GRAVIMETRIC DETERMINATION OF MOISTURE AND MOISTURE AND PHOSPHORUS IN FERTILIZER SAMPLE
A.L. BULIGAN 1INSTITUTE OF CHEMISTRY , COLLEGE OF SCIENCE 2DEPARTMENT
OF FOOD SCIENCE AND TECHNOLOGY, COLLEGE OF HOME ECONOMICS
, DILIMAN , QUEZON CITY , PHILIPINES UNIVERSITY OF THE PHILIPPINES DATE PERFORMED: AUG. 27, 2014 INSTRUCTOR’S NAME: COLLEEN ROSALES
ABSTRACT Determination of %moisture, %phosphorus and %phosphorus pentoxide content of a fertilizer sample is the main goal of the study. The experiment uses the principle of gravimetric analysis. The sample undergoes repeated heating and cooling process in an oven at constant temperature at 110 OC and a desiccator respectively. A precipitation of MgNH 4PO4*6H 2O was also observed to determine the final weight of the sample. A % moisture of 2.6250±0.01% was obtained which is within range of the accepted value in the market. An approximate of 10% and 24% of %P and % P2O5 respectively was also obtained in this experiment which is also within the range of the accepted value in the fertilizer market.
INTRODUCTION Gravimetric analysis is a quantitative analysis in which it can determine the amount of species in a material by converting it to a product that we can isolate and weigh completely [1]. This can be classified classified into two types: precipitation precipitation and volatilization. Under precipitation method, you will separate the specie thru precipitation reaction, filtering, washing the precipitate and conversion to a product of known composition. In volatilization method, the specie that is being studied is volatilize at a suitable temperature, collected then weighed.
Fertilizers contain essential nutrients that are necessary for plant growth such as phosphorus, nitrogen and potassium. In particular, phosphorus when applied to soil helps promote root growth, winter hardiness, stimulates tillering, and often hastens maturity and it also and aids in maturity photosynthesis under Calvin Cycle [2]. In this experiment, the moisture content of the fertilizer sample was determine by oven drying the sample to a constant temperature at 110 OC. Also in this
experiment, the phosphorus content of the fertilizer was measure by precipitating the fertilizer sample as magnesium ammonium phosphate hexahydrate, MgNH 4PO4*6H2O. The chemical reaction is given as follows:
A 500ml of 10% (w/v) MgSO 4*7H2O solution was prepare and three – 500ml of 2M NH3 solution.
A fluted filter paper was prepare and was weigh. The dried sample was place in a clean and dry beaker. This was dissolve with 40ml of distilled water. The resulting solution was filter afterwards using a different filter paper which is not weighed. Discard the filter residue. After filtration, 45ml of 10% MgSO 4 · 7H2O was added to the filtrate. 150ml of 2M NH3 was added slowly while stirring. The solution was covered with a watch glass and was allowed to precipitate for 15mins in the fume hood. The solution was filter using the pre-weighed filter paper. The filter paper was wash with 5ml distilled water twice and was wash again with 5ml of 95% ethanol. The filter paper with the fertilizer sample was place in a beaker and was to the oven at 110OC for 1 hour. It was then transferred to the desiccator for 15mins. This was weigh using an analytical balance and the weigh was recorded.
