Making Phosphoric Acid
Here we are going to make phosphoric acid to be use in carbonate digestions for isotopic analysis. The goal of this method is to produce phophoric acid that has very little water and, as an isogeochemistry community, an agreed upon specific gravity. This particular recipe was obtained from the Eiler lab and has the salient steps from original methods as in Coplen (1983). Differences from original methods are, we do not add chromium oxide or hydrogen peroxide. These were used to remove organics that may have originated from vacuum pump oil when acid was prepared by heating under vacuum. We also don’t allow the acid to age. Its ready as soon as the specific gravity is appropriate.
- Phosphorus pentoxide P2O5 (under hood in 302A)
- 85% ortho-phosphoric acid (under hood in 302A)
- stir bar
- stirring hot plate
- 2 L beaker
- low profile glass beaker
- spatula / scoupula
- watch glass
- manual glass thermometer (not digital thermometer)
- notepad / pen
- portable balance
- space in a hood
- add 800 mL of acid to the 2 L beaker
- stir and heat to approximately 80 °C
- add 700 g P2O5 slowly until it all dissolves - this takes several hours. Try adding in 50 to 100 g sessions where you weigh out 100 g into the low profile pyrex beaker, slowly add it to the stirring acid using the spatula, wait until this 100 g fully dissolves before adding another round.
- take a known-volume sample of the acid, allow it to cool, measure its specific gravity, it should be between 1.90 and 1.92 g/mL
- if the density is too high, add more acid, heat and stir; if the density is too low, add more P2O5, heat and stir
- pour the finished acid into one of the glass-stoppered phosphoric acid glass containers, label it with your name, date, and specific gravity
Coplen TB, Kendall C, Hopple J. 1983. Comparison of stable isotope reference samples. Nature 302, p 236 – 238.