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.
Wash anything that comes in contact with poison ivy to remove the oil. As you know, the rash is very itchy. It usually lasts about three weeks. For mild cases, you can use cool compresses, over-the-counter oral antihistamines, and hydrocortisone cream applied to the skin. For more severe cases, see your doctor.