Loss on ignition
Loss on ignition (LOI) is a test used in inorganic analytical chemistry and soil science, particularly in the analysis of minerals and the chemical makeup of soil. It consists of strongly heating ("igniting") a sample of the material at a specified temperature, allowing volatile substances to escape, until its mass ceases to change. This may be done in air, or in some other reactive or inert atmosphere. The simple test typically consists of placing a few grams of the material in a tared, pre-ignited crucible and determining its mass, placing it in a temperature-controlled furnace for a set time, cooling it in a controlled (e.g. water-free, CO2-free) atmosphere, and redetermining the mass. The process may be repeated to show that mass-change is complete. A variant of the test in which mass-change is continually monitored as the temperature is changed, is thermogravimetry.
The loss on ignition is reported as part of an elemental or oxide analysis of a mineral. The volatile materials lost usually consist of "combined water" (hydrates and labile hydroxy-compounds) and carbon dioxide from carbonates. It may be used as a quality test, commonly carried out for minerals such as iron ore. For example, the loss on ignition of fly ash consists of contaminant unburnt fuel.
In pyroprocessing industries such as lime, calcined bauxite, refractories or cement manufacture, the loss on ignition of the raw material is roughly equivalent to the loss in mass that it will undergo in a kiln. Similarly, for minerals, the loss on ignition represents the actual material lost during smelting or refining in a furnace or smelter. The loss on ignition of the product indicates the extent to which the pyroprocessing was incomplete. ASTM tests are defined for limestone and lime and cement among others.
Soil is composed of living organisms, water, carbonates, carbon containing material, decomposing matter and much more. To determine how much one of these soil components make up the entire soil mass, the LOI procedure is implemented. Initially, the researcher will take the mass of the sample prior to LOI and then place the sample into a heating device. Depending on what the researcher is trying to determine in the soil, the temperature of the device can be set to the corresponding temperature. The soil sample is kept at this temperature for an extended period of time after which it is removed and allowed to cool down before re-weighing the sample. The amount of mass lost after the LOI treatment is equal to the mass of the component the researcher is trying to determine. The typical set of materials needed to use LOI include: a high precision mass balance, a drying oven, temperature controlled furnace, preheated crucibles and soil sample from the location of interest. Typically, this method is used to determine water content levels, carbon levels, amount of organic matter levels, amount of volatile compounds. LOI is also used in the cement industry which operates the furnace in the 950° Celsius range, combustion engineers also use LOI but at temperatures lower than 950° Celsius range. A complete procedure for LOI can be found below, keep in mind however that some of the steps may need to be modified depending on the intended use of LOI.
There are many ways to properly utilize loss on ignition for scientific research. A soil sample left overnight in a drying oven at 100° Celsius would have its water content completely evaporated by morning. This could allow the researchers to determine the amount of water initially in the soil sample and its porosity by comparing the change in weight of the sample before and after the evaporation. This new weight of the sample is called the dry weight and its previous weight is called the wet weight. A general procedure of how to perform a loss on ignition is as follows:
- Weigh the empty crucible that the sample is to be placed in and record its weight in a lab book.
- Place the sample in the empty crucible and weigh the crucible again with the sample in it. The new weight minus the empty crucible weight is the sample’s wet weight.
- Place the sample in the drying oven or blast furnace as required.
- Set the oven or furnace to the desired temperature. If the researcher wants to find the dry weight of the soil then the furnace would need to be 100° Celsius.
- Leave the sample in the furnace for the desired length of time. If the researcher wanted to know the sample's dry weight and is using a furnace set at 100° Celsius, then the researcher would usually leave the furnace on overnight.
- Open the oven but also back away from it at the same time since the hot air escaping from the furnace can burn bare skin.
- Allow the oven and sample to cool down before removing the sample from the oven.
- Weigh the crucible with the sample again. Subtract the empty crucible weight from this new weight and that is the sample's dry weight.
In many research labs, the use of asbestos gloves is required when operating the furnace as it can reach very hot temperatures. The use of face masks is also recommended at higher temperatures to ensure safety of researchers and junior lab members. It is also recommended that researchers performing LOI procedure remove all jewelry and watches as they are great conductors of heat. When removing samples at high temperatures, these accessories can easily heat up and result in burns.
The cement industry uses the LOI method by heating up a cement sample to 900-1000° Celsius, and it is heated until the mass of the sample stabilizes. Once the mass has stabilized the mass lost due to LOI is determined. This is usually done to determine the high water content in the cement or carbonation as these reduce the quality of cement. High losses are usually due to poor storage conditions of cement or manipulation of cement quality by suppliers. This ensures that the cement used in a site is of the correct composition to meet safety protocols and requirements of customers.
In the mining industry, the use of LOI is vital in determining moisture and volatile material present in the rock. Hence when performing whole-rock analysis to determine total volatiles the LOI method is used. In order to remove all volatiles and to oxidize all iron into iron oxides, the temperature of the LOI is set to 900-1000° Celsius.
- ASTM C 25 Standard test methods for chemical analysis of limestone, quicklime and hydrated lime
- ASTM C 114 Standard test methods for chemical analysis of hydraulic cement
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