Coulometer

Content
1. What is coulometer?
2. Application of coulometer
3. Coulometer working principle
4. Composition of the coulometer
5. Types of coulometer
    5.1. General Coulometer
    5.2. Micro Coulometer
    5.3. Karl Fischer coulometer
6. Coulometer specifications
7. Coulometer titration cell
8. Precautions for using coulometer
9. How to order coulometer?

Coulometer adopts the principle of dynamic coulometry, when the sample is burned in the conversion tube, the redox reaction occurs and is carried by the carrier gas into the titration cell for titration.
The coulometer is a "zero equilibrium" closed-loop negative feedback system consisting of a coulometric amplifier, a titration cell, and a suitable electrolytic system using the principle of micro coulometric titration technology. The bias data acquisition and cracking furnace temperature control are performed by a microcontroller, which is connected to a computer by serial communication, thus realizing the automatic control of the whole system. 

Coulometers are widely used in the analysis of total sulfur or total chlorine content of samples in production, research, and monitoring fields such as petroleum, petrochemical, pharmaceutical, health, environmental protection, coal, geology, metallurgy, commercial inspection, quality inspection, and schools. Because coulometric titration is currently the most accurate method for the analysis of macronutrients and a highly sensitive method for the determination of trace components. Since both time and current can be accurately measured, the precision of coulometric titration is very high, and precision in the determination of macronutrient components can be expected to reach a few parts per 200,000. The method is superior to general volumetric analysis in the situations where it can be applied. It does not require the preparation of standard solutions and thus does not have the problem of stability of standard solutions. It does not need to measure the volume, and there is no error in this regard. It is easier to automate than the general macro metric method. In the coulometric electrolytic cell, the titrants produced by electrolysis are H+, OH-, Cl2, Br2, I2, Ce(Ⅳ), Ti(Ⅲ), Fe(Ⅱ), Mn(Ⅱ), Ag(Ⅰ) and so on. They can titrate many inorganic and organic substances. Coulometric titration is most suitable for the analysis of chemical reagents that are used as basic standards in volumetric analysis.

Coulometry works on the principle that the reference electrode in the titration cell supplies a constant reference potential and, together with the measuring electrode, forms an indicator electrode pair to generate a voltage signal. This signal is added to the input of the Coulomb amplifier in reverse series with the applied given bias voltage. When the two voltages are equal, the amplifier input is zero and the output is also zero, no current passes between the electrolytic electrode pairs and the instrument display shows a smooth baseline. When the sample is injected from the syringe into the lysis tube, the measured substance in the sample is reacted into titratable ions and carried by the carrier gas into the titration cell, which consumes the titrant in the electrolyte. The change in titrant concentration causes a change in the potential of the indicator electrode pair in the titration cell, and the change in its value is fed to a microcomputer-controlled micro Coulomb amplifier, which is amplified and added to the electrolytic electrode pair (cathode and anode), where titrant ions are generated electrically to replenish the consumed titrant. The above process continues continuously with the consumption of titrant ions until no substance consuming titrant ions enters and enough titrant ions have been electrically generated so that the value of the indicator electrode pair is again equal to the given bias value and the instrument returns to equilibrium. During the process of depletion and replenishment of titrant ions, the amount of electricity generated during the electro-generation of titrant is measured and the sample content is calculated by processing the data according to Faraday's law.

Currently on the market, coulometers are composed of a main unit, a temperature gas flow control unit, a stirrer, a feeder, a computer, a printer and other components.
Among other things, the stirrer is the device that holds the electrolysis cell and the housing should be well grounded, protected from light and shielded. The temperature gas flow control unit provides heat to the lysis tube used for sample lysis and supplies carrier gas and oxygen for sample combustion. The sample feeder automatically injects the sample into the lysis tube, and it can freely adjust the feed rate and distance to meet the analysis requirements of different samples.

For solid and gaseous samples, there are also heavy oil and gas injectors respectively, which can be used for different analytical requirements during experiments. The mainframe is the place for data acquisition and analysis control, and is the core of the whole instrument. Good grounding is required. The computer and printer are the equipment for processing and storing the analysis data and output.

