Introduction:
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.
Carbon dioxide does dissolve in water; however the system is somewhat complex. First the CO2 dissolves according to:
CO2 (g) = CO2 (l) 5.1
At room temperature, the solubility of carbon dioxide is about 90 cm 3 of CO2 per 100 ml water (cl/cg = 0.8). Any water-soluble gas becomes more soluble as the temperature decreases, due to the thermodynamics of the reaction:
GAS (l) = GAS (g) 5.2
This effect is particularly large for gases like CO2 that undergo specific reactions with water. Equilibrium is established between the dissolved CO2 and H2CO3, carbonic acid.
CO2 (l) + H2O (l) = H2CO3 (l) 5.3
Carbonic acid is a weak acid that dissociates in two steps.
The test for determination of free carbon dioxide in water is based on the titration of water sample with standard Sodium Hydroxide solution in the presence of phenolphthalein indicator. The CO2 reacts with NaOH to form Sodium Bicarbonate with a consequent increase in pH. The reaction of NaOH with free CO2 reach a completion at a pH of 8.3.
Environmental significance:
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 concentrations of carbon dioxide in the water itself.
Corrosion is the principal difficulty caused by carbon dioxide. This gas on solution in water produces carbonic acid resulting in lowering of pH. With a decrease in pH corrosive characteristics is induced in water resulting severe corrosion of heat exchanger, pipes, valves etc. Corrosion in boiler system takes place due to the presence of carbonate and bicarbonate although Carbon dioxide is not present in this case.
Guidelines:
Bangladesh Environment Conservation Rules (1997) does not set any limits for the presence of CO2 in water.
Precaution:
Since excess CO2, if present in water, easily escapes to the atmosphere, tests for presence of CO2 in water should be performed immediately after collection of water sample, especially for ground water samples which usually contain high CO2. If this is not possible, the sample bottle should be completely filled and capped and the sample should be kept at a temperature lower than that at which it was collected.
Where the free CO2 of the water sample is high, there may be some loss of CO2 to the atmosphere during the titration process. To check this, it is advisable to secure a second sample. Upon complete determination of CO2 on the first sample, then take the second sample (100ml) and immediately add the full amount of N/44 sodium hydroxide solution used in the titration of the first sample. Add 10 drops of phenolphthalein indicator and if the sample remain colourless, add additional N/44 sodium hydroxide to the end point (till the slight pink colour appears) and accepting this second test as more accurate titration.
Apparatus:
Beaker
Measuring cylinder
Dropper
Stirrer
Burette
Reagent:
Standard N/44 Sodium Hydroxide
Phenolphthalein Indicator **
**An indicator is a substance that undergoes a change in color when the end-point of a titration is reached. Acid-base indicators are used to signal the end of acid-base titrations. An acid-base indicator is itself a weak acid (or its conjugate base).
Phenolphthalein is a commonly used indicator for titrations, and is a weak acid. The weak acid is colorless and its ion is bright pink. Adding extra hydrogen ions shifts the position of equilibrium to the left, and turns the indicator colorless. Adding hydroxide ions removes the hydrogen ions from the equilibrium which tips to the right to replace them - turning the indicator pink. The half-way stage happens at pH 8.3. Since a mixture of pink and colorless is simply a paler pink, this is difficult to detect with any accuracy.
Procedure:
Take a 100 ml of sample in a beaker and add 10 drops of Phenolphthalein indicator. If a pink color develops, no carbon dioxide is present in the water sample.
Add N/44 sodium Hydroxide solution from a burette to the sample and stir gently until a slight permanent pink color appears as compared with distilled water. Record ml of sodium hydroxide used. Since excess CO2, if present easily escapes to atmosphere, so tests should be performed immediately after collection of water sample. If this is not possible sample bottle should be completely filled and stoppered and be kept at a temperature lower than that at which it was collected.
Calculation:
Carbon dioxide (mg/L) =mL of N/44 NaOH used x Multiplying Factor (M.F.)
Where, M.F.
Determination of CO2 acidity:
Phenolphthalein acidity (often called CO2 acidity) of water is defined as the amount of standard base (usually 1/50 N NaOH) required raising the pH of a sample of water to the phenolphthalein end point of 8.3. CO2 acidity is expressed as CaCO3 (calcium carbonate) required to neutralize H2CO3)
Hence, Acidity could be easily determined from the results of CO2 determination as follows:
Phenolphthalein Acidity as mg/L CaCO3 = CO2 (mg/L)
