Water Analysis: Dissolved Carbon Dioxide (CO2)

Carbon Dioxide is present in water in the form of a dissolved gas. Surface waters normally contain less than 10 ppm free carbon dioxide, while some ground waters may easily exceed that concentration. Carbon dioxide is readily soluble in water. Over the ordinary temperature range (0-30 C) the solubility is about 200 times that of oxygen. Calcium and magnesium combine with carbon dioxide to form carbonates and bicarbonates.

Aquatic plant life depends upon carbon dioxide and bicarbonates in water for growth. Microscopic plant life suspended in the water, phytoplankton, as well as large rooted plants, utilize carbon dioxide in the photosynthesis of plant materials; starches, sugars, oils, proteins. The carbon in all these materials comes from the carbon dioxide in water.

When the oxygen concentration in waters containing organic matter is reduced, the carbon dioxide concentration rises. The rise in carbon dioxide makes it more difficult for fish to use the limited amount of oxygen present. To take on fresh oxygen, fish must first discharge the carbon dioxide in their blood streams and this is a much slower process when there are high concentration of carbon dioxide in the water itself.

Carbon dioxide test using Hach kit

Low range test

1. Fill the mixing bottle to the 23- ml mark with the water sample
   
2. Add one drop of the phenophthalein indicator solution to the sample.
   
3. Add the sodium hydroxide solution drop by drop to the sample. Count each drop as it is added. Swirl the bottle to mix after each drop is added. Continue adding drops until a light pink color forms, and persists for 30 seconds.
   
4. Each drop of sodium hydroxide solution used equals 1.25 mg/l carbon dioxide.

Medium range test

1. Fill the mixing bottle to the 15 ml mark with the water sample.
   
2. Proceed as in the low range test.
   
3. Each drop of sodium hydroxide solution used equals 2 mg/l carbon dioxide.

High range test

1. Fill the plastic measuring tube level full with the water to be tested. Transfer to the mixing bottle by placing the mixing bottle over the tube and then turning the bottle right-side up.
   
2. Add one drop of phenolphthalein indicator to the mixing bottle.
   
3. Add sodium hydroxide solution drop by drop.
   
4. Each drop sodium hydroxide used equals 5 mg/l carbon dioxide.