Basic Laboratory Techniques Pre Lab Questions Answers

Basic Laboratory Techniques: Pre-Lab Questions & Answers

Working in a laboratory, whether for research, education, or industrial purposes, requires a solid understanding of basic laboratory techniques. These techniques ensure the safety of the researcher, the accuracy of experiments, and the reproducibility of results. Before embarking on any lab work, it's crucial to thoroughly understand the procedures and answer pre-lab questions. This article will delve into several fundamental techniques, accompanied by example pre-lab questions and detailed answers.

Section 1: Measurement and Pipetting

Accurate measurement is paramount in any laboratory setting. This section covers volumetric glassware and pipetting techniques.

Pre-Lab Question 1: What are the differences between a beaker, Erlenmeyer flask, and volumetric flask? Which one would you use for precise measurements and why?

Answer:

• Beaker: A cylindrical container with a lip for pouring, primarily used for mixing and heating liquids. It's not designed for precise measurements due to its relatively large tolerance.
• Erlenmeyer flask (conical flask): A conical container with a narrow neck, used for mixing, heating, and storing liquids. It offers better accuracy than a beaker but still isn't ideal for precise measurements.
• Volumetric flask: A pear-shaped flask with a long, narrow neck marked with a single calibration line indicating a precise volume. It's specifically designed for preparing solutions of a known concentration with high accuracy.

For precise measurements, you would use a volumetric flask. Its narrow neck minimizes errors in reading the meniscus, and the single calibration line ensures that the contained volume is accurate to within a very small tolerance.

Pre-Lab Question 2: Describe the proper technique for using a micropipette. What are potential sources of error?

Answer:

Using a micropipette requires precision and careful technique:

1. Select the correct pipette: Choose a pipette with a volume range appropriate for the desired volume.
2. Attach a sterile tip: Firmly attach a sterile tip to the pipette.
3. Set the desired volume: Rotate the volume adjustment dial to the desired volume.
4. Aspirate: Depress the plunger to the first stop, then immerse the tip into the liquid and slowly release the plunger to draw up the liquid.
5. Dispense: Place the tip against the side wall of the receiving vessel and slowly depress the plunger to the first stop to dispense the liquid. Then, depress the plunger to the second stop to expel any remaining liquid in the tip.
6. Eject the tip: Press the tip ejector button to discard the tip into an appropriate waste container.

Potential sources of error:

• Incorrect volume setting.
• Air bubbles in the liquid.
• Incorrect angle during aspiration or dispensing.
• Improper tip attachment.
• Forgetting to pre-wet the pipette tip (especially crucial for viscous liquids).
• Not allowing the liquid to drain completely.
• Contamination of the pipette tip.

Pre-Lab Question 3: What is a meniscus, and how do you correctly read a meniscus when measuring liquids?

Answer:

The meniscus is the curved upper surface of a liquid in a tube or container. It's caused by the surface tension of the liquid and its interaction with the container walls. For most liquids (like water), the meniscus is concave (curved downwards).

To correctly read a meniscus:

1. Position your eye level with the meniscus.
2. Read the bottom of the meniscus for liquids that form concave menisci (e.g., water).
3. Read the top of the meniscus for liquids that form convex menisci (e.g., mercury).

Failure to read the meniscus correctly can lead to significant measurement errors.

Section 2: Heating and Mixing Techniques

Heating and mixing are crucial steps in many laboratory procedures. Understanding the appropriate techniques is essential for safety and accuracy.

Pre-Lab Question 4: Explain the differences between heating a liquid in a beaker versus an Erlenmeyer flask. When would you choose one over the other?

Answer:

Heating a liquid in a beaker poses a higher risk of bumping (sudden, violent boiling) and splashing due to its wide opening. An Erlenmeyer flask is generally preferred for heating liquids because its narrow neck reduces the risk of splashing and provides better control over boiling. The sloped sides also help prevent loss due to evaporation.

You would choose a beaker for heating if rapid mixing or visual observation of the solution is needed, or if the risk of bumping is minimal (e.g., when heating a relatively small volume of a low-boiling liquid). You would choose an Erlenmeyer flask if careful control of boiling, minimized splashing, and reduced evaporation are crucial.

Pre-Lab Question 5: Describe the proper technique for using a Bunsen burner. What are the safety precautions you should take?

Answer:

Using a Bunsen burner requires careful attention to safety:

1. Ensure that the gas supply is turned off before lighting the burner.
2. Check that the air intake is closed.
3. Strike a match or use a lighter and hold it near the top of the burner.
4. Turn the gas supply on slowly.
5. Adjust the air intake to obtain a non-luminous (blue) flame, which is hotter and more efficient.
6. Never leave a lit Bunsen burner unattended.
7. Always turn off the gas supply and allow the burner to cool completely before storing.

