chemistry ii honors portfolio: - pc\|macimages.pcmac.org/sisfiles/schools/al/mobilecounty... · web...

$: Chemistry II Honors Portfolio: - PC\|MACimages.pcmac.org/SiSFiles/Schools/AL/MobileCounty... · Web viewIt will still be contaminated with harsh chemicals from the reaction. Objectives$
Chemistry II Honors Portfolio:Aspirin Synthesis, Analysis and Presentation

3rd Quarter Spring 2006Instructor: Mrs. V. Hall

GROUP #: ___________________________________

STUDENTS: ____________________________________________________________________________________________________________________________________________

STATION: ___________________________________

SPEC-20 #: ___________________________________

GC #: ___________________________________

Tentative Guidelines and Schedule:

Pre Week: Choose Groups (3 students per group), go over tentative schedule and needed ‘outside’ supplies for lab.

Week 1: Aspirin Synthesis and Mel-Temp Analysis of Aspirin

Week 2 and 3: Make standard and test samples. Spec-20 analysis of standard and samples.

Week 4: GC analysis of standard and samples

Week 5 and 6: PowerPoint presentation(s), abstracts, and papers. Presentation order to be determined.

Week 7 and 8: Presentations, abstracts and papers are to be complete. Presentations are to start (one per day). Guest graders are to be invited to participate in the presentations.

Paper Guidelines:

TitleParticipantsAbstractIntroduction

Background informationAny previous research Objectives of the researchCite references (follow paper example)

MethodsLabs usedOther calculations necessary to obtain dataCite references (follow paper example)

ResultsIndividual lab results with data to support resultsCharts and graphs (referenced, titled) are appropriate in this sectionStatistical interpretationsPercent relationshipsCite references (follow paper example)

DiscussionExplanation of results and its impact on the objectives of the resultsCharts and graphs (referenced, titled) are appropriate in this sectionExplanation of contradictive results based on direct observations/conclusionsExplanation of contradictive results based on drawn conclusionsLimitations to the researchStrengths of the researchCite references (follow paper example)

ConclusionRelate to observed data and objectives of researchRecommendations with examples Cite references (follow paper example)

ReferencesAlphabetical order and numberedHelpful Information: http://info.lib.uh.edu/research/help/apaonline.html

Presentation Guidelines:

http://info.lib.uh.edu/research/help/apaonline.html

Slide information 40 PowerPoint slides minimum cover slide

o titleo presenters nameso classo instructoro date

abstract slide reference slide Slides should contain the following

o Some animationo data chartso data graphso photos of actual lab worko molecular model of acetylsalicylic acid

Presentation Guidelines 30-60 minute presentation All group members must participate in the presentation 1 print out of the PowerPoint presentation (6 slides per page) Handouts of the abstract for observers (if you would like me to photocopy your

abstracts, they must be given to me 2 days prior to your presentation date)

Abstract Guidelines:

Introduction

Writing in chemistry is similar to writing in other disciplines in that your paper must have a clear purpose that explains why you are writing, a thesis statement or main idea that defines the problem to be addressed, and background information wherever necessary. In addition, you should include evidence in the form of figures, graphs, and tables to support your argument.

AbstractsYou will be asked to write an abstract -- a single-spaced paragraph summary that briefly states the purpose of the experiment, important results (and how the results were obtained), and conclusions. Ideally, the abstract can be thought of as one or two sentences from each section of the paper that form a cohesive paragraph that summarizes the entire paper. The abstract should be single spaced unless you receive other instructions from your professor. An example of an abstract with an alternative layout is provided for your use below.When writing an abstract, you should avoid too much experimental detail (e.g. concentration of stock solutions used) or preliminary results (i.e. "raw" data). In addition, make certain that the purpose of the experiment is stated clearly and early in the abstract. Ideally, it should be stated in the first or second sentence.Discipline-Specific Strategies Chemistry papers should be written in passive voice (unless you receive other instructions from your professor).Abbreviations or acronyms must be explained the first time they are used.Figures, graphs, and tables must be titled and referenced in the text.References (including textbooks and lab manuals) must be cited and numbered consecutively with the superscript number corresponding to that reference in the reference section of the paper. The use of superscript suffices as the mode of reference because it eliminates the need for in-text citations and footnotes.Watch Out for… Organization: As for all lab reports, chemistry reports are very structured and must be highly organized in a logical way. Organization of results is especially important. Your results and discussion sections, as well as tables and figures, should be organized in a way that leads the reader to draw the same conclusion that you did based on your data. Don't just tack on a graph at the end of the paper or arbitrarily put your results into a table. Think about how you can use tables to make comparisons between your data and literature or reference values. Think about the format of your tables and the chronology of your results section. How can you present your results so that the reader is already convinced of your conclusion before you explicitly state it? Repetition: If you've already said it once, don't say it again. You can refer to other parts of your paper instead of repeating explanations or facts. If you've already written an experimental methods section, you've already explained your procedure; there is no need to provide procedural details again when you talk about results. If the procedure you used came from a published article, provide a short summary, explain any alterations, and then give the citation. Also, if you explain someone else's experimental results in the introduction, it is acceptable to write statements like, "As discussed above, Khmelnitsky, et al. found contradictory results" in your results section. Repetitious or unnecessary words or figures are unwelcome.Distraction: Remember that the whole point of writing a chemistry paper is to present results and prove your conclusion based on those results. There are a lot of numbers, facts, and procedure information that you can easily get bogged down by. Just remember that ultimately you have to convince the reader that your conclusion is accurate. If you feel overwhelmed by the amount of information you have to include, try making a flow chart that shows the logical progression of your procedure. Or create your figures and tables first, and then use them as an outline or guide to write your results section. Take a look at published articles to get a sense of how others organize papers and what kinds of phrases and sentence structure are useful and accepted.

