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Although it sounds odd, one of the best ways to discriminate fiber specimens is by attempting to dissolve them in various solvents. For example, you may have identified a questioned fiber as artificial by the burning test, but you do not yet know what type of artificial fiber it is. By attempting to dissolve a small specimen of that fiber in each of several solvents, you can determine if it is, for example, nylon, acetate, or polyester.

Solubility tests are particularly important for discriminating artificial and reconstituted fibers, because they are produced by machinery and so lack the microscopic variations characteristic of natural fibers. For example, although it is easily possible to discriminate wool or cotton from an artificial fiber microscopically, it may be impossible to determine whether an artificial fiber is acetate or polyester without performing solubility or other chemical tests.

Professional forensic scientists use a wide variety of solvents for solubility tests, but many of those solvents are expensive or difficult to obtain for a home lab. In this lab session, we’ll use the following common solvents to discriminate among many different natural, artificial, and reconstituted fibers:

Acetic acid

At 20 °C, glacial (100%) acetic acid dissolves acetates.

At 100 °C, glacial acetic acid dissolves acetates and triacetates. Nylon dissolves, but very slowly.

Acetone

At 20 °C, acetone dissolves acetates, triacetates, modacrylics, and vinyons.

Hydrochloric acid

At 20 °C, concentrated (12 M) hydrochloric acid dissolves acetates, triacetates, saponified cellulose acetate (Fortisan), nylon, and silk. Rayons dissolve slowly. Less concentrated (6 M) hydrochloric acid dissolves nylon, but not the other fibers listed.

Sodium hydroxide

At 100 °C, a 5% solution of sodium hydroxide dissolves nytrils, wool and other animal hair, and cultivated silk. Acetates, triacetates, wild (Tussah) silk, and reconstituted protein fibers dissolve only partially and very slowly.

At 100 °C, a 50% solution of sodium hydroxide dissolves all of the above fibers readily, and slowly (30 to 60 minutes) dissolves polyesters. Acrylics dissolve partially and very slowly. Modacrylics and sarans (Enjay, Saran) melt but do not dissolve.

Sulfuric acid

At 20 °C, 60% sulfuric acid dissolves acetates, triacetates, rayons (cuprammonium and viscose), silk, and nylon, but not cotton, linen, or other cellulosic plant fibers. At 20 °C, 70% sulfuric acid dissolves all of the above fibers. The 70% acid can be used to discriminate reconstituted cellulose fibers from cotton and other natural cellulose fibers.

Zinc chloride

At 45 °C, 75% zinc chloride solution dissolves acetates, triacetates, acrylics, silk, cotton that has not been mercerized, and rayons (cuprammonium and viscose).

In this lab session, we’ll attempt to dissolve fiber specimens in various solvents and record our observations.

Required Equipment and Supplies

  • goggles, gloves, and protective clothing
  • hot water bath (see Substitutions and Modifications)
  • thermometer
  • graduated cylinder, 10 mL
  • test tubes (as required; 12+)
  • test tube rack
  • test tube holder
  • stirring rod
  • distilled or deionized water
  • acetic acid, 100% glacial (~ 25 mL)
  • acetone, 100% (~ 25 mL)
  • hydrochloric acid, concentrated (~ 25 mL)
  • hydrochloric acid, 6 M (~ 25 mL)
  • sodium hydroxide, 1.25 M (5% w/v) (~ 25 mL)
  • sodium hydroxide, 12.5 M (50% w/v) (~ 25 mL)
  • sodium hypochlorite, 5.25% (~ 25 mL)
  • sulfuric acid, 60% (see Substitutions and Modifications) (~ 25 mL)
  • sulfuric acid, 70% (see Substitutions and Modifications) (~ 25 mL)
  • zinc chloride solution, 75% w/w (see Substitutions and Modifications) (~ 25 mL)
  • fiber specimens (see Substitutions and Modifications)

The Maker Shed carries a variety of chemistry supplies and equipment. Check out the Science Room section to see what’s currently available.

sciRoomCAUTION2.gif CAUTIONS

Read the MSDS for each of the chemicals you use and follow the handling precautions noted. Acetone is extremely flammable. Avoid open flames. When diluting acids, add the concentrated acid to water, not vice versa. Acetic acid and hydrochloric acid are corrosive and produce strong fumes. Use adequate ventilation or work outdoors. Sodium hydroxide produces intense heat when it dissolves, and solutions of sodium hydroxide are corrosive. Sodium hypochlorite is corrosive, and reacts with acids to form toxic chlorine gas. Sulfuric acid is corrosive. Zinc chloride is corrosive and toxic to many organisms, and local laws may require handling solutions of it as toxic waste. Wear splash goggles, gloves, and protective clothing at all times.

