Lab Report: Percentage of Nitrogen in Lawn Fertilizer

Categories: Chemistry

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Abstract

This lab report aims to determine the percentage by mass of available nitrogen in Bunnings Green Up brand of lawn fertilizer, specifically in the form of the ammonium ion (NH4+). The experiment involves a back titration method, where excess sodium hydroxide (NaOH) is reacted with the fertilizer, and the unreacted NaOH is titrated with hydrochloric acid (HCl). The calculated percentage of nitrogen is then compared to the manufacturer's specification of 12.3%. The result obtained was 13.9%, which is very close to the manufacturer's specification.

Introduction

Fertilizers play a crucial role in enriching soil and promoting plant growth.

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Among the essential nutrients for plants, nitrogen is one of the most important. It influences the color, stress tolerance, weed control, and disease resistance of grass and plants. Lawn fertilizers often contain nitrogen in various forms, and it is essential to verify their actual nitrogen content compared to the manufacturer's claims.

This experiment aims to determine the percentage by mass of available nitrogen, specifically in the form of the ammonium ion (NH4+), in Bunnings Green Up brand of lawn fertilizer.

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The manufacturer specifies that this fertilizer contains 12.3% nitrogen. To achieve this, a back titration method is employed due to the difficulty of obtaining a sharp endpoint in direct titrations involving weak acids.

Materials and Methods

Materials

100 cm³ of 0.1 mol dm⁻³ NaOH
100 cm³ of 0.1 mol dm⁻³ HCl
1.5 grams of Bunnings Green Up lawn fertilizer
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 due to the use of sodium hydroxide, which can react with proteins in the eyes.
Wear a lab coat to protect against skin irritation from hydrochloric acid.
Ammonia vapors irritate the skin, eyes, and respiratory system; handle with care.

Procedure

Weigh approximately 1.2 grams of Bunnings Green Up lawn fertilizer and record both the brand and the manufacturer's specified nitrogen content (12.3%).
Dissolve the fertilizer in 250 cm³ of distilled water in a volumetric flask, ensuring that all fertilizer is transferred.
Dispense 20 cm³ aliquots of the fertilizer solution into three conical flasks.
Rinse the pipette thoroughly and rinse it with a small volume of standard NaOH solution.

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Add 20 cm³ of NaOH solution to each flask containing the fertilizer solution. Record the concentration of the standard NaOH solution.
In one of the flasks, add 50 cm³ of distilled water and boil the mixture for about 10 minutes, testing for ammonia with red litmus paper until no ammonia is detected. Cool the flask under running water.
Repeat the boiling step with the other two flasks containing fertilizer solution.
Rinse and fill a burette with standard HCl solution, recording the concentration and initial burette reading.
Add two to three drops of methyl red indicator to each conical flask containing the fertilizer mixture, and titrate with HCl. The endpoint is reached when the yellow solution turns orange. Record the burette reading for each flask at the endpoint.

Data Analysis

Table 1: Titration of Excess NaOH (0.084 mol dm⁻³) against HCl Using Methyl Red Indicator

Conical Flask	Final Burette Reading (cm³)	Volume of HCl Used (cm³)
1	24.50	24.50
2	23.80	23.80
3	24.20	24.20

Discussion

The goal of this experiment was to determine the percentage by mass of available nitrogen, in the form of the ammonium ion (NH4+), in Bunnings Green Up lawn fertilizer and compare it to the manufacturer's specified value of 12.3%. The back titration method was chosen due to the difficulty of obtaining a sharp endpoint in direct titrations involving weak acids.

The results obtained showed that the calculated percentage of nitrogen in the fertilizer was 13.9% (± 1.00%), which is very close to the manufacturer's specification. This indicates that the fertilizer contains a slightly higher percentage of nitrogen than claimed. However, it's essential to consider potential sources of error and limitations in the experiment.

Sources of Error and Limitations

Several sources of error and limitations may have affected the accuracy of the results. One potential source of error is the accuracy of weighing the fertilizer sample. Even a small measurement error in the mass of the sample could lead to significant discrepancies in the final percentage calculation. Ensuring precise weighing techniques and transferring all sample material into the volumetric flask is crucial to minimize this error.

Another potential source of error is the calibration of the volumetric flask. If the meniscus of the solution was not exactly on the calibration line, it could have resulted in an inaccurate volume measurement. Careful attention to the calibration line and using eye level to check it can reduce this parallax error.

The use of red litmus paper to detect ammonia may have introduced some uncertainty. The subjective judgment of the shade of orange during the titration could have affected the results. Using an indicator that provides a clearer and more distinguishable color change may improve the accuracy of the endpoint detection.

Lastly, there may have been some residual ammonia present in the conical flasks even after boiling. This could lead to additional HCl consumption during titration and affect the calculated percentage. Ensuring thorough boiling and evaporation of ammonia is critical to minimize this error.

Conclusion

In conclusion, this experiment aimed to determine the percentage by mass of available nitrogen in Bunnings Green Up lawn fertilizer and compare it to the manufacturer's specification of 12.3%. The back titration method was employed to overcome the challenges of titrating weak acids directly. The calculated percentage of nitrogen in the fertilizer was found to be 13.9% (± 1.00%), which is very close to the manufacturer's specification. The slight discrepancy may be attributed to sources of error and limitations in the experiment, such as measurement errors and subjective color detection. Overall, the experiment successfully determined the nitrogen content in the fertilizer and verified the manufacturer's claim.

Recommendations

To improve the accuracy of future experiments, the following recommendations are suggested:

Ensure precise weighing techniques and complete transfer of sample material into the volumetric flask.
Pay careful attention to the calibration line when preparing solutions in volumetric flasks.
Consider using a different indicator that provides a clearer and more distinguishable color change during titration.
Thoroughly boil and evaporate ammonia during the experiment to minimize its interference in titration.

Lab Report: Percentage of Nitrogen in Lawn Fertilizer. (2017, Nov 17). Retrieved from https://studymoose.com/document/analysis-of-the-nitrogen-content-of-lawn-fertiliser