Ch. 2: Tools of the Trade

Transcription

1 Ch. 2: Tools of the Trade Outline: 2-1 Safety 2-2 Notebook 2-3 Analytical Balance 2-4 Burets 2-5 Volumetric Flasks 2-6 Pipets and Syringes 2-7 Filtration 2-8 Drying 2-9 Calibration of Volumetric Glassware 2-10 Excel - this will be a separate handout

2 Hazards & Safety 1. Identify all hazards - do not proceed until you are comfortable! 2. Safety glasses (or goggles) - worn at all times, no contact lenses! 3. Fumehoods - concentrated acids and bases; organic solvents 4. Spills - flood with water and clean up; skin or eyes, flush with water and seek treatment 5. Fires - fire extinguishers and fire blankets 6. First aid kits - locations and contents 7. Labels - label everything that you work with 8. Waste - dispense of in appropriate bottles Why we wear lab coats: In 2008, 23-year-old University of California research assistant Sheharbano Sangji was withdrawing t-butyllithium with a syringe from a bottle. The plunger came out of the syringe and the pyrophoric liquid burst into flames, igniting her sweater and gloves. Burns on 40% of her body proved fatal. A flame-resistant coat might have protected her. Limitations of gloves: In 1997, Dartmouth College professor Karen Wetterhahn, age 48, died from a drop of dimethylmercury absorbed through her latex gloves. Many organic compounds readily penetrate latex. Wetterhahn was an expert in the biochemistry of metals, the first female professor of chemistry at Dartmouth, and played a major role in bringing more women into science.

3 Chemical Hazard Labels 4. Deadly 3. Extreme Danger 2. Hazardous 1. Slight haz. 0. Normal material Flash points 4. < 73 F 3. < 100 F F F 1. > 200 F 0. will not burn ACID ALK COR OXY P (polymerization) W (use no water) (radioactive) 4. May detonate 3. Shock or heat may detonate 2. Violent chemical change 1. Unstable if heated 0. Stable

4 Lab notebook Your lab notebook is used to record an official record of what you have done. It must: describe what you did and when you did it describe what you observed be readable by others As beginners, it is best to record information on the purpose of the experiment, and many of the experimental details. One future day, you or one of your classmates will make an important discovery and will seek a patent. The lab notebook is your legal record of your discovery. For this purpose, each page in your notebook should be signed and dated. Anything of potential importance should also be signed and dated by a second person. Most importantly, have the notebook ready to accept all of the incoming numerical data for your experiment!

5 Analytical Balance Modern analytical balances typically have upper capacities from g, and have a readability (i.e., smallest increment of mass that can be measured) ranging from mg. Top-loading balances are typically used for measurement of less approximate masses. Electronic balances are almost used exclusively in all modern laboratories, and utilize EMF compensation for load balancing. The casing on the analytical balance requires one to load from the side; the balance is extremely sensitive to temperature and subtle drafts of air (hence, the closed environment). Weighing (general procedure): 1. Place a clean receiving vessel on the pan. 2. The mass of the empty vessel is known as the tare. One may record the mass reading, or more likely, press the tare button, which sets the digital readout to g. The doors must be shut, and the readout must be stable to ± g. 3. Open a side door and carefully add the chemical to the vessel, either directly from the bottle or with a spatula/scoopula. 4. Close the door, and allow stabilization to occur. 5. Record the mass.

6 Weighing by Difference Weighing by difference is the most best methods for obtaining precise measurements of mass. It is especially important when you have hygroscopic samples, or you are concerned about sample contamination, cross contamination, etc. Step 1: Tare the empty balance with the balance doors closed. Always handle your sample bottle with a kimwipe or equivalent holder! Finger oils, dirt, moisture, etc. can slightly alter the mass of your sample vessel. Step 2: Push the tare button so that the reading on the balance reads g. Open one of the balance doors and place the weighing bottle together with the cap on the balance pan. Ensure all the balance doors are closed before recording the mass of the weighing bottle and cap with its contents to ± g. Remove from the balance. Always allow time for stabilization!

7 Weighing by Difference, 2 Step 3: Remove the cap from the sample bottle and tip the bottle over the receiving container (i.e., beaker, flask, etc.). Allow a small amount of the sample to fall into the receiving container by gently tapping the sample bottle on the lip of the container (try for less sample than you actually require, otherwise you have to start all over again! Note: Avoid any loss of the sample to the bench top! Do not use a spatula or scoopula to aid in the transfer of the sample! Step 4: Tip the weighing bottle back to its vertical position and gently tap the weighing bottle on the counter to redistribute the sample back to the bottom of the bottle. Try to remove and redistribute any sample left along the rim of the bottle before re-weighing. Do not use a spatula!! Step 5: Cap the weighing bottle, open the door to the balance, and carefully place the sample bottle back on the pan. Record the mass of the weighing bottle and remaining sample. The difference between the first and second weighing represents the amount of sample transferred. Step 6: Continue the procedure above until you are close to the desired mass. (N.B.: In Step 2, you may tare the balance, and work w.r.t g).

