The Color of Chemistry
Student Laboratory Kit
Materials Included In Kit
Aluminum foil, 25-foot roll Aluminum potassium sulfate (alum), AlK(SO4)2•12H2O, 2 g Alizarin red, 1% solution, 60 mL Calcium oxide, CaO, 1 g Congo red, 0.1% solution, 150 mL Crystal violet, 1% solution, 25 mL Malachite green, 1% solution, 25 mL
Methyl orange, 2 g Sodium carbonate, Na2CO3, 4 g Sodium sulfate decahydrate, Na2SO4•10H2O, 10 g Sulfuric acid solution, H2SO4, 1 M, 25 mL Boiling stones, 10 g Multifiber test fabric, 4 ft* *Cut into 12-cm strips and distribute.
Additional Materials Required
Water, distilled Beakers, 400-mL, 12* Forceps or tongs, 30 Hot plates, 6* Paper towels Pencils, 10
Permanent markers, 10 Scissors, 10 Stirring rods, 10 Wash bottles, 30 *See Lab Hints.
Prelab Preparation
Dye and Mordant Solutions Directions are given for preparing 200 mL of each solution. Prepare two baths for each dye. Dye baths may be used continuously during the day by different class sections.
Alizarin Red: Dilute 25 mL of 1% alizarin red solution with 175 mL of distilled or deionized water in a 400-mL beaker. Place a boiling stone in the dye solution and heat to near boiling on a hot plate.
Aluminum Potassium Sulfate (Alum): Dissolve 0.7 g of alum [AlK(SO4)2•12H2O] in 200 mL of distilled or deionized water in a 400-mL beaker. Add 0.4 g of calcium oxide and stir to dissolve. Place a boiling stone in the solution and heat to near boiling on a hot plate.
Congo Red: Dilute 70 mL of 0.1% congo red solution with 130 mL of distilled or deionized water in a 400-mL beaker. Add 2-g of sodium sulfate decahydrate (Na2SO4•10H2O) and 1.5 g of anhydrous sodium carbonate (Na2CO3) and stir to dissolve. Place a boiling stone in the dye solution and heat to near boiling on a hot plate.
Crystal Violet: Dilute 10 mL of 1% crystal violet solution with 190 mL of distilled or deionized water in a 400-mL beaker. Place a boiling stone in the dye solution and heat to near boiling on a hot plate.
Malachite Green: Dilute 10 mL of 1% malachite green solution with 190 mL of distilled or deionized water. Place a boiling stone in the dye solution and heat to near boiling on a hot plate.
Methyl Orange: Dissolve 0.7 g of methyl orange in 200 mL of distilled or deionized water. Add 2.5 g of sodium sulfate decahydrate (Na2SO4•10H2O) and 5 mL of 1 M sulfuric acid and stir to dissolve. Place a boiling stone in the dye solution heat to near boiling on a hot plate.
Safety Precautions
All of the dyes are strong stains and will stain skin and clothing. Methyl orange, crystal violet and malachite green are toxic by ingestion and irritating to body tissue. Sulfuric acid is corrosive and toxic by ingestion. Alizarin red is a body tissue irritant. The dye baths are very hot, near boiling. Exercise care to avoid scalding and skin burns. Avoid contact of all chemicals with eyes and skin. Remind students to wash their hands thoroughly with soap and water before leaving the lab. Wear chemical splash goggles and chemical-resistant gloves and apron. Please consult current Safety Data Sheets for additional safety, handling and disposal information.
Disposal
Consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The dye solutions may be washed down the drain with plenty of excess water according to Flinn Suggested Disposal Method #26b.
Lab Hints
- The laboratory work for this activity can be completed in a typical 50-minute lab period if students work collaboratively in groups of three as described in the Prelab Questions. The experiment is far too long for completion if only one student does the actual lab work and the others merely observe or record results. Students should work on different parts of the procedure and then share results to complete the data table and answer the Post-Lab Questions.
- If three students are working together, each student may be responsible for testing two dyes. For best results, allow the fabrics to dry overnight before recording the final results. Students may test the colorfastness of the dyed fabrics at home, if desired. Make sure excess dye has been completely rinsed out of the fabric strips before allowing students to take the samples home.
- Place lots of paper towels, absorbent lab mats or newspaper all around the dye baths. This will help keep the room clean. Instruct students to store books, bags, and other personal items away from the lab area to avoid staining them.
