And I quote from the "Taylor K-2005C" test kit manual:

Like all perishables, reagents need to be stored under controlled conditions.
Exposure to extremes of heat or strong ultraviolet light(sunlight) should be avoided. Follow these helpful storage techniques will help increase the shelf life of our reagents:
~ store out of direct sunlight between 36-85 F (2-29C).
~ Tighten bottle caps after use
~ keep reagents in case (if appropriate)



I keep mine in the fridge, until one hour before testing!