Making an isotonic saline solution is a precise, but simple, science. The prefix "iso" refers to something that is equal and means that the concentration of saline -- or the amount of salt -- is roughly equal to the salt concentration in body fluids. This amount is about 0.9 per cent or 9g per 1000ml of fluid. It is about 1/4 tsp of salt per 236ml. of water. Using more salt makes a hypertonic solution, and using less salt makes a hypotonic solution.
Decide how much isotonic saline solution you want to make. The solution generally lasts for months in a refrigerator. It is easy to make, however, and, depending on your intended use, you may want to make a fresh batch prior to each use. Making fresh solution allows you to have room temperature or warmer solutions rather than refrigerated solutions.
Measure the amounts of water and salt needed for the solution. For each 227gr. of tap water or filtered water, use 1/4 tsp of salt. Use kosher, macrobiotic or table salt (NaCl).
Heat the desired amount of water on a stove or in a microwave-safe container in a microwave oven until it is warm but not boiling.
Transfer the water to a glass or mixing bowl, and stir in the salt. Mix the combination with a spoon until the salt dissolves fully.
Store any unused portion of the solution in a glass storage jar in a refrigerator. You may want to mark the date on its container and check for "floaters" before using the solution. Generally, an isotonic solution stays fresh in a refrigerator for a number of months. If the solution becomes contaminated, though, you will see particles floating in the solution. If you see floating particles, discard the solution, and make a fresh batch.
Some isotonic saline solutions also contain 1/4 tsp of baking soda per 227gr., which improves the mucus-solvent properties of the solution and acts as a buffer. Depending on the use of the solution, you may or may not want to add baking soda. Some nasal irrigation solutions include both salt and baking soda. If you include baking soda in the solution, add it to the water when you add the salt.
Use sterile water and kosher salt if you make an isotonic solution for contact lenses. Do not add baking soda to the solution if you use the solution for contact lenses.