The cleanliness of glassware directly affects experimental accuracy. Residual contaminants can cause cross-contamination, abnormal reactions, or even incorrect experimental results. Different contaminants require different cleaning methods. This article details cleaning solutions for various contaminants, cleaning steps, and precautions.
The first principle of glassware cleaning is: choose the cleaning agent according to the contaminant type. Use organic solvent for organic stains; use acid/alkali cleaning agents for inorganic stains; use chelating agents for insoluble substances. Do not blindly use chromic acid cleaning solution (toxic, environment-polluting).
1. Common Cleaning Agents & Selection
According to the contaminant type, choose the appropriate cleaning agent:
| Cleaning Agent | Main Ingredient | Suitable Stains | Precautions |
|---|---|---|---|
| Chromic Acid Solution | Potassium dichromate + conc. sulfuric acid | Organic residues, insoluble substances | ❌ Toxic, carcinogenic. Prefer other agents |
| Alkali Cleaning Agent | Sodium hydroxide, sodium carbonate | Oils, grease, organic matter | ✅ Safe, but do not soak glass too long |
| Organic Solvent | Acetone, ethanol, methanol | Organic stains, grease | ✅ Safe, but flammable. Use in fume hood |
| Acid Cleaning Agent | Hydrochloric acid, nitric acid, aqua regia | Inorganic stains, metal ions | ✅ Effective, but note corrosion resistance |
| Detergent (Household) | Surfactant | Light oil stains | ✅ Safe, suitable for daily cleaning |
Chromic acid solution contains hexavalent chromium (Cr&sup6;+), which is toxic and carcinogenic, and harmful to the environment. Many laboratories have now banned or restricted its use. It is recommended to prioritize alkali cleaning agents or organic solvents as alternatives.
2. Cleaning Methods for Different Stains
Recommended Cleaning Agents
✅ Organic solvent (acetone, ethanol, ether) → oils easily dissolve in organic solvents
✅ Alkali cleaning agent (5%~10% sodium hydroxide solution) → oils saponify in alkaline solution
✅ Household detergent (e.g., dish soap) → contains surfactants, can emulsify oils
Cleaning Steps
- Use paper towel or fat-free cotton to wipe off visible grease as much as possible (reduces organic solvent usage)
- Use acetone or ethanol to rinse 2~3 times (about 5mL each time, gently shake)
- Use detergent solution to scrub (use soft brush)
- Use tap water to rinse 5~10 times
- Use distilled water to rinse 3 times (the last rinse should make water evenly distribute on the wall, not bead up)
Recommended Cleaning Agents
✅ Proteinase K solution (or household protease) → decomposes proteins
✅ Sodium hypochlorite solution (bleach) → oxidatively decomposes organic matter
✅ Conc. sulfuric-nitric acid mixture → strong oxidation decomposition (use with caution)
Cleaning Steps
- Use tap water to rinse off most residues
- Fill with proteinase solution, soak at room temperature for 30 minutes (or 50°C water bath for 10 minutes)
- Pour out enzyme solution, use tap water to rinse 5 times
- Use detergent to scrub, then rinse with tap water
- Use distilled water to rinse 3 times
Recommended Cleaning Agents
✅ Dilute hydrochloric acid (5%~10% HCl) → dissolves most metal oxides
✅ Dilute nitric acid (5%~10% HNO&sub3;) → dissolves more stubborn metal stains
✅ EDTA solution → chelates metal ions, excellent effect
Cleaning Steps
- Use dilute hydrochloric acid to soak for 30 minutes (for stubborn stains, use dilute nitric acid)
- Pour out acid, use tap water to rinse 10 times (fully wash off acid)
- Use detergent to scrub, removing possible organic residues
- Use distilled water to rinse 3 times
Recommended Cleaning Agents
✅ EDTA solution → chelates calcium ions, dissolves barium sulfate
✅ Ammonia water → dissolves silver chloride, silver hydroxide
✅ Aqua regia (conc. HCl:conc. HNO&sub3; = 3:1) → dissolves extremely insoluble substances (use with caution)
Cleaning Steps
- According to the insoluble substance type, choose the corresponding chelating agent or solvent
- Soak for 1~2 hours (or warm to accelerate dissolution)
- Pour out soaking solution, use plenty of tap water to rinse
- Use distilled water to rinse 3 times
3. Special Cleaning Requirements for Precision Measuring Instruments
The inner wall cleanliness of precision measuring instruments directly affects liquid outflow volume. If there is oil stain on the inner wall, the liquid will bead up and cannot flow down evenly, causing inaccurate volume.
