Green Chemistry Workshop #4:

Analytical Chemistry II

Green Chemistry Workshop #4:

Analytical Chemistry II

US – Thai Distance Learning Program

February 24, 2010

Professor Kenneth M. Doxsee

University of Oregon

The Principles of Green Chemistry

1. Prevention

2. Atom Economy

3. Less Hazardous Synthesis

4. Designing Safer Chemicals

5. Safer Solvents and Auxiliaries

6. Design for Energy Efficiency

7. Renewable Feedstocks

8. Reduce Derivatives

9. Catalysis

10. Design for Degradation

11. Real-time Analysis

12. Inherently Safer Chemistry

Cementation Experiment from Session 3

Determination of Acetylsalicylic Acid in an Aspirin Tablet

Premise• Acid-base titration can be used to determine

the quantity of acetylsalicylic acid.

Chemical Concepts • Acid-base chemistry; titration; volumetric

analysis.

Aspirin image: http://www.bayerhealthcare.com/html/images/upload/presse/aspirin_tablette_205.jpg

Background

• Aspirin is derived from salicylic acid, originally obtained from the bark of a willow tree.

• Commercial aspirin tablets contain acetylsalicylic acid as the active ingredient.

• Typical tablets also contains “binders” and “fillers” such as corn starch or cellulose.

Tree image: http://www.janiceboling.com/herbs/images/willow.jpg

HC

HCCH

C

C

HC C

OH

O

O

CCH3O

+ NaOH + H2OHC

HCCH

C

C

HC C

O- Na+

O

O

CCH3O

O

HO

OH

O

O O- Na+

CO2- Na+

+ NaOH

Reaction Chemistry

Phenolphthalein

Acetylsalicylic acid

colorless pink

Pre-lab Preparations

• Weigh the tablet, then grind to a powder.• Transfer the powder to the volumetric flask,

weighing to determine the amount transferred.• Add 20 mL H2O and 20 mL ethanol.

• Swirl, then dilute to the mark with H2O.• It is helpful but not necessary to filter the mixture

before transferring to the volumetric flask.Aspirin image: http://www.bayerhealthcare.com/html/images/upload/presse/aspirin_tablette_205.jpgMortar and pestle image: http://www.ancientlight.info/products/images_stones/LMORL.jpgVolumetric flask image: http://www.jaytecglass.co.uk/images/VolumetricFlaskB250ml.jpg

Item AmountCommercial aspirin tablet 1Ethanol 20 mLDistilled or deionized water 80 mL100-mL volumetric flask 1

Typical Apparatus

Schematic image: http://www.fao.org/docrep/v5380e/V5380E17.GIFBurette image: http://www.thesciencefair.com/Merchant2/graphics/00000001/5x8SingleSheet50mlTeflon_M.jpgBurette close-up images: http://www.pelletlab.com/v5Files/pellet/144992/640/veegee_burette.jpg

Microscale Titration

Automatic delivery pipet tip

1 mL syringe

(for titration)

1 mL syringe

(for refilling)

Three-way stopcock

THE MEXICAN MICROSCALE

TITRATION SYSTEM

Syringe image: http://www.jolly-medical.com/doc/1/syringe%20picture/syringe%201ml%20insulin.jpgNeedle image: http://www.1st-product.com/uploads/10/45-1/Dispsable_Syringe_Needle_CE_ISO_273.jpgPipette tip image: http://image.made-in-china.com/2f0j00bCkQEqMyEhoe/Pipette-Tips.jpg

A SIMPLER SYSTEM

Procedure

• Place 1.0 mL of the aspirin solution in a small Erlenmeyer flask.

• Add a few drops of phenolphthalein solution as an indicator.

• Using a 1.0 mL syringe, carefully titrate with 0.05 M NaOH solution, recording the initial and final volume.

Data Table

Laboratory Safety

Accident: An unexpected and undesirable event, especially one resulting in damage or harm.

Safety glasses image from: http://facilitysigns.wordpress.com/category/signs-and-labels/

LABORATORY SESSIONBreak for ….

