1、 Classification of common desiccants
Desiccants used in middle school can be divided into three types based on their acidity and alkalinity:
1. Acidic desiccant: concentrated sulfuric acid, phosphorus pentoxide, silica gel
① Concentrated sulfuric acid (strong oxidizing acid)
② Phosphorus pentoxide (acidic white powder)
③ Silicone (It is a semi transparent, porous solid with a large inner surface area and good adsorption properties. It has a strong adsorption effect on water. Silicone containing cobalt salts is called color changing silicone, which appears blue when not absorbing water and pink when saturated with water.)
2. Alkaline desiccant: Alkali lime Ca0
① Alkali lime (it is a white solid mainly composed of CaO and NaOH)
② Ca0 (it is a white solid, alkaline oxide)
3. Neutral desiccant: anhydrous calcium chloride
2、 Classification of Common Gases
The common gases in middle school can be roughly divided into three types based on their acidity and alkalinity:
1. Acidic gases: carbon dioxide, sulfur dioxide, nitrogen dioxide, chlorine gas, hydrogen sulfide, hydrogen chloride, hydrogen bromide, hydrogen iodide, etc
2. Alkaline gas: Ammonia gas
3. Neutral gases: nitrogen, oxygen, hydrogen, methane, etc
3、 Selection of Desiccant
The general principle for selecting desiccants is that the selected desiccant should not react with the substance being dried, and no new substances should be introduced during the drying process.
Specifically, it can be considered from three perspectives:
1. The acidity and alkalinity of the desiccant and the dried substance should be consistent
① Acidic desiccants cannot dry alkaline gases, but can dry acidic and neutral gases
For example, alkaline gases such as ammonia cannot be dried with concentrated sulfuric acid or phosphorus pentoxide
② Alkaline desiccants cannot dry acidic gases, but can dry alkaline and neutral gases
For example, alkaline lime cannot be used to dry acidic gases such as carbon dioxide and hydrogen sulfide
③ Neutral desiccants can dry various gases (only considering acidity and alkalinity).
2. There should be no oxidation-reduction reaction between the desiccant and the dried substance
For example, concentrated sulfuric acid cannot be used to dry reducing gases such as hydrogen sulfide, hydrogen bromide, and hydrogen iodide.