Preparation and Characterization of … (place your title here)(names of all group members listed here)

Introduction: The required styles illustrated in this Microsoft WORD document, must be used as a template for production of abstracts, by replacing the relevant text with your own. The easiest way to use this abstract form is by cutting and pasting of unformatted text to maintain the present format.

Matherials and Methods. The top margin should be set to I in, bottom margin to 1 in, 1eft and right margins to 1 in. The body of the document should be set in 11 pt Times New Roman, justified, with single line-spacing. Do not use fonts other than Times New Roman (or Times) and Symbol (excluding equations and figures). Title should be in bold all-caps. Author should be listed consecutively (as shown above) by initials and last name. Sample reference entries [l-3] are given in the References section. References should be set as one block, as below, and a maximum of four main references may be used in the abstract (a complete list of references must be included with your paper).

Results and Discussion. Figures should use Figure style and have the caption below them. Tables should use Table style and have the caption above them. Figure and table caption styles are FigCaption and TableCaption, respectively. Scanned figures should have a minimum resolution of 300 dpi for an 80 mm figure width. JPEG is the preferred format. Equations should use Equation style:

(1)

and should be tab centered, with the number in parentheses on the right.

(a) (b)Fig.1 The SEM micrographs of air-facing surface (a) and glass-facing surface (b) for sample CHI.

Table 1 The charge distribution at the atoms implied in 3+3 DTD for the most stable conformers of BPPBM.

C8 N13 N12 C11 C14

Ia 0.139 -0.387 0.867 -0.499 -0.617IIa 0.085 -0.237 0.330 -0.224 -0.246Ib 0.154 -0.359 0.874 -0.501 -0.606IIb 0.024 -0.153 0.465 -0.335 -0.310

Conclusions. Abstracts must not exceed ONE PAGE. Please submit the abstract as a Word document. Only typed submissions will be accepted. The abstracts may be submitted via e-mail (attachment) labeled with group number as the file name.

References1. M. T. Qurashi, H. S. Blair, and S. J. Allen, Journal Appl. Polym.Sci., vol 46, nr.2, 255-261 (1992).2. C. Ciobanu, P. Afloarei, C. Bîrlădeanu and C. V. Culic, Brevet Ro. 93590/25.06,1987. 3. Zugrăvescu and M. Petrovanu, “N-Ylid Chemistry”, Ed. Academic Press, Mc. Graw Hill, London, (1976).4. J.W. Boretos, in “Polyurethanes in Biomedical Engineering”, eds. H. Plank, G. Egbers, and I. Syre, Ed. Elsevier, Amsterdam,

1984, p.135.

Melting Point Data Collection Sheet:

Temperature (oC)

Temperature (oC)

Salicylic Acid

Beg --------End YOUR Sample Beg --------End

Capillary A Capillary ACapillary B Capillary B

Capillary C Capillary C

Average Average

Store Brand : ___________

Beg --------End Chem I Sample Beg --------End

Capillary A Capillary ACapillary B Capillary BCapillary C Capillary C

Average Average

Store Brand : ___________

Beg --------End Acetyl Salicylic Acid

Beg --------End

Capillary A Capillary ACapillary B Capillary BCapillary C Capillary C

Average Average

Mathematical Calculation sheet for the making of the samples and standard:

Standard Ratios: Aspirin: NaOH: Water (0.4:10:250)

Your Aspirin Sample

Store Brand: _______________

Store Brand: __________________

Chemistry I Sample

WeighboatSample weight

Formula for # of samples: Sample weight / # of samples

Your Aspirin Sample

Store Brand: _______________

Store Brand: __________________

Chemistry I Sample

Sample 1

Sample 2

Sample 3

Sample 4

Sample 5

Formula for divider: 0.4 / sample mass

Formula for Amount of NaOH needed per sample: 10/divider

Your Aspirin Sample

Store Brand: _______________

Store Brand: __________________

Chemistry I Sample

NaOH needed

Formula for Amount of H2O needed per sample: 250/divider

Your Aspirin Sample

Store Brand: _______________

Store Brand: __________________

Chemistry I Sample

H2O needed

Calculated Data sheet for the making of the samples and standard:

ACTUAL Data Sheet for the making of the samples and standards

Data Sheets for Spec-20 analysis: Sample __________

Wavelength

transmittance

absorbance

concentration

Wavelength

transmittance

absorbance

concentration

Wavelength

transmittance

absorbance

concentration

Wavelength

transmittance

absorbance

concentration

Lab Equipment Page:

