Charles Ellicott Commentary Exodus 24:6

"And Moses took half of the blood, and put it in basins; and half of the blood he sprinkled on the altar." — Exodus 24:6 (ASV)

Put it in basins. —Reserving it for the purpose stated in Exodus 24:8.

Half of the blood he sprinkled on the altar. —This was the most essential part of every sacrifice—the act by which the victim, the representative of the offerer, was made over and delivered up to God. Usually all the blood was thus devoted; here there was need of some for another purpose.