How do you measure the nitrogen content of a urea fertilizer?

Answer 1

Given below is an experiment, which should be brought out in a laboratory.

Apparatus and ingredients

1 molar NaOH solution; 0.2 molar HCl solution; Phenolphthalein and Methyl Orange; 250 cm3 volumetric flask; titration flask; burette; 25 ml pipette; beakers

Procedure

Measure accurately the mass of a fertilizer sample which weighs 1.00 g approximately using an electronic balance.

Add 25.00 ml of the NaOH solution using the pipette to a beaker. Add the measured portion of the sample to the NaOH solution. Warm it thoroughly and stir it using a glass rod.

After warming the solution for about a half an hour, let it to be cool and add it to the volumetric flask. Wash the beaker using distilled water and add it too to the flask, and then dilute it till 250 cm3.

Take a portion of 25 ml from the flask from the pipette to the titration flask. Add phenolphthalein as the indicator and titrate it against the HCl solution. Observe and record the end point.

Next, add few drops of methyl orange to the titration flask with the same solution and continue the titration. Record the end point.

Theory and calculations

The reactions involving are;

CO(NH2)2 + 2NaOH --> Na2CO3 + 2NH3 --- (1)

NaOH + HCl --> NaCl + H2O --- (2)

Na2CO3 + HCl --> NaHCO3 + NaCl ---(3)

NaHCO3 + HCl --> NaCl + H2O + CO2 ---(4)

where the first three reactions occur during the first titration.

Suppose the two burette readings are V1 and V2 respectively. Then the HCl involved in the last reaction is given by V1 - V2. Hence, we can find the number of NaHCO3 moles involved in the second titration, which is equivalent to the moles of Na2CO3 in reaction (3). Finally moles of sodium carbonate is equal to half the moles of N atom moles present in urea. By multiplying the moles of N atoms by its molar mass, the weight of N present in the 25 ml can be calculated.

Now, if a mass m is there in 25 ml, we can calculate the mass of N atoms dissolved in 250 ml in the volumetric flask.

By now, we know the entire mass of the examined sample and the mass of the nitrogen content, so the percentage can be calculated.