Gravimetric Determination of Phosphorus in Fertilizer Samples
More than 50% of fertilizers are composed of nitrogen, phosphorus and potassium because these three nutrients are essential but are deficient in soils. To ensure good quality of fertilizers, it is significant that the amounts of these nutrients are quantified. In this light, gravimetric principles are of much use. The objective of the experiment is to determine the percentage of phosphorus and diphosphorus pentoxide in fertilizer samples. Gravimetric analysis follows only a few fundamental steps.
Here, the quantity of the analyte is determined by separating it physically from the system. To do this, it should be converted to an insoluble substance. In the experiment, the phosphorus of the fertilizer sample was precipitated as magnesium ammonium phosphate hexahydrate. It follows the equation, 5H2O(l) + HPO42-(aq) + NH4+(aq) + Mg2+(aq) + OH-(aq)> MgNH4PO4• 6H2O(s) (1) The precipitating reagent used is ammonia, NH3. Ammonia is used instead of ammonium chloride because the latter produces Cl- ions that may react with the Mg2+ ions found in the solution, thus, orming MgCl2(s). This can interfere to the production of the desired precipitate.
Forming the precipitate is critical. First and foremost, the precipitate formed should be free from impurities. Also, the size of the particles (making up the precipitate) should be considered. In gravimetric analysis, relatively few crystals are preferred over many small ones. Experimentally, it is found that particle size is affected by experimental variables such as precipitate solubility, reactant concentrations in the precipitating solution, the rate of addition and mixing of reactants, and the temperature.
Mathematically, it is reflected in the Von- Weimarn equation, which states that R=Q-SS (2) where Q is the concentration of the solute at any instant and S is the equilibrium solubility. It is seen from the equation that it is most favorable when the supersaturation ratio is at its smallest finite value. This implies that Q should be relatively higher than S. This entails mixing the reagents rapidly while the precipitant is being added slowly. This is the principle behind the slow addition of the precipitating agent, NH3 to the filtrate in the experiment.
There are cases where, no matter how much effort we exert, the supersaturation ratio cannot be maintained to avoid the production of a colloidal suspension. Fortunately, these particles can be made to coagulate to bigger and more filterable ones. In the experiment, stirring was done while adding NH3. This is done because it reduces the volume of the electrical double layer. This eventually results to higher energies of the ions in the counter-ion layer. Due to their higher energies, the ions tend to move closely to the ions in the primary layer. This attraction makes particles to coagulate.
After the precipitate was formed, it was allowed to stand at room temperature. This completes the precipitation process. As is said, forming the precipitate is critical. Often, impurities are present in the precipitate because the process of producing it, sometimes, is too fast to get such impurities out of the way. The process of allowing the precipitate to stand or heating it is called digestion. Digestion gives rise to bigger and purer particles. During digestion, crystals or parts of crystals that have great surface area or those that have lattice imperfections are most likely to dissolve.
Filterability improves as the crystal size increases. Larger particles have less surface area per unit volume and therefore, lesser absorbed impurities. After digestion, the precipitate was filtered and was rinsed with water and 95% ethanol. Rinsing the precipitate with water also removes impurities. Much of the electrolyte in the counter-ion layer is removed. As this happens, the size of this layer increases and the strong repulsive forces, which make the original colloidal state, are reestablished. These end up in the loss of colloidal particles.
On the other hand, rinsing the precipitate with ethanol avoids a process called peptization to occur. Here, coagulated colloids (precipitate) revert to its original dispersed state. This is caused by the aggregate repulsions between the ions in the electric double layer and the counter-ion layer. Rinsing the precipitate with ethanol lessens the repulsions by somehow neutralizing the system, hence, lowering the charges on it. Figure 1. An Ethanol Molecule Then, the precipitate was then put into an oven. During this process, the moisture and other volatile components of the precipitate are removed.