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PIPES is an acid. Don't get this stuff on your skin because it will irritate it. We use acid PIPES because there was a discussion with Koch about what type of ions are needed in solution for motility to work. Some say that you need potassium and others, sodium. Since we have a free acid form of PIPES, we can add either NaOH or KOH to pH the buffer to PIPES's pKa of 6.89, thus adding the appropriate counter ion we want.
PIPES should be stored in the desiccator at room temperature in its original container. Also, when using PIPES, make sure you add it to water and not the other way around.