[Telamond]

Wasn't sure where to put this (inorganic or organic). But the question is in the subject line "How do I dry K3PO4?". I think the one that I use absorbed some water.

[enahs]

Heat it in a crucible to drive water off.

[Telamond]

How high temperature would I usually need?

[enahs]

Hot fast. Make the bottom of the crucible red hot. Not super bright red all over, but have it glowing red a little.

[cheese (MSW)]

The CRC lists the mp of K3PO4 as 1340 °C without dec.  But it is hygroscopic!  Cooling after heating in an open crucible will cause it to reabsorb moisture.
You need to put it into a Schlenk tube and heat it on on a high vac line at say 150 °C (silicone oil bath) overnight and dry to const weight (check IR for O-H).
And keep your dried product under N2. You might even get away with putting the sample under high vac at RT in a desiccator over P2O5.  Good luck!