Question

GRAVIMETRIC ANALYSIS OF A PHOSPHORUS-CONTAINING FERTILIZER 1. How would adding too little MgSO4 solution affect your results? Explain. 2. If the solution is too basic (too much ammonia is added), there is the possibility of forming magnesium hydroxide. How would this affect your results (you may want to consult the solubility rules)? Explain.

Answer 1

phosphorous is one of the main component of fertilizers (NPK i.e. nitrogen, phosphorous and potassium) present in the form of P₂O₅.

Phosphorus content in the fertilizers can be measured by gravimetric analysis wherein it is converted to MgNH₄PO₄.6H₂O as shown in the reaction equation

Mg²⁺(aq) + NH₃(aq) + HPO₄^2-(aq) + 6H₂O ---------> MgNH₄PO₄.6H₂O.

in order to measure the exact content of phsphorous in fertilizers, all of the P is ensured to convert to MgNH₄PO₄.6H₂O using excess of MgSO₄ and then the salt is weighed accurately.

(i) If we add too little MgSO₄, then all of the phosphorous will not convert completely to MgNH₄PO₄.6H₂O and will lead to erroneous results. The analysis would then show the lower amount of P than actually it is present in the fertilizer sample.

(ii) If the solution is too basic, there is the possibility that Mg²⁺ ions will form magnesium hydroxide Mg(OH)₂ (it has a low solubility in water). So, There would not be enough Mg²⁺ to form complexes with P. The analysis would then show the lower amount of P than actually it is present in the fertilizer sample.

Incase if Mg(OH)₂ co-precipitates along with MgNH₄PO₄.6H₂O (depending on the conditions), it would lead to higher weight than actullay it should be. so, the amount of P from the calculation be higher than actually it is present in the fertilizer sample.