D. Harvey - Modern Analytical Chemistry (794078), страница 12
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Higher temperatures, up to 1700° C, can be achievedusing a muffle furnace (Figure 2.8).After drying or decomposing a sample, it should be cooled to room temperature in a desiccator to avoid the readsorption of moisture. A desiccator (Figure 2.9)is a closed container that isolates the sample from the atmosphere. A drying agent,called a desiccant, is placed in the bottom of the container. Typical desiccants include calcium chloride and silica gel. A perforated plate sits above the desiccant,providing a shelf for storing samples. Some desiccators are equipped with stopcocksthat allow them to be evacuated.Figure 2.7Conventional laboratory oven used fordrying materials.Figure 2.8Example of a muffle furnace used forheating samples to maximum temperaturesof 1100–1700 °C.Courtesy of Fisher Scientific.desiccatorA closed container containing adesiccant; used to store samples in amoisture-free environment.(a)(b)Figure 2.9(a) Desiccator.
(b) Desiccator with stopcock for evacuating the desiccator.Photos courtesy of Fisher Scientific.desiccantA drying agent.1400-CH02 9/8/99 3:48 PM Page 3030Modern Analytical Chemistry2E Preparing SolutionsPreparing a solution of known concentration is perhaps the most common activityin any analytical lab. The method for measuring out the solute and solvent dependon the desired concentration units, and how exact the solution’s concentrationneeds to be known.
Pipets and volumetric flasks are used when a solution’s concentration must be exact; graduated cylinders, beakers, and reagent bottles suffice whenconcentrations need only be approximate. Two methods for preparing solutions aredescribed in this section.2E.1 Preparing Stock Solutionsstock solutionA solution of known concentration fromwhich other solutions are prepared.A stock solution is prepared by weighing out an appropriate portion of a pure solidor by measuring out an appropriate volume of a pure liquid and diluting to aknown volume.
Exactly how this is done depends on the required concentrationunits. For example, to prepare a solution with a desired molarity you would weighout an appropriate mass of the reagent, dissolve it in a portion of solvent, and bringto the desired volume. To prepare a solution where the solute’s concentration isgiven as a volume percent, you would measure out an appropriate volume of soluteand add sufficient solvent to obtain the desired total volume.EXAMPLE 2.9Describe how you would prepare the following three solutions: (a) 500 mL ofapproximately 0.20 M NaOH using solid NaOH; (b) 1 L of 150.0 ppm Cu2+using Cu metal; and (c) 2 L of 4% v/v acetic acid using concentrated glacialacetic acid.SOLUTION(a) Since the concentration only needs to be known to two significant figures,the mass of NaOH and volume of solution do not need to be measuredexactly.
The desired mass of NaOH is0.20 mol 40.0 g×× 0.50 L = 4.0 gLmolTo prepare the solution we place 4.0 g of NaOH, weighed to the nearest tenthof a gram, in a bottle or beaker and add approximately 500 mL of water.(b) Since the concentration of Cu2+ needs to be exact, the mass of Cu metaland the final solution volume must be measured exactly.
The desired massof Cu metal is150.0 mg× 1.000 L = 150.0 mg = 0.1500 gLquantitative transferThe process of moving a sample fromone container to another in a mannerthat ensures all material is transferred.To prepare the solution we measure out exactly 0.1500 g of Cu into a smallbeaker.
To dissolve the Cu we add a small portion of concentrated HNO3and gently heat until it completely dissolves. The resulting solution ispoured into a 1-L volumetric flask. The beaker is rinsed repeatedly withsmall portions of water, which are added to the volumetric flask. Thisprocess, which is called a quantitative transfer, ensures that the Cu2+ iscompletely transferred to the volumetric flask. Finally, additional water isadded to the volumetric flask’s calibration mark.1400-CH02 9/8/99 3:48 PM Page 31Chapter 2 Basic Tools of Analytical Chemistry(c) The concentration of this solution is only approximate, so volumes do notneed to be measured exactly. The necessary volume of glacial acetic acid is4 mL CH3COOH× 2000 mL = 80 mL CH3COOH100 mLTo prepare the solution we use a graduated cylinder to transfer 80 mL ofglacial acetic acid to a container that holds approximately 2 L, and we thenadd sufficient water to bring the solution to the desired volume.2E.2 Preparing Solutions by DilutionSolutions with small concentrations are often prepared by diluting a more concentrated stock solution.
