Laboratory Technique

Be courteous to your colleagues--CLEAN UP! Return all of your equipment CLEAN, swab your bench each day before you leave, and keep the common work areas clean. Do not remove reagent bottles from the hood or balance area. Place and tighten the cap on a reagent bottle when you are finished with it. Place all caps top-down on the bench or hood surface so that dirt from these areas does not contaminate the reagents.

• Do not waste reagents.
• Do not waste Pasteur pipets. Rinse them with water immediately after use, and aspirate or oven dry.
• Do not waste graduated pipets. Rinse them with water immediately after use, and aspirate or oven dry.

• Wear goggles or safety glasses at all times.
• Handle organic solvents in the hoods. To obtain solvent from a solvent bottle, pour slightly more than you need into a clean beaker or Erlenmeyer flask, then transfer from that to the desired container. DO NOT CONTAMINATE THE SOLVENT SUPPLY. You can look here for details on transferring liquids.
• Avoid abrupt movements; avoid moving before looking. Keep movements slow and smooth.
• If you bring backpacks/book bags into the lab, store them in the under-the-counter cabinets.

• Keeping a clean work area. Place strips of paper toweling on your work area at the beginning of each period. Glassware that must be protected from contamination should be placed on the toweling. Discard toweling when it is dirty.
• Measuring small volumes of liquids. For volumes of 2 mL or less, use a syringe or a syringe pipet pump with 1- or 2-mL graduated pipet. To make a syringe pipet pump, use a short piece (1.5-2") of tygon tubing to connect a 1 or 2-mL plastic syringe to a disposable 1- or 2-mL graduated pipet. Use the syringe as a pipet pump to pull up, level, and dispense precise volumes.
• Drying glassware by aspiration. Insert a clean Pasteur pipet into a length of vacuum tubing connected to an aspirator trap, and turn on the water to maximum flow rate. Insert the end of the pipet into the item of glassware to be dried. Continue to pull air through the item of glassware until all solvent has evaporated (rinsing with a volatile solvent such as acetone is recommended prior to drying by aspiration). Turn off the water flow. UNDER NO CIRCUMSTANCES SHOULD YOU DRY GLASSWARE USING LABORATORY COMPRESSED AIR; THE AIR LINES CONTAIN DIRT AND GREASE.
• Small-scale filtration. Small volumes of solution can be filtered in a couple of ways:
  • Insert a wad of Kimwipe (or glass wool) into a Pasteur pipet and use a "ramrod" to wedge it into the narrowing portion of the pipet. Introduce the solution to be filtered into the top of the pipet. If necessary, force the liquid thru the filter using a Pasteur pipet bulb.
  • Plug the narrow portion of a funnel with a wadded piece of Kimwipe; add the solution to be filtered to the funnel in the usual way.
• Lighting a Bunsen Burner. Strike a match. Turn on the gas, and bring the lighted match up along the barrel of the burner from below the burner gas exit nozzle. The burner should light smoothly. Adjust the flame height and temperature.
• Using an Aspirator Trap. An aspirator trap is a device used when creating a vacuum using water-flow aspiration. It's proper use involves the following steps:
  • Attach tube A to the aspirator sidearm of a faucet in the sink. Note that tube A is connected to the glass tube that extends to the bottom of the trap.
  • Attach tube C to the sidearm of a suction flask or suction trap tube.
  • Turn on the water flow to full speed.
  • When ready to apply suction, plug the end of tube B, the atmosphere vent, with a glass or plastic plug, or clamp tube B with a screw clamp. Be aware that it is difficult to completely close off tube B with a clamp unless you fold it over before clamping. Note that tube B is connected to the glass tube that extends only a short way into the trap tube.
  • To release suction, open tube B to the atmosphere, then turn off the water flow. DO NOT TURN OFF THE WATER FLOW BEFORE OPENING TUBE B. DOING THIS WILL CAUSE BACKUP OF TAP WATER INTO YOUR SYSTEM.
• Suction Filtration. This is probably more complicated than it seems. The steps in performing a suction filtration:
  • Fit the funnel with proper size filter aper. The paper should lie flat against the bottom.
  • Mount the funnel in a suction flask, using a rubber ring or cone.
  • Clamp the flask securely.
  • Attach the flask to an aspirator trap connected to an aspirator.
  • Apply full suction.
  • Wet the filter paper with a small volume of the same solvent that is in the mixture to be filtered. This should cause the filter paper to flatten snugly on the bottom of the funnel. Check to make sure that this occurs. Check for gaps or creases in the paper that will lead to incomplete retention of the solid on the paper.
  • Without releasing suction, pour the mixture to be filtered into the funnel.
  • Maintain suction until all liquid has been pulled through.
  To wash the solid while it is in the funnel:
  • Gently release the suction by slowly opening the atmosphere vent, NOT by turning off the aspirator water.
  • Pour in the wash solvent.
  • Use a stirring rod (not a spatula) to break up the mat of solid completely. Be careful not to tear the filter paper. Suspend the solid in the liquid using gentle agitation with the stirring rod.
  • Reapply suction to pull wash liquid through.
  To dry the solid by suction, keep pulling air through the solid in the funnel until the funnel is no longer cold to the touch (usually 1/2 hour). (This works better with porcelain and glass funnels than with plastic funnels.)