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METHODOLOGY A. Preparation and Constant Weighing of Crucibles Crucible with cover was thoroughly wash and clean. This was place in the oven at 110OC with the cover slightly ajar and was kept until the next period. The crucible was handle using crucible tongs and a watch glass throughout the experiment. The crucible was transfer to a desiccator and was allow to cool for 15minutes. The weight was was then obtained using an analytical balance. After weighing, the crucible with cover was transfer back to the oven at 110 OC for 30mins to dry. The cover was left ajar. The cooling and heating process was repeat until a constant weight was obtained. B. Moisture Analysis The fertilizer was weigh up to 3.0g to the nearest 0.1mg into the constantly weigh crucible using the technique weighing by difference. This was then placed back to the oven at 110 OC for 1 hour with the cover slightly ajar. The crucible was then transferred to the desiccator and was allow to cool for 15mins. The crucible with the fertilizer was weigh. After weighing, it was transfer back to the oven with the same temperature for 30mins. This process of cooling and heating was repeat until a constant weigh was obtained. C. Preparation of Solutions
D. Precipitation of MgNH 4PO4*6H2O
RESULTS AND DISCUSSION The whole experiment consists of two parts; the first one is to measure the moisture content of the sample and the second one is to obtain the phosphorus content of the sample. This goal was achieve using gravimetric analysis. The first first step step of the experiment was done by taking the constant weight of the crucible. This was place in the oven at 110OC for 30mins to dry then cooled for 15mins in the desiccator. The cooling of the crucible is crucial to avoid errors caused by air currents. A buoyant effect on the pan and the object was exert by convection currents within the balance. Warm air inside inside the crucible is lighter compare to the same
volume at a lower temperature [3]. Using crucible tongs is also important in this experiment because handling the crucibles with our bare hands may affect the result of the data. Our skin contains moisture that will adhere to the crucible itself. After the initial weigh in, the whole heating-cooling-weighing is repeated until a constant weight was observed. A constant weight will be obtain if the next weight is within the range of ±0.0003 from the previous recorded weight. This implies that no more moisture can be remove from the crucible. This assumption is based on the uncertainty of the analytical balance: (1)
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Table 1. Constant Weighing of the Crucible Weighing # Weighing 1 Weighing 2 Weighing 3 Constant weight
Weight (g) 32.4820±0. 0002 32.4813±0. 0002 32.4813±0. 0002 32.4813±0. 0002
After getting the constant weight of the crucible, 3.0171grams of the fertilizer sample was obtained using the technique weighing by difference. This technique is done by noting the initial mass of the container holding the fertilizer sample then noting the mass again after transferring the sample to the constant weighed crucible. The mass decrease of the container is then recorded as the mass of the sample obtained. This technique also prolongs the lifetime of the analytical balance by weighing light vials instead of heavy Erlenmeyer flasks since the balance can only hold up to 200 g. This was use instead of the direct method because of the amount of the sample might not be transferred completely due to intermediate objects like watch glass. The crucible was then heated in the oven 110 OC for an hour
with the cover slightly ajar. The cover was place slightly ajar so that the moisture in the sample can evaporate. It was then transferred to the desiccator for 15mins to cool. After cooling, the crucible was weigh. Table 2. Constant Weighing of Crucible with Sample
Weighing 1 Weighing 2 Weighing 3 Weighing 4 Constant weight of crucible + sample Net weight of dried sample
Weight 35.5011±0.0002 35.4675±0.0002 35.4665±0.0002 35.4192±0.0002 35.4192±0.0002 2.9379±0.0002
The mass of water which is 0.079200±0.0003 was also obtained by subtracting the initial amount of the sample to the net weight of the dried sample. The percent moisture can be calculated using the formula: %moisture =
A % moisture of 2.6250 ± 0.01 was obtain using the formula above. Aside from the oven drying method of determining the moisture, there is also a different method which is the direct method of moisture determination. The direct method may use a moisture balance, which is faster and more accurate, compared to an oven drying, which takes many hours to complete. A moisture balance also weighs and heats a sample simultaneously, reducing errors caused by moisture evaporation during transport and weighing. The second part of the experiment took place after obtaining the %moisture of
the sample. A fluted filter paper was made because the filtrate is much more important than the residue. After dissolving the sample using distilled water, it was then filtered. 45ml of 10% MgSO4·7H2O was added to the filtrate. 150 mL of 2M NH 3 solution was then added slowly while stirring. This now is the precipitation part of the gravimetric analysis. Slowly addition of NH 3 avoids the supersaturation which is a requirement in lowering the Von Weimarn ratio. Supersaturation varies inversely with the particle size [4]. It means that a small magnitude of supersaturation means having a smaller magnitude for the Von Weimarn Equation:
where: Q = concentration of reactants before precipitation. S = solubility of precipitate in the medium from which it is being precipitated. precipitated.
