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 is an effective poison ivy rash natural treatment. It contains medicinal properties and works by drawing out toxins from the rash. This way apple cider vinegar helps to tame he allergic reaction as well as the itch. Start off by thoroughly cleaning the area using lukewarm water and soap.