Many industries, including the food, chemical, and pharmaceutical product industries, rely heavily on precise information of the moisture content of their products as a part of quality control. Such measurements required to be fast and reproducible in order to permit for immediate correction of the work flow 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 alternative indirect indicators.
Direct Methods :
Many devices for LOD measurement use the thermo gravimetric 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.
Infrared radiation is used in many moisture analysers, such as halogen moisture analysers 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.
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.
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.
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.
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.
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