Thus resulting to a larger particle size of the precipitate. A small magnitude for R is required so that a larger precipitate will form. NH4Cl was not used instead of NH 3 because the digestion process requires a basic substance wherein NH 4Cl is acidic. The beaker was placed under a fume hood for about 15mins. This part of the the experiment is called digestion. It promotes slow recrystallization of the precipitate. Digestion of crystalline precipitates for some time after formation frequently yields a purer, more filterable product [4]. This was then filtered using the fluted filter paper and was wash twice using distilled water and 95% ethanol. Ethanol was used because the precipitate formed is insoluble to ethanol which is a nonpolar substance. Ethanol also displaces water and helps to promote the drying of precipitate since it can easily evaporate.
The filter paper was place in a beaker and was put inside the oven at 110 OC for 1 hour. This was done so that the filter paper together with the sample will dry completely. It was allow to cool inside the desiccator for 15mins. This was then weighed and recorded. Table 3. Sample Size / Weight of Filter Paper / Weight of Precipitate
Sample weight (as received) Sample weight (dried) Weight of filter paper Weight of filter paper + precipitate Net weight of dried precipitate
Weight (g) 2.9379±0.0002 3.0171±0.0002 0.7426±0.0002 3.2224±0.0002 2.4798±0.0002
After obtaining these data, %phosphorus (%P) and %phosphorus pentoxide (%P2O5) in a dry and wet basis using the following formulas:
Using the formula following data were obtained:
above,
the
Table 4. Reported Values %P (wet) %P (dry) %P2O5 (dry) %P2O5 (wet)
10.653±0.006 10.374±0.06 24.411±0.006 23.771±0.06
By law, fertilizer sold in the market should have a label guaranteeing the percentage of nutrients. Three combinations (fertilizer grade or analysis) on the product identify percentages of nitrogen (N), phosphate (P2O5), and potash (K2O), respectively [5]. For example, a 20-10-5 fertilizer contains 20% nitrogen, 10% phosphate, and 5% potash. With the data obtain in the experiment; it can be identify what is the ratio of the nutrient inside the fertilizer. CONCLUSION AND RECOMMENDATION Determination of %moisture, %P, %P2O5 was carried out properly in the whole experiment. % moisture of 2.6250% + 0.01 is within the accepted value of 1-10%. In this experiment, a value of %P which is 10% and a value of 23-23% for %P 2O5 were obtained. Since there is no accepted value of %P and % P2O5 that is being used by fertilizer manufacturers and the amount that is being used depends on the amount of the other nutrients [6]. Sources of error in this experiment would be incorrect weighing technique since there’s a possibi lity that the sample might get spill during the transferring from the desiccator to the oven and analytical balance. The temperature of the surrounding may not be optimal. One source of error could also be the time used for the precipitate to form. The 15mins allotted might not be enough for the large precipitate to form since it could still be seen that some smaller precipitate passed through the filter paper. It
could be recommended that the direct method which is faster and more accurate could be used instead of oven-heating method to determine the moisture content of the sample. This would have a lesser error propagation. REFERENCES [1] Ebbing, Darrell ; Gammon, Steven, General Chemistry, Enhanced Edition , 2011 [2] Soils - Part 6: Phosphorus and Potassium in the Soil. Plant and Soil Sciences eLibrary [3] Harris, D. (2007). Quantitative Chemical [3] Analysis. (8th ed.).New York:W.H. Freeman and Company [4] Skoog, D., West, D., Holler, F. J., & Crouch, S. (2004).Fundamentals (2004).Fundamentals of analytical th chemistry . (8 ed.). Canada: Brooks/ColeThomson Learning. [5] Understanding Fertilizers. Colorado State University. http://www.ext.colostate.edu/mg/gardennot es/232.html [6] The Basics of Fertilizer Calculations for Greenhouse Crops. Virginia Cooperative Extension. http://pubs.ext.vt.edu/430/430-100/430100.html
