Hiya guys.
Ive come up with couple of other possible solutions.

1> Empty ALL the bottles into a huge vat. Stir well.
Pour the contents of the vat back into the bottles and re-cork them.
The poison SHOULD be safely diluted.
You can test this on the prisoners by giving them a whole bottle to share.
(assuming you didnt drown them in the full vat just for kicks)

2> Depending on HOW evil your ruler is...
Test 20 bottles with the prisoners in one of the suggested methods.
If NONE die, keep those bottles for your PERSONAL USE, and host the party.
If one does die, grab 20 UNOPENED bottles from the untested 980 for yourself,
and put the 20 tested ones back into the pile.
(Who really cares if you lose a few minions here and there, eh?)