Gravimetric Determination of Moisture and Phosphorus in Fertilizer Samples Lourdes Lyn Amarillo , Alyanna Patricia Angulo, Mareah Karlene Chelsey Mac aspac Institute of Chemistry, University of the Philippines, Diliman, Quezon City Aug - Sept 2, 2014 September 2014 Abstract The experiment aims to determine the moisture and Phosphorus content of sample fertilizers using gravimetric analysis which involves constant weighing, heating and cooling of the samples. The percent moisture, phosphorus and P2O5 were determined by using the constant weight of the dry precipitate and are as follows: %moisture: 2.67%, %Pdry: 5.95%, %Pwet= 5.78, %P2O5dry: 13.64 and %P2O5wet= 13.27%. The data goes to show the content of the fertilizer samples. I. Introduction
As an agricultural country, the Philippines largely depends on plant cultivation to sustain its economy[1]. Plants require certain essential nutrients for it to grow fully, however most soils lack these essential nutrients which leads to lesser food production. To compensate for the lack of nutrients, fertilizers have been aiding farmers and plant growers by supplying these essential nutrients, such as Nitrogen, Phosphorus and Potassium (N-P-K). Fertilizer labels usually have a set of numbers such as 15-30-15 which indicates that the fertilizer contains at least 1 5% Nitrogen, 30% Phosphorus(as P2O5) and 15% Potassium (as K20)on a dry basis. Knowing the levels of t hese nutrients can ensure better product yie lds which is why it is important to know them. The experiment uses gravimetry, specifically volatilization and precipitation, to determine the Phosphorus content of sample fertilizers.. The term “gravimetric” refers to a method that determines the amount of an “analyte” by either isolating it in i ts pure form or reacting it to form a compound of known chemical composition.[2] The gravimetric methods suffers in some ways such as being tedious or time consuming but a large amount of sample can be tested simultaneously. [2]
One clean crucible was set in an oven at 110° with its lid slightly ajar for two days. With a pair of crucible tongs, the crucible was moved to a desiccator for 15 minutes to dry. It was w as then weighed by using weighing by difference in an analytical balance. balance. The procedure was repeated until a constant w eight was obtained. Handling the crucibles with bare hands is strictly forbidden because it adds moisture. After obtaining a constant weight a 3.0 g fertilizer sample was added, again by using weighing by difference. Also, in handling the fertilizer sample vial, paper tongs were used instead of handling them with bare hands. The crucible was placed again in an oven at 110°, with its cover slightly ajar, for two days. After placing in the oven, the crucible was set in the desiccator to dry for 15 minutes and was weighed. The cycle was repeated until constant weight was achieved. A filter paper was pre-weighed and kept inside the lockers. The now-dried fertilizers were transferred to a beaker and 40 mL of distilled water was added to dissolve the fertilizer samples. It was then filtered and 45 mL of 10% (w/v) MgSO4-7h2O was added to the filtrate. 50 drops of the pre-prepared NH3 was added slowly into the solution. For the next 100 drops the drop rate was increased. Once the white precipitate precipitate formed it was allowed to stand for 15 minutes. This is the 'digestion step'.
II. Methodology
A 500 mL solution of 10% (w/v) MgSO4-7h2O and three solutions of 500 mL 2M NH3 we re prepared before hand and stored in PET bottles except for the NH3 solutions. These were used for the latter parts of the experiment.
The solution was filtered into the pre w eighed filter paper and washed with 10 mL o f distilled water and two 10 mL portions of 95% ethanol. The precipitate was then blow dried and set into the oven at 110° for one hour. It was placed inside the desiccator for 15 minutes to dry and then weighed.
All excess solutions were flushed down the sink with copious running water and the precipitates were thrown in the trash bin.
Table 1.1 Constant Weights
Object(s) weighed
Weight (g)
Crucible
32.1654 ±.0002
III. Results and Discussion
Wet Fertilizer
3.006 ±.0002
The experiment made use of the process of constant weighing and heating to determine the moisture sample in the fertilizers.
Weight of Crucible + fertilizer
35.0896 ±.0002
Filter paper + fertilizer
2.6075 ±.0002
Filter paper
1.2274 ±.0002
Dried Fertilizer
2.9242 ±.0004
The quality of a fertilizer depends somewhat on its moisture content as it determines the quality of the fertilizer. If the fertilizer exceeds or lacks the recommended or ideal amount of moisture it may lead to unwanted caking and/or dust formation. [3] The experiment makes use of constant w eighing which involves a cycle of heating using an oven, cooling in a desiccator and weighing using an analytical balance, to determine the true weight of the sample analyzed and reproduce these values which, in turn, will lessen the error. The reason for the use of constant weighing deals with the property of some materials and solids, such as hydrated ones, to capture w ater present in the air. Once the sample is exposed to air, it will gain weight because of the moisture they absorb. Humidity and temperature vary constantly meaning the sample composition may also vary during different measurements. [5] Before weighing the sample fertilizer, it was first ground by using a small spatula in order to increase the surface area of the sample which in turn releases the occluded water and exposes the adsorbed water, making the water vaporize easier inside the oven. The sample fertilizers were transferred into the crucible and weighed by using the technique we ighing by difference. This technique provides a relatively smaller error propagation during weighing. Deviations from the true weight caused by spilling is lessened because the transfer of the sample from the vial to the crucible are performed outside the balance. Furthermore it is advantageous because the vial, wh ich holds the fertilizers, is exposed to the air less frequently. [4] Table 1.1 shows the results obtained from constant weighing.
