Pizza, casseroles, pasta -- some things work great as leftovers. Hair toner and developer mixed together, however, aren’t meant to be a leftover. When you have a toner and developer mix you would like to use a day after mixing it, the developer will either have lost its effectiveness or may have resulted in an unwanted explosion. Since using toner a day after mixing with developer won’t work for you, always mix your toner and developer immediately before using. Significance Toner is a semi-permanent hair color option that can “tone,” or soften, your just-applied hair color to eliminate brassiness in blond hair. Without developer, putting toner on your hair would seem pretty useless. Another name for developer is hydrogen peroxide, which helps to generate the chemical reaction that lightens your hair when a mix of toner and developer is applied. Your stylist mixes these two together typically in a 1-to-1 ratio and applies them to your hair long enough for the toner to lighten your hair one to two shades. All About the Chemistry You need one other factor besides toner and developer to complete the chemical reaction process: oxygen. When you apply the toner and developer to your hair, the chemicals react with the air to release products called aniline derivatives in the toner that deposits the desired hair color shade on your hair. Oxygen is why you can’t use toner and developer the next day -- the oxygen gets into the mixture and cause chemical reactions, making the mix ineffective by the time it gets to your hair. Airtight = Not Right You might be thinking about putting the toner and developer mixture in a sealed container to make the oxygen less of a factor. Chemically speaking, this isn’t as good of an idea as it may seem. Toner and developer together in an airtight bottle produce a chemical reaction that causes gas to build up in the container. The result is an explosion in a bottle, which definitely won’t help you or your hair. In other words: just don’t leave the mixture in an airtight container. What Will Work If you are going to use toner on your hair, shake the mixture together just before you are going to apply it whenever possible. Once you’ve applied the mixture to your hair, rinse the toner and developer out as soon as possible to keep those gas bubbles from building up. Reserve leftovers only for food, not your hair color.