Safety precautions:

• Tie back long hair.
• Wear appropriate eye protection.
• Keep flammable materials away from the burner.
• Never reach across a lit Bunsen burner.
• Be aware of the hot areas around the burner.
• Know the location of fire extinguishers and safety showers.

Pre-Lab Question 6: What are different mixing techniques used in a laboratory setting? Discuss their advantages and disadvantages.

Answer:

Several mixing techniques exist, each with its own advantages and disadvantages:

• Manual stirring with a glass rod: Simple and inexpensive. However, it can be inefficient for large volumes or viscous liquids and may not provide uniform mixing.
• Magnetic stirrer with a stir bar: Provides efficient and uniform mixing, especially for viscous liquids. It requires specialized equipment and is not suitable for all types of containers.
• Vortex mixer: A rapid and efficient method for mixing small volumes of liquids in tubes or vials. Not suitable for large volumes or delicate samples.
• Shaking: Effective for mixing liquids in flasks or bottles. Can be used manually or with automated shaking machines. The effectiveness depends on the viscosity and volume of the liquid.

The choice of mixing technique depends on the volume, viscosity, and nature of the liquid, as well as the available equipment.

Section 3: Centrifugation and Filtration

Separation techniques like centrifugation and filtration are fundamental in isolating and purifying substances.

Pre-Lab Question 7: Explain the principle of centrifugation. What factors affect the separation of components in a centrifuge?

Answer:

Centrifugation separates components of a mixture based on their density and size. A centrifuge spins samples at high speeds, generating centrifugal force that pushes denser components outwards, towards the bottom of the tube, while lighter components remain closer to the top.

Factors affecting separation:

• Speed (RPM or G-force): Higher speeds lead to better separation, but excessive speeds can damage samples.
• Time: Longer centrifugation times allow for more complete separation.
• Sample volume: Overfilling tubes can hinder efficient separation.
• Density difference between components: The greater the density difference, the easier it is to separate the components.

Pre-Lab Question 8: What are the different types of filtration techniques used in a laboratory? When would you use each type?

Answer:

Several filtration techniques exist, each suited for different applications:

• Gravity filtration: Simple and inexpensive; utilizes gravity to pull liquid through a filter paper. Used for separating solids from liquids where high purity isn't crucial.
• Vacuum filtration: Faster than gravity filtration; utilizes a vacuum to accelerate the filtration process. Used for separating solids from liquids where rapid separation is needed or when dealing with fine particles.
• Membrane filtration: Uses membranes with specific pore sizes to separate particles based on size. Used for sterilization, clarification, and separating particles of a specific size.

The choice depends on the desired speed of separation, the size of the particles to be removed, and the required purity of the filtrate.

Section 4: Spectrophotometry and other analytical techniques

Many lab techniques involve using instruments to measure properties of a substance.

Pre-Lab Question 9: What is a spectrophotometer, and how does it work? What are some of its applications?

Answer:

A spectrophotometer is an instrument used to measure the absorbance or transmission of light through a solution. It works by passing a beam of light of a specific wavelength through a sample and measuring the amount of light that passes through (transmittance) or is absorbed (absorbance). The absorbance is directly proportional to the concentration of the substance in the solution (Beer-Lambert Law).

Applications:

• Quantitative analysis of solutions.
• Measuring the purity of chemicals.
• Studying enzyme kinetics.
• Analyzing protein concentrations.
• Monitoring chemical reactions.

Pre-Lab Question 10: What are some other common laboratory analytical techniques? Briefly describe each.

Answer:

Many other analytical techniques are used in laboratories, including:

• Titration: A quantitative chemical analysis method used to determine the concentration of an unknown solution using a solution of known concentration (titrant).
• Chromatography: A separation technique used to separate components of a mixture based on their differential interaction with a stationary and mobile phase. Different types exist, such as gas chromatography (GC) and high-performance liquid chromatography (HPLC).
• Electrophoresis: A technique used to separate charged molecules, like proteins or DNA, based on their size and charge in an electric field.
• Mass spectrometry: A technique used to identify and quantify molecules based on their mass-to-charge ratio. It’s commonly coupled with chromatography (e.g., GC-MS, LC-MS).

Conclusion

Mastering basic laboratory techniques is crucial for any scientist or researcher. Thoroughly understanding the procedures, safety precautions, and potential sources of error is paramount to ensure accurate, reproducible results and maintain a safe working environment. By carefully addressing pre-lab questions and actively seeking to understand the principles behind each technique, researchers can greatly enhance their experimental success. Always consult appropriate laboratory manuals and follow safety guidelines provided by your institution. Remember, accuracy, precision and safety are always paramount in laboratory work.