Gravimetric Determination of Phosphorus in Plant Food

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Gravimetric Determination of Phosphorus in Plant Food

Abstract:

        Gravimetric analysis can be used to determine the percentage of phosphorus in plant food. A precipitant of know composition is produced and weighed to find percent of phosphorus in compound. From the mass and known composition of the precipitate, the amount of the original ion can be determined. In doing so, percents of phosphorus and average percent phosphorus of sample plant food were determined.

Introduction:        

        Gravimetric analysis is a quantitative method of classical analysis. The element to be determined is isolated in a solid compound of known identity and definite composition. The mass  of the element that was present in the original sample can be determined from the mass of this compound. Plant foods contain three essential nutrients that are not readily available from soils. These are soluble compounds of nitrogen, phosphorus, and potassium. A typical label on a plant food will have a set of numbers such as 15-30-15. These numbers mean that the plant food is guaranteed to contain at least 15% nitrogen, 30% phosphorus (expressed as P2O5) and 15 % potassium (expressed as K2O). The remaining of the product is fillers, dyes and other anions and cations to balance the charge in the chemical compounds. In this experiment, we will illustrate aquality control analysis for the determination of phosphorus in plant food by gravimetric analysis. Phosphorus will be determined by precipitation of the insoluble salt magnesium ammonium phosphate hexahydrate according to the reaction:

5H2O(l)   +   HPO42(aq)   +   NH4+(aq)   +   Mg2+(aq)    +   OH-(aq)     →    MgNH4PO4 · 6H2O(s)

The %P and %P2O5 in the initial sample can be calculated from the mass of MgNH4PO4 · 6H2O(s)

obtained using the following method:

               mass of MgNH4PO4 · 6H2O(s)  →   moles of MgNH4PO4 · 6H2O(s)  →  moles of P  →  mass of P  →  %P

                                                and

mass of MgNH4PO4 · 6H2O(s)  →  moles of MgNH4PO4 · 6H2O(s) →  moles of P →  moles of P2O5 →  mass of P2O5 →  %P2O5

        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.

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         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,  MgNH4PO4•6H2O. [The “•6H2O” 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 ...

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