Exp 05: Elemental Analysis

Your job is to determine the phosphorus content in a sample of plant food.

Report two numbers: As a percent of phosphorus in the original sample. As a percent of diphosphorus pentoxide that would be produced, if we burned the original sample.

You will use the mol ratio to determine and justify your answer.

Exp 05: Elemental Analysis

We used to do elemental analysis of plant food mostly by combustion analysis. Folks cared less about knowing exactly how much phosphorus was in the plant food, than whether this sample or that had “more.”

So we took a short cut and started reporting phosphorus composition as the amount of P2O5 we captured when analyzing it (as a percent of the sample we burned).

Phosphorus is reported as % P2O5 — even though no P2O5 exists in the original sample. It’s just a convention.

52.17% C 13.04% H 34.78% O

C2H5OH?+

Exp 05: Elemental AnalysisProcedure

1. If your instructor provides one, use the report form with data table to record data for this experiment. The flow chart in the included illustrations provides an overview of the procedure described below.

2. Obtain a sample of plant food and record the brand name and type of plant food in the report form. Also record the amount of phosphorous listed on the label as % P2O5. Weigh a clean 400 mL or beaker and record the mass to the nearest 0.01 g. Put a little more than 10 g of plant food in the beaker and weigh again and record the mass to the nearest 0.01 g. Mix the plant food with about 150 mL of water in the beaker. Stir until it dissolves.

3. Weigh a clean, dry 250 mL beaker and add to it about 20 g of Epsom salts (magnesium sulfate). It is not necessary to know the exact mass as about 20 g is a considerable excess. Add approximately 200 mL of water and stir to dissolve.

4. You should now have a plant food solution and an Epsom salt solution in two separate beakers. In order to make sure we precipitate all the phosphorus from the plant food, we need to calculate how much of the Epsom salt solution we should use based on the amount of P2O5 in our particular plant food sample. Using the mass of your plant food sample and the percent P2O5 on the label of the plant food, calculate the grams of P2O5 in your sample. You will need 50 mL of Epsom salt solution for every 1.0 g of P2O5 in the sample. Calculate the total volume needed and add this volume to your plant food solution. Stir to mix.

Exp 05: Elemental Analysis

Procedure

5. The solution is acidic at this point and must be neutralized to precipitate MgNH4PO4·6H2O. Add 15 mL of ammonia (NH3) to your plant food and Epsom salt mixture (using a graduated cylinder). A white precipitate of MgNH4PO4·6H2O will form. The solution must be basic for precipitation to be complete. Stir well and test the solution with pH paper. As long as the solution remains acidic continue to add ammonia (in 15 mL increments) until a pH between 9.0 and 10.0 is achieved. Allow the solution to stand for fifteen minutes before filtering.

6. Prepare a vacuum filtration apparatus using a Buchner funnel (a Buchner funnel apparatus setup is shown in the included illustrations). Obtain a piece of filter paper for your sample, place it flat in the funnel and wet it with distilled water. Turn on the vacuum. Transfer all the solution and the precipitate from the 400 mL beaker to the funnel using a rubber policeman. If needed, when the level of the filtrate nears the top of the filter flask, turn off the vacuum, remove the Buchner funnel, dispose of the filtrate in the sink, replace the funnel on the filtering flask and continue filtering. After transferring all of the contents to the funnel wash the precipitate with two or three 5-mL portions of distilled water. Do this by adding each portion to the beaker in which you did the precipitation to transfer any remaining precipitate; then pour over the solid in the funnel.

7. Finally, pour two 10-mL portions of 75% isopropyl alcohol through the filter paper. When all the liquid has gone through, disconnect the hose and then turn the vacuum off. (Warning: if you turn the vacuum off before disconnecting the hose debris in the vacuum system may be pulled into your beaker!) Carefully remove the filter paper with the precipitate and place on a marked paper towel. Blot your samples dry with a second paper towel. Allow it to dry until the next lab period.

8. When the product is dry, weigh a clean, dry watch glass to the nearest 0.01 g and record the mass. Carefully scrape all the precipitate off the filter paper onto the watch glass and weigh it and record the mass.

Exp 05: Elemental Analysis

C1: Show how you calculated the mass of MgNH4PO4·6H2O(s) from the weights recorded in your data.

C2: Use dimensional analysis and molar mass of MgNH4PO4·6H2O(s) to calculate the moles of MgNH4PO4·6H2O(s) you recovered and then use dimensional analysis to show how many moles of P were recovered.

C3: What mass of P was in your original sample? Using the weight of your original sample and the mass of P in that sample you just calculated, determine the %P by weight in your original sample.

C4: If that sample was analyzed by combustion analysis instead of gravimetric analysis, all the phosphorus in the original sample would have been recovered as P2O5. Traditionally phosphorus content is reported as the ratio of that weight to the weight of the original sample, or "%P2O5". Using your data, calculate the percent P2O5 that would have been reported for your sample.

Interpreting Data (Calculations & Reasoning):

Questions?