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What do I do with my leftover monomer?
Answer: Nail products can be quite expensive, so it makes sense that nobody wants to waste them. Unfortunately, any leftover monomer liquid at the end of a service cannot go back into its bottle. It is usually contaminated with polymer powder particles, and it will contaminate the rest of the product. You should not use it for your next appointment either, for the same reason (contaminated liquid will affect polymerization). Your best solution is to safely dispose of it.
Never pour monomer liquid or other nail products down the sink or flush them down the toilet. Wastewater from the sink and toilet goes through a local treatment plant where it is generally treated for bacteria. Water treatment plants are not set up to filter out chemicals that should not be in the water in the first place. These chemicals can poison plants and wildlife. They can also get into crops that may eventually end up on your table.
If you have a small amount of monomer liquid that is leftover in your dappen dish, you can combine it with an equal amount of polymer powder, and allow it to harden into a small ball, then dispose of it in the trash. However, never mix larger quantities (i.e., more than 20-30ml) because the exothermic reaction will be more intense, so the mixture can become very hot, and may cause burns.
Another acceptable disposal method is to use a paper towel to absorb the extra monomer liquid. Then wipe the dappen dish clean, seal the paper towel in a plastic bag, and put it in the trash. Wear gloves to make sure you have no skin contact with the monomer liquid; repeated contact may cause skin irritation and/or an allergy.
The best solution, of course, would be to control the quantity of monomer liquid you decant in your dappen dish, to begin with. It is better to start with a smaller quantity and add as you go. Pay attention to how much liquid you usually need for a set of nails. Keep that quantity in mind as your standard measurement. Use a pipette to help you measure the liquid, and keep everything clean. I have seen many nail professionals pour the monomer liquid against their brush to help guide the liquid to the dappen dish with no spills, but then they grab that same brush to work with and get liquid all over their fingers.