A, the classification of common desiccant

Desiccants in middle school can be divided into three types according to their acidity and alkalinity:

1. Acid desiccant: concentrated sulfuric acid, phosphorus pentoxide and silica gel.

① concentrated sulfuric acid (strong oxidizing acid)

② Phosphorus pentoxide (acidic white powder)

(3) Silica gel (it is a translucent porous solid with large internal surface area, which has good adsorption and strong adsorption on water. Silica gel containing cobalt salt is called chromotropic silica gel, which is blue when it does not absorb water and pink when it is saturated with water. )

2. Alkaline desiccant: alkali lime, Ca0

(1) alkali lime (it is a white solid, mainly composed of CaO and NaOH)

(2) ②Ca0 (it is a white solid, alkaline oxide),

3. Neutral desiccant: anhydrous calcium chloride

Second, the classification of common gases

Common gases in middle school can be roughly divided into three types according to their acidity and alkalinity:

1. Acid gases: carbon dioxide, sulfur dioxide, nitrogen dioxide, chlorine, hydrogen sulfide, hydrogen chloride, hydrogen bromide, hydrogen iodide, etc.

2. Alkaline gas: ammonia

3. Neutral gases: nitrogen, oxygen, hydrogen, methane, etc.

Third, the choice of desiccant

The general principle of desiccant selection is: the selected desiccant cannot react with the dried substance, and no new substance can be introduced during the drying process.

Specifically, it can be considered from three angles:

1. The drying agent should be consistent with the acidity and alkalinity of the material to be dried.

① Acid desiccant can not dry alkaline gas, but can dry acid gas and neutral gas.

For example, alkaline gases such as ammonia cannot be dried with concentrated sulfuric acid or phosphorus pentoxide.

② Alkaline desiccant can not dry acidic gas, but can dry alkaline gas and neutral gas.

For example, acid gases such as carbon dioxide and hydrogen sulfide cannot be dried with alkaline lime.

③ Neutral desiccant can dry all kinds of gases (only from the point of view of acidity and alkalinity).

2. Redox reaction cannot occur between the desiccant and the dried substance.

For example, reducing gases such as hydrogen sulfide, hydrogen bromide and hydrogen iodide cannot be dried with concentrated sulfuric acid.