- Students may bring suitable fabrics (e.g., cotton T-shirts, acrylic socks or yarn, polyester sheets) from home to dye. Scavenge fabric stores for inexpensive bolts of white cloth (read the labels!). Pure (100%) white cotton, polyester, acetate and nylon are easy to find and relatively inexpensive. Wash fabrics before dyeing to remove sizing and other fabric finishes.
- Other multifiber test fabrics containing 8 or 13 different fabrics are available from Testfabrics, Inc. See their website at www.testfabrics.com.
- Congo red is an acid–base indicator. The red color of fabrics dyed with congo red will turn blue when placed in a mild acid solution, such as 0.1 M HCl. The blue color disappears and the red color returns when the congo red–dyed fabric is placed in a washing soda (sodium carbonate) bath.
- Malachite green and crystal violet are often paired with tannic acid as a mordant for difficult-to-dye fabrics. The toxic heavy metal salt antimony potassium tartrate is required as an adjunct to “fix” the tannic acid to the fabric. Since using antimony in the lab would necessitate dedicated heavy metal waste disposal, the use of tannic acid as a mordant was omitted from this experiment. Fabrics mordanted with tannic acid must be thoroughly dried before dyeing.
- Students may experiment with other metal salts as mordants for alizarin—some interesting color changes result. Using iron(II) sulfate as the mordant imparts a rich brown color to the dyed fabric.
- To ease congestion and improve safety, set up several dyeing stations around the lab. We recommend using 200 mL of dye solution in 400-mL beakers for each dye bath. To achieve even dyeing of fabrics, do not immerse more than three pieces of fabric in any one dye bath at the same time. Large (7" x 7") hot plates will accommodate two 400-mL dye baths.
Teacher Tips
- The “Mystery Nylon Factory” demonstration kit available from Flinn Scientific (Catalog No. AP2088) provides a good lead-in to this activity. Use the nylon rope trick demonstration to introduce polymers and their unique characteristics. One point worth mentioning is that individual polymer molecules do not all have the same molecular weight. Polymers are polydisperse—the molecular weight is an average based on the number of molecules having different molecular weights in a sample (the so-called number-average molecular weight).
- One of the most famous dyes is indigo, which is used to dye blue jeans. Indigo is a so-called vat dye—the dye is first reduced to a colorless, water-soluble form, which is then applied to a fabric. The ingrained dye is re-oxidized back to its colored form when the fabric is exposed to air. The history and chemistry of dyeing with indigo are investigated in “Dyeing with Indigo,” a student laboratory kit available from Flinn Scientific (Catalog No. AP6166).
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering Practices
Developing and using models Planning and carrying out investigations Analyzing and interpreting data Constructing explanations and designing solutions
Disciplinary Core Ideas
MS-PS1.A: Structure and Properties of Matter HS-PS1.A: Structure and Properties of Matter
Crosscutting Concepts
Patterns Systems and system models Structure and function Stability and change
Answers to Prelab Questions
- Redraw the structure of methyl orange (see Figure 2), and identify the groups in the dye that will bind to ionic and polar sites in a fabric.
{13962_PreLabAnswers_Figure_1}
- Complete the following “If/then” hypothesis to explain how the structure of a fabric will influence the relative color intensity produced by methyl orange.
If a fabric contains more ionic and polar groups in its structure, then the intensity of the dye color due to methyl orange should increase, because there will be more sites on the fabric for the dye molecules to bind to.
- Using this hypothesis, predict the relative color intensity that will be produced by methyl orange on the six fibers in the multifiber test fabric. Rank the fabrics from 1 = lightest color to 6 = darkest color.
{13962_PreLabAnswers_Figure_3_Composition of the multifiber test fabric}
Note: Ask students to arrange the fabric molecules from least polar to most polar. See the Background section.
- Form a working group with two or three other students. Read the entire Procedure and the recommended Safety Precautions. This is a cooperative lab activity—decide how you will divide up the lab work and write out an action plan. You will not have time to complete the activity if one of the partners does all the work.
Note: Ask to see the students’ plans!
Sample Data
Data Table
{13962_Data_Table_1}
( Optional) Use the following space to write down any observations concerning the colorfastness of the dyes.
All of the dyes were colorfast. There were a few exceptions:
- The color of malachite green on cotton faded after washing.
- The color of crystal violet on both cotton and polyester also faded after washing.
