Karl Fischer water content tester volumetric and Coulometric methods

Karl fischer method is divided into karl fischer volumetric method and coulometric method.

The volumetric method is suitable for samples with higher water content (0.1 to 500 mg).

Coulometric method is suitable for samples with low water content (10 μg ~ 100 mg)

The volumetric Karl Fischer moisture analyzer is a precise titration of Karl Fischer reagents to the test sample through a precision metering system.

In the sealed titration cell of the sample, the total amount of water reacted in the sample is calculated by calculating the total amount of Karl Fischer reagent consumed when reaching the end point, and then the water content in the sample is obtained.

advantage:

1. The judgment of the accurate reaction end point can accurately detect the moisture content in various liquids, solids and some gas samples;

2. As long as there is an appropriate amount of water in the titration tank, the water in the sample can be measured quickly and accurately;

3. The time for moisture measurement is shorter than that of the heating method rapid moisture analyzer, from tens of seconds to several minutes on average;

4. For some insoluble and insoluble substances, as well as samples with other volatile substances that are not suitable for heating detection. Accurate detection can be carried out by auxiliary equipment, such as cassette headspace sampler (cassette furnace);

Disadvantage:

1. The detection accuracy is related to many factors of the instrument, and the requirements for the design and manufacture of the instrument are high;

2. Auxiliary equipment and toxic Karl Fischer reagents are required in the detection process; it is easy to cause environmental pollution;

Compared with the volumetric method, the operation mode of the coulometric Karl Fischer moisture analyzer is reversed.

The reagent (divided into catholyte and anolyte) is placed in a closed coulometric titration cell in advance. After equilibration, the tested sample is added to the Karl Fischer reagent environment of the sealed titration cell, and the iodine produced by electrolysis of the water in the titration cell is calculated by calculating The electricity of electrolytic iodine consumed after the complete reaction is used to calculate the total amount of water that has been reacted. Because the unit of electricity is coulomb, it is called a coulometric moisture meter, and it can also be called a coulometric moisture meter.

Advantage:

1) Coulometric Karl Fischer moisture analyzer detects moisture content by calculating the amount of electricity generated by anode electrolysis of iodine. In the case of reliable quality, the moisture content can be determined very accurately,

2) When detecting trace moisture, the electrolysis speed can meet the requirements, and it is often possible to detect a sample in ten to ten seconds. High detection efficiency;

3) Compared with the volumetric Karl Fischer moisture analyzer, the coulometric Karl Fischer moisture analyzer does not require a precision titration system.

System, no transmission structure and high-precision measurement system, the possible instrument failures are also greatly reduced;

Disadvantage:

1) Since the coulometric method measures the water content by calculating the electricity generated by the electrolysis of iodine, it must be ensured that the sample to be tested cannot have the following reaction:

A, react with Karl Fischer reagent to generate water;

B. The sample cannot consume iodine or react to release iodine in the detection pool; affected by this: some samples such as iron salts, nitrites, ketone salts, oxides, hydroxides, aldehydes, ketones, metals Peroxides, strong oxidants, strong acids, boron-containing compounds, etc. cannot be detected by coulometric method;

2) Since the electrolysis speed of iodine in the electrolysis cell is very slow, such as when the water content in the sample is high, the electrolysis speed cannot keep up with the reaction speed of water, which will lead to a significant decrease in detection efficiency, long electrolysis time and shorten the working life of the instrument. , and if it is detected by reducing the sample volume, the reliability of the sample results will be reduced, so the coulometric Karl Fischer moisture analyzer is recommended to be used for the detection of moisture content below 1000PPM