- Phosphric acid is quite hygroscopic and can absorb water from H2O vapor in the air. Leave the storage bottle capped at all times except when removing acid

- During carbonate sample digestion, any free water in the phosphoric acid can exchange its oxygen with oxygen in the carbonate, and affect the measured d18O

- To "remove" the water, either of two methods can be used, 1) Heat to 150C under vacuum until a density of 1.91 is reached (at 150C. It's 1.83 at room T, but you want to measure at 150 in case more treatment is needed), or 2) Heat to 150C and then add phosphorous pentoxide (P2O5), a very fine white powder, until no more will dissolve. We used method 1) only once, and a brownish color was noted due to back-streaming of pump oil vapor/mist into the vacuum oven and the acid.

- Since then we have used method 2). Now, the acid will form very fine grained opaque white patches on the glass bottle inner surface in just a few weeks, along with the suspended translucent crystals. This suggests that the acid is supersaturated with P2O5, and there is still good potential to react with any water to yield the acid. And thus better than merely anhydrous.

 VACUUM METHOD

- Under a vacuum, which lowers the partial pressure and fugacity of H2O gas to almost zero, these polymer-forming dehydration reactions are driven to the right:

     2 H3PO4 --> H4P2O7 + H2O

     H3PO4 + H4P2O7 --> H5P3O10 + H2O

     H3PO4 + H5P3O10 --> H6P4O13 + H2O

     etc.

     factoid: the polymers occur in lesser amounts with increasing length, but "fully dehydrated" phophoric acid contains ~15% pyrohposphoric acid H4P2O7 and the balance mostly H3PO4, with only tiny amounts of longer polymers

- Glacially slow at room temperature, the reactions can be sped up greatly by heating to 150 °C

- Density increases with water removal, the target being 1.92 g/cc

- The polymers allow the acid to absorb moisture from the air, but it will still contain no free water. The reactions above going in reverse.

 P2O5 METHOD

- But wait! P2O5 can be used for dehydration:

     P2O5 + 3 H2O --> 2 H3PO4

     P2O5 + 2 H3PO4 --> 2 H4P2O7

     etc.

     Noting the absence of H2O in these reactions, its presence as vapor in air is irrelevant, but...

- The temperature needs to be hot enough so the reactions proceed much faster than the rate of absorbtion of H2O from air. In the range of 110-150 °C appears to work

- Helpful of course is that heat also slows the rate of H2O absorbtion from air

- Recently the density of the H3PO4 product was actually measured, and appearing that 1.92 density is easily obtained

- NOTE: The P2O5 powder is *extremely* hygroscopic. Left uncapped, the jar contents would form a solid block in just a few hours. SO, open the jar only long enough to remove a small amount, 10g or so at a time into a small beaker. Rinse any leftover powder into the acid waste jug rather than putting it back into the jar.

Other info:

The Fluka brand phosphoric acid delivered with the Gasbench was anhydrous and ready for immediate use, the Thermo service engineer assured us. It was in the form of a solid block in a glass bottle, so it did need to be melted first on a hot plate. After a few months, it will solidify again.


Related-
   https://en.wikipedia.org/wiki/Phosphoric_acid
   https://en.wikipedia.org/wiki/Pyrophosphoric_acid