Name of Equipment Cost Amount ($) Quantity (#) Total Price ($ x # =)

10 mL graduated cylinder 11.77

250 mL volumetric flask 19.12

500 mL round bottom flask 9.90

250 mL Erlenmeyer flask 3.68

Dropper bottles 2.49

Cap for volumetric flask 2.33

Stand for graduated cylinder 4.50

Other: Box for keeping supplies

Other: 3 film canisters

Other: CD (R or RW)

Other: Disc

Other: Thumbdrive (portable memory)

Other: digital camera / camera phone

Other: camera (develop film to CD)

Other: graphing paper

Other: overhead transparency film

Other:

Other:

Other:

Other:

Other:

Other:

Other:

Other:

Other:

Other:

Lab Write-up Procedures:

1. Cover Page: a. title of labb. your namec. your partners name(s)d. date lab was performede. teachers namef. block

2. Objective (s)

3. Materiala. Chemical materialb. Equipment

4. Safety

5. Procedure (s)

6. Data a. Chartsb. Graphsc. Picturesd. Photose. Writtenf. Mathematical

7. Questions with answers highlighted or in bold

8. Conclusiona. Explanation of results b. Why the laboratory went wrongc. Modification / Improvements to laboratory

Lab Write-ups may be handwritten or may be computerized (typed) for EXTRA CREDIT.

Lab Write-ups are due on the day assigned. If they are turned in late it is 10 points off your write-up PER DAY (INCLUDING Saturday and Sunday). You are given at least a week turn around on labs; therefore, I expect them on time.

Introduction

The melting point of a compound is the temperature at which it changes from a solid to a liquid. This is a physical property often used to identify compounds or to check the compound’s purity. Strangely, it is difficult to find an actual melting point for most compounds, even with sophisticated equipment; usually, chemists settle on a melting range of 2 – 3 C. Fortunately, this is sufficient for most purposes.

Objectives

By doing this lab, students will be able to:

define melting point demonstrate proper use of the melting-point apparatus use melting point as a physical property in the identification of unknown compounds

Materials and Equipment

Mel-Temp® apparatus AcetoneThermometer Capillary tubesPestle Samples of solid organic compoundsWatch glass

Safety Considerations

As always, you should wear safety glasses/goggles when doing this experiment. Be careful when handling glass thermometers – they aren’t that difficult to break. Capillary tubes are very easy to break – and when they do, they have a tendency to puncture fingers or hands or

leave small, sharp pieces of glass that are easy to miss when cleaning up the work area. Handle them with care! The parts on the top of the Mel-Temp are hot when the unit is on. Do not touch these parts – you will get

burned! Do not place your eye on the eyepiece of the Mel-Temp – you will get burned! Be careful when handling these compounds – some of them are skin and eye irritants. Wash your hands thoroughly with soap and water when you’re finished with the lab.

Procedure A: Practice Compound

1. Your instructor will tell you which compound to use for practice. Place a small amount – no more than a few grains – of the practice compound on a watch glass. Use the pestle to gently grind it into a fine powder.

2. Take the pestle over to the fume hood – away from any heat source – and rinse any excess compound off the pestle with acetone. Dry the pestle thoroughly with a paper towel and bring it back to your lab station.

Microscale Labs – 4

Determination of Melting Points©1999, 2003 by Science in Motion. All rights reserved.

3. Push the open end of the capillary tube into the compound on your watch glass. Some of the compound will now be in the top of the tube. Gently tap the closed end of the tube on the bench top. This will pack the compound down in the closed end of the tube. Repeat this a couple of times until you have about 0.5 cm of the compound in the bottom of the tube (about half the width of your little finger, unless you have tiny hands).

4. Place the capillary tube in the chamber of the Mel-Temp apparatus and turn on the unit. Start with a setting of 1 or 2 and increase the power only in small increments as needed, and never above 5. Heating your com-pound slowly is the key to getting good results!

5. Observe the sample chamber frequently to make sure you don’t miss the melting point of your material. Heat the compound slowly and record the temperatures at which the compound begins to melt and the temperature when all of it has melted.

6. Use the table on the data sheet to identify your practice compound.

Procedure B: Identifying an Unknown Compound

After everything has had a chance to cool down a bit, repeat the above procedure for an unknown compound.Make sure you record which sample you have!

Disposal and Cleanup

Pour any excess compound left on your watch glass into the trash can. Take your watch glass over to the vent hood and rinse off any remaining compound with acetone. Dry the glass thoroughly with a paper towel.

If you used a digital thermometer, make sure you turn it off before leaving the lab area. This helps prolong the battery life (and these batteries are expensive).

Make sure the Mel-Temp apparatus is turned off before you leave the lab area. Throw away any used capillary tubes. Put everything where it was before you started the lab. Wash your hands thoroughly with soap and water before leaving the lab area.

NAME Microscale Labs – 4

Determination of Melting Points©1999, 2003 by Science in Motion. All rights reserved.

