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User Guide Contents

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Introduction

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IODP's UIC Coulometrics CM5011 CM5015 coulometer provides absolute determination of the concentration of carbon dioxide (CO₂) evolved from an acidification process. The coulometer cell is filled with a proprietary solution containing monoethanolamine and a colorimetric pH indicator. A platinum cathode and silver anode are positioned in the cell, and the assembly is located between a light source and a photodetector. When a gas stream passes through the solution, CO₂ is quantitatively absorbed, reacting with the monoethanolamine to form a titratable acid. This acid causes the color indicator to fade. A spectrophotometer monitors the change in the solution's percent transmittance (%T). As %T increases, the titration current is automatically adjusted to generate a base at a rate proportional to the reduction of %T. When the solution returns to its original color (original %T), the current stops. The amount of CO₂ evolved is quantitated calculated from the duration and magnitude of the current required to balance the acid by CO₂ evolution. Based on the principle of Faraday's Law of Electrolysis (the quantity of a substance produced by electrolysis is proportional to the quantity of electricity used), each mole of electrons added to the solution is equivalent to 1 mole of CO₂ titrated.

Chemical reactions occurring in the coulometer cell follow:

Absorption of CO₂ by the cathode solution (cathode reaction):

CO₂ + HOCH₂CH₂NH₂ —> HOCH₂CH₂NHCOOH

Electrochemical generation of OH^– (cathode reaction):

2H₂O + 2e^– —> H_{2 (g)} + 2OH^–

Neutralization of absorbed CO₂ reaction product by electrochemically generated OH^–:

HOCH₂CH₂NHCOOH + OH^– —> HOCH₂CH₂NHCOO^– + H₂O

Anode reaction:

AgO —> Ag⁺ + e^–

Interferences

A variety of carrier gases can be used for coulometry (O₂, N₂, He, and dry air). The JRSO uses N₂ for the measurement. Interferences caused by compounds such as SO₂, SO₃, H₂S, HCl, HBr, HI, and Cl₂ are removed with KOH and AgNO₃ scrubbers.

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• Coulometer unit (UIC CM5011CM5015) with titration cell (Figure 1)
• Acidification module (similar to UIC CM5030) (Figure 2)
• Dual balance system, motion-compensated, with control software

Dual Balance System Hardware

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• Nitrogen (99.995% or better) is used as carrier gas to minimize the amount of CO₂ the scrubber (KOH) must absorb

Reagent Solutions

• 45% KOH ([%w/v]: add 90 g KOH pellets to water and make up to 200 mL once fully dissolved. Warning!This procedure liberates caustic fumes and heat. Perform in a fume hood.)
• 3% AgNO₃ ([%w/v]: dissolve 3 g silver nitrate in water and make up to 100 mL when fully dissolved.)
• Caution! Concentrated acids should always be added to water, and may generate significant heat.
• 2N H₂SO₄: add 55.5 mL concentrated sulfuric acid to water and make up to 1L
• 2N HCl: add 166 mL concentrated hydrochloric acid to water and make up to 1L

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• Sample tubes: rinse sample tubes with DI water and place into the oven to dry. They do not need to be acid washed.
• Cell: clean the cathode/anode cell in a fume hood by adding acetone to the anode cell. The acetone will leach through the bridge between the cells and clean it. Follow the acetone rinse by placing DI water in the anode cell and letting that leach through.
• Platinum electrodes: Electrodes can acquire surface coatings from the solutions. Remove this coating by placing the electrode in a solution of 1:1 concentrated nitric acid solution for 1 hr: water for 20 seconds. Rinse with DI water immediately.

Data Handling

Weight percent calcium carbonate is calculated from µg carbon measured during the titration as follows:
%CaCO³ = µg C x 8.333/sample mass
Sample mass is stored in LIMS associated with the container ID that the coulometer analysis is associated with.

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–Incompatible materials: alkaloid salts, chloral hydrate, potassium chlorate, metallic salts, tartaric and other acids, bromine trifluoride, fluorine perchlorate

Waste Management

Waste may be washed down drain with flowing waterof cathode and anode solutions should be collected in a bottle until it can be removed during the next port call. The potassium hydroxide and silver nitrate solutions may be disposed of in the sink.

Maintenance/Troubleshooting

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The calculated % Difference for any of the Analysis Types should be below ± 0.15%. If any of the values are > 0.15%, contact UIC for a bench calibration of the instrument.

Thorough Cleaning

At times, component parts may require a more thorough cleaning. To clean the frit, fill the cell with enough 1:1 concentrated nitric acid to water solution to cover the frit and allow the acid to clean the frit overnight. Dispose of the acid and rinse the cell and frit completely with water before re-use. If the potassium iodide solution turns brown after refilling the anode compartment, the frit has not been sufficiently rinsed.

1. With no cell in coulometer, install a shorting strap and turn on current.
2. Set coulometer as follows:

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• Clean compressed air (oil-free; zero grade preferred) ¿ 40 psi
• Two 110 V outlets; 8 A peak
• Vent for reaction effluent (preferred as effluent smells bad, releasing amine derivatives)
• Counter area ¿ 2 ft x 2 ft
• Cooling capacity = 800 btu

Hardware Setup

Be certain that the CM5015 is running in CM5011 emulation mode for proper interface with the JRSO software. Also note that the "latch" commands used with the CM5011 are not applicable to the CM5015.

Coulometer Serial Port Jumper Configuration

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