Phosphoric Acid Preparation

Method: Take 500g of 99% crystalline, ortho-phosphoric acid (e.g., Fluka Cat. no. 79622). Leave the acid in its the original bottle but exchange the cap with a rubber stopper fitted with inflow and outflow tubes for a purge gas (Fig.1). Do not place these tubes into the acid. Place the ortho-phosphoric acid in a 50-60°C oven and attach the inflow tube to a tank of UHP He. Set the He flow to a few milliliters per minute. Wait 24 hours for the acid to melt.

Once the acid is melted transfer it to a clean glass flask and attach it to a vacuum line. Heat the acid up to 100-130°C under vacuum for at least 2 days (Fig. 2).

Test the specific gravity of the acid. If it is between 1.89 and 1.92 it is suitable for use. If the specific gravity is <1.89 continue to heat under vacuum for another day. We also recommend testing the new acid by running some standards before analyzing samples.

Figure 1.

Figure 2.