NAME

NAME

DATE PERIOD

Compound Melting Point / RangeAcetylsalicylic acid 135 – 136 C

Benzoic acid 122 CBHT (2, 6-di-t-butyl-4-methylphenol) 69 – 71 C

Citric acid 153 CEthyl p-aminobenzoate 88 – 90 C

Oxalic acid 101.5 CPalmitic acid 61 – 64 C

Potassium iodide 680 CSodium chloride 804 C

Stearic acid 71.2 CTOP (4-(t-octyl)phenol) 79 – 82 C

Urea 132 CVanillin 81 – 83 C

Practice Compounds Data

Practice Compound Observed Melting Range, C

Unknown Compound Data

Observed melting range C

Identity of unknown compound

Questions

1. Define the “melting point” of a substance.

2. What is the purpose of determining melting points?

3. Why is this method not used for finding the melting points of inorganic compounds?

4. Why could the rate of heating influence the observed melting point?

5. The melting ranges of TOP (4-(t-octyl)phenol) and vanillin overlap. If you could narrow down your choices so that you knew your unknown was one of these two compounds, how might you distinguish between the two?


Synthesis of Acetylsalicylic Acid©2000, 2003 by Science in Motion. All rights reserved.

Introduction

Acetylsalicylic acid is a wonder drug par excellence. It is widely used as an analgesic (pain reliever) and fever depressant; it also reduces inflammation and may even prevent heart attacks. It has a few side effects for some people, yet it is safe enough to be sold without a prescription. Because it is easy to prepare, acetylsalicylic acid – better known by its trade name, aspirin – is one of the most inexpensive drugs available and is produced in vast amounts. In fact, the drug industry makes about 20,000 metric tons (43 million pounds!) of aspirin every year.

In this microscale experiment, you will make acetylsalicylic acid – aspirin – by reacting salicylic acid with acetic anhydride. Structurally, the reaction looks like this:

+ +

salicylic acid + acetic anhydride acetylsalicylic acid + acetic acid

You will be making acetylsalicylic acid equivalent to half of an aspirin tablet, but your aspirin will not be in a form that can be taken. It will still be contaminated with harsh chemicals from the reaction.

Objectives


synthesize acetylsalicylic acid on a microscale basis describe the dangers of ingesting substances made in the lab demonstrate proper use of microscale chemistry equipment


Microscale organic kit Thermometer Concentrated phosphoric acidAnalytical balance Styrofoam® cup Distilled waterWeighing paper Stirring rod IceHot plate Salicylic acid Filter paper100-mL beaker Acetic anhydride Vacuum pump


As always, you should wear safety goggles/glasses when working in the lab area. If aprons are available, wear one.

Several of these chemicals will irritate your skin and eyes. If you spill any of the chemicals on you, wash the area thoroughly with soap and water.

Do not rub your face or eyes, because you may get chemicals in your eyes. If your eyes start to burn, rinse them at the eyewash station.

Do not ingest the aspirin you produce. It still contains harmful chemicals and is not fit to be taken.Procedure

1. Pour about 40 mL of tap water into the 100-mL beaker. Place the beaker on the hot plate and turn on the hot plate to a power setting of 5 (or about halfway).

2. Using a pencil, write your name on a piece of filter paper. Determine the mass of the paper using an analytical balance and record the mass on the data table.

3. With the filter paper still on the balance, press the TARE button. Weigh out between 135 and 140 milligrams (0.1350 – 0.140 g) of salicylic acid directly onto the filter paper. Record the mass (all four decimal places) in the data table. Place the acid in a test tube from the microscale kit.

4. Add one drop of concentrated phosphoric acid to the test tube.

5. Using the syringe from the microscale kit, add 0.3 mL of acetic anhydride to the test tube. Try to rinse all the other ingredients to the bottom of the tube when adding the anhydride.

6. Use a stirring rod to mix the reactants thoroughly.

7. When the water bath – the water in the beaker – is between 70 and 90 C, place the test tube in the water bath to heat its contents. You are trying to dissolve the salicylic acid and may need to stir the contents of the test tube while it is in the water bath.

8. Once the acid is dissolved, heat the tube for 2 more minutes. Then, carefully add 0.5 mL of distilled water to the tube.

9. Remove the test tube from the water bath and allow it to cool to room temperature. Crystals of acetylsali-cylic acid should form inside the tube. If crystallization does not occur at room temperature, use a stirring rod to scratch the inside of the test tube.

10. Once crystallization has begun, cool the test tube in a cup of ice water for several minutes, until crystalliza-tion is complete. Be careful not to let the tube tip over and spill into the ice water.

11. Line the Hirsch funnel from your microscale kit with the filter paper you weighed earlier. Using a spatula, transfer the contents of the test tube to the funnel. Rinse the test tube with ice water to make sure you transfer all your product to the funnel.

12. Using the vacuum pump, filter your product. Your instructor will demonstrate how to do this.

13. Leave the filter paper and acetylsalicylic acid to dry overnight. Make sure to leave it in a place where it will not be disturbed.

14. Once your product is dry, weigh the filter paper and product and record the mass in the data table. Calculate the mass of the dry product.