The weight of the dried fertilizer was obtained by subtracting the weight of crucible from the weight of the crucible with the sample. The sample is said to be at constant weight when the values from the reading differ by a margin of .0003g. Since the analytical balance reports values to a precision of ±.0002, the calculated error propagation is ±.0003. Table 1.2 summarizes the moisture content of the sample. Table 1.2 Mass and Percent of Water
Object Weighed
Weight
Mass of water
.0804
Percent Moisture
2.67 %
The mass of the water was obtained by subtracting the weight of the dried fertilizer from the weight of the wet fertilizer sample. The percent moisture was obtained by dividing the mass of water by the wet fertilizer and multiplying by 100. Table 1.3 shows the P and P2O5 co ntent Table 1.3 P and P2O4 Content
Object Weighed
Weight
Grams P
.174 g
%Pdry
5.95 %
%P wet
5.78 %
Grams p2O5
.399
%P2O5 dry
13.64 %
%P2O5 wet
13.27 %
The grams of P were obtained by using the constant weight obtained from Table 1.1 and by using a gravimetric factor. The %P was obtained by dividing the mass of the P by the dry sample. The same procedures apply to P2O5. It is important to cool the sample first before weighing them instantly while they are still hot because warmer samples creates convection currents that may contribute to more errors. Warm samples tend to become lighter than the true weight due to convective air currents. [4] While adding the NH3 to the solution it is important to add the Nh3 and to constantly stir the solution to avoid co-precipitation of Mg(OH).
The main objective of the experiment which was to determine the Phosphorus and moisture content was successfully obtained. Furthermore data from other experiments do not stray too far from the data obtained. The researcher suggests more care in handling the crucibles because spillage and touching the crucibles causes more errors. Also, it may be more convenient to work in a place where the temperature is just right, not too cold nor too hot, as temperature may affect the weight of the samples.
V. References
[1]Encyclopedia of the Nations. (2007). Retrieved September 6, 2014, from Philippines: http://www.nationsencyclopedia.com/economies/Asiaand-the-Pacific/Philippines-AGRICULTURE.html [2] Gravimetric Determination of Moisture and Phosphorus in Fertilizer Samples. In Analytical
Errors in the weight measurement may have been due to several factors. One is that the sample was spilled while transferring the crucible from the ove n to the desiccator. Another is that the crucible may have absorbed moisture from the watch glass. Also, while grinding the pellets, some of the fe rtilizer samples may have stuck to the spatula.
Chemistry Laboratory Manual. Quezon City.
Apart from successive cooling and heating there are also other ways of to determine the moisture content of fertilizers. presence Thermogravimetric analysis generates a plot of mass as a function of time using a thermobalance. On the other hand, differential thermal analysis measures the temperature difference between the sample and a reference material as a function of temperature that is being controlled. [6]
[4] Harris D. Quantitative Chemical Analysis, 8th ed.; W.H. Freeman and Company: New York, 2010.
IV. Conclusion
The 5.95% and 5.78% indicates the percent Phosphorus present in the dry and wet sample respectively, while the 13.64 % and 13.27 indicates the presence of P2O5 in the dry and wet sample respectively. This means that the sample contains more or less 13-14% of P2O5 and it contains about 6% Phosphorus. The moisture content of the sample is shown to be 2.67%.
[3] International Fertilizer Industry Association. Determination of Moisture in Fertilizers. http://www.fertilizer.org/ifa/layout/set/print/content/ download/99401/1454692/version/1/file/2013_H_rio_ determination-moisture.pdf (accessed Sept. 5,2014)
[5 ]Skoog, D.; West, D.; Holler, F.; Crouch S.Fundamentals of Analytical Chemistry, 9th ed.; Brooks/Cole: California, 2013 [6] . Skoog, D.; Holler, F.; Crouch, S. Principles of Instrumental Analysis, 6th ed.; Brooks/Cole: California, 2007.
VI. Appendix
A. Data Sheet Crucible weight Trial #
Weight of the Crucible
1
32.1654 ±.0002 C. Answers to Questions (unanswered)
2
32.1654 ±.0002
Weight of the wet fertilizer: 3.006 g Crucible + Fertilizer Weight
Data from the other groups are closely related to the data obtained in our experiment.
Trial #
Weight
1
35.1133 ±.0002
2
35.0994 ±.0002
3
35.0974 ±.0002
4
35.0899 ±.0002
5
35.0896 ±.0002
Weight of filter paper: 1.2274 g Weight of Filter paper and fertilizer: 2.6075 g B. Sample Calculations and Equations Equation 2-
2+
1. How do the results of your group correlate with those of other groups?
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5H2O + HPOa + NH4 + Mg + OH -> MgNH4PO4-6 H2O Solution preparation: 500 mL solution of 10% (w/v) MgSO4-7h2O = .1 g MgSO4-7h2O x 500 ml = 50 g [(500mL) (2M) NH3]/ 14.8 M Nh3 = 67.6 mL %moisture: [mass water/wet fertilizer]x 100 = 2.67 Grams P = 1.3801 g MgNH4PO4-6H2O x [FW P / FW MgNH4PO4-6H2O] = .174 g %Pdry = [GramsP/Dry Sple]x 100 = 5.95% %Pwet = [GramsP/wet Sple]x 100 = 5.78% Grams p205 = grams P x [2 FW p2o5/ fw p] = .399 %P2O5 dry = [GramsP2O5/Dry Sple]x 100 = 13.64 % %P2O5 wet = [GramsP2O5/Wet Sple]x 100 = 13.27 %
