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How to Test the Solubility of Carbon Dioxide in Salt Water

Carbon dioxide is a naturally occurring gas present in our oceans -- and it's also readily dissolved in salt-water aquariums. The rate of dissolution is also known as solubility, and is typically notated in milligrams of solute per liter of water. To get a full description of the amount of carbon dioxide dissolved in salt water, you'll want to take water samples from your tank, or from various depths within a column of ocean water. One of the best ways to test how much dissolved carbon dioxide is in your sample is to use a specialized lab kit made for that purpose.

Things You'll Need

Test kit
Note pad
Pen

Instructions

- 1
  Scoop samples of salt water from your aquarium tank or from ocean locations of interest to you. Use the specialized container that comes with your kit so you can trap water samples taken from the desired depth.
- 2
  Pour salt water into each of the three mixing bottles included in the carbon-dioxide testing kit. Use the appropriate amount of water required for each of the three different titrations. Refer to the manufacturer's instructions, included with your kit, to determine how many milliliters of salt water are needed to fill the three bottles for the low-, medium- and high-level carbon-dioxide saturation tests.
- 3
  Add the recommended about of indicator solution, usually only one drop, of phenolphthalein or other similar chemical provided with your supplies
- 4
  Do a low-range carbon-dioxide test. Mix in the active ingredient from your test kit, such as sodium-hydroxide solution, one drop at a time. Ensure you gently stir or swirl the container after each drop. Add solution until the mixture turns a light pink color that's maintained for at least 30 seconds. Continue to add sodium hydroxide, or similar supplied chemicals, until this is achieved. Keep track of the number of drops of sodium hydroxide used, by writing it down on a sheet of paper.
- 5
  Perform the medium- and high-range carbon-dioxide solubility tests, one at a time, per requirements prescribed by the lab-kit manufacturer.
- 6
  Calculate the amount of carbon-dioxide solubility for each of the salt-water samples. Take the number of drops of sodium hydroxide used above, and multiply that by the factor provided from the lab manufacturer. Test kits, such as those from Hach, equate one drop of sodium hydroxide with 1.25 mg/l CO2 at the low end, 2.0 mg/l at the medium range and 5.0 mg/l at the high range.

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