Cleanup and Disposal

To clean up your lab station, rinse all the glassware and the funnel and return them to the microscale kit. Once you have recorded the mass of the filter paper and acetylsalicylic acid, throw the paper and acid away in

the trash can. Under no circumstances should you even think about ingesting the product! Wash your hands thoroughly with soap and water when you finish making the aspirin – and when you’re

through weighing it.

NAME

NAME

NAME

DATE PERIOD

Data

Mass of salicylic acid used g


Synthesis of Acetylsalicylic Acid©2000, 2003 by Science in Motion. All rights reserved.

Mass of filter paper and product g

Mass of filter paper g

Mass of product after drying g

Questions

1. What chemicals may still be present that would contaminate your aspirin product?

2. How could you use a Mel-Temp® (a melting-point apparatus) to check the purity of the aspirin you produced?

3. Write the word equation for the synthesis of acetylsalicylic acid.

4. Determine the mass of acetic anhydride used. (Danhydride = 1.0820 g / mL)

5. Determine the limiting reactant in this reaction.

6. Calculate the theoretical yield and compare it to your experimental yield in terms of percent yield.

7. On the back of this sheet, list several sources of experimental error.

SPECTROPHOTOMETRY LABS

Spectrophotometry is a quantitative method of analysis involving the principles associated with how visible light interacts with atoms. Visible light is a small portion of the electromagnetic spectrum and includes the colors we observe – red, orange, yellow, green, blue, and violet. Visible light consists of electromagnetic radiation whose wavelengths range from 700 nm (red) down to 400 nm (violet).

When we observe “white light,” what we are actually seeing is all the colors of light combined. When this light passes through a substance, certain energies (colors) of light are absorbed while the other color(s) are trans-mitted. This is why some substances appear colored. The color we see is the combination of the energies of visible light that are not absorbed by the sample. If the substance does not absorb any light, it appears white or colorless. A solution appears a certain color due to the absorbance and transmittance of visible light. For example, an orange solution appears orange

because it is absorbing all colors of light except orange. Conversely, a solution may appear orange because all colors of light except blue are being transmitted. This is because blue and orange are said to be complementary colors (see Figure 1).

Figure 1. A “color wheel,” showing approximate wavelength “boundaries” in nanometers (nm).Colors opposite one another are said to be complementary colors.

The wavelength associated with the complementary color is known as the wavelength of maximum absorbance, or max. The wavelength of maximum absorbance is used when determining the concentration of a colored solution, since at this wavelength a slight change in concentration allows for a significant change in the absorbance of light.

Many compounds involving transition elements are colored. This is because the transition metals included orbitals in their atomic structures. The spacing of these d orbitals allows for electronic transitions within the energy range of the visible portion of the electromagnetic spectrum. Compounds containing the alkali and alkaline-earth metals are white because they only involve s electron transitions. More energy is required to cause this type of tran-sition and, thus, light of shorter wavelength is involved.

The amount of light absorbed by a solution is dependent upon the ability of the compound to absorb light (its molar absorptivity), the distance through which the light must pass through the sample (the path length), and the molar concentration of the compound in the solution. This relationship is known as the Beer-Lambert Law (or, more commonly, “Beer’s Law”) and is represented by the equation

A = abc

where A is the absorbance, a is the molar absorptivity, b is the path length, and c is the molar concentration. If the same compound is being used and the path length is kept constant, then the absorbance is directly proportional to the molar concentration of the sample.

A spectrophotometer is used to provide light of certain energy (and, therefore, wavelength) and to measure the absorption of that light. The basic operation of the spectrophotometer includes a white-light radiation source which sends white light to a monochromator. The monochromator is either a prism or a diffraction grating, which separates light into its colored components and allows only light of a particular wavelength to strike the sample. The sample is poured into a cuvette, which is similar to a small test tube. It is marked so that it can be positioned in the light beam the same way each time to avoid variations due to differences in the composition and thickness of the glass. The light passes through the sample, and the unabsorbed portion strikes a photodetector, which produces an electrical signal proportional to the intensity of the light. The signal is then converted to a readable output, which is used in the analysis of the sample. See Figure 2 for a simplified diagram of this process.

Source

GratingCuvette withSample

Detector

I oI

Figure 2. Diagram of what happens inside a spectrophotometer. Light of intensity I0 passes throughthe cuvette, where some of it is absorbed; light of intensity I reaches the detector, which then converts

that into a digital or analog readout. Moving the diffraction grating changes the wavelength of thelight that strikes the sample.

Rationale

Spectrophotometry uses visible light to analyze colored compounds in solution. Students will collect and analyze data from a set of standards and find the concentration of an unknown solution.

Central Question

How do you use a spectrophotometer for quantitative analysis?

General Objectives

The student will:

1. demonstrate the ability to operate a spectrophotometer to find the wavelength of maximum absorbance.

2. calculate absorbance from percent transmittance.

3. plot a calibration curve.

4. determine the amount of a particular substance in two different samples of an unknown

Specific Objectives

Skills

The student will:

1. know how to prepare standards for the calibration curve.

2. plot a standard curve.

3. handle and clean the cuvettes.

4. adjust the wavelength of the spectrophotometer and set it to zero.

