Gravimetric Determination of Phosphorus in Plant Food.

Essay by dlindley November 2005

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Abstract: Gravimetric analysis can be used to determine the percentage of phosphorus in plant food. A precipitant of known composition is produced and weighed to find percent of phosphorus in compound. From the mass and known composition of the precipitate, the amount of original ion can be determined.

Introduction: Gravimetric analysis is based on the measurement of mass. Gravimetric analysis involves isolation of an ion in solution by a precipitation reaction, filtering, washing the precipitate free of contaminants, conversion of the precipitate to a product of known composition, and finally weighing the precipitate and determining its mass by difference. From the mass and known composition of the precipitate, the amount of the original ion can be determined. Theodore W. Richards (1868-1928) and his graduate students at Harvard developed or defined many of the techniques of gravimetric analysis of silver and chlorine. These techniques were used to determine the atomic weights of 25 of the elements, decomposing known weights of the compounds, and determining the chloride content by gravimetric methods.

Purpose: To find the percent of phosphorus in known compound MgNH4PO4*6H2O using gravimetric analysis.


1. Weigh by difference to the nearest hundreth gram .75 of your unknown sample, using weighing paper. Using solualbe Expert plant food label percentate 30% P2O5.

2. Transfer the sample quantitatively to a 250-ml beaker and record the sample mass.

3. Add 18 ml of distilled water and stir the misture with a glass stirring rod to dissolve the sample.

4. To the filtrate add about 45ml of 10% MgSO4*7H2Osolution.

5. Then add approximately 150ml of 2 M NH3(aq) while stirring. A white precipitate of MgNH4PO4*6H20will form. Allow the mixture to sit at room temperature for 15 minutes to complete the precipitation.

6. Collect the precipitate on a preweighed piece of filter paper.

7. Obtain a...