Gravimetric Determination of Phosphorus in Fertilizer Samples E.M.M. Medrano Medrano,, J.M. Pasco Department of Mining, Metallurgical and Materials Engineering, College of Engineering University of the Philippines, Diliman, Quezon City, Philippines Date Due: 7 January 2014 Date Submitted: 7 January 2014
Abstract Concepts in precipitation gravimetry were applied in this experiment. Phosphorus from fertilizer samples was dissolved in water, ammonia and magnesium sulfate heptahydrate solution. A white precipitate, magnesium ammonium phosphate hexahydrate, was formed. The solid contained the phosphorus from the fertilizer sample. It was weighed and then results compared to the amount of phosphorus indicated in the package. The outcome showed that fertilizers actually have lower phosphorus content than indicated in the package. Keywords: gravimetric analysis, digestion, phosphorus, precipitate, ethanol, stirring INTRODUCTION Phosphorus is one of the main components in fertilizers. To determine how much phosphorus is present in a fertilizer sample, gravimetric analysis through precipitation of magnesium ammonium phosphate hexahydrate, MgNH4PO4 • 6H2O, was used. Gravimetric analysis through precipitation works with soluble analyte.[1] In this experiment, soluble phosphorus was the analyte which was precipitated and measured. This experiment aimed to demonstrate proper gravimetric steps in order to quantitatively determine the amount of phosphorus in fertilizers. METHODOLOGY Filter paper was weighed then kept in the desiccator prior to use in the latter part of the experiment. Powdered fertilizer weighing 3.0056 grams when wet and 2.9159 grams after drying was
dissolved in 40 mL water. A different filter paper was used to filter the mixture. 45 mL of 10% MgSO4 • 7H2O was then added to the filtrate. Then, 150 mL of 2M NH3 was slowly added while stirring. White precipitate was formed. The mixture was then allowed to stand at room temperature for 15 minutes. The precipitate was then quantitatively transferred to the pre-weighed filter paper then washed with two (2) 5 mL portions of distilled water. Two (2) 10 mL portions of 95% ethanol was used to wash the precipitate. The filter paper was then spread on a watch glass and stored in the locker until the next laboratory period. The filter paper with precipitate was then dried in the oven at 100°C for 1 hour. It was then cooled for 15 minutes and then weighed. Results were tabulated. RESULTS AND DISCUSSION The amount of phosphorus was quantitatively determined. The results pooled from 10 trials were as follows:
amount of phosphorus in fertilizers especially because phosphorus is one of the main components of fertilizers.
Table 3.1 %P and %P2O5 4.5455% ± 0.0226% %P 10.3918% ± 0.05167% %P2O5 The phosphorus content of fertilizers is expressed in percent phosphate. The sample used has 20% P2O5. Compared to the results, there is a relative error of 48%. This error may be attributed to several factors like the forming of MgOH2 or magnesium hydroxide. The formation of MgOH2 will cause less precipitate to be formed. The constant stirring and slow addition of ammonia aimed to lower the formation of magnesium hydroxide. Another one is the acidity of the solution. Since a change in acidity will affect the amount of HPO42− , a solution that is too basic or too acidic will cause less precipitate to be formed. In precipitation gravimetric analysis, the first step is to convert the analyte into a sparingly soluble precipitate. [2] The phosphorus in the sample was precipitated using water, ammonia and magnesium ions as shown in the reaction below: 2− 4
