In air and waste gas monitoring in environmental monitoring, standard gases are as important as standard solutions and standard substances. They are the basis for testing and testing methods, evaluating sampling efficiency, drawing standard curves, calibrating analytical instruments, and performing testing quality control. The method of preparing low-concentration standard gas is usually divided into: static gas distribution method and dynamic gas distribution method.
The static gas distribution method is to add a certain amount of gaseous or vaporous raw material gas into a container of known volume, then fill it with diluent gas, and mix it evenly. The concentration of the standard gas is calculated according to the amount of diluent gas added to the raw gas and the volume of the container. The raw material gas used can be pure gas or mixed gas of known concentration, and its purity needs to be determined by a suitable analytical method.
The advantages of the static gas distribution method are that the equipment used is simple and easy to operate. However, because some gases have more active chemical properties, chemical reactions may occur when they contact the container wall for a long time. At the same time, the container wall also has an adsorption effect, which will cause the concentration of the prepared gas to be inaccurate. Or its concentration changes with the storage time, especially the preparation of low-concentration standard gas, which often causes larger errors. For standard gases that are chemically inactive and do not use large amounts, it is easier to prepare with this method.