D. Harvey - Modern Analytical Chemistry (794078), страница 11
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The sample to be weighed is placed on the sample pan, displacing thepan downward by a force equal to the product of the sample’s mass and the acceleration due to gravity. The balance detects this downward movement and generates acounterbalancing force using an electromagnet. The current needed to produce thisforce is proportional to the object’s mass. A typical electronic balance has a capacityof 100–200 g and can measure mass to the nearest ±0.01 to ±1 mg.Another type of balance is the single-pan, unequal arm balance (Figure 2.3). Inthis mechanical balance the balance pan and a set of removable standard weights onone side of a beam are balanced against a fixed counterweight on the beam’s otherside.
The beam itself is balanced on a fulcrum consisting of a sharp knife edge. Addinga sample to the balance pan tilts the beam away from its balance point. Selected standard weights are then removed until the beam is brought back into balance. The combined mass of the removed weights equals the sample’s mass. The capacities and measurement limits of these balances are comparable to an electronic balance.DetectorbalanceAn apparatus used to measure mass.Light sourceBalancepanSNSFigure 2.2ControlcircuitryElectromagneticservomotor(a)(b)(a) Photo of a typical electronic balance.(b) Schematic diagram of electronic balance;adding a sample moves the balance pandown, allowing more light to reach thedetector. The control circuitry directs theelectromagnetic servomotor to generate anopposing force, raising the sample up untilthe original intensity of light at the detectoris restored.Photo courtesy of Fisher Scientific.1400-CH02 9/8/99 3:48 PM Page 2626Modern Analytical ChemistryFulcrumBalance beamFulcrumCounterweightRemovable weightsFigure 2.3Schematic diagram of single-arm mechanicalbalance.Balance panThe mass of a sample is determined by difference.
If the material being weighedis not moisture-sensitive, a clean and dry container is placed on the balance. Themass of this container is called the tare. Most balances allow the tare to be automatically adjusted to read a mass of zero. The sample is then transferred to the container, the new mass is measured and the sample’s mass determined by subtractingthe tare. Samples that absorb moisture from the air are weighed differently.
Thesample is placed in a covered weighing bottle and their combined mass is determined. A portion of the sample is removed, and the weighing bottle and remainingsample are reweighed. The difference between the two masses gives the mass of thetransferred sample.Several important precautions help to minimize errors in measuring an object’smass. Balances should be placed on heavy surfaces to minimize the effect of vibrations in the surrounding environment and should be maintained in a level position.Analytical balances are sensitive enough that they can measure the mass of a fingerprint. For this reason, materials placed on a balance should normally be handledusing tongs or laboratory tissues.
Volatile liquid samples should be weighed in acovered container to avoid the loss of sample by evaporation. Air currents can significantly affect a sample’s mass. To avoid air currents, the balance’s glass doorsshould be closed, or the balance’s wind shield should be in place. A sample that iscooler or warmer than the surrounding air will create convective air currents thatadversely affect the measurement of its mass. Finally, samples dried in an ovenshould be stored in a desiccator to prevent them from reabsorbing moisture fromthe atmosphere.2D.2 Equipment for Measuring Volumevolumetric flaskGlassware designed to contain a specificvolume of solution when filled to itscalibration mark.Analytical chemists use a variety of glassware to measure volume, several examplesof which are shown in Figure 2.4. The type of glassware used depends on how exactthe volume needs to be. Beakers, dropping pipets, and graduated cylinders are usedto measure volumes approximately, typically with errors of several percent.Pipets and volumetric flasks provide a more accurate means for measuring volume.
When filled to its calibration mark, a volumetric flask is designed to contain aspecified volume of solution at a stated temperature, usually 20 °C. The actual vol-1400-CH02 9/8/99 3:48 PM Page 27Chapter 2 Basic Tools of Analytical Chemistry(a)Figure 2.4Common examples of glasswareused to measure volume:(a) beaker; (b) graduated cylinder;(c) volumetric flask; (d) pipet;(e) dropping pipet.Photos courtesy of Fisher Scientific.(b)(c)ume contained by the volumetric flask is usually within 0.03–0.2% of the statedvalue. Volumetric flasks containing less than 100 mL generally measure volumes tothe hundredth of a milliliter, whereas larger volumetric flasks measure volumes tothe tenth of a milliliter.
