[Teri]

I need to add some ammonium phosphate to my ICP multi-element standard solutions (for use in a DCP), but I'm not sure how long I need to dry the ammonium phosphate in a desiccator before weighing (ammonium phosphate is hygroscopic).  Is there a general rule for how long you should dry a hygroscopic chemical?  Thanks for your *delete me*

[Dude]

The efficiency of drying is very poor in a desiccator.  The drying occurs when you place the chemical in an oven with heat or vacuum.  The general rule for water is 1 h at 105 C if the chemical can tolerate those conditions.  Otherwise, some lower temperature with a vacuum can be used.  The desiccator is usually just a dry storage chamber that you store the chemical in until you are ready to use it.  

[lemonoman]

Agreed.

You'll have to look up the melting/decomposition point of your chemical, and judge where to go from there.

And, as a side note, make sure you know which 'ammonium phosphate' you're using...there's monobasic (NH₄H₂PO₄), dibasic ((NH₄)₂HPO₄), and tribasic (the REAL one, (NH₄)₃PO₄)...and each has its own property

Each can be called "Ammonium Phosphate"...even though TECHNICALLY only the tribasic one is...some chemists are lazy  

[Teri]

Thanks for the information!  I put the monobasic ammonium phosphate in the drying oven this afternoon.  Although it might be overkill, I went for the overnight approach since the MSDS said the chemical should be fine at 105C.

[AWK]

Dessicating monoammonium phosphate by heating  is not a good idea since some loss of ammonia is possible, and you can obtain a vitrous product that is the mixture of your chemical with phosphoric acid