Carbonate system chemistry
NIVA has two VINDTA systems for high precision and accuracy analysis of total alkalinity and dissolved inorganic carbon. These are VINDTA 3C instruments fabricated by Marianda. The left half of the instrument (picture below) makes an automated total alkalinity titration run on a seawater sample, while the right half makes a total dissolved inorganic carbon analysis on an acidified seawater sample in conjunction with a UIC Inc. coulometer. The VINDTA 3C can requires ~250 ml of seawater, can analyze ~3 samples per hour, and make measurements with an uncertainity of ~+/- 1-3 micromol/kg. This is the state of the art technique for high precision and high accuracy total alkalinity and dissolved inorganic carbon measurements that are core variables for ocean acidification studies. Using these two carbonate system variables (with temperature, salinity, and nutrients), a calculation can be performed to estimate two other carbonate system variables: pH and pCO2.
Instruments to measure pH and pCO2 directly are also in use and under development. Unlike the VINDTA system that is designed to measure samples in a relatively controlled laboratory setting with low sample throughput, we are currently using and developing pH and pCO2 sensors that make underway measurements on seawater that is continuously pumped into ships. These are both designed to be part of a FerryBox underway sampling setup and continuously analyze samples autonomously.
Briefly, the pH system is based on adding a pH sensitive colorimetric dye to a seawater sample that is then analzyed for light absorption at various wavelengths.
The pCO2 system (manufactured by Franatech AS) measures CO2 that is equilibrated across a gas permeable membrane from flowing seawater to an analytical air mass. The concentration of CO2 in the air mass is then analyzed with non-dispersive infrared detector.