Calculations(Experimental data for a 20 mL sample of aspirin (ASA) solution, titrating with 0.0462 M NaOH)

 

(11.20 mL)(0.0462 mol/L) / 20 mL = 0.0259 mol/L

 

The amount of ASA per 20-mL sample is then:

 

(0.0259 mol/L)(180 g/mol)(0.020 L) = 0.093 g

In the 100-mL solution originally prepared:

0.093 g x 5 = 0.465 g

Calculations

The original tablet weighed 0.5819 g, and the amount of tablet used was 0.5716 g, so the ASA content of the original tablet is:

 

(0.5819/0.5716) x 0.465 g = 0.473 g

or

(0.473 / 0.5819) * 100% = 81.4%

 (US regulations require each tablet to contain 500 ± 50 mg.)

Other Approaches

• Back-titration – add excess base, then titrate with acid.• Electrochemical methods.• Spectrophotometric – Fe(III) complex (hydrolysis in 1 M NaOH).• Total fluorescence spectroscopy - 1% acetic acid in CHCl3.• Raman spectroscopy.• High performance liquid chromatography

Image: http://www.scielo.br/img/revistas/jbchs/v20n2/a22fig04.gif

Larger volumes of chemicals. Hazardous waste. Specialized equipment. Hazardous processing steps.

Green Relevance

Green Concepts• Safer reagents.

• Waste prevention.

• Consider Green Principles

– #1 (prevent waste)

– #3 (use less hazardous substances)

– #4 (safer chemicals)

– #5 (safer solvents and auxiliaries)

– #7 (renewable feedstocks)

– #11 (real-time analysis)

Botanic image: http://www.therampantgardener.co.uk/salix_alba_caerulea-cricket_bat_willow.html

Connecting Solubility, Equilibrium, and Periodicity

Premise• We can use chemical knowledge to make

informed predictions about properties of materials too dangerous to work with.

Chemical Concepts • Equilibrium; solubility; solubility product;

periodic properties; acid/base titration.

Background

• Solubilities of metal hydroxides vary with row in the periodic table – Be < Mg < Ca < Sr < Ba.

Other experiments to illustrate periodic trends can be unsafe – e.g., reactivities of Na, K.

Beryllium compounds are particularly toxic. Barium compounds should be used with caution

(though used as medical imaging agent). Strontium compounds are not too bad.

Image: http://www.chemicalconnection.org.uk/chemistry/topics/images/lmw9d.jpg

Reaction Chemistry

• Titration of M(OH)2 with HCl:

M(OH)2 + 2 HCl = M2+(aq) + 2 Cl-

(aq) + 2 H2O

(M = Mg, Ca, Sr)

Pre-lab Preparations

Magnesium hydroxide image: http://sustainability.ucsb.edu/LARS/programs/chem_db/photos/663.jpgCalcium hydroxide image: http://www.aquariumlighting.com/Wholesale_Only/images/ESV_Calcium_Hydroxide.jpg

• Stir 2 days, then allow to stand for 1 week.• Decant into clean container just before use.

Sr(OH)2 is easy to make from SrCl2 (aq) and NaOH (aq) - just mix the solutions, filter off the product, and dry in an oven.

Sample Solution Sample V Titration VMg(OH)2 0.75 g/1.0 L 5.0 mL 0.6 – 0.8 mLCa(OH)2 0.50 g/0.1 L 1.00 mL 20.0 – 22.0 mLSr(OH)2 5.0 g/50 mL 0.50 mL 30.0 –33.0 mL

Procedure• Place the desired M(OH)2 solution in an Erlenmeyer flask.

oMg(OH)2 -- 5.0 mLoCa(OH)2 -- 1.0 mL (also add 50 mL H2O)o Sr(OH)2 -- 0.5 mL (also add 50 mL H2O)

• Add a few drops of phenolphthalein solution as an indicator.

