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CHEMICAL ANALYSIS

CARBONATE ANALYSIS

coulometer.gif (130460 bytes)

Principles

The acidification module acidifies bone samples to evolve inorganic carbon as CO2.   The potassium hydroxide (KOH) scrubber removes CO2 from air.  Air free of CO2 sweeps the evolved CO2 into the coulometer.  The coulometer determines carbon content in any CO2-containing gas stream.  The coulometer cell contains a solution of monoethanolamine (MEA) and a pH indicator, a platinum cathode and a silver anode.  The assembly is positioned between a light source and a photodetector in the coulometer.  The CO2 in the gas stream from the acidification module enters the cell.  It reacts with the MEA to form a titratable acid, indicated by a fading of colour of the coulometric solution:

CO2 + HOCH2CH2NH2    ----->   HOCH2CH2NHCOOH

Photodetection records the change solution colour as percent transmittance (%T). As %T increases, the titration current electrochemically generates base, with e- generated at the silver anode. The reactions are:

2H2O + 2e -   ---->    H2(g) + 2OH -

Ago   ---->   Ag+ + e -

The silver anode is gradually used up. The silver ion reacts with excess potassium iodide.  Neutralization occurs as:

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