Gravimetric Determination of Moisture in Fertilizer Samples

Gravimetric Determination of Moisture in Fertilizer Samples

Theo Victor A. Cavite

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Department of Chemical Engineering, College of Engineering, University of the Philippines Diliman Quezon City 1101 Philippines

Results and Discussion

Gravimetric analysis utilizes the final mass of the product to infer for the mass of the analyte- for this experiment, the moisture content of the fertilizer samples.

The first part of the experiment was the determination of the mass of the crucibles. This was done by a repeated process of oven drying, desiccator cooling, and balance weighing until a constant mass is attained. For this matter, an observed difference of at most 0.0003 g between any two successive weighing indicates that the crucible’s mass is already constant. This assumption is valid since the utilized weighing scale, the analytical balance, has a tolerance of 0.0002 g.[9] Hence, any successive weighing would propagate a reading uncertainty of 2.8284 x 10-4 g that is roughly equal to 0.0003 g.

Every time the crucible is removed from the oven for weighing, it is first stored in the desiccator to prevent it from absorbing further moisture present in the air while it acclimatizes to the room temperature. If this particular step will not be carried out, the uncorrected temperature difference between the crucible and its surrounding area would create convection currents within the balance enclosure. In effect, these air currents apply a net downward force on colder samples while an upward force is observed on hotter samples. This phenomenon, coined as dynamic buoyancy, further suggests that colder objects appear to weigh more while hotter ones weigh less.[8] However, this does not mean that the sample’s actual mass changed- only its apparent mass did.

Once the crucible’s constant mass was already known, the investigators prepared a homogenized fertilizer sample through grinding. With the aid of such method,...