• Carefully titrate with 0.002 M HCl solution, recording the initial and final volume.oMg(OH)2 -- use syringe (approx. 0.5 – 0.8 mL)oCa(OH)2 -- use burette (approx. 20 – 22 mL)o Sr(OH)2 -- use burette (approx. 30 – 33 mL)

Data

Laboratory Safety

Accident: An unexpected and undesirable event, especially one resulting in damage or harm.

Image from: http://facilitysigns.wordpress.com/category/signs-and-labels/

LABORATORY SESSIONBreak for ….

Calculations

• Solubility product = Ksp = [M2+][OH-]2

• [M2+] = 0.5 [OH-]

• Ksp = 0.5 [OH-]3

• [OH-] = mL HCl x 0.002 mmol-mL-1 / Vsample (mL)

Results

• Reported Ksp values (note variability in literature)

o Mg(OH)2 -- 5.6 x 10-12 – 1.8 x 10-11

o Ca(OH)2 -- 4.7 x 10-6 – 5.5 x 10-6

o Sr(OH)2 -- 6.4 x 10-3

o Be(OH)2 -- 6.9 x 10-22

o Ba(OH)2 -- 5.0 x 10-3

Other Approaches

• Radiochemical determination, using Ba-137 (, t1/2 = 153 sec).

• PbCl2, AgCrO4, …

• Reduced quantities.

• Protective equipment.

• Hope for no accidents!

• Simulations.

Risk of accidental exposure. Disposal of toxic waste. Loss of opportunity to explore important chemical

issues.Image from: http://img.brothersoft.com/screenshots/softimage/s/salts__solubility-305259-1258523535.jpeg

Green Relevance

Green Concepts• Waste prevention.

• Avoiding hazardous chemicals

• Consider Green Principles

– #1 (prevent waste)

– #3 (use less hazardous substances)

– #11 (real time monitoring)

– #12 (use substances in forms that minimize hazards)

Questions and Discussion

Image from: http://www.legis.state.wi.us/senate/sen11/news/images/questions.jpg

Preparation for the Next Workshop

• Greening the Blue Bottle

Methylene blue image: http://www.petsmart.com/graphics/product_images/pPETS-3758964t400.jpg

Packet reports:

600+ mL container with cap4.80 g ascorbic acid 1.00 g NaCl150 mg NaHCO3 45 mg CuSO4

5H2O

3.6 mg methylene blue 600 mL H2O

(We will try smaller scale – ½ - ¼ of all amounts.)

(Demonstrations)

Preparation for the Next Workshop

• Factors Influencing the Rate of a Chemical Reaction

Cigarette image: http://www.prlog.org/10426002-cigarette-serial-killer.jpgYeast image: http://ninecooks.typepad.com/photos/uncategorized/2008/04/08/yeast1.jpgVial image: http://ecx.images-amazon.com/images/I/316wTFca6BL._SL500_AA280_.jpgBag image: http://www.supplierlist.com/photo_images/66082/LDPE_Ziplock_Bag_Plastic_Bag_Packaging_Bag_Zip_loc.jpg

• One packet dry yeast• One pint of 3% hydrogen peroxide• Two clear kitchen storage bags (quart size)• Two small vials with caps• Matches and cigarette (or other objectto produce glowing embers)

Preparation for the Next Workshop

• A Chemical Clock Reaction

• Starch solution (1/2 teaspoon of starch in 100 mL of boiling water)• Vitamin C (100 mg in 6 mL of water, or 1-g effervescent pill in 60 mL

of water)• 2% Tincture of iodine (1.8 – 2.2 g of I2 and 2.1 – 2.6 g of NaI in 50 mL

ethanol, diluted to 100 mL with water.)• Hydrogen peroxide, H2O2 (3%)

• 6 10- or 20-mL beakers• 3 20-mL Erlenmeyer flasks• 1 10-mL graduated cylinder• 4 plastic Beral pipets• 1 timer (or a watch that measures seconds)

Tincture image: http://periodictable.com/Samples/053.12/s12s.JPG

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