I. Maintenance of Common Electrical Instruments
1.1
Electronic Balances
What to Do When an Electronic Balance Gives Unstable Readings? Let’s Take a Closer Look!
Electronic balances are essential weighing tools in our chemistry labs, and while we generally know how to use them, we often neglect the finer details of lab procedures and cleaning and maintenance. As a result, residue remains under the weighing pan after experiments, and stains are left on the pan itself. This reduces the balance’s sensitivity and accuracy and shortens its service life.
An electronic balance must be preheated for 10 minutes before use, but some students start weighing immediately after turning it on. The temperature of the item being weighed must match that of the balance; do not place objects that are too hot or too cold on the weighing pan. Volatile or corrosive reagents should be weighed in sealed containers. When weighing objects, be sure to close the balance’s side door, and promptly clean the top and bottom of the weighing pan as well as all corners after weighing is complete.
In humid conditions where substances have formed scale deposits, mechanical rust removal should be used: first, remove the rust layers from components by scraping, chiseling, or scraping; then sand and polish the surface with sandpaper; and finally, apply a protective coating. After the experiment, close the side door, cover the balance with a protective cloth, and store it in a dry, dark place.
How to Properly Maintain a Laboratory Ultrapure Water System?
How to Properly Maintain Your Centrifuge?
1.2
Electric Heating Constant-Temperature Drying Oven
The electric heating constant-temperature drying oven is a standard instrument used in chemical experiments for baking and drying items. The drying oven operates at 220 V and a temperature range of 100–110 degrees; the temperature should not be set too high. Never place flammable or volatile chemicals inside the oven. Items should be placed toward the center of the shelves without obstructing airflow to ensure uniform temperature distribution.
While the oven is in use, do not touch the oven body or switches with wet hands. If any abnormalities, odors, or smoke occur during normal operation, immediately turn off the power and have a professional inspect and repair the unit. Wipe down the inner chamber walls and the exterior surfaces of the equipment regularly to maintain cleanliness and enhance the clarity of the glass. If the drying oven will not be used for an extended period, unplug the power cord to prevent injury, and periodically run it for 2–3 days under normal operating conditions to remove moisture from the electrical components and prevent damage to related parts.
1.3
Constant-Temperature Water Bath
Take a look at your constant-temperature shaker—just look at the state of corrosion it’s in!
The constant-temperature water bath is primarily used in laboratories for distillation, drying, and concentration of chemical reagents. It can also be used for constant-temperature heating and other temperature tests, making it an essential tool for chemistry laboratories, analytical labs, and educational and research institutions. Users should be familiar with its operating procedures and precautions to ensure experiments proceed smoothly.
Never plug in the unit before adding water. It is best to use distilled water to prevent scale buildup. Do not overfill the bath, as water may overflow when it boils; however, the water level must not fall below half the bath’s capacity, and the heating element must remain submerged. Never operate the unit without water or with the water level below the partition, as this will damage the heating element.
When adding water, ensure that no water enters the control box to prevent electric shock. If the temperature control malfunctions, polish the silver contacts on the controller with fine sandpaper. After use, remove the sample being heated, turn off the power, drain the water from the chamber, and maintain accurate usage records for the instrument.
II. Use and Maintenance of Common Glassware
2.1 Beakers
What principles should be followed when cleaning glassware?
Because beakers have a uniform diameter throughout, they are the most commonly used reaction vessels for handling liquids, preparing solutions, and conducting simple chemical reactions. However, beakers should not be used in place of graduated cylinders for measuring liquids. When heating a beaker, place an asbestos mesh under it. Do not heat a beaker directly over a flame. Because beakers have a large base, direct heating with a flame will only heat a small area, causing uneven heating of the glass and potentially leading to shattering.
When heating a liquid in a beaker, the volume of liquid should not exceed one-third of the beaker’s capacity to prevent overflow when it boils. Before heating, the outer wall of the beaker must be wiped dry.
When heating corrosive chemicals, place a watch glass over the mouth of the beaker to prevent splashing. Do not store chemical reagents in beakers for extended periods to prevent dust from entering and to minimize evaporation of water from the solution. When stirring with a glass rod during dissolution or dilution, do not touch the bottom or sides of the beaker.
2.2 Test Tubes
What precautions should be taken when managing laboratory chemical reagents? Do you know?
Test tubes are laboratory containers used to hold small amounts of chemicals and to conduct small-scale reactions under heating conditions. When filling with a solution, do not exceed half the test tube’s capacity; when heating, do not exceed one-third of the test tube’s capacity. Use tweezers to place solid chunks into a test tube; do not allow solids to fall directly into the tube to prevent the bottom from breaking. Use a test tube clamp when heating; the clamp should be positioned one-third of the way from the mouth of the tube. Do not point the mouth of the test tube toward anyone.
Test tubes should not be heated directly; they must be preheated first. When heating a test tube containing a solid, tilt the opening slightly downward; when heating a liquid, tilt the tube at an angle of approximately 45°. During heating, ensure there are no water droplets on the outer wall of the test tube to prevent uneven heating and subsequent breakage. Do not cool the test tube abruptly after heating to prevent breakage. Do not wash the test tube before it has cooled to room temperature. Use the outer flame when heating.
2.3 Volumetric Flasks
How do we select volumetric flasks at home?
Volumetric flasks are primarily used to accurately prepare solutions of a specific molar concentration. They are slender, pear-shaped glass flasks with a flat bottom and a ground-glass stopper. The neck is marked with a calibration line; when the liquid inside reaches this line at the specified temperature, its volume equals the capacity indicated on the flask.