5H 2O + HPO + NH MgNH 4 PO4 • 6 H 2O (s)
+ 4
2+
-
+ Mg + OH --> (1)
Ammonium chloride, NH4Cl, may not be used in place of ammonia, NH3, because it will cause the formation of MgCl2 and the precipitate will not be formed. For 15 minutes, the solution was left to digest. This step helped to improve the purity and filterability of the precipitate. [3] The precipitate was then washed with ethanol which was polar in nature so that any polar impurities would dissolve and be removed from the precipitate. CONCLUSION The experiment showed that the actual amount of phosphorus in fertilizers is lower than the amount indicated in the package. This illustrates how important it is to determine the
Through gravimetric analysis, the percent phosphate was determined and compared against the amount indicated on the package. The experiment was able to fully use the concepts in precipitation gravimetry. REFERENCES [1-2] Crouch, S.; Holler, F.; Skoog, D.; West, D. Fundamentals of Analytic Chemistry, 8th ed.; Harris: Canada, 2004; p. 336 [3] Crouch, S.; Holler, F.; Skoog, D.; West, D. Fundamentals of Analytic Chemistry, 8th ed.; Harris: Canada, 2004; p. 341 APPENDIX 1.1 Data Trial Sample weight (as received), g* Sample weight (dried), g* Weight of filter paper, g Weight of air-dried filter paper with precipitate, g Net weight of dried precipitate, g Trial Sample weight (as received), g* Sample weight (dried), g* Weight of filter paper, g Weight of air-dried filter paper with precipitate, g Net weight of dried precipitate, g
1 3.0 29 3.0 188 1.4 251
2 3.1 625 2.9 434 2.9 187
2.6 14
3
2.9 434 1.2 653
4 3.0 001 2.8 905 1.0 64
5 2.9 958 2.9 025 1.1 661
3.2 666
2.4 236
1.9 515
2.2 265
1.1 889
0.3 479
1.1 583
0.8 875
1.0 604
6 3.0 063 2.9 087 1.1 554
7 3.0 771 2.9 961 0.9 315
8 3.0 056 2.9 159 0.9 424
9 3.0 001 2.9 074 1.11 37
10 3.0 781 2.9 93 1.1 312
2.2 705
2.1 411
2.1 326
2.4 369
2.3 737
1.11 51
1.2 096
1.1 902
1.3 232
1.2 425
3
FM P2O5 1 m ppt × × 2 FM MgNH 4 PO4 •6 H 2O
1.2 %P
mwetfert
%P was calculated in excel using the following formulas:
m ppt × %P (dry)=
%P(wet)=
FM P
The results were as follows:
FM MgNH 4 PO4 •6 H 2O
mdryfert FM P m ppt × FM MgNH 4 PO4 •6 H 2O mwetfert
× 100% (2)
× 100%
(3)
1 4.96 5198 4.98 1975 4.97 3586 0.01 1863 6 4.69 2151 4.84 9594 4.77 0873 0.111 329
Average Standard Deviation Trial %P (wet basis) %P (dry basis) Average Standard Deviation
2 1.39 16 1.49 5188 1.44 3394 0.07 3247 7 4.97 2682 5.10 7119 5.03 9901 0.09 5061
3 4.88 4165 4.97 8085 4.93 1125 0.06 6411 8 5.00 9326 5.16 3425 5.08 6375 0.10 8964
4 3.74 2167 3.88 406 3.81 3113 0.10 0333 9 5.57 9307 5.75 7199 5.66 8253 0.12 5788
5 4.47 7622 4.62 1554 4.54 9588 0.10 1775 10 5.10 6275 5.25 1462 5.17 8868 0.10 2663
1.3 %P2O5 %P 2O5was calculated in excel using the following formulas: %P2O5(dry)=
FM P2O5 1 m ppt × × 2 FM MgNH 4 PO4 •6 H 2O mdryfert %P2O5(wet)=
Trial %P2O5 (wet basis) %P2O5 (dry basis) Average
The results were as follows: Trial %P (wet basis) %P (dry basis)
× 100% (4)
× 100% (4)
Standard Deviation Trial %P2O5 (wet basis) %P2O5 (dry basis) Average Standard Deviation
1 11.3 5127 11.3 8962 11.3 7044 0.02 712 6 10.7 2704 11.0 8698 10.9 0701 0.25 4516
2 3.18 1429 3.41 8247 3.29 9838 0.16 7456 7 11.3 6838 11.6 7572 11.5 2205 0.21 7326
3 11.1 6601 11.3 8073 11.2 7337 0.15 1827 8 11.4 5215 11.8 0445 11.6 283 0.24 911
4 8.55 5215 8.87 9606 8.71 741 0.22 9379 9 12.7 5522 13.1 6191 12.9 5857 0.28 7573
5 10.2 3659 10.5 6564 10.4 0111 0.23 2675 10 11.6 7379 12.0 0571 11.8 3975 0.23 4704
