Analysis of the Nitrogen Content of Lawn Fertiliser

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2nd April 2008

                Atika M.Ismail 12B

CHEMISTRY INVESTIGATION

Analysis of the Nitrogen Content of Lawn Fertiliser

Research Question: What is the percentage by mass of nitrogen (from the ammonium ion NH4+) in Bunnings Green Up brand of lawn fertilizer compared to the manufacturer’s specification of 12.3%?

Aim: To determine the percentage by mass of available nitrogen, present as the ammonium ion, in Bunnings Green Up brand of lawn fertiliser, then comparing it with the one specified by the manufacturer.

Hypothesis: Since the ammonium ion (NH4+) present in fertilisers is a weak acid, it is hard to obtain an exact end point. Hence, back titration can be used to determine the percentage by mass composition of nitrogen in the fertiliser. To deduce the amount of nitrogen in ammonia, the NH4+ ion is reacted with an excess of NaOH, and the amount of unreacted NaOH found by titration with HCl is used to deduce the amount of NH4+ ion present by subtracting it from the initial amount of NaOH to find the amount that reacted with it. From this, the percentage by mass of nitrogen present in the fertiliser can be found and subsequently compared to the manufacturer’s specification.  

It is hypothesized that the percentage by mass of nitrogen present in the fertiliser will be similar or close to that specified by the manufacturer (12.3%).

Materials:

  • 100 cm3 of 0.1 mol dm-3 NaOH
  • 100 cm3 of 0.1 mol dm-3 HCl
  • 1.5 grams of fertiliser
  • 250 cm3 volumetric flask
  • 3 × 250 cm3 conical flasks
  • 20 cm3 pipette
  • Burette and retort stand
  • 50 cm3 measuring cylinder
  • Bunsen burner, tripod, gauze mat
  • Mortar and pestle
  • Electronic balance
  • White tile, bench mat
  • Funnel
  • Spatula
  • Red Litmus paper
  • Methyl red indicator

 

Safety Measures:

  • Safety goggles must be worn at all times. Sodium hydroxide can react with the proteins in the eye.
  • Wear a lab coat. Hydrochloric acid can cause severe skin irritation. If any is spilt on the skin, wash it off with water immediately.
  • Ammonia vapour irritates the skin, eyes, and respiratory system.

Method:

  1. Accurately weigh about 1.2 grams of fertilizer. Record the brand of the fertiliser and the nitrogen content as specified by the manufacturer.
  2. Dissolve the fertiliser in 250 cm3 of distilled water in a volumetric flask. Add the fertiliser first using a funnel and then add the water, allowing it to take with it the fertiliser remaining on the funnel, to the calibration line, stopper, and swirl the contents to dissolve the ammonium compounds in the fertilizer.
  3. Using a pipette, dispense 20 cm3 aliquots of the fertilizer solution into three conical flasks.
  4. Thoroughly wash the pipette and rinse it with a small volume of standard NaOH solution. Place a 20 cm3 aliquot of NaOH solution in each of the flasks containing fertilizer solution. Record the concentration of the standard solution of NaOH.
  5. To one of the flasks, add 50 cm3 of distilled water. Boil the mixture for about 10 minutes. If necessary, add more water to maintain a constant volume of solution whilst boiling. Test the vapour at the neck of the flask with a moist strip of red litmus paper. If ammonia is present the paper will turn blue and heating should be continued until the point where litmus paper does not detect the presence of ammonia. Once this point has been reached, cool the flask under running water.
  6. Repeat Step 5 with each of the other two conical flasks.
  7. Rinse and fill a burette with standard HCl. Record the concentration of the acid and the initial burette reading.
  8. Add two to three drops of methyl red indicator to each conical flask containing the fertilizer mixture, and titrate. The end point occurs when the initially yellow solution becomes orange. For each flask, record the burette reading once the end point has been reached.
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Data Collection: 

Table 1: Titration of the excess NaOH (0.084 mol dm-3) against HCl using Methyl Red Indicator

*Colourless solution turned a shade of orange upon endpoint.

Data Processing and Presentation:

  1. Find the number of moles of hydrochloric acid

The reaction between hydrochloric acid and sodium hydroxide is given in this equation:

HCl (aq) + NaOH (aq) → NaCl (aq) + H2O (l)

c(HCl) = 0.1 mol dm-3

v(HCl) = 7.9 cm3 → 0.0079 dm3

n(HCl) = c ...

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