Answers to Questions
- Describe the colors produced by methyl orange on the different fabrics in the multifiber test fabric. Compare the results with the relative color intensities predicted in the Prelab Questions. Explain any differences between the predicted and actual results.
Observed color intensity: Wool > nylon > acetate > cotton > polyester > acrylic The color of methyl orange ranged from dark red-orange on wool and bright orange on nylon to essentially colorless (white) on acrylic. Acetate was dyed lemon yellow, while cotton and polyester were light yellow and pale yellow, respectively. Note: Student predictions in the Prelab Questions will vary. Most should predict that wool will show the greatest affinity for the dye, and thus the most intense color with methyl orange. The results for nylon may be a surprise, since there are no charged groups shown in the structure of nylon. Students may notice, however, that both nylon and wool contain amide-linking groups in their repeating units—maybe the polar amide groups interact very strongly with the dye via hydrogen bonding.
- Compare the general ease of dyeing the six different fabrics in the multifiber test fabric. Which fabric(s) consistently developed the most intense colors, regardless of the type of dye used? Which fabric was the most difficult to dye?
Wool consistently developed the most intense colors with all of the dyes except congo red. Even with congo red, however, wool was only a shade paler than cotton, which gave the most intense color. Nylon, cotton and acetate were also relatively easy to dye. They gave fairly intense colors with at least four out of the six dyes tested. Polyester was the most difficult fabric to dye.
- Consult Figure 1: What feature stands out as unique in the structure of the fabric that was the easiest to dye? What feature stands out as unique in the structure of the fabric that was hardest to dye?
Wool contains many charged groups in its structure. None of the other fabrics show any charged groups in their normal repeating units. Polyester is unique in that it appears to be the least polar of all the fabrics. Polyester has no –X–H (where X = O or N) groups capable of forming hydrogen bonds with electron donor sites in dye molecules. Note: Students may notice that acrylic fiber is similar to polyester in that is lacks polar groups capable of hydrogen bonding to electron donor sites in dye molecules. The dyeability of acrylic is improved commercially by incorporating small amounts of charged monomers, such as AMPS, into the growing polymer.
{13962_Answers_Figure_1}
- Consult Figure 2: Which two dyes have very similar structures? Compare the relative color intensities produced by these dyes on the different fabrics in the multifiber test fabric. Are the color patterns (from lightest to darkest) similar for these two dyes? Explain.
Crystal violet and malachite green have similar structures and produced similar color patterns with the six fabrics in the multifiber test fabric. The observed color intensity produced by crystal violet and malachite green was: Wool > cotton, acrylic, and acetate > nylon >> polyester.
- Compare the color patterns produced on the different types of fabrics by methyl orange (a direct dye) and congo red (a substantive dye). Suggest a possible reason for any differences based on the chemical bonding interactions of direct versus substantive dyes (see the Background section).
Congo red dyed every fabric! It gave nice bright reds of almost equal color intensity with four of the fabrics (wool, nylon, cotton, and acetate) and light pink colors with acrylic and polyester. Methyl orange showed a much greater variability in the colors that it produced on different fabrics (see Question 1). Binding of methyl orange may depend on its ability to form ionic bonds with fabric molecules. Congo red binds to fabrics via hydrogen bonding. More fabrics are capable of hydrogen bonding than ionic bonding.
- Show by means of a diagram one hydrogen bond that might form between a glucose unit in cotton and congo red. Hint: Hydrogen bonds have the general form X–H --- :Y, where X and Y are highly electronegative atoms, such as N, O, F and Y, has an unshared pair of electrons.
{13962_Answers_Figure_2}
- Compare the colors produced by alizarin on the untreated and mordanted test strips. What is the principal advantage of using a mordant? What fabric was almost impossible to dye except with a mordant?
The effect of the mordant was unique—it produced almost equal color intensity (shades of purple) on every fabric in the multifiber test fabric. The untreated test strip showed large variations in the color shade produced with alizarin. The mordanted test strip was simply purple almost all the way across. The only exception was wool, which was darker than the rest of the fabrics on the mordanted test strip. The effect of the mordant was most significant on polyester, which was almost impossible to dye using any other dyes.
References
This activity was adapted from Flinn ChemTopic™ Labs, Volume 5, Chemical Bonding; Cesa, I., Editor; Flinn Scientific: Batavia, IL, 2004.
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