Many industries, including the food, chemical, and pharmaceutical product industries, depend heavily on exact knowledge of the moisture content of their products as part of quality control.
Such measurements need to be rapid and reproducible in order to allow for immediate correction of the workflow if any flaw appears in the final product.
Both direct and indirect methods are used. Only the direct methods actually measure the moisture content; the indirect techniques only calculate it from other indirect indicators.
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Direct Methods
Thermogravimetric Measurement
Many devices for LOD measurement use the thermogravimetric principle. This uses the total loss of weight incurred by the sample on drying to calculate the moisture content.
Drying ovens are based on convection heating of the sample by circulating hot air. Sometimes a vacuum is added to speed up the drying process.
While capable of processing multiple samples, and handling large weights of a sample, this technique does take a long time to give the results, typically hours, and requires expert operator input.
The margin for error is therefore larger. This method is very often the reference or official method in a number of moisture content standards.
Infrared radiation is used in many moisture analyzers, such as halogen moisture analyzers which are used to produce infrared radiation from a halogen lamp. The weight of the sample is measured and recorded continuously and once it becomes constant the drying is stopped.
The difference in the weight of the sample at the end of drying is used to calculate the moisture percentage. Halogen lamps are used in preference to ordinary infrared generators as they are much lighter and therefore achieve maximal heat output very fast, and allow excellent control of the heating process as they heat up and cool down rapidly.
They also distribute the heat uniformly over the sample surface which promotes good reproducibility. The infrared radiation in such devices is absorbed by the moisture analyzer and this further reduces the time taken to heat up the sample. The infrared and halogen moisture analyzers are destructive to the sample.
Microwave radiation is also an extremely rapid method of drying up a sample but the temperatures achieved are very high, making it suitable only for very thermostable materials.
Larger samples can be used but the level of control of heating is reduced. Like the infrared method, the sample is typically destroyed by the analysis. It is also not useful if the moisture content is below 2%.
Developing a Method
In all these cases, a method must be developed. This refers to setting the parameters such as time, drying temperature, and the weight of the sample, according to the requirement for each sample. This can be saved as a program for the next measurement of a similar sample.
This type of adjustment is necessary to achieve values which agree with those set by the reference methods, such as the oven drying method or the Karl Fischer titration method, whichever is used. In other cases, a deviation is acceptable as long as it is measured and repeatable with each measurement.
The LOD method does not selectively determine the water content of a sample, only the moisture content.
Other Physical Methods
Phosphorous Pentoxide Method
In this method, phosphorous pentoxide is placed with the sample in a closed container which is then heated. Phosphorous pentoxide is a powerful desiccating agent if placed in proximity to materials with which it is chemically non-reactive. It absorbs water from the sample.
The final increase in weight of the chemical is measured to give the moisture content of the desiccated sample. Phosphorous pentoxide is a dangerous chemical.
Distillation
Another method of moisture determination is inexpensive but uses toxic solvents while yielding only relatively accurate results. It is based upon separating and measuring the moisture directly after separating it from the sample by heating.
Chemical Methods for Moisture Analysis
Karl Fischer Titration
The most accurate and specific method for determining the water content of a substance is Karl Fischer (KF) titration. It is based upon the reaction of iodine with sample water, in presence of alcohol solvent, sulfur dioxide, and a base. It uses up the total sample water, including the water of crystallization and surface absorbed water, in the redox reaction.
The reagent titer required until this end point is used is the basis on which the water content is determined. Both volumetric and coulometric variants are available, differing in the size of the sample, the water content measured, the accuracy and the way in which the total water used up is calculated.
Automated KF titration has made this technique convenient and rapid for precise determination of the water in a sample. The volumetric method is more appropriate for higher water content but is slightly more labor intensive. All solids must be dissolved in an appropriate solvent for analysis, and this may be difficult with some solids.
Calcium Carbide Method
The calcium carbide method is cost-effective and uses a combination of materials to react with water. The end product is potentially explosive, and so the method requires great care. Moreover, the total water in the sample does not take part in the reaction and this means that repeated calibration is a necessity.
Spectroscopic Methods
The use of spectroscopy in moisture analysis includes infrared, microwave, and nuclear magnetic resonance (NMR) spectroscopy to determine surface and total moisture, respectively. These are indirect methods of measurement can take a long time for determination because of the need to calibrate using multiple samples, and are therefore not much used in industrial moisture analysis.
Other Methods
These include gas chromatography, density determination, and refractometry.
Indirect Methods
These are based upon taking measurements of moisture in different grains of the sample by a moisture sensor, for example, which are then used to calculate the moisture content of the sample. An advantage is that instant measurements are taken and thus changes in the moisture content can be monitored over time.
The moisture sensing technology may involve the use of capacitive, microwave, infrared, neutron or gamma ray or conductive methods. They are not suitable if the water content is below 0.1 percent.
- The capacitive method is convenient, reproducible, and accurate, depending upon the shift in the dielectric constants of water and the test material, which is between the sensor capacitor electrodes.
- Infrared sensors measure the moisture content by passing an infrared beam through the material, and measuring the ratio of absorbed to reflected wavelengths.
- Microwave sensors measure the loss of energy of microwaves passed through the sample at the point of emission, and the difference in their speed, to calculate the moisture content. They are very accurate in measuring moisture in very fine uniformly distributed materials.
- A conductive sensor uses the difference in conductivity from the specific conductivity caused by moisture content, which is measured between two electrodes which are thrust into the material to calculate the moisture content.
- Neutron ray and gamma ray beams can measure water content in thick and dense materials but are quite expensive. They are also strictly regulated due to their radioactive nature.
Further Reading