Gravimetric Determination of Phosphorus in Fertilizer Samples

Topics: Solubility, Precipitation, Gravimetric analysis Pages: 13 (3085 words) Published: February 15, 2013

Alyssa Erika L Borras Date Due: 18 January 2013
Date Submitted: 18 January 2013

Experiment No. 3:

I. Abstract

The purpose of this experiment is to determinethe Phophorus content of fertilizer samples using Gravimetric Analysis. It involves dissolution of the fertilizer sample, precipitation and heating and cooling of the sample. Pooled standard deviations of the two data sets for % Pwet, %Pdry, %P2O5 wet and %P2O5 dry were 5.1448, 5.1472, 11.8436 and 11.8504 respectvely. During the experiment a data rejection for %P and %P2O5 was concluded at approximately 20% and 47%. Determination of the Phosphorus content in fertilizer is vital in quality assurance and in the choosing of a fertilizer grade important for agriculture.

II. Introduction

Plants require essential nutrients such as Nitrogen, Phosphorus, Potassium or Sulfur to grow. However, most soils lack these macronutrients thus a decrease in food production. In connection with this, the use of fertilizers containing these macronutrients has been part of the agricultural tradition. Fertilizers have in its labels a set of numbers, known as the fertilizer grade that always appear in the form N-P-K, indicates the proportions of essential nutrients present (ie 10-10-10 which means 10% Nitrogen, 10% Phosphate in the form of P2O5, 10% Potassium in the form of K2O).

Different organizations worldwide like Association of American Plant Food Control Officials (AAPFCO) in the United States, conduct different tests to assess the accuracy and consistency of the nutrient content with regards to that of the product label and also to ensure quality assurance as part of consumer rights. Analysis of fertilizer can be conducted using Gravimetric Analysis.

In Gravimetric Analysis, the product with known composition is resulted from the conversion of the analyte to a soluble precipitate that can be easily filtered and is free from impurities by subjecting it to heat. This product of known composition will then be weighed and the percentage of the analyte in the sample can be determined. In this experiment, the analyte, Phosphorus in the fertilizer will be precipitated as Magnesium Ammonium Phosphate Hexahydrate, MgNH4PO4 ( 6H2O:

5H2O + HPO42- + NH4+ + Mg2+ + OH- ( MgNH4PO4 ( 6H2O (s) (1)

The purpose of this experiment is to determine the percentage of Phosphorus (P) and Diphosphorus (P2O5) in the fertilizer sample by the means of Gravimetric Analysis.

III. Data and Results

In this experiment, the wet (as received) and dry weights of the fertilizer sample, obtained from the previous experiment of moisture determination of the same fertilizer sample were used. Weights were divided into two data sets, Data Set 1 having four (4) trials and Data Set 2 having three (3) trials.

Table 1 Wet and Dry Sample Weights
| |Data Set 1 |Data Set 2 | | |1 |2 |3 |4 |6 |7 |8 | |Wet Sample, g |3.0001 ± 0.0002 |3.0144 ± 0.0002 |3.0618 ± 0.0002 |3.0071 ± 0.0002 |3.0007 ± 0.0002 |3.0630 ± 0.0002 |3.0054 ± 0.0002 | |Dry Sample, g |2.9720 ± 0.0003 |3.0004 ± 0.0003 |3.0145 ± 0.0003 |2.9533 ± 0.0003 |2.9516 ± 0.0003 |3.0136 ± 0.0003 |2.9630 ± 0.0003 |

The dried sample from the previous experiment from which moisture was determined, was dissolved and is converted into a sparingly soluble precipitate, MgNH4PO4 ( 6H2O which is then filtered and weighed. The filter paper used was weighed to easily determine the weight of the precipitate. The precipitate was then dried and weighed.

Table 2 Filter Paper and Precipitate Weights
| |Data Set 1...

References: Fischer, R.B. and Peters, D.G. Quantitative Chemical Analysis 3rd ed. 1968; pp42-43;49
Food and Agriculture Organization of the United Nations
Rehm, G. et al. Understanding Phosphorus Fertilizers: Phophorus in the Agricultural Environment. University of Minnesota: Extension. 2010. <> Accessed 17 January 2013.
Skoog, D. et al. Fundamentals of Analytical Chemistry 6th ed. Saunder College Publishing. 1992; pp72-80
= 1.6214 x (30.973761/245.406451) x 100
Sample weight (dried)
= 1.6214 x (30.973761/245.406451) x 100
= 1.6214 x (141.944522/490.812902) x 100
Sample weight (dried)
= 1.6214 x (141.944522/490.812902) x 100
Relative Standard Deviation (RSD) = [pic]x 1000 ppt
= (6.39406/10.9817) x 1000 ppt
= 10.9817 ± [(3.18)(6.3941)/√4]
= 10.9817 ± 10.1666
Q- test = (Xq - Xn)/ R
= (13.5746-11.4727)/ 4.4960
= √[(122.6521-9.6949)/ (4+3-2)]
= 5.1448
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