For example, a 10-mL volumetric flask contains 10.00 mL,but a 250-mL volumetric flask holds 250.0 mL (this is important when keepingtrack of significant figures).Because a volumetric flask contains a solution, it is useful in preparing solutions with exact concentrations. The reagent is transferred to the volumetric flask,and enough solvent is added to dissolve the reagent.
After the reagent is dissolved,additional solvent is added in several portions, mixing the solution after each addition. The final adjustment of volume to the flask’s calibration mark is made using adropping pipet. To complete the mixing process, the volumetric flask should be inverted at least ten times.A pipet is used to deliver a specified volume of solution. Several differentstyles of pipets are available (Figure 2.5). Transfer pipets provide the most accuratemeans for delivering a known volume of solution; their volume error is similar tothat from an equivalent volumetric flask.
A 250-mL transfer pipet, for instance,will deliver 250.0 mL. To fill a transfer pipet, suction from a rubber bulb is used topull the liquid up past the calibration mark (never use your mouth to suck a solution into a pipet). After replacing the bulb with your finger, the liquid’s level is adjusted to the calibration mark, and the outside of the pipet is wiped dry.
Thepipet’s contents are allowed to drain into the receiving container with the tip of thepipet touching the container walls. A small portion of the liquid remains in thepipet’s tip and should not be blown out. Measuring pipets are used to deliver variable volumes, but with less accuracy than transfer pipets. With some measuring(d)(e)pipetGlassware designed to deliver a specificvolume of solution when filled to itscalibration mark.271400-CH02 9/8/99 3:48 PM Page 2828Modern Analytical Chemistry(c)(d)Figure 2.5Common types of pipets and syringes: (a) transfer pipet; (b) measuring pipet;(c) digital pipet; (d) syringe.Photos courtesy of Fisher Scientific.(a)(b)MeniscusCalibrationmarkFigure 2.6Proper means of reading the meniscus on avolumetric flask or pipet.pipets, delivery of the calibrated volume requires that any solution remaining inthe tip be blown out. Digital pipets and syringes can be used to deliver volumes assmall as a microliter.Three important precautions are needed when working with pipets andvolumetric flasks.
First, the volume delivered by a pipet or contained by a volumetric flask assumes that the glassware is clean. Dirt and grease on the innerglass surface prevents liquids from draining evenly, leaving droplets of the liquidon the container’s walls. For a pipet this means that the delivered volume isless than the calibrated volume, whereas drops of liquid above the calibrationmark mean that a volumetric flask contains more than its calibrated volume.Commercially available cleaning solutions can be used to clean pipets and volumetric flasks.1400-CH02 9/8/99 3:48 PM Page 29Chapter 2 Basic Tools of Analytical ChemistrySecond, when filling a pipet or volumetric flask, set the liquid’s level exactlyat the calibration mark. The liquid’s top surface is curved into a meniscus, thebottom of which should be exactly even with the glassware’s calibration mark(Figure 2.6).
The meniscus should be adjusted with the calibration mark at eyelevel to avoid parallax errors. If your eye level is above the calibration mark thepipet or volumetric flask will be overfilled. The pipet or volumetric flask will beunderfilled if your eye level is below the calibration mark.Finally, before using a pipet or volumetric flask you should rinse it with severalsmall portions of the solution whose volume is being measured. This ensures thatany residual liquid remaining in the pipet or volumetric flask is removed.29meniscusThe curved surface of a liquid containedin a tube.2D.3 Equipment for Drying SamplesMany materials need to be dried prior to their analysis to remove residual moisture.Depending on the material, heating to a temperature of 110–140 °C is usually sufficient.
Other materials need to be heated to much higher temperatures to initiatethermal decomposition. Both processes can be accomplished using a laboratoryoven capable of providing the required temperature.Commercial laboratory ovens (Figure 2.7) are used when the maximum desired temperature is 160–325 °C (depending on the model). Some ovens include theability to circulate heated air, allowing for a more efficient removal of moisture andshorter drying times. Other ovens provide a tight seal for the door, allowing theoven to be evacuated. In some situations a conventional laboratory oven can be replaced with a microwave oven.
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