I want to measure dissolved Co2 in seawater. At the moment we have only an autotitrator available but it is very costly. If anyone can tell me a titration method for DIC determination and/or refer to a corresponding protocol.
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)
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 Phenolphthalien indicator. The CO2 reacts with
NaOH to form Sodium Bicarbonate with a consequent increase in pH. The reaction of NaOH with free
CO2 reache a completion at a pH of 8.3
CO2 + H2O = H2CO3
H2CO3 + NaOH = HCO3
- + Na+ + H2O
HCO3 -+ NaOH = CO3
2- + Na+ + H2O
Reagent
1. Standard N/44 Sodium Hydroxide,
2. Phelophthalein Indicator
Procedure
(1) Take a 100 ml of sample in a beaker and add 10 drops of Phenolpthalein indicator. If a pink color
develops, no carbon dioxide is present in the water sample.
(2) 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 lowar than that at which it was collected.
Calculation
Carbon dioxide present (mg/L) = (normality of NAOH * equivalent wt of CO2 * 1000 * milliliter of of N/44
NaOH added) / milliliter of sample taken
The Guide to Best Practices for ocean CO2 measuements (http://cdiac.ornl.gov/oceans/Handbook_2007.html) describes the best methods for measuring all four carbon parameters.
Coulometry is the most precise method for DIC, but infrared analysis is also common. A closed cell total alkalinity titration can also provide pH and DIC, but provides its own difficulities, particularly building a closed cell that won't allow escape of CO2 to the atmosphere and with a precisely known volumn. Measurements of certified reference material are extremely important no matter what method you use.
can i get the reference of the procedure you have recommended..
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