Dissolved Inorganic Carbon (DIC) Measurements

The carbon dissolved in sea water is generally divided into two major categories. Dissolved carbon derived from the decay of plants and animals as well as from their waste products, is referred to as dissolved organic carbon, or DOC. DOC mostly consists of long-chain organic molecules, but smaller species, such as methane, are also considered organic. The second form of dissolved carbon in sea water is dissolved inorganic carbon, or DIC. DIC in sea water consists of carbon in three forms: carbon dioxide (CO₂), bicarbonate (HCO₃), and carbonate (CO₃). The relative abundance of these three carbon compounds is controlled by the pH of the water, while the total amount of DIC is controlled by a much more complicated set of factors.

Most origins of DIC are biological in nature, but the one we are interested in here at the Scripps CO₂ program is anthropogenic. As the concentration of CO₂ in our atmosphere increases, so does the amount of CO₂ in the ocean's surface waters. Why is this important? While the oceans are able to remove some of the increasing carbon from the atmosphere, just how much they are able to remove has vital implications in determining the potential severity of global warming. Here at the Scripps CO₂ program, our laboratory DIC measurements are accomplished in what is essentially a two step process.

Step 1: Vacuum Extraction

The first step is the physical extraction of the DIC from the liquid sea water. Our extraction procedure is relatively straight forward. In an evacuated extraction flask, we acidify an aliquot of sea water with dilute phosphoric acid, which converts all forms of inorganic carbon (dissolve carbonate and bicarbonate) into CO₂ gas. The extraction flask is then opened to a vacuum line, and the gasses evolved (mostly water vapor, nitrogen, oxygen and CO₂) are drawn through a series of cryogenic traps. First, water vapor is removed from the extracted stream of gasses with a dry ice/ethanol trap (about -90º C) and the CO₂ is collected using a trap filled with liquid nitrogen (about -190º C). The remaining gasses do not freeze at the temperatures of the traps, and are therefore allowed to simply pass through the line and are pumped away.
 
Extraction Line Schematic:

The aliquot of extracted CO₂ is then further purified via sublimation using "plastic ethanol" (ethanol chilled to about -100º C) and cryogenically transferred into an ampule of about 1cc volume, which is sealed under vacuum and stored for later quantitative analysis.

Step 2: Manometric Analysis

After extraction, the CO₂ ampules are gathered and run on our Electronic Constant-volume Manometer (ECM). This instrument is designed to measure the pressure of a small amount of gas very precisely. The pressure sensing device consists of a Ruska DDR6000 Quartz Spiral manometer and a Differential Pressure Indicator (DPI), which are mated to a confined volume of glass into which our samples are transferred at the time of measurement. The DPI and the volume are both contained in a constant temperature air bath which is kept at 40º C. The Quartz spiral itself is kept at a constant temperature in a chamber of it's own. The sample volume is calibrated by the introduction of a precisely defined amount of gas into the volume itself. The pressure produced by this gas is measured, and, using the non-ideal gas law, the volume of the chamber is calculated. These calibration gases are produced by filling small (1 to 2.5 cc) glass "plenums" with CO₂ at a precisely known pressure supplied by a Ruska Dead Weight Tester. The volumes of the plenums themselves were determined by weighing the volumes filled with degassed water on a micro-balance.

The Scripps CO₂ Progrm Electronic Contstant-Volume Manometer:

The pressure measured for each unknown sea water is combined with the calibrated volume of the ECM, again using the non- ideal gas law, to arrive at the number of moles of CO₂. This quantity is then divided by the weight of the aliquot of water from which this CO₂ was extracted to give us the DIC content of the sample. To eliminate variations in concentration caused by short term dilution of the water mass due to precipitation/evaporation cycles, the raw DIC is often normalized to a constant salinity (salinity is quite a conservative property of sea water, meaning that is usually unaffected by anything other than precipitation/evaporation cycles).