As starting point to continue the discussion:
a) Wilmad Glass Company sells as reference standards for 31P: 100% H3PO4 and/or
100% trimethylphosphite.
2) In "Handbook of High Resolution Multinuclear NMR" authors: C. Brevard and P.
Granger; Editor: John Wiley and Sons; year: 1981; page 103. Is defined as
reference for 31P chemical shifts H3PO4 85 % H2O. They cited two references
about 31P:
1)Progress in NMR Spectroscopy, Pergamon, London Vol. 6 (1971)
2)Annual Reports of NMR Spectroscopy, Vol. 5B (1973).
If I get more specific information about it I will let you know.
Have a nice day,
JOE