8 How a balance works The pan pushes down on a load receptor attached to parallel guides, with the left side of the lever down, and the right side up. The null sensor detects displacement of the lever away from the null position (0 mass), and the servo amplifier sends electric current through the force compensation wire coil in the field of a permanent magnet. Current flowing through the coil creates a voltage across the precision resistor, which is converted to a digital signal (at the ADC) and to a readout in grams (via the microprocessor).

9 Weighing Errors Temperature is an important factor: if the measurement is not made at ambient temperatures, there is the potential for air currents to influence the measured mass. Even the warmth of ones hand can be a factor! The side doors also prevent the inward flow of external air currents which can create massing errors. Fingerprints, oil, dirt and grease can also influence the measured mass, so be careful to handle the sample bottle that is being used with a kimwipe or some other clean implement. Moisture can also affect the measured mass; hence, samples should be kept in a desiccator when not in use. Balances are set up on levelled, heavy, stable tables or benches, so as to avoid the influences of external vibrations, movement, bumps, etc. The balance must also be levelled (using the bubble meter) to ensure accurate weighing. Continuous fluctuation of the mass readout (up and down) is a sure sign of a problem, perhaps in one of the categories listed above. Balances should be regularly checked for accuracy (i.e., calibrated) via the use of standard masses.

10 Burets Burets are very precisely manufactured glass tubes that are marked with graduations, which allow one to measure very accurate volumes of liquid that are delivered to a vessel through the stopcock at the bottom. The tolerance of a buret is the error in the volume if liquid delivered. For a 50 ml class A buret (as you will be using), this tolerance is ±0.05 ml, even though the smallest graduations are 0.1 ml. Example: If the reading of a 50-mL buret is ml, the true volume can be anywhere in the range to ml and still be within the tolerance of ±0.05 ml. To accurately read the volume level of a buret, your eye must be exactly at the level of the meniscus; if this is not the case, errors arise which are said to be the result of parallax.

11 Burets: operation tips Clean your buret, and wash several times with titrant solution. Eliminate air bubble before use This is crucial - if it is there, all of the volumes will be off! Drain liquid slowly Deliver fraction of a drop near end point Wash the end of the tip with distilled water, and/or touching the tip to the edge of the flask into which you are titrating. Read bottom of concave meniscus Use black tape on a white card as a background for locating the precise position of the meniscus Estimate reading to 1/10 of a division Thickness of the markings is ca ml; so, if meniscus is at the top of the mark it is level, and at the bottom of the mark, it is 0.02 ml less. Avoid parallax - make sure your eyes are level!

12 Volumetric Flasks A volumetric flask is calibrated to contain a particular volume of solution at 20 C when the bottom of the meniscus is adjusted to the centre of the mark on the neck of the flask. Such flasks are labeled TC 20 o C. Pipettes and burets are often labeled TD 20 o C, which means that they deliver a particular volume of solution at 20 o C. This temperature is important due to expansion (heating) and contraction (cooling) of liquid.

13 Volumetric Flasks: Tips Dissolve the reagent in the flask by swirling with less than the final volume of liquid. Add more liquid and swirl the solution again. Adjust the final volume with as much well-mixed liquid in the flask as possible. When two different liquids are mixed, there is generally a small volume change - remember physical chemistry! The total volume is not the sum of the two volumes that were mixed. By swirling the liquid in a nearly full volumetric flask before liquid reaches the thin neck, you minimize the change in volume when the last liquid is added. Add the final drops of liquid with a pipet, not a squirt bottle. After adjusting the liquid to the correct level, hold the cap firmly in place and invert the flask several times to complete mixing. Before the liquid is homogeneous, we observe streaks (called schlieren) arising from regions that refract light differently. After the schlieren are gone, invert the flask a few more times to ensure complete mixing. To avoid adsorption of chemicals by the glassware (i.e., sticking to the surface), you may acid wash the flask to replace surface cations with H + ions. Soak thoroughly cleaned glassware in 3 6 M HCl or HNO3 (in a fume hood) for 1 h. Then rinse it well with distilled water and, finally, soak it in distilled water. Acid can be reused many times, as long as it is only used for clean glassware

14 Pipets Pipets deliver known amounts of liquid (very accurately). The transfer pipet (a) is the most accurate type, and delivers only one fixed volume which is calibrated for a particular temperature. The last drop does not drain out, and should never be blown out! The measuring pipet (or Mohr pipet) works in a similar manner to a buret, and delivers a variable volume (e.g., to deliver 5.5 ml, one can start at 1.0 ml and finish at 6.5 ml). Never drain liquid below the bottom calibration mark (10 ml above). Pipettes must be very clean! Soak in distilled water when not in use, and do not allow solutions to dry (this leaves residues).

