Chapter 2 Tools of the Trade

Overview 2-1 Safe, Ethical Handling of chemicals and Waste 2-2 The Lab 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 Introduction to Microsoft Excel ® 2-11 Graphing with Microsoft Excel ®

2-1: Safety Safety requires you to plan in advance about what you will do and to consider the hazards of each operation before executing it. Do not carry out a procedure until adequate safety precautions are in place. Know how to use safety equipment such as goggles, fume hood, lab coat, gloves, emergency shower, eyewash, and fire extinguisher. Chemicals should be stored and used in a manner that minimizes human contact. Environmentally acceptable disposal procedures should be established in advance for every chemical that you use. Label all containers that contain chemicals.

CHAPTER 02: Figure 2.1

2-2: The Lab Notebook Your lab notebook tells what you did and what you observed; it should be understandable to other people. It also should allow you (or others) to repeat an experiment in the same manner in the future. Scientific “truth” 1. State what was done 2. State what was observed 3. Be understandable to someone else Greatest error made by even experienced scientists is writing incomplete or unintelligible notes.

2-3: Electronic Balance Understand the principle of operation of an electronic balance and treat it as delicate equipment.

2-3: How an Electronic Balance Works An object (having a mass m) placed on the electronic balance pushes the pan down with a force mg, where g is the acceleration of gravity. The balance generates an electric current to cancel exactly the motion of the pan. The magnitude of the current tells us how much mass was placed on the pan.

2-3: Weighing by Difference An alternative procedure, called weighing by difference, is necessary for hygroscopic reagents, which rapidly absorb moisture from the air. The sample to be used is dried in a weighing bottle. Set the initial mass of the weighing bottle to zero with the tare button. Quickly deliver the dry reagent from the bottle to the receiver. Cap the weighing bottle and reweigh the bottle. The negative reading on the balance is the mass of reagent delivered from the bottle.

2-3: Minimizing Errors for an Analytical Balance Locate on a heavy table (marble slab) to minimize vibrations. Keep level. Recalibrate after the level is adjusted. Keep the object to be weighed close to the center of the pan. Samples must be at ambient temperature. Place the sample in a desiccator during cooling to prevent accumulation of moisture. Close the glass doors to prevent drafts from affecting the reading. Fingerprints can affect the apparent mass of an object, so tweezers or a tissue are recommended for placing objects on a balance.

2-3: Buoyancy Corrections Buoyancy corrections are required in accurate work. The true mass m can be calculated from the observed mass, m’. where da is the density of air (0.001 2 g/mL at 1 bar and 25ºC) dw is the density of the calibration weights (8.0 g/mL) do is the density of the object being weighed.

2-5: Measuring Volume Volumetric flasks are used to prepare solutions with known volume. Transfer pipets deliver fixed volumes; less accurate measuring pipets deliver variable volumes. The larger the volumetric glassware, the smaller its relative uncertainty. You should understand how to design a serial dilution to make a less concentrated solution from a more concentrated solution by using transfer pipets and volumetric flasks.

2-4: Burets Burets should be read in a reproducible manner and drained slowly for best results. Interpolate between markings to obtain accuracy one decimal place beyond the graduations.

2-4: Operating a Buret Wash buret with new solution (aliquots) Eliminate air bubble before use Drain liquid slowly Deliver a fraction of a drop near end point Read bottom of concave meniscus Estimate reading to 1/10 of a division Avoid parallax Account for graduation thickness in reading

2-4: Relative Uncertainty (%) Relative uncertainty is the uncertainty in a quantity divided by the magnitude of the quantity. We usually express relative uncertainty as a percentage: Use larger volumes to decrease the % relative uncertainty.

2-4: % Relative Uncertainty A certain buret has an uncertainty of ±0.05 mL. If you deliver 20 mL and then 40 mL each, then the % relative uncertainty in the volume delivered is shown below. Volume delivered Relative uncertainty (%) 20 mL (0.050/20)  100% 0.25% 40 mL (0.050/40)  100% 0.12%

2-6: Pipets Pipets deliver known volumes of liquid. The transfer pipet is calibrated to deliver one fixed volume. The last drop does not drain out of the pipet and should not be blown out. The measuring pipet is calibrated like a buret. It is used to deliver a variable volume, such as 5.6 mL, by starting delivery at the 1.0-mL mark and terminating at the 6.6-mL mark. The transfer pipet is more accurate.

2-6: Pipets

2-6: Micropipets Micropipets deliver volumes of 1 to 1000 mL Liquid is contained in the disposable polypropylene tip. Stable to most aqueous solutions and many organic solvents except chloroform (CHCl3). Not resistant to concentrated nitric or sulfuric acids.

2-6: Never Trust a Pipet Do not be lulled into complacency by the nice digital reading on a micropipet. Unless your pipet has been calibrated recently and your personal technique tested, micropipets can have gross errors.

2-6: Never Trust a Pipet

CHAPTER 02: Figure 2.15

CHAPTER 02: Figure 2.16

2-9: Volumetric Glassware Volumetric glassware is calibrated by weighing water contained in or delivered by the vessel. In the most careful work, solution concentrations and volumes of vessels should be corrected for changes in temperature.

2-7: Filtration Filtration and collection of precipitates require careful technique, as does the drying of reagents, precipitates, and glassware in ovens and desiccators.

2-7: Gravity Filtration Filtration and collection of precipitates require careful technique, as does the drying of reagents, precipitates, and glassware in ovens and desiccators.

2-7: Filtration Filtration and collection of precipitates require careful technique, as does the drying of reagents, precipitates, and glassware in ovens and desiccators.

2-7: Filtration Fritted-glass funnel (Gooch filter crucible) filtrate mother liquor

2-7: Filtration Conical funnel (with filter paper)

2-8: Drying Dry reagents, precipitates, and glassware in an oven at 110ºC. Anything that you put in the oven should be labeled. Dry to constant mass. Use a beaker and watch glass to minimize contamination by dust during drying. Cool in a desiccator.

2-8: Desiccator

2-10: Spreadsheets Spreadsheets are an excellent means to organize data and perform calculations. Learn how to enter formulas in a spreadsheet and how to draw a graph of data from a spreadsheet. Formulas begin with an equal (=) sign. Arithmetic operations in a spreadsheet are: + addition, - subtraction, * multiplication / division, ^ exponentiation

2-10: Spreadsheets

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