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.
The best way in how to get rid of poison ivy with these herbicides is to apply the herbicide to the leaves of the poison ivy plant. Like pulling, the poison ivy will regrow, as even the most powerful herbicide will not kill all of the roots. But as the poison ivy plant regrows, spray the herbicide on any new growth. A few applications on new growth will deplete the poison ivy plantâs ability to regrow and the plant will die back completely.