Originally, exactly as much as a volume of water equal to the cube of the hundredth part of the metre, at the temperature of melting ice. I don't find that terribly helpful. Not many did. That's why, in 1889, the mass of the kilogram was redefined as being equal to the IPK.
Boiling water is also an effective poison ivy killer. If the area where you will be killing poison ivy has no other plants you wish to keep, pour boiling water over the poison ivy plant. Boiling water will kill any part of a plant that it comes in contact with, so be careful using this around desirable plants.
Apple cider vinegar seems to be helpful for just about everything and poison ivy is no exception. This vinegar helps to draw the toxins out of the rash to accelerate healing and reduce the discomfort. Take apply cider vinegar and a brown paper bag, soak the bag and then apply this directly to the rash.