Chem 261 Lab – Gravimetric Analysis of Phosphorous



Chem 261 Lab – Gravimetric Analysis of Phosphorous

Introduction

I am sure that at one time or another, everyone has seen the ingredients list on a commercial product. In some cases, especially where an active ingredient is involved, a weight or mass percent is also provided in the listing, such as 5.0 wt% sodium hypochlorite in bleach. Many manufacturing companies employ lab technicians that test products to ensure the company is producing the correct material before placing it on market. Depending on the product, there are many methods used by these lab technicians who are involved in “quality control”. Some methods require sophisticated instrumentation, while others may rely on some simple bench top chemistry such as titrations, or as in the case of this experiment, gravimetric analysis. In gravimetric analysis, the component of the mixture is isolated, generally in a solid form, and its mass is compared to the original mass of the sample to determine a quantitative mass percent.

Plant food is an example of a commercial product where the buyer needs to know the mass percent of the active ingredient in order to apply the right amount to their crop or garden. Of course one does not want to purchase a product that says one thing, but delivers another. Many plant food products are marked with the mass percent of available nitrogen, phosphorous, and potassium, three essential nutrients for plant growth. A package of plant food is generally marked with a set of numbers indicating these mass percents. For example, the plant food analyzed in this experiment consists of the numbers 15-30-15, for 15% N (may involve two or more nitrogen containing compounds), 30% P2O5, and 15% K2O respectively. In the case of phosphorous, 100 g of plant food contains enough available phosphorous that would equate to having at least 30 g of P2O5. In this experiment, the % P will be verified gravimetrically. In order to do this, the phosphorous must be converted to a form that can be isolated as a solid.

Since the plant food is water soluble, the available phosphorous will have to be converted to a form that will precipitate from the mixture in a pure form without precipitating other components. In an aqueous solution, the phosphorous likely exists in the form of hydrogen phosphate, HPO42-. It is known that upon the addition of magnesium sulfate, MgSO4, and aqueous ammonia, NH4OH, an insoluble hydrated salt, MgNH4PO4 · 6 H2O, will form and precipitate from the mixture, illustrated by the reaction below. After filtering, drying, and weighing this solid, one can calculate the mass of phosphorus present in the precipitate and then determine the mass percent of P and/or P2O5 in the plant food. Note that for every one phosphorus atom in the precipitate, there are two P atoms in P2O5.

HPO42- (aq) + NH4+ (aq) + OH- (aq) + Mg2+ (aq) + 5 H2O (l) ( MgNH4PO4 · 6H2O (s)

Procedure

Obtain three, 3g samples of plant food and dissolve each in 35 mL water. After dissolving, filter the solution by gravity filtration to remove any insoluble contaminants. Slowly add 45 mL of MgSO4(magnesium sulfate) solution and 150 mL of 2 M NH3 (aq) (aqueous ammonia or NH4OH or ammonium hydroxide) while stirring to each sample. Allow the mixtures to stand approximately 15 minutes to permit complete precipitation and settling. Using a Büchner funnel with aspirator vacuum and pre-weighed filter paper, filter the three samples and wash each precipitate with three 5 mL portions of water and two 10 mL portions of isopropanol. Place the collected solids and filter paper in your drawer to dry over night (until the next lab period).

Chem 261 Lab – Gravimetric Analysis of Phosphorous

Pre-lab questions

1. What are the ingredients in common home garden fertilizers? Cite the sources for your answer and justify the reliability.

2. How are the quantities of the ingredients reported? Cite the sources for your answer and justify the reliability.

3. List several experimental methods that can be used to measure the amount of phosphorus in a sample of fertilizer? Cite the sources for your answer and justify the reliability.

4. What is the mass percent of P in H3PO4?

5. Determine the average and standard deviation for the following set of numbers.

45.6 g, 52.1 g, and 53.8 g

Report:

Provide a data table that includes mass of plant food, mass of precipitate, mass % P in precipitate, gram equivalent of P in the precipitate, gram equivalent of P2O5 in plant food, and mass percent of P2O5 in plant food for each of the three samples. Report the average and the standard deviation mass percent of P2O5 for your samples.

Post-lab questions:

1. Do your results support the package label that the plant food contains at least 30% P2O5? Briefly explain.

2. If the precipitate was slightly soluble in water, how would this affect your results?

3. If good precision were based on +/- one standard deviation, were your determinations precise?

4. Ignition (not combustion) of MgNH4PO4 · 6H2O produces Mg2P2O7 and ammonia and water. Write the balanced chemical equation.

5. What is the mass percent of P2O5 in Mg2P2O7?

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