After cleaning, use distilled water to rinse the wall, and observe whether the water evenly distributes and does not bead up. If there are still water beads, it means oil stain is not fully removed, and needs re-cleaning (use detergent or organic solvent again).
The inner wall of precision measuring instruments (volumetric flasks, pipettes, burettes) needs to be smooth. Do not use hard brushes to scrub, otherwise it will scratch the inner wall and affect volume accuracy. Should use soft brushes or soak in detergent solution.
The tip (nozzle) of pipettes and burettes is the easiest place to retain stains, and is also the key part affecting volume accuracy. When cleaning, should repeatedly suck and discharge cleaning agent/water to ensure the inside of the tip is also cleaned.
4. Cleaning Precautions
- Do not use chromic acid solution to clean precision measuring instruments: Chromic acid will corrode the glass surface, reduce mechanical strength, and is difficult to fully wash off (chromium residue will affect experiments).
- Alkali cleaning agents should not soak glass for too long: Alkali will corrode glass (especially borosilicate glass), making the surface rough. Soaking time should be < 2 hours.
- Organic solvents need to be used in a fume hood: Acetone, ether, and other organic solvents are flammable and toxic, must be operated in a fume hood, away from open flames.
- Must thoroughly rinse after cleaning: Any cleaning agent residue will affect experiments. Alkaline cleaning agent residue needs dilute acid neutralization before rinsing; acidic cleaning agent residue needs dilute alkali neutralization before rinsing.
- Do not use a dishwasher to clean precision glassware: The high temperature, high-pressure water flow, and alkaline detergent in dishwashers will damage the glass surface and reduce accuracy.
- Cleaned glassware should be air-dried naturally: Do not use an oven to dry (uneven temperature will cause breakage), and also do not directly use cloth or paper towel to wipe dry (will leave fiber residue). Should be placed upside down on a drying rack to air-dry naturally.
5. Frequently Asked Questions (FAQ)
1. Inner wall evenly wetted: After distiled water rinsing, water should evenly distribute on the wall, not bead up. If there are water beads, it means oil stain is not removed.
2. No visible stains: Observe against light, should be no visible residues.
3. No cleaning agent residue: After distiled water rinsing, should be no foam (if there is foam, it means detergent is not fully rinsed off).
1. Alkali cleaning agent + organic solvent: Suitable for most organic stains.
2. Commercial glass cleaning agent: Such as "glassware cleaning agent" (chromium-free, safe and environmentally friendly).
3. Dilute nitric acid + EDTA: Suitable for inorganic stains and metal ions.
4. Ultrasonic cleaning: With ordinary detergent, works very well for instruments with complex shapes.
• PTFE (Teflon): Resists almost all chemical reagents, can be cleaned with acid, alkali, and organic solvents.
• Polypropylene (PP): Resists acid and alkali, but not high temperature resistant (cannot be boiled for disinfection), also not resistant to strong oxidants (e.g., chromic acid).
• Polystyrene (PS): Not resistant to most organic solvents (will dissolve or craze).
Before cleaning plastic ware, should confirm the material and its chemical resistance.
1. Precision measuring instruments (e.g., volumetric flasks) are not recommended for ultrasonic cleaning, may affect volume accuracy.
2. Ultrasonic cleaning should be preceded by manual pre-cleaning, to remove most stains and avoid re-deposition on the wall.
3. Choose the right cleaning agent: Ultrasonic cleaning usually uses detergent solution.
1. Use dilute acid or dilute alkali to soak: Neutralize odor molecules.
2. Use activated carbon adsorption: Put a small amount of activated carbon inside the instrument, add water, soak for 24 hours.
3. Use ethanol or acetpone to rinse: Organic solvents can dissolve odor molecules.
4. Fully air-dry: Odors sometimes disappear as moisture evaporates. Place the instrument upside down in a ventilated area, fully air-dry for 2~3 days.