15 Transfer pipet tips Using a rubber bulb (not your mouth) suck liquid up past the calibration mark. Discard one or two volumes of liquid to rinse traces of previous reagents from the pipet. After taking up a third volume past the calibration mark, quickly replace the bulb with your index finger (not your thumb) at the end of the pipet. Gently pressing the pipet against the bottom of the vessel while removing the rubber bulb helps prevent liquid from draining below the mark while you put your finger in place. Wipe the excess liquid off the outside of the pipet with a clean kimwipe. Touch the tip of the pipet to the side of a beaker and drain the liquid until the bottom of the meniscus just reaches the center of the mark. Touching the beaker draws liquid from the pipet without leaving part of a drop hanging when the liquid reaches the calibration mark. Transfer the pipet to a receiving vessel and drain it by gravity while holding the tip against the wall of the vessel. After liquid stops draining, hold the pipet to the wall for a few more seconds to complete draining. Do not blow out the last drop. The pipet should be nearly vertical at the end of delivery.

16 Syringes Microlitre syringes come in sizes from 1 to 500 μl and have an accuracy and precision near 1%. When using a syringe, take up and discard several volumes of liquid to wash the glass walls and to remove air bubbles from the barrel. The steel needle is attacked by strong acid and will contaminate strongly acidic solutions with iron. A syringe is more reliable than a micropipet (commonly used in biochemistry laboratories), but the syringe requires more care in handling and cleaning.

17 Filtration Gravimetric analyses, in which the mass of product from a reaction is measured to determine how much unknown is present, almost always rely on the collection/isolation of a precipitate via filtration. The precipitate is filtered, washed and dried prior to massing. Most often the fritted-glass funnel (or Gooch filter crucible) is used in combination with suction (to make the filtration fast). The liquid that passes through the filter into the suction flask is called the filtrate, and the liquid from which a substance precipitates or crystallizes is called the mother liquor.

18 Filtration, 2 In cases where an ignition is required to form a precipitate (of a known, constant composition), ashless filter paper is used (fairly resistant to decomposition), and filtration is often done through a conical funnel. Filtering via a conical funnel can be done for many types of analyses: Pour the slurry (suspension of solid in liquid) of precipitate down a glass rod to prevent splattering. Particles adhering to the beaker or rod can be dislodged with a rubber policeman (a flattened piece of rubber at the end of a glass rod). Use a jet of appropriate wash liquid from a squirt bottle to transfer particles from the rubber and glassware to the filter.

19 Drying All reagents, products, etc. may be dried in an oven at 110 o C. All samples in the oven should be labeled, and covered at all times (protect from dust, etc.). The mass of a gravimetric precipitate is measured by weighing a dry, empty filter crucible before the procedure and reweighing the same crucible filled with dry product after the procedure. 1. Bring it to constant mass by drying it in the oven for 1 hour or longer and then cooling for 30 min in a desiccator. 2. Weigh the crucible and then heat it again for about 30 min. Cool it and reweigh it. 3. When successive weighings agree to ±0.3 mg, the filter has reached constant mass. You can use a microwave oven instead of an electric oven for drying reagents and crucibles. Try an initial heating time of 4 min, with subsequent 2-min heatings. Use a 15-min cooldown before weighing. If the crucible is warm when it is weighed, it creates convection currents that give a false weight.

20 Drying, 2 A desiccator (Figure 2-21) is a closed chamber containing a drying agent called a desiccant. The lid is greased to make an airtight seal, and desiccant is placed in the bottom beneath the perforated disk. After placing a hot object in the desiccator, leave the lid cracked open for a minute until the object has cooled slightly. This practice prevents the lid from popping open when the air inside warms up. To open a desiccator, slide the lid sideways rather than trying to pull it straight up. (a) regular and (b) vacuum desiccators are available in the lab!

21 Calibration of Volumetric Glassware The manufacturers of pipets, burettes and volumetric flasks certify that the true measurement of volume lies within a certain tolerance from the true quantity (e.g., a class A transfer pipet is certified to deliver ± 0.02 ml - if it is used properly!) A specific pipet (perhaps the one in your locker) might always deliver ± ml in a series of trials (i.e., an average of ml more than the indicated volume). To ensure accuracy, volumetric glassware is calibrated to measure the volume actually contained in or delivered by measuring the mass of water contained or delivered by the vessel and using the density of water to convert mass into volume. For very careful work, thermal expansion must also be taken into account. Since water expands 0.02% per degree (near 20 oc - see Table 2.9, 8th edition), and because the concentration of solution is proportional to density, we can easily correct for thermal expansion. For two temperatures T and T : c d = c d