In addition, coulometer is equipped with a lysis tube and a titration cell. The function of the lysis tube is to convert the organic sulfur, nitrogen, chlorine, carbon and hydrogen in the sample into SO2, NH3, HCl and non-reacting CO2, H2O, CH4, etc., which can interact with the titration ions in the electrolyte.
CO2, H2O, CH4 and other compounds. Figure 3 shows a typical cracking tube for the determination of sulfur and chlorine by the oxidation method. The tube is made of quartz and the sample is vaporized by injecting it into the cracking tube inlet with a syringe through a silicone rubber plug. The nitrogen, helium or argon enters the gasification chamber through a spiral tube near the plug after preheating and mixing with the sample gas phase, and together they enter the combustion chamber and burn in the oxygen supplied by the tube on the other side.

The general coulometer has four kinds of indicator electrode endpoint detection methods: current method, potential method, equivalence point rise, equivalence point fall, according to different requirements, choose electrode and electrolyte, can complete different experiments, is a new type of general coulometer for scientific research and teaching and chemical analysis. It is widely used in scientific research institutes, universities and colleges for scientific research and teaching.

Micro coulometer is suitable for the analysis of total chlorine content of samples in the fields of petroleum, petrochemical, medicine, health, environmental protection, coal, geology, metallurgy, commercial inspection, quality inspection, schools and other production, scientific research, and monitoring.

Karl Fischer coulometer is an instrument for testing moisture by electrochemical method, and the electrolyte can be used repeatedly, featuring fast analysis, simple operation, and high accuracy.

Features:
1. Micro coulometer adopts integrated structure, compact design, beautiful appearance, high-temperature furnace heat preservation, and heat dissipation set up good effect.
2. The stirrer is integrated with the main machine, effectively avoiding the quartz tube jamming.

Titration cell definition
Titration cell is the heart of the microcoulometric titration reaction, which plays the role of introducing the measured substance produced by the lysis tube into the titration cell and reacting with the titrant in the electrolyte.

Cleaning of the titration cell
Immerse the entire titration cell with fresh chromic acid wash for 5-10 minutes, then wash and blow dry with tap water, deionized water, and acetone, respectively, apply a little grease to the side arm piston and secure it with a rubber band.

Installation of titration cell
Close the piston on both sides, fill the titration cell with electrolyte, open the piston of the reference arm, and let the electrolyte flow into the reference side arm to drive out the air bubbles, after the air bubbles are removed, let the electrolyte fill the reference electrode chamber, gently put 20-40 purpose iodine in the side arm with a small spoon, drive out the air bubbles with a through a needle, apply a little vacuum silicone grease on the grinding mouth of the reference electrode, insert the platinum wire into the iodine carefully, be careful not to bend the platinum wire, at this time, you should check the reference electrode chamber carefully to ensure that the platinum wire of the reference electrode is all buried in iodine and that there is no residual air bubble in the whole reference electrode side arm, otherwise it must be reassembled, and after being sure that it is correct, fix it with a rubber band.

Titration cell maintenance
Sulfur titration cells should be kept in a cool place without air pollution;
The electrolyte cell should always be stored with a certain amount of electrolyte, and make the platinum sheet below the liquid level; never pull the reference electrode;
Electrolyte should be prepared frequently and kept fresh;
If there is no deionized water, use secondary distilled water instead, the resistance value should be above 2 megohms;
Always keep the electrolytic cell clean;
Under no circumstances, do not touch the platinum electrode with your hands;
Do not let the washing solution or acetone penetrate the reference side arm when cleaning, otherwise, the whole electrolytic cell should be reinstalled.

1. When using a coulometer, the buret and buret cell of the coulometer should be well sealed.

2. The size of the reagent titration should be determined by the amount of water in the test solution.

3.The titration should be well stirred.

4. When feeding the sample, the syringe tip should be protected from external contamination that may affect the measurement results, such as the operator's exhalation and contamination when wiping the syringe tip.

5. When the concentration of sulfuric acid is higher than 0.05%, a reverse reaction may occur and affect the measurement results.

6. A desiccator should be installed at the inlet of the reagent bottle to prevent the reagents from absorbing moisture in the air and causing the titration to drop, resulting in serious measurement errors.

7. In the process of titration by coulometric instrument, sometimes the phenomenon of a false endpoint may occur, i.e., the endpoint is reached in advance, resulting in low determination results.

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