A volumetric flask cannot be used to prepare a solution with a volume less than its own capacity; that is, it can only be used to prepare a solution equal to its stated volume. Before use, check the volumetric flask for leaks, then rinse it thoroughly with tap water followed by distilled water and set it aside. Dissolve the solute in a beaker first, then transfer the solution to the volumetric flask; do not dissolve the solute directly in the volumetric flask. Control the amount of distilled water used to dissolve the solute. A glass rod must be used to guide the flow during transfer.
Volumetric flasks must not be heated. If the solute releases heat during dissolution, wait until the solution has cooled before transferring it, as an increase in temperature will cause the flask to expand, resulting in an inaccurate volume measurement. The total volume of solvent used to rinse the beaker must not exceed the volumetric flask’s mark; if it does, the preparation must be repeated. The prepared solution should be transferred to a reagent bottle for storage. A volumetric flask is intended only for preparing solutions, not for storing them, as the solution may corrode the flask, thereby affecting its accuracy.
After use, the volumetric flask should be rinsed thoroughly with water, capped, and a strip of paper inserted between the stopper and the mouth of the flask to prevent the stopper from sticking to the mouth. Finally, when recording the volume of the solution, generally retain four significant figures (XXX.0 mL).
2.4 Burette
A burette is a volumetric glass instrument capable of dispensing variable volumes of liquid; it is primarily used in titration analysis to measure the volume of a titrant. It can measure with an accuracy of 0.01 mL. A burette equipped with a glass stopper is an acid burette, while one fitted with a rubber tube containing a glass ball is a base burette.
Acid and base burettes must not be used interchangeably. Acid burettes are used for acidic substances and most neutral substances (especially strong oxidizing agents), while base burettes are used for alkaline substances (including bases and alkaline salts). Before use, check the burette for leaks; if water leaks, reapply Vaseline to the stopcock. Before filling, rinse the burette thoroughly with a washing solution and then with water, and rinse it with the solution to be used. Pour the working solution into the burette until it is filled to a level above the zero mark.
Be sure to check that the outlet tube of the burette is completely filled with solution; if bubbles are present, perform a degassing procedure. When adjusting the liquid level, keep it at “0” or at a specific mark below “0.” When titrating, clamp the burette vertically in the burette clamp. If using a burette with a glass bead, use your thumb and index finger to pinch the rubber tube next to the glass bead—do not pinch the glass bead itself, nor allow it to move up or down. Add the solution drop by drop in a continuous stream; as you approach the endpoint, add only one drop or half a drop until a distinct color change appears in the solution. When taking a reading, hold the upper, unmarked portion of the burette to keep it vertical. Look straight at the concave meniscus of the solution and read to the second decimal place, which corresponds to an estimate of 0.01 mL. After titration is complete, discard the remaining solution in the burette—do not pour it back into the original bottle. Immediately rinse the burette and hang it upside down on a wire rack.
III. Cleaning Common Glassware
Common glassware used in laboratories includes beakers, test tubes, burettes, pipettes, and volumetric flasks. During use, these instruments can become contaminated with grease, limescale, rust, and other residues. Failure to clean them thoroughly and promptly after use can lead to measurement errors and may even have a severely adverse impact on the instruments’ lifespan and performance. Therefore, glassware used in chemistry experiments must be cleaned thoroughly. The following section outlines some methods for cleaning glassware.
Simple instruments with easily removable substances, such as test tubes and beakers, should first be rinsed with tap water, then scrubbed with a test tube brush dipped in synthetic detergent, and finally rinsed with tap water. The instrument is considered clean when, upon inverting it, a uniform film of water forms on the walls and does not run down in streams.
For glassware with oil stains, first wash it with an alkaline alcohol solution, then wash it with a solution of laundry detergent or soap, and finally rinse it thoroughly with tap water.
For glassware with rust or limescale, soak it in a (1:3) hydrochloric acid solution, then rinse it thoroughly with tap water.
For glassware with Vaseline residue, first wipe off the Vaseline, then wash and boil the item in a solution of laundry detergent or soap, and finally rinse thoroughly with tap water after removing it.
For some glassware with delicate or complex structures, such as volumetric flasks and pipettes, which cannot be scrubbed with a brush, soak them in a cleaning solution for a certain period of time before washing.
Do not wipe clean glassware with your hands, a cloth, or paper to avoid recontamination.
IV. Maintenance and Cleaning of Common Optical Instruments
Optical instruments commonly used in laboratories—such as spectrophotometers, refractometers, prisms, and glass slides—are all precision optical instruments. Care and caution must be exercised during their use and maintenance; they must be operated strictly in accordance with the instructions. Do not loosen any of the instrument’s connecting parts without authorization, and do not drop or bump the instruments, to prevent damage to optical components and ensure accuracy. Test samples must be free of hard impurities. When testing solid samples, care must be taken to prevent scratching or indenting the surface of the refractive prism.
After use, never place the instrument directly in water for cleaning to prevent water from entering the optical system’s tubing. Open the prism and gently dry it with lens paper. Under no circumstances should anything other than lens paper come into contact with the prism, as this may damage its optical surfaces. The instrument should be stored in a dry, dust-free, grease-free environment free of harmful, flammable, or explosive gases to prevent corrosion or mold growth on optical components.
Optical instruments are prone to accumulating oil, moisture, and fingerprints during use, which can affect image quality and light transmittance. When cleaning the prisms and plane mirrors of a refractometer or the lenses of a microscope, first rinse them with distilled water. If there are stains on the mirror surfaces, they can be cleaned with a mixture of ethanol and diethyl ether. When cleaning, use specialized lens-cleaning paper or a cotton swab moistened with a small amount of cleaning solution. Wipe in one direction, using circular motions from the center of the lens outward. Never immerse these lenses in cleaning solution. Do not rub the lenses vigorously, as this may scratch the anti-reflective coating and damage the lens. After cleaning, dry them with lens-cleaning paper and store them away from light.
