm3)Volumes of 0.1M HCl
Required (cm3) Mean volume of 0.1M HCl required (cm3) (1dp) 0.329.2, 9.3, 9.39.3...
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