5. know how to prepare samples for analysis.

6. know the purpose of a blank

7. analyze an unknown using a spectrophotometer and a standard curve.

Understandings

The student will be able to:

1. explain why different substances being analyzed absorb at different wavelengths.

2. demonstrate understanding that the spectrophotometer may be used to analyze a variety of compounds by adjusting the wavelength and using different standards.

3. see that substances absorb in different regions in the electromagnetic spectrum, e.g., infrared and ultraviolet.

Evaluation

The student will:

1. plot a calibration curve.

2. determine the concentration of a substance in an unknown solution.

3. write a brief explanation of spectrophotometry.

Introduction

The Digital Spectrophotometer 20 – also known as the “Spec-20D” or simply the “Spec-20” – is a workhorse of the chemistry lab. With it, we can perform a number of analytical procedures, including the determination of the concentration of a solution of colored ions and measuring the rate of a chemical reaction.

This lab is designed to familiarize you with the basic terminology and workings of the Spec-20D – how to prepare a sample, which knob or button does what, and how to read and understand the display.

Spectrophotometry Labs – 2

Introduction to the Digital Spectrophotometer20 [“Spec-20D”]

ObjectivesAfter doing this lab, students will be able to:

name each of the controls on the Spec-20D and describe what they do set the Spec-20D to a given wavelength and zero it using a blank

Materials and EquipmentSpectrophotometer 20-D water3 cuvettes (also spelled “cuvets”) Kimwipes®

chalk

Safety Considerations As always, you should wear safety glasses/goggles when working in the lab area. Avoid dropping the cuvettes – they can be expensive. Do not empty the cuvettes into the sample compartment! This is an electronic instrument, and water will

damage it – and you as well.

Procedure[NOTE: In this procedure, steps that require a response on the data sheet are marked with an asterisk (*).]

1. To turn on the Spec-20D, turn the left front knob to the right until it clicks and the display and fan comeon.

* 2. Turn the large knob on top of the machine slowly and observe what happens on the display.

* 3. Wavelength is measured in nanometers. One nanometer is equal to 10–9 meter, or one billionth of a meter.Adjust the wavelength to 520 nm. Open the lid of the sample compartment. Place a cuvette with chalk intothe compartment. The slope of the chalk should face toward the wavelength control knob. Do not empty thecuvette into the compartment!

* 4. While looking at the chalk, slowly turn the right front knob to the right and then to the left.

5. Adjust the transmittance control knob until you observe a bright band of light on the chalk (see Step 4).

* 6. Using the large knob on top of the Spec-20D, adjust the wavelength to the lowest possible setting. Then,while watching the chalk in the cuvette, increase the wavelength until the light first appears on the chalk.This is the lowest visible wavelength. Record this number on the data sheet.

* 7. While watching the chalk, turn the wavelength control slowly through its entire range.

* 8. Adjust the wavelength control knob to its lowest possible setting. While looking at the chalk, slowly increase the wavelength. Record the wavelength in nanometers at which you first see the color violet. Con-

tinue to record the wavelength at which you first see each color. Then determine the highest visible wave-length (the wavelength at which the color red disappears).

* 9. Use your data to describe the relationship between color and wavelength (on the data sheet).

*10. Remove the chalk cuvette and close the lid on the sample compartment. There should be a light by the wordTRANSMITTANCE on the display; if not, press the MODE button until there is. Turn the left front knob untilthe data value is 0.0 (the minus sign may flash on and off – this is normal).

*11. Fill a cuvette with distilled or deionized water to within 2 cm of the top. Since the water in this cuvette hasnothing in it, it is called a blank. Wipe the bottom of the cuvette with a Kimwipe to make sure there is nodirt on it. Always handle cuvettes by the top to avoid getting dirt or fingerprints on them. Notice that thereis a line at the top of the cuvette; this mark should line up with the raised ridge on the front of the samplecompartment. Insert the cuvette and close the lid on the compartment.

*12. Adjust the transmittance control (right front knob) until the right number reads 100.0. Press the MODE buttononce. The spectrophotometer should now be set for absorbance, and the data display should read 0.0.

*13. Press the MODE button until the light next to TRANSMITTANCE is lit. Adjust the transmittance to 10.0. Pressthe MODE button once. Record the value for absorbance. (Absorbance is not a linear function, as is trans-mittance. It has no units; it is merely a number.)

*14. Remove your cuvette from the spectrophotometer and set the MODE to TRANSMITTANCE. Close the lid on thesample compartment. Set the wavelength to 425 nm. Use the zero knob to set the instrument to zero if it isnot already reading zero. Insert a blank, close the lid, and set the transmittance to 100.0 using the right frontknob. Adjust the wavelength to 500 nm.

*15. Because the instrument is not equally sensitive to all wavelengths, it must be adjusted every time you changewavelengths. On the data sheet, record the value(s) to which the transmittance or absorbance should be ad-justed each time you change wavelengths.

Disposal and Cleanup Place the chalk cuvette back into its box. Empty your blank cuvette into the sink. Dry it out and put it back in the box. Turn off the Spec-20D if instructed to do so.

