PCT Controller described a loss of separation event resulting from issuing one altitude but thinking another; the reporter adding that ZDC should have been placed into holding much sooner.

Narrative

Giving Instruction during the main arrival bank of the day. ZDC was late on getting aircraft down and switching to my position and was the initial reason our aircraft were being vectored abnormally high into our sector for sequence on the arrival. The complexity of the sector was increasing so I took over from the developmental; and gave an altitude of 9000 to a south bound ACR Y; when I meant to restate 8000; because of traffic opposite direction at 7000. ACR X was opposite direction to ACR Y as well and some 10 miles away. I thought I had lateral and vertical; but when I noticed the situation; I turned ACR Y right to a 270 heading. No response from ACR Y. I turned ACR X East to 090 with no response. I again did this to each aircraft with no response from either one. Finally after issuing a Traffic Alert to ACR X; who acknowledged turning East to 090. It was a case of meaning one thing and saying another. Recommendation; should have put ZDC into a hold earlier than I did. Should have re-scanned my altitude assignments again; especially during high complex situations. Focusing more on all prospective situations. Make sure I say what I mean and mean what I say.