Question

i was assigned to leach a phosphate rock to come up with phosphoric acid. am having problems with determining the grade of the phosphoric acid or how much phosphate went to the filtrate or pregnant leach solution using a UV microscope

Answer 1

ANSWER:

There are several ways to calculate the amount of P that was leached and resulted in phosphoric acid. The following two are the easiest ones.

1. Standard addition method:

Prepare several stock solutions of phosphoric acid and measure its absorption intensity using UV-Vis spectrophotometer. Then, make a calibration curve plotting the concentration of the stock solutions against the absorption intensity. Now, if you measure the unknown, leached solution's intensity, it may fall anywhere within the calibration curve obtained. Using this, you can find the concentration of phosphoric acid in the filtrate and the amount of P that was actually leached out.

Limitations of this method: Applicable only to transparent solutions of lower concentrations.

2. Annealing method: This is the best method provided that your filtrate contains only phosphoric acid.

Weigh exactly a desired amount of filtrate in a crucible (must be clean and free of impurities and of known weight) and then heat it up to 773 K in an oven until all the water molecules are evaporated and the phosphoric acid gives its anhydrate (P4O10). Then, allow the crucible too get cooled naturally and measure the weight again. From the difference in the weight, the amount of P that was leached out can be easily calculated.