Re: why K2HPO4 is hygroscopic? where as Na2HPO4 is not?

Lange's Handbook of Chemistry 9th Ed.  states that Na2HPO4 is hygroscopic 
and K2HPO4 is deliquesent[actually dissolves in the absorbed water] so it 
is possibly a matter of degree.  The anhydrous Na salt uas a lower 
solubility than the K salt12g/100g vs 33g/100g. This does not seem 
significant.  The Na salt also lists 2 hydrates .7H2O and .12 H20 while 
the K salt doesn't list any hydrates.  What might be happening is that the 
sodium salt is absorbing water and forming the solid hydrates which are 
then not hygroscopic while the K salt doesn't form a solid hydrate but 
solvated ions. This could be related to the smaller size of the Na+ ion 
giving more space in the crystal latice for the water molecules.  One of 
the uses of the Na salt is as an anticaking agent which lends credence to 
that scenario.  It is possible that Xray diffraction studies have been 
done on these compounds to investigate this.