Differences Between Coulometric and Volumetric Moisture Analyzers

What is the difference between coulometric and volumetric moisture analyzers? Both are Karl Fischer moisture analyzers based on the method proposed by Karl Fischer in 1935. Below we explain the differences in detail:

volumetricKarl Fischer Moisture Analyzers

A precision metering system titrates Karl Fischer reagent into the sealed cell containing the sample; the reagent consumed at the endpoint yields the water reacted and hence the moisture content.

Advantages:

1. Accurate endpoint detection enables precise moisture determination in various liquids, solids, and some gases;

2. With an appropriate total water amount in the cell, sample moisture is measured quickly and accurately;

3. Measurement time is shorter than heated rapid analyzers—typically tens of seconds to a few minutes;

4. Hard-to-dissolve or volatile-containing samples can be measured precisely with auxiliary equipment such as a Karl Fischer headspace sampler (Karl Fischer oven);

Disadvantages:

1. Accuracy depends on multiple instrument factors, placing high demands on design and manufacturing;

2. Auxiliary equipment and toxic Karl Fischer reagents are required, which can pollute the environment;

coulometricKarl Fischer Moisture Analyzers

The coulometric method works in reverse: the sealed cell is pre-charged with Karl Fischer reagent (catholyte and anolyte); after equilibrium the sample is added, and the water reacted is calculated from the electricity consumed by electrolytically generated iodine—hence the name coulometric (electrolytic) moisture analyzer.

Advantages:

1. A coulometric analyzer determines moisture from the charge used to electrogenerate iodine; with a well-built instrument, accuracy is very high.

2. For trace moisture, a sample is often measured in ten to tens of seconds, with extremely high efficiency;

3. Unlike volumetric instruments, the coulometric version needs no precision titration system or high-precision metering, greatly reducing potential failures;

Disadvantages:

1. Because moisture is calculated from the electrogeneration charge, the sample must be free of these side reactions:

A. Reactions with Karl Fischer reagent that generate water;

B. The sample must neither consume nor release iodine in the cell. Hence iron salts, nitrites, ketone salts, oxides, hydroxides, aldehydes, ketones, metal peroxides, strong oxidants, strong acids, and boron compounds cannot be tested coulometrically;

2. Iodine electrolysis is slow: with high-moisture samples it cannot keep pace, cutting efficiency and shortening instrument life. Coulometric analyzers are therefore recommended below 1000PPM.

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