  To remove the funnel from the receiver tube

  • Release suction by opening the atmosphere vent.
  • Turn off the aspirator water.
  • Lift the funnel from the sidearm tube.
• Volume reduction. There are several ways to reduce the volume of a solution:
  • Evaporation of solvent by heating (essentially distillation). This is SLOW.
  • Rotary evaporation. This is quick, but you need the apparatus!
  • Suction Flask method. Put the solution and a stir bar in a suction flask, connect the flask to an aspirator, and stopper the flask. With stirring, use aspirator suction to pull off solvent. Flask can be heated under suction on a hot plate if desired.
• Preparation of NMR samples. Place a few milligrams of the sample in a new (or clean) 1-dram vial. Add 0.5 mL of the selected deuterated solvent using a new Pasteur pipet. Repeatedly pull the solvent up into the Pasteur pipet and expel it to dissolve the maximum amount of sample. Filter the solution through a Kimwipe-plugged Pasteur pipet into an NMR tube. Cap the tube. After running the spectrum, empty the tube, rinse it several times with a solvent known to dissolve the sample, and aspirate dry (see above).
• Preparation of IR Samples.
  • KBr pellets. The following instructions depend on the availability of a Delta Press or a standard die for making KBr pellets.
    1. Place a few mg of IR-grade KBr in an agate mortar. An amount of KBr sufficient to cover an area of about 20 mm² to a depth of 1 mm is sufficient.
    2. Grind the KBr in the mortar until there is no evidence of crystallinity.
    3. Add a very small quantity of sample. An amount of sample sufficient to cover an area of about 4 mm² to an average depth of 0.5 mm is sufficient.
    4. Grind the sample into the KBr until it is uniformly distributed throughout the KBr.
    5. Transfer some of the ground mix to a Delta Press or some other pellet making die and make a pellet.
    6. Obtain the FTIR spectrum of the pellet.
    7. Clean the mortar and pestle and the pellet die.
  • Nujol mulls.
    1. Add a very small quantity of sample to an agate mortar. An amount of sample sufficient to cover an area of about 4 mm² to an average depth of 0.5 mm is sufficient.
    2. Grind the sample in the mortar until there is no evidence of crystallinity.
    3. Add 1 drop of Nujol (light mineral oil) to the mortar. Grind with the pestle until the solid is uniformly distributed in the Nujol. This should produce a suspension with the viscosity of wet paste. The suspension is called a "mull".
    4. Use the end of a flat spatula to scrape some of the mull from the mortar. You should see a "blob" of the mull on the end of the spatula.
    5. Carefully transfer this to a clean dry NaCl plate (a disc about 2 cm in diameter and 5 mm thick made of highly polished NaCl).
    6. Place a second NaCl plate so as to sandwich the sample between the plates. Rotate the top plate gently to spread the sample.
    7. Place the plates in the sample compartment of an IR spectrometer and obtain the FTIR spectrum of the pellet.
    8. Separate the plates and wipe off the sample with a Kimwipe. Make sure you leave the plates clean. If necessary, squirt a little acetone onto the Kimwipe and wipe the plates. UNDER NO CIRCUMSTANCES SHOULD YOU USE WATER TO CLEAN THE NaCl PLATES--IT WILL DISSOLVE THE SALT, RUINING THE PLATES.
    9. Return the plates to the storage desiccator.
• Drawing TLC Capillaries. Light a Bunsen burner and adjust the flame for medium temperature. Hold a melting point capillary horizontally in the flame above the inverted cone. When the glass just starts to sag, withdraw the capillary from the flame while rapidly pulling the ends of the capillary apart. Bend the capillary; it will break near the center of the drawn portion.
• Using a Buret. A buret is an item of volumetric glassware. It is a long glass tube, open on one end, and with a stopcock (valve) and a narrow glass tip at the other end. The buret capacity is normally 50, 25, or 10 mL. It is graduated to 0.1 mL, meaning that there are major marks for each mL, and 9 minor marks between adjacent mL marks. A buret is an essential piece of equipment for performing a titration, a process in which a standard solution of precisely known concentration is dispensed in a controlled way from a buret into a solution of a substance that reacts quantitatively with the standard solution. The standard solution in the buret is normally called the titrant, and the substance to which this is added is called the analyte. Addition of titrant is continued until all of the analyte has reacted. The volume of titrant added combined with its precise molarity then allows the calculation of the number of moles of analyte present in the titrated sample.

  A critical aspect of using a buret properly is reading the liquid level in the buret. The volume level of liquid is read by matching up the lowest part of the liquid meniscus with the graduation marks on the buret. Normally the meniscus will be positioned between two adjacent 0.1 mL graduation marks; estimation of the volume level to hundredths of an mL is therefore possible, based on the position of the meniscus between the two marks. For example, in this illustration, the menicus is approximately 0.6 of the way from the 1.5 mL mark to the 1.6 mL mark, so the volume level is read as 1.56 mL. Liquid volume levels in burets must always be read and written to the nearest 0.01 mL. Thus if the meniscus happens to coincide exactly with, say, the 10.7 mL mark, the volume level would be read and reported as 10.70 mL. The difference between the volume levels at the end and at the beginning of the titration is the volume of titrant added to the analyte.