Phosphorous(phosphate) determination in Plant Food *

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1 Phosphorous(phosphate) determination in Plant Food * This experiment uses a technique known as gravimetric analysis to determine how much phosphorous (as a weight % P 2 O 5 ) there is in samples of plant food. You haven t covered all of this in class yet, so don t get bogged down in the details of what is going on just keep in mind that we are analyzing the contents of a sample of plant food by reacting it with magnesium and ammonia, and weighing the product. Background: Plant foods and fertilizers are commonly characterized by three numbers, a weight percent Nitrogen, a weight percent P 2 O 5, and a weight percent K 2 O. The common vernacular identifies these numbers as the percent nitrogen, phosphorus, and potassium, but the numbers are really derived from 1) nitrogen, usually in the form of amines and nitrates, 2) P 2 O 5 or at least the amount of it if all the phosphorus in the sample existed as P 2 O 5, and 3) K 2 O or at least the amount of it if all of the potassium in the sample existed as K 2 O. In this experiment we will be interested in the number corresponding to the phosphorus content. P 2 O 5 is the acid anhydride of H 3 PO 4 (phosphoric acid, the acid that has phosphate ion, PO 4 3-, as its anion). An acid anhydride is the substance that combines with water to make a particular acid. P 2 O H 2 O 2 H 3 PO 4 (1) Or conversely, the acid anhydride is the substance one gets by taking water out of (dehydrating) an acid. It is a common term when discussing oxyanions (polyatomic anions that include oxygen atoms) and the related oxide species. Consequently, when a plant food is dissolved in water, the phosphorouscontaining species is converted to something more like H 3 PO 4 than P 2 O 5. The reason we don t say that the species is H 3 PO 4, is that in solutions that have a range of acidities, the actual species in solution could be H 3 PO 4, H 2 PO 4, HPO 4 2, or PO 4 3, with more acidic solutions favoring the protonated (Hcontaining) species, and more basic solutions favoring the deprotonated (without H) species. In the slight acidities and basicities experienced in this experiment we will actually be dealing with HPO 4 2, and we will utilize a property of these ions that will allow us to precipitate them (cause a solid to form) and separate them from solution.

2 Technique: In a gravimetric analysis one utilizes a property of certain species that they precipitate (i.e., form an insoluble solid product) when mixed in a solution. The solid precipitate can then be separated from the surrounding liquid by filtering. By weighing the dried precipitate one can deduce something about the composition of the original solution. It is common, for instance, to take a solution that contains Ag + ion and add an excessive amount of Cl to it; this enables the formation of the insoluble precipitate AgCl which falls to the bottom of the container as a powder. The key here is that Ag + and Cl can t stay in solution together they will come together and form AgCl, which does not stay dissolved in water, so it precipitates. After filtering the AgCl from the solution, the sample is dried and weighed. A knowledge of the weight of the collected AgCl can be related to the amount of Ag + ion in the original solution. In general, the key to successful gravimetric analyses is having components in two or more solutions, which when mixed together will react to form an insoluble product that will be able to be separated from the solution by filtration. Today we will take one of the phosphorous-containing ions present when plant foods dissolve and combine it with magnesium and ammonia to make a substance that is insoluble, MgNH 4 PO 4 6H 2 O. [The 6H 2 O refers to the waters of hydration, i.e. there are 6 water molecules which are part of the crystal structure they aren t covalently bound to the rest of the species, but they are held rigidly in place.] Mg 2+ (aq) + NH 3 (aq) + HPO 4 2 (aq) + 6 H 2 O(l) MgNH 4 PO 4 6H 2 O(s) (2) The calculation goes something like this: - A mass of MgNH 4 PO 4 6H 2 O is collected, and it can be related to the number of moles of MgNH 4 PO 4 6H 2 O by knowing the molecular weight (you figure it out). - The precipitation equation indicates that the number of moles of MgNH 4 PO 4 6H 2 O is the same as the number of moles of HPO 4 2, - The number of moles of HPO 4 2 is twice the number of moles of P 2 O 5 present in the plant food sample originally. (See eqn (1)) - One can find the mass of P 2 O 5 in the plant food sample by knowing its molecular weight (you figure that one out too). - Finally, knowing the mass of the P 2 O 5 and the mass of the original plant food sample, one can easily determine the weight percent P 2 O 5 in the plant food. The difficult part of this experiment is in controlling the acidity of the solution so that the only phosphate species is the HPO 4 2 ion. If we make the solution too basic, there will be PO 4 3 rather than HPO 4 2, and we won t form the desired precipitate. Additionally, if there are too many OH ions (i.e., the solution is too basic) they will precipitate with Mg 2+ to form Mg(OH) 2, and we don t want that to happen either. If the solution is not basic enough, H 2 PO 4 will be formed, and it

