You can purchase 3% hydrogen peroxide at any local grocery store.
Ideally, you should use 1 US tbsp (20 grams) of baking soda for every 2 cups (470 mL) of liquid that your thermos can hold. [3] X Research source
The mixture will foam slightly.
If you would like to clean the lid of the Thermos, place the lid sideways on the top of the Thermos so that the side you drink from is submerged in the liquid and allow it to sit overnight.
After rinsing out your Thermos, you can wash it as normal, but it is not required. To remove any leftover baking soda, scrub the bottom of the Thermos with a bottle brush.
Any brand of dental cleaner will work just fine. If you have a Thermos with a smaller neck, break the tablet in half to fit it into the bottle.
You will notice the fizz begin to settle in about 5-10 minutes.
You can use a bottle brush to clean the inside of the Thermos to ensure the entire dental tablet has been removed. For tougher stains you may need to repeat this process.
If you no longer have a cap on your vinegar bottle, add a splash of vinegar to the bottom of your Thermos.
Lukewarm water is preferred, however, any temperature of water will work.
If you do not have a bottle brush available you can use any other type of cleaning brush that has soft bristles and fits into your Thermos.
You may opt to wash your Thermos with dish soap and water, but you do not need too. The bottle will not taste or smell like vinegar. If you still notice any stains, odors, or discoloration, go ahead and repeat these steps.
