Coulometry is an analytical technique that uses electricity as a means to
electrochemically generate and measure the amount of titrant used in a titration.
Apparatus consists primarily of an electrochemical cell, wherein the titration takes place,
and the associated electronics used to generate titration current, measure the total
charge (coulombs) passing through the cell, and digitally display the results,
The titration cell consists of a cathode compartment containing a platinum electrode, an
anode compartment containing a silver electrode, and the titration solutions. The cathode
chemistry is a semi-aqueous solution containing an acidv-base indicator and 2aminoethanol.
As the CO2 produced from the acidification reaction of surface carbon enters the cathode
solution, it reacts quantitatively with 2-aminoethanol to form 2-hydroxyethylcarbamic acid:
As the acid forms within the cell solution, the indicator’s color fades from blue to clear. A
photometric detector sensing the change in light transmission through the electrochemical
cell automatically switches the cell current to “ON.” While current flows through the cell,
water in the cathode solution is electrolytically reduced at the platinum cathode to form the
hydroxide ion titrant:
2H2O + 2e- H2 + 20HAs the OH- titrant reacts with the 2-hydroxyethylcarbamic acid analyte, an acid-base
neutralization reaction occurs:
OH- + HO(CH2)2 NHCOOH H2O + HO(CH2)2NHCOO
which restores the cathode solution to its original end-point color, causing the photometric
detector to switch the cell current to “OFF.”
Faraday’s Law
The amount of hydroxide ion generated and consequently the amount of CO2 titrated as 2hydroxyethylcarbamic acid relates to the number of coulombs which pass through the cell
during the titration. The weight of carbon titrated is determined by Faraday’s Law.
The instrument electronically measures the total charge and, using the above equation,
calculates and digitally displays the weight of carbon detected to the nearest tenth of a
microgram. Since the fundamental units of coulombs are measured during this loo-percent
efficient coulometric process, chemical standard calibration is not required.
Sensitivity of the CO2 analyzer is better than 1 µg C. Overall accuracy is limited by the
reproducibility of successive background measurements. Using standard materials,
deviations are typically better than 0.15 percent relative or + 1 to 2 µg of C (whichever is
greater). Sample homogeneity normally limits the accuracy of the analysis rather than the
instrument’s detection capability.