How to determine free carbon dioxide
Free Carbon Dioxide: Direct Titration Method, (0-100 ppm)
Carbon dioxide exists in aqueous solutions as “free” carbon dioxide and in the combined forms as carbonate and bicarbonate ions. “Free” carbon dioxide refers to the uncombined dissolved gas. Its presence in solutions is due either to absorption from the atmosphere or to the decay of organic matter. Since the carbon dioxide content of the normal atmosphere is quite low (less than 0.04 per cent), surface waters typically contain less than 10 ppm free carbon dioxide. Well waters, however, may contain several hundred ppm of carbon dioxide.
Corrosion is the principal effect of dissolved carbon dioxide. The gas will ionize in water, producing corrosive carbonic acid.
H20 +CO2 --> H2CO3 --> H+ + HCO3-
<-- <--
Water Carbon Carbonic acid Hydrogen Bicarbonate ion
dioxide ion
The low pH resulting from this reaction also enhances the corrosive effects of oxygen. Therefore, carbon dioxide should be removed from raw waters before use in boiler systems.
In boiler systems, corrosion resulting form carbon dioxide is most often encountered in steam and return condensate lines. Since feedwater deaeration normally removes carbon dioxide from the boiler feedwater, the presence of the gas in condensate line is typically due to carbonate and bicarbonate decomposition under boiler condition. Sodium hydroxide, or caustic soda, is a by- product of this reaction. Corrosion of steam and return lines often occurs with boiler feedwater of high methyl orange alkalinity. For such water, the carbon dioxide content of the steam may be approximated by multiplying the feedwater methyl orange alkalinity by a factor of 0.79 which takes into account 80 per cent sodium carbonate decomposition. For example, feed water with a methyl orange alkalinity of 100 ppm as calcium carbonate could be expected to generate a carbon dioxide level of 79 ppm in the steam. Such a high carbon dioxide level would create extremely corrosive conditions in units where the gas accumulates such as jacketed kettles, unit heaters, etc.
Theory of Test
Free carbon dioxide reacts with sodium carbonate or with alkaline solutions to form sodium bicarbonate. In this test, sodium carbonate is added to react with free carbon dioxide. When all the carbon dioxide has reacted, excess carbonate is present. At this equivalence point, pH 8.3 the phenolphthalein indicator develops a pink color.
Appratus Required
Burette, Automatic, 25 ml - 1
Cylinder, Graduated, 100 ml - 1
Flask, Erlenmeyer, 250 ml gum rubber tubing. - 1
Stirring rod, Glass - 1
Chemicals Required
Phenolphthalein Indicator
Sodium Carbonate, 0.045N
Procedure for Test
Reliable determinations of free carbon dioxide are very dependent on sampling, handling techniques and sample aging. For this reason the following sampling method should be used.
Obtain the sample through gum rubber tubing discharging at the base of the graduated cylinder. Allow the sample to overflow for a few minutes. Remove the tubing from the cylinder before stopping the sample flow. Flick the cylinder to remove excess sample above the 100-ml mark.
Transfer the sample to the Erlenmeyer flask by pouring the sample carefully down the side of the flask. Add 5 to 10 drops of phenolphthalein indicator. If a red color develops no free carbon dioxide is present in the sample. If the solution remains colorless, titrate rapidly with 0.045N sodium carbonate with gentle stirring. The endpoint is reached when a pink color persists for 30 seconds.
When the free carbon dioxide levels are high, this technique may suffer from losses of carbon dioxide to the atmosphere. If this is the case, a second sample should be obtained and treated as follows. Immediately after obtaining a 100-ml sample, add the titrant volume obtained in the first titration. Now add 5 to 10 drops of indicator. If the sample remains colorless add additional 0.045N sodium carbonate until the endpoint persists. Use this corrected titrant volume for more accurate result. If a red color develops, the first titration was correct.
Calculation of Results
FORMULA:
ppm free carbon dioxide as CO2 = ml 0.045N sodium carbonate x 1000 ml sample
When using a 100-ml sample, the free carbon dioxide is equal to the ml 0.045N sodium carbonate multiplied by 10.
Limitations of Test
Free mineral acidity, if present, will be measured by this method. Heavy metal ions or high total dissolved solids may cause erroneous results. Free carbon dioxide may be lost by excessive sample agitation during sampling or titration.
Estimates of carbon dioxide levels may be obtained for systems whose pH and total alkalinity (methyl orange) are known.
To determine the free carbon dioxide, locate the intersection of the vertical line for the system pH with the slanting line for the system methyl orange alkalinity. Read horizontally to the left to determine the ppm free carbon dioxide.
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