NAME

NAME

NAME

DATE PERIOD

[NOTE: On this data sheet, the numbers match the step numbers in the Procedure.]

2. What happens to the display when you turn the large knob on top of the Spec-20D?

What was the lowest number you could get? nm

What was the highest number you could get? nm

What does this number represent?

What is a good name for this control?


Introduction to the Digital Spectrophoto-meter 20 [“Spec-20D”]

©2003 by Science in Motion. All rights reserved.

3. What do you see when you look into the sample compartment?

4. What do you observe when you turn the right front knob clockwise and counterclockwise?

Since this knob controls the amount of light transmitted through your sample, what might be a good name for it?

6. What is the lowest visible wavelength? nm

7. What do you observe as you turn the wavelength control knob through its entire range?

8. Record the wavelength at which you first see each color.

Color Red Orange Yellow Green Blue Violet

Wavelength, nm

What is the highest visible wavelength? nm

9. What is the relationship between color and wavelength? In your experience, where else have you seen colors appear in this order?

10. Since the left front knob sets the Spec-20D to zero, what might be a good name for it?

11. Why is it important to keep cuvettes clean?

12. Having set the spectrophotometer to 100.0% transmittance, complete the following relation:

100.0% transmittance = absorbance

13. Having set the spectrophotometer to 10.0% transmittance, complete the following relation:

10.0% transmittance = absorbance.

Remember that absorbance is not a linear function, as is transmittance. Absorbance has no units; it is merely a number!

14. What happened to the percent transmittance when you changed the wavelength from 425 to 500 nm?

15. When you change the wavelength, the transmittance control should be adjusted so that the instrument

reads transmittance or absorbance.

Now that you’ve finished your tour of the controls of the Spec-20D, answer the following questions about the instrument:

A When there is no cuvette in the sample compartment, which knob do you turn to zero the instrument?

B In addition to the markings, how do you think cuvettes might be different from ordinary test tubes? Is there anything that has to be made more carefully on a cuvette?

C Different solutions absorb light of different colors or wavelengths. Therefore, the wavelength of light that a solution absorbs best depends on … what?

Introduction

One of the most effective uses of the Spec-20D® spectrophotometer is in determining the concentration of an unknown solution. This technique can be used in forensics to determine the amount of poison in a murder victim’s tissues, in pharmacology to determine the safe and effective dosage of a new drug, or in industry to determine the con-centration of pollutants in wastewater.

This spectrophotometric technique makes use of the Beer-Lambert law, frequently known as “Beer’s Law.” We represent the Beer-Lambert law as

A = abc

where A is the absorbance, a is the molar absorptivity of the substance in question, b is the length of the light path inside the spectrophotometer, and c is the concentration of the solution.

In this experiment, we will apply the Beer-Lambert law to a series of aqueous solutions of cobalt(II) chloride, CoCl2 , which have a pink color due to the presence of Co2+ ions. We will identify the wavelength of maximum ab-sorbance in the absorption spectrum of Co2+, then use that wavelength in determining the absorbances of a series of solutions of known concentration.

When we plot the measured absorbances against concentration, we will observe a linear relationship; the slope of the line will correspond to ab. Since the path length, b, can be measured, we can calculate the value of a. As long as the wavelength is not changed, a will be a constant for any solution of CoCl2 . Thus, we can measure the ab-sorbance of a solution of Co2+ and calculate its concentration from the information we already have. We report the concentration as “[Co2+] = x”; this is notation you will see frequently in future labs.


Beer’s Law I: Determining the Concentration of a Cobalt(II) Chloride Solution

©1999, 2003 by Science in Motion. All rights reserved.

This may all sound terribly complex, but it really isn’t. And to make matters better, obtaining the absorption curve is something you only have to do once for each substance.

Objectives


state the Beer-Lambert law and define each variable in it use an absorption curve to determine the concentration of Co2+ in a solution of unknown concentration extrapolate from what they learn in this lab to determine the concentrations of ions in other solutions


0.15 M cobalt(II) chloride solution cobalt(II) chloride solution, concentration unknown6 cuvettes Eppendorf® micropipetterstirring rod graph paperSpec-20D spectrophotometer ruler


As always, you should wear safety glasses or goggles when working in the lab area. Cobalt(II) chloride is toxic by ingestion. If you have open wounds on your hands, you should wear gloves during this experiment in order to prevent

blood damage. Avoid dropping or breaking the cuvettes – they can be quite expensive.

Procedure A: Obtaining the Absorption Spectrum

1. Turn on the Spec-20D and allow it to warm up. This takes about ten minutes; your instructor may have already done this for you.

2. Prepare a “blank” cuvette by placing 5.0 mL of distilled/deionized water into a cuvette.

3. Obtain approximately 20 mL of 0.15 M cobalt(II) chloride solution.

4. Using the micropipetter, transfer 5.0 mL of the cobalt(II) chloride solution to another cuvette.

5. Using the large knob on top of the Spec-20D, adjust the wavelength to 400 nm (nanometers). The light next to TRANSMITTANCE should be lit; if it is not, press the MODE button until it is.

