## Common ground
All options are types of silica gel, a porous material used primarily for adsorption and drying.
## Options at a glance
**2811.22 – Pure silica gel**
- **Defining trait:** Unmodified, inorganic silica gel without added indicators or organic modifiers.
- **Typical examples:** Desiccant packets in electronics, pharmaceuticals.
- **Best fit when:** Used purely for moisture absorption with no visual indicator needed.
**3824.99 – Cobalt-doped humidity indicator gel**
- ****Defining trait:** Contains cobalt salts that change color (blue to pink) when hydrated, indicating moisture levels.
- **Typical examples:** Desiccants in sealed packaging with visible status indicators.
- **Best fit when:** Visual monitoring of humidity is required.
**3910.00 – Methyl silica gel**
- **Defining trait:** Chemically modified with methyl groups (organosilica), altering surface properties for specific adsorption.
- **Typical examples:** Adsorbing non-polar compounds, chromatography, specialty drying.
- **Best fit when:** Requires selective adsorption of organic or non-polar substances.
## Key differences
| Feature | 2811.22 (Pure) | 3824.99 (Cobalt-doped) | 3910.00 (Methyl-modified) |
|-----------------------|----------------|------------------------|----------------------------|
| Color change | No | Yes (blue → pink) | No |
| Chemical modification | None | Cobalt salts added | Methyl groups grafted |
| Primary use | General drying | Humidity indication | Selective adsorption |
## How to choose
- Does it change color when wet? → 3824.99
- Is it modified with organic groups (e.g., methyl)? → 3910.00
- Is it unmodified and used for general drying? → 2811.22
## Overlaps & pitfalls
- A methyl silica gel may also be colored, but the defining trait is chemical modification, not color.
- Cobalt-doped gel can be pure silica gel *with* additives—so it’s a subset of 2811.22 in base material but distinct due to function.
- **Priority:** Choose based on the product's MAIN function: visual indicator → 3824.99; selective adsorption → 3910.00; general drying → 2811.22