3 will also not precipitate. You will slowly add ammonia until it just begins to become basic. Adding too fast or too much can lead to the coprecipitation of Mg(OH) 2. Vacuum filtration is done with a filter flask, a buchner funnel, a rubber cone adapter, filter paper, a piece of vacuum tubing, and a source of vacuum (usually a connection on the side of a faucet. Figure 2.1 below shows the basic set-up. There is a tendency for the filter flask to fall over, so it should be supported with a clamp attached to a ring stand (not shown in the figure). Figure 2.1. Set-up for vacuum filtration. A piece of filter paper is placed in the buchner funnel, then with the vacuum on (water flowing), one wets the filter paper with an appropriate liquid (water, in this experiment) so that it is pulled tight against the holes in the funnel. In any filtration you should pour off the supernatant (transparent) liquid first, so the filter paper doesn t get clogged early in the process. Eventually you will then pour off the solid-containing portion. To transfer the solid remaining in the flask after pouring off what could be poured, use a stir rod and rubber policeman. You can also use small portions of the liquid being used. Once all the solid has been transferred to the filter, it can be rinsed by a few additional small portions of the liquid. So you don t have water back up from the faucet into the filter flask, you should disconnect the vacuum tubing before turning off the faucet when stopping the filtration.

4 Procedure: Select one of the plant food samples available on the back bench. Note the name as well as the stated percentage of P 2 O 5. Accurately weigh about 3g of your plant food and dissolve it in about 50mL of water. Knowing the actual weight of plant food is important knowing the volume of water is not. Use vacuum filtration to remove any undissolved material from the sample solution. Remember that you will be keeping the liquid filtrate, not the solid collected in the filter paper. Also remember to rinse the material collected in the filter with several portions of water; this helps ensure that no P 2 O 5 remains on the material collected on the filter paper. Add an excess of Mg 2+ by mixing the sample solution with about 50mL of the approximately 0.4M MgSO 4 solution. Slowly add 2-3mL portions of a 3% ammonia solution (2-3 ml at a time) in such a fashion that you can see where it comes in contact with the supernatant liquid (the transparent solution above the settling solid). Repeat the additions until you do not see any precipitate form when the ammonia is added. Allow the precipitate to settle for at least 15 minutes. Weigh a piece of filter paper, then use it to filter the precipitate using vacuum filtration. Remember to pour off the supernatant liquid, as much as possible, before pouring off the part containing the precipitate. Wash the solid with several small portions of water. Unlike the previous filtration, this time you are keeping the solid collected in the filter While the precipitate is still in the filter funnel, use your microspatula to gently break it apart into smaller pieces so it will dry easier. Allow it to dry with the vacuum still operating for 10 minutes. Finally, stop the vacuum, remove the precipitate with the paper from the buchner funnel. Place it on a labeled watch glass and put the watch glass in drawer #2 for further drying. Return before tomorrow s lab and weigh the precipitate and paper. All solutions used in this lab are low hazard household chemicals. Dispose of them in the appropriate container.

5 Calculations: Follow the calculation outlined in the Technique section to determine the fraction or weight percent P 2 O 5 in the original sample of plant food. Compare the result to the value on the box. Put your results on the website immediately after you weigh the final product. Collect the classes results later that day. Make a plot or tabulate the measured %P 2 O 5 vs the expected %P 2 O 5. You should attempt to conclude how closely the results correspond to the amounts given on the packages. Does it look like manufacturers put a little more, a little less, or close to the advertised amount of phosphorous in their plant food? Be sure your lab report includes. (this is not necessarily an exhaustive list) an objective which mentions the general technique as well as the precipitation equation. data tables with your key data: sample mass, precipitate mass, and the results of calculations. A plot or table showing the class s data for %P 2 O 5 vs the manufacturer s claim for %P 2 O 5. mention of any mistakes that were likely and what effect they would have had on your calculated %P 2 O 5 relative to the true %P 2 O 5 (i.e. higher, lower, unknown relation). statements addressing the various questions asked throughout the procedure. * See also Solomon, S. ; Lee, A.; and Bates, D. J.Chem. Ed. 70, 1993,