6. With nothing in the sample compartment and the lid closed, set the transmittance to zero using the front left knob.

7. Press the MODE button once so that the light next to ABSORBANCE is lit.

8. Place the blank cuvette in the sample compartment and close the lid.

9. Set the absorbance to zero using the front right knob.

10. Remove the blank and replace it with the cuvette containing the cobalt(II) chloride solution. Close the lid. Record the absorbance on the data table.

11. Change the wavelength to 425 nm and repeat Steps 8 – 10.

12. Continue to change the wavelength by 25 nm and repeat Steps 8 – 10 until you reach 600 nm. Remember that each time you change the wavelength, you must adjust the absorbance to zero using the blank.

13. Graph the absorbance (A) on the y-axis versus wavelength on the x-axis. Draw a smooth curve to fit the ex-perimental points. Identify the maximum in the absorption curve to the nearest multiple of 25 nm and record this on your data sheet.

Procedure B: Validating the Beer-Lambert Law

1. Using the micropipetter, prepare a series of cobalt(II) chloride solutions using the following table. Do all the CoCl2 transfers before adding water to any of the cuvettes; this means you use only two micropipetter tips in the preparation. Mix the contents of each cuvette using a clean, dry stirring rod.

Note: You prepared tube 1 in Procedure A. Keep in mind that careful measurement is important!

Tube # 1 2 3 4 5Volume of 0.15 M CoCl2 (mL) 5.0 4.0 3.0 2.0 1.0Volume of distilled water (mL) 0.0 1.0 2.0 3.0 4.0

2. Set the wavelength of the Spec-20D to the wavelength of maximum absorbance you found in Procedure A. Use the blank cuvette to adjust the absorbance to zero.

3. Measure and record the absorbances of tubes 2 through 5 (you already have the absorbance for tube 1).

4. Calculate the concentration of each of the solutions as a percentage or ratio of tube 1.

5. Graph the solutions’ absorbances versus [Co2+]. Use a ruler to draw a straight line through the origin and as close as possible to all the experimental points. Calculate the slope of this line.

Procedure C: Determining the Concentration of an Unknown Solution

1. Obtain about 5 mL of a cobalt(II) chloride solution of unknown concentration and record its letter/number on your data sheet.

2. Place the sample in the Spec-20D and record its absorbance on your data sheet.

3. Calculate the concentration of the unknown using Beer’s Law and the data you gathered earlier (see the sidebar below for help).

Disposal and Cleanup

Pour your cobalt(II) chloride cuvettes into the waste container provided by your instructor. Pour the distilled water down the drain.

Thoroughly rinse out the cuvettes with tap water and dry them with a Kimwipe or three. Wash your hands thoroughly with soap and water.

Calculating the Concentration of the Unknown

We can use the numbers we’ve obtained in the procedure to calculate the concentration of the unknown. Re-member that Beer’s Law is A = abc, where

A = absorbance [from the Spec-20D]a = absorptivity, which is a constant for each individual compound = slope of lineb = path length, which is a constant when using the same Spec-20D = 1 cm in this experimentc = concentration

Comparing one standard (s) and one unknown (u),

Since a and b are the same for both the standard and the unknown (and can be canceled out),

If you’re worried about this, check the algebra yourself on a piece of scratch paper before you plug in the numbers.

NAME

NAME

NAME

DATE PERIOD

Data for Procedure A: The Absorption Spectrum

Wavelength, nm Absorbance400

425

450

475

500

525

550

575

600

The wavelength of maximum absorbance (to the nearest 25 nm) occurs at nm.

Data for Procedure B: Validating the Beer-Lambert Law


Beer’s Law I: Determining the Concentra-tion of a Cobalt(II) Chloride Solution

©1999, 2003 by Science in Motion. All rights reserved.

Tube # Absorbance [Co2+]Calculated from concentration of Tube 1

1

2

3

4

5

Data for Procedure C: Determining the Concentration of an Unknown

Number/letter of unknown:

Absorbance of unknown:

[Co2+] of unknown: (see sidebar for details of calculation)

[Questions on back]Questions

1. Calculate the concentration of chloride ion (written “[Cl–]”) in the unknown sample. An equation for the dissociation of CoCl2 would be very helpful here.

2. In Procedure B, why must the straight line pass through the origin?

3. What experimental problems would confront you if you attempted to determine the concentration of Cr3+ in a solution? Cr3+ has a maximum absorbance at 407 nm with a = 15 M –1 cm–1 and a second maximum at 574 nm with a = 13 M –1 cm–1.

4. What experimental problems would you have in trying to determine the concentration of Mn2+, which has a maximum absorbance at 530 nm with a = 0.050 M –1 cm–1 ?

Standardization of Pipette

Directions:

Take your pipette and count the number of drops it takes to make 1mL. Repeat twice more. DO NOT loose your pipette or you will be standardizing a new pipette.

Number of Drops per 1mLTrial 1Trial 2Trial 3

Formula for Average # of drops per 1mL = T1 + T2 + T3 / 3

Average: _______________________

Formula for # of mL per drop: 1 drop x 1mL 1 # of drops

1 drop: __________________mL

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