A known volume of the stock solution is transferred to a newcontainer and brought to a new volume. Since the total amount of solute is thesame before and after dilution, we know thatCo × Vo = Cd × Vd2.4where Co is the concentration of the stock solution, Vo is the volume of the stocksolution being diluted, Cd is the concentration of the dilute solution, and Vd is thevolume of the dilute solution. Again, the type of glassware used to measure Vo andVd depends on how exact the solution’s concentration must be known.EXAMPLE 2.10A laboratory procedure calls for 250 mL of an approximately 0.10 M solutionof NH3. Describe how you would prepare this solution using a stock solution ofconcentrated NH3 (14.8 M).SOLUTIONSubstituting known volumes in equation 2.414.8 M × Vo = 0.10 M × 0.25 Land solving for Vo gives 1.69 × 10–3 L, or 1.7 mL.
Since we are trying to make asolution that is approximately 0.10 M NH3, we can measure the appropriateamount of concentrated NH3 using a graduated cylinder, transfer the NH3 to abeaker, and add sufficient water to bring the total solution volume toapproximately 250 mL.As shown in the following example, equation 2.4 also can be used to calculate asolution’s original concentration using its known concentration after dilution.EXAMPLE 2.11A sample of an ore was analyzed for Cu2+ as follows. A 1.25-g sample of the orewas dissolved in acid and diluted to volume in a 250-mL volumetric flask. A20-mL portion of the resulting solution was transferred by pipet to a 50-mLvolumetric flask and diluted to volume. An analysis showed that theconcentration of Cu2+ in the final solution was 4.62 ppm.
What is the weightpercent of Cu in the original ore?dilutionThe process of preparing a lessconcentrated solution from a moreconcentrated solution.311400-CH02 9/8/99 3:48 PM Page 3232Modern Analytical ChemistrySOLUTIONSubstituting known volumes (with significant figures appropriate for pipetsand volumetric flasks) into equation 2.4(ppm Cu2+)o × 20.00 mL = 4.62 ppm × 50.00 mLand solving for (ppm Cu2+)o gives the original solution concentration as 11.55ppm. To calculate the grams of Cu2+ we multiply this concentration by the totalvolume11.55 µg Cu2+1g× 250.0 mL × 6= 2.888 × 10 –3 g Cu2+mL10 µgThe weight percent Cu is then given by2.888 × 10 –3 g Cu2+× 100 = 0.231% w/w Cu1.25 g sample2F The Laboratory NotebookFinally, we cannot end a chapter on the basic tools of analytical chemistry withoutmentioning the laboratory notebook. Your laboratory notebook is your most important tool when working in the lab, providing a complete record of all your work.If kept properly, you should be able to look back at your laboratory notebook several years from now and reconstruct the experiments on which you worked.Your instructor will probably provide you with detailed instructions on how heor she wants you to maintain your notebook.
Of course, you should expect to bringyour notebook to the lab. Everything you do, measure, or observe while working inthe lab should be recorded in your notebook as it takes place. Preparing data tablesto organize your data will help ensure that you record the data you need and thatyou can find the data when it is time to calculate and analyze your results. Writing anarrative to accompany your data will help you remember what you did, why youdid it, and why you thought it was significant.
Reserve space for your calculations,for analyzing your data, and for interpreting your results. Take your notebook withyou when you do research in the library.Maintaining a laboratory notebook may seem like a great deal of effort, but ifyou do it well you have a permanent record of your work. Scientists working in academic, industrial, and governmental research labs rely on their notebooks to provide a written record of their work.
Questions about research carried out at sometime in the past can be answered by finding the appropriate pages in the laboratorynotebook. A laboratory notebook is also a legal document that helps establishpatent rights and proof of discovery.2GKEY TERMSbalance (p. 25)concentration (p. 15)desiccant (p. 29)desiccator (p. 29)dilution (p.
31)equivalent (p. 17)equivalent weight (p. 17)formality (p. 15)formula weight (p. 17)meniscus (p. 29)molality (p. 18)molarity (p. 15)1400-CH02 9/8/99 3:49 PM Page 33Chapter 2 Basic Tools of Analytical Chemistrynormality (p. 16)parts per billion (p. 18)parts per million (p. 18)p-function (p. 19)pipet (p. 27)quantitative transfer (p. 30)scientific notation (p. 12)significant figures (p. 13)SI units (p. 12)stock solution (p. 30)volume percent (p. 18)volumetric flask (p. 26)weight percent (p.
18)weight-to-volume percent33(p. 18)2H SUMMARYThere are a few basic numerical and experimental tools withwhich you must be familiar. Fundamental measurements in analytical chemistry, such as mass and volume, use base SI units, suchas the kilogram (kg) and the liter (L).
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