1. 2. 3.
At the end of the experiment, experiment, the student student should be able to: to: Perform proper gravimetric steps in order to obtain pure and filterable precipitate suitable for weighing. Relate the experiment to analyte separation by precipitation. Do a quantitative determination of phosphorus phosphorus in fertilizer and similar samples using gravimetric method of analysis.
Fertilizer samples contain soluble compounds of nitrogen, phosphorus and potassiu potassium m which which are considere considered d essential essential nutrients nutrients that maybe maybe lacking lacking in soils. soils. Labels on fertilizer packages usually have a set of numbers such as 15-30-15. These numbers indicate that the fertilizer is guaranteed to contain at least 15% nitrogen, 30% phosphorus (expressed as P 2O5) and 15% potassium (expressed as K 2O). The rest of the sample is composed of counter-ions and dyes to serve as fillers and to provide pleasing colors, respectively. To ensure ensure good quality quality of fertilizers for consumers, consumers, it is imperative imperative that the levels of these nutrients be determined. Analysis of these nutrients, however, does not req requir uire e highly highly accura accurate te result resultss so that that the choice choice of fast fast and inexp inexpens ensive ive gravimetric analysis is suitable for its purposes. In this experiment, the soluble form of phosphorus in plant food will be precipitat precipitated ed as magnesiu magnesium m ammonium ammonium phosphate phosphate hexahydra hexahydrate, te, MgNH MgNH4PO4∙6H2O, according to the reaction: 5H2O + HPO42- + NH4+ + Mg2+ + OH-
→
MgNH4PO4∙6H2O(s)
From From the consta constant nt weigh weightt of the pre precip cipita itate, te, the per percen centag tage e phosp phosphor horus us and percentage diphosphorus tentoxide will be calculated using gravimetric factors.
filter papers iron stands
pair of scissors iron clamps
Magnesium sulfate pentahydrate crystals, MgSO4∙7H2O concentrated ammonia, NH4OH 95% ethanol, C2H5OH
250-mL beakers 100-mL volumetric flasks 1000-mL volumetric flasks 5-mL volumetric pipets 10-mL volumetric pipets
graduated cylinder watch glass funnels dessicator spatulas
analytical balance oven
1. 2.
Prepare the following solutions quantitatively: 100 mL of 10% (w/v) MgSO4∙7H2O solution 1000 mL of 2 M NH3 solution
1. Take the weight of the filter papers to be used in filtering the precipitate during the latter part of the experiment (Step 8). Perform two (2) trials and record the weights in your data sheet. Keep the filter papers in a desiccator while not in use. 2. Record the wet and dry weights of the fertilizer samples used for moisture determination in your data sheet and quantitatively transfer the dried samples into two (2) separate clean and dry 250-mL beakers. 3. Add 40 mL of distilled water and stir to dissolve the sample. 4. If the sample does not dissolve completely, filter the mixture such that a clear filtrate is obtained. The filter paper to be used in this part need not be weighed. 5. Add about 45 mL of 10% MgSO4∙7H2O solution to the filtrate. 6. Then, slowly add 150 mL of 2 M NH 3 while stirring. A white precipitate is expected to form. 7. Allow the mixture to stand at room temperature for 15 minutes to complete the precipitation process. (Note 2) 8. Quantitatively transfer the precipitate to the pre-weighed filter paper and wash the precipitate with two (2) 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 filter paper. 9. Wash the precipitate again, this time, with two (2) 10 mL portions of 95% ethanol. 10. Spread the filter paper on a watch glass and store in the locker until the next period. 11. On the next period, place the filter paper with the precipitate in the oven set at 110oC for 1 hour. Cool for 15 minutes inside a desiccator and weigh. Record all data in your data sheet.
1. Filtrate may be colored depending on the possible presence of dyes in the original sample. 2. This is the digestion step. It is a process in which the precipitate is allowed to stand in contact with the mother liquor for some time before filtration.
Dispose all excess solutions into the waste bottle. Never dispose your chemical waste into the sink.
Calculate the: 1. Amount of phosphorus in the fertilizer sample as %P and %P2O5 on a dry and wet basis. 2. Mean, median, range, relative standard deviation (in ppt), coefficient of variation, pooled standard deviation, and confidence limits (95% confidence level).
Discuss: 1. The principle behind gravimetric analysis of phosphorus in the fertilizer sample (describe the steps involved in a gravimetric analysis and describe how each step is applied in the experiment). 2. The significance of digestion in a gravimetric analysis. 3. Possible sources of errors and their effect on phosphorus determination. 4. How the result of your group correlate to those obtained by the other groups.
Paper No. Weighing 1 Weighing 2 Weighing 3 Weighing 4 Constant Weight, g
1
2
3
Sample No. Sample Wet Weight, g Weighing 1 Weighing 2 Weighing 3 Weighing 4 Constant Weight, g % Moisture Sample Weight, g
1
2
3
Sample No. Weighing 1 Weighing 2 Weighing 3 Weighing 4 Constant Weight of filter paper + precipitate, g Constant Weight of filter paper, g Net Weight of Precipitate, g
1
2
3
Sample No. Mass of P in original sample % P in the sample % P2O5 in the sample Average % P2O5
1
2
3
