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Quantitative Analysis of Phosphorus in Plant Food

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Erica Brown

on 28 November 2012

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Transcript of Quantitative Analysis of Phosphorus in Plant Food

Chemistry 141, November 2012 Quantitative Analysis of
Phosphorus in Plant Food What is plant food? Plant food is primarily comprised of
Nitrogen (promotes healthy green color)
Phosphorus (helps establish strong root system)
Potassium (regulates plants, produces larger fruit)
The amount of each compound varies by the type
and purpose of the plant food. Phosphorus In fertilizer, phosphorus takes the form
P2O5 (empirical formula)
P4O10 (molecular formula) Procedure Procedure Procedure Filtering the precipitate The purpose of this lab was to determine the phosphorus content in two different kinds of plant food. What are we looking for? Ball-and-stick diagram of P4O10 This procedure precipitated the
phosphorus found in fertilizer and
allowed us to determine what
percent comprised it, and the
variances in percent composition
between different brands. Prepare magnesium sulfate (MgSO4) solution Add 30 g Epsom salts to 300 mL deionized water
Stir vigorously until the salt is completely dissolved This creates enough MgSO4 solution for the entire procedure,
so no more will have to be mixed. Measure 2 g of fertilizer In a mortar and pestle, pulverize fertilizer into a fine powder.
Measure 2 g fertilizer.
Filter out any precipitate that forms. Preparing the fertilizer solution Add 30 mL of MgSO4 solution and stir
Add 40 mL NH4OH (ammonia) while stirring
Allow the solution to sit for 15 minutes Preparing the Buchner flask/funnel While solution is sitting for 15 minutes, label and weigh filter paper with fertilizer type and trial number
Wet filter paper with deionized water and quickly turn on aspirator to create a seal (ensuring that no precipitate will be lost) Types of fertilizer
we used! A Buchner funnel Turn the aspirator on, slowly pouring the fertilizer solution into the Buchner flask
Wait for all the liquid to pass through (this may take a while)
Once the solution has been completely filtered twice, rinse it again with one or two pipets of acetone to expedite the precipitation of the phosphorus.
When this is complete, take the filter paper out with tweezers and place it into a labeled, weighed boat to dry out. The six trials were left to dry for three days to allow for the most precipitate possible to be collected. Measuring Results After the precipitate and filter paper were suitably dry, each trial was weighed. The mass of the filter paper and plastic boat was subtracted from the combined mass of the filter paper, plastic boat, and precipitate. This gave the mass obtained of the precipitate. What is the precipitate? Magnesium, ammonium, phosphate, and water react to form the anhydrous compound struvite. Data Data (Part 2) Calculations with Precipitate
Obtained Subtract the mass of the filter paper/boat/precipitate from the mass of the filter paper/boat. We are looking for moles of phosphorus pentoxide (P2O5), the form of phosphorus found in plant food. Find moles of precipitate. The molar mass of the precipitate (struvite) is 245.41 grams/mole. The moles of precipitate are 1:1 with phosphorus (phosphate in this equation). Divide moles phosphorus by 2 to obtain the moles of phosphorus pentoxide. Use the molar mass of P2O5 to find grams obtained. Red phosphorus! Results This is the data from all six trials, with percent composition calculated, and average percent composition (as well as adjusted average percent composition. Calculating Percent Error Percent error = | Calculated value - actual value | _________________________ Actual value x 100 But what do these results mean? Interpreting Results Obviously the percent error for this lab was very high. There are a few possible sources of error. Sources of Error Vigoro was very difficult to grind and lots of precipitate in the DI water-ammonia solution had to be filtered out. The composition of this precipitate was indeterminate and it is unknown if it contained some of the phosphorus.
Due to the limited number of Buchner funnels/aspirators, most of the samples had to sit for more than the recommended 15 minutes. This could have affected the amount of precipitate collected. Skill of students - given only a basic understanding of chemistry lab, and basic materials, the probability for error was very high Conclusion The percent error was very high, especially on certain trials, but in this context (student lab, relatively unguided, limited materials, etc), what matters more is the consistency of the results. The results can be said to be very consistent, with all the results centering fairly closely around a common number. One way to think about it: if the spectrum of results was a dartboard, with the center point being exactly the percentage of phosphorus in the plant food, the darts would be clustered together in a pattern! Getting consistent results is perhaps more telling of preparedness and skill for lab than acquiring a low percent error. The Miracle-Gro trials had a fairly decent percent error, as well. With the experience, time, and supplies taken into consideration, the lab to determine phosphorus content was a success. Sources Body of presentation http://www.ehow.com/about_4902638_what-does-potassium-do-plants.htmlhttp://www.ehow.com/about_4587203_what-does-phosphorus-do-plants.htmlhttp://en.wikipedia.org/wiki/Nitrogen#Applications_of_nitrogen_compounds
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