Substitutions and Modifications

  • You need a hot water bath to run tests at 45 °C and 100 °C. The easiest solution is to use a beaker large enough to hold all of your test tubes. Fill the beaker about half full of water, and place it on a hotplate or other flameless heat source. (Do not use a flame around an open container of acetone!) Heat the water to 45 °C and complete the zinc chloride solubility tests described in the procedure section. Then heat the water to boiling and complete the tests with glacial acetic acid, 5% sodium hydroxide, and 50% sodium hydroxide.
  • You can substitute hardware store 31.45% (10.2 M) muriatic acid for concentrated hydrochloric acid.
  • To make up 25 mL of 6 M hydrochloric acid, carefully add 12.5 mL of concentrated (12 M) hydrochloric acid to 12.5 mL of distilled or deionized water. Alternatively, add 14.7 mL of hardware store muriatic acid to 10.3 mL of water.
  • To make up 25 mL of 1.25 M sodium hydroxide, dissolve 1.25 g of sodium hydroxide in 20 mL of distilled or deionized water and make up the final volume to 25 mL.
  • To make up 25 mL of 12.5 M sodium hydroxide, dissolve 12.50 g of sodium hydroxide in 20 mL of distilled or deionized water and make up the final volume to 25 mL.
  • Ordinary chlorine laundry bleach is 5.25% sodium hypochlorite.
  • To make up 25 mL of a 60% sulfuric acid solution, carefully add 15.3 mL of concentrated (98%) sulfuric acid to 9.7 mL of distilled or deionized water.
  • To make up 25 mL of a 70% sulfuric acid solution, carefully add 17.9 mL of concentrated (98%) sulfuric acid to 7.1 mL of distilled or deionized water.
  • To make up a 75% w/w solution of zinc chloride, dissolve 75 grams of anhydrous zinc chloride in 25 mL of distilled or deionized water.
  • Obtain known fiber specimens, ideally one of each of the following pure fibers: acetate, acrylic, cotton, modacrylic, nylon, nytril, polyester, rayon, silk, triacetate, vinyon, and wool. Ideally, all of the specimens should be white or of the same color. Ask someone to choose a piece of one of the known specimens as your questioned specimen.

Standard Fiber specimens

Professional forensics labs use standard fiber specimens, such as those made by Testfabrics, Inc. For a home forensics lab, the most useful Testfabrics fiber specimen is their Multifiber Fabric #43. MFF #43 includes 8 mm stripes of 13 different fibers in an 11.5 cm wide ribbon, including spun diacetate, SEF (modacrylic), filament triacetate, bleached cotton, Creslan 61 (acrylic), Dacron 54 (polyester), Dacron 64 (polyester), Nylon 66 (polyamide), Orlon 75 (acrylic), spun silk, polypropylene (polyolefin), viscose (rayon), and worsted wool.

Small specimens of Multifiber Fabric #43, suitable for use in the lab sessions in this chapter, can be purchased directly from Testfabrics, Inc.

Procedure

  1. If you have not done so already, put on your goggles, gloves, and protective clothing.
  2. Label one test tube for each of your knowns and for your questioned specimen. (We simply labeled our tubes 1 through 12 and Q.)
  3. Make sure there is no exposed open flame, and then transfer about 2 mL of acetone to each of the tubes.
  4. Place the tubes in sequence in the rack, and then transfer a small piece (about 5mm square) of each of the known specimens and the questioned specimen to the corresponding tubes. If necessary, use a clean stirring rod to make sure the specimen is fully submerged in the liquid.
  5. Observe the tubes carefully for at least five minutes or until you are certain that any change that will occur has occurred. (Figure 6-13 shows some of our samples in acetone. The specimen in the left tube is unaffected; that in the second tube has dissolved completely; those in the third and fourth tubes have dissolved partially.) Record the results in your lab notebook and/or in Table 6-11. Use “S” to indicate that the specimen dissolved readily; SS to indicate that the specimen dissolved slowly; PS to indicate that the specimen dissolved only partially, and so on.

Figure 6-13

Figure 6-13. Examining the solubility of various fiber specimens

  1. Empty the tubes into the sink with copious running water, and rinse the tubes and the stirring rod thoroughly with distilled or deionized water.
  2. Repeat steps 3 through six for each of the other solvents that are used at room temperature (glacial acetic acid, 6 M and 12 M hydrochloric acid, sodium hypochlorite, 60% and 70% sulfuric acid). Flush the sodium hypochlorite solution down the sink with plenty of water; empty the other tubes into a waste container that already contains a couple liters or more of tap water.
  3. Heat the water bath to 45 °C, transfer about 2 mL of zinc chloride solution to each of the test tubes, place a specimen of each known and the questioned specimen in the corresponding tubes, and place the tubes in the bath. After 10 minutes or so, examine the tubes and record your results in your lab notebook and/or in Table 6-11.

Figure 6-14

Figure 6-14. Testing fiber specimen solubility with a hot water bath

  1. Allow the tubes to cool, empty the contents into your waste container, and rinse the tubes and stirring rod thoroughly.
  2. Bring the water bath to a gentle boil (about 100 °C), as shown in Figure 6-14.
  3. Transfer about 2 mL of glacial acetic acid to each of the test tubes, place a specimen of each known and the questioned specimen in the corresponding tubes, and place the tubes in the bath. After 10 minutes or so, examine the tubes and record your results in your lab notebook and/or in Table 6-11.
  4. Allow the tubes to cool, empty the contents into your waste container, and rinse the tubes and stirring rod thoroughly.
  5. Repeat steps 11 and 12 with the 5% sodium hydroxide solution and the 50% sodium hydroxide solution.
  6. The waste container contains a mixture of neutralized acids and bases and a small amount of zinc chloride (which is toxic to many aquatic organisms). If your local laws permit you to do so, flush the contents of the waste container down the drain with plenty of water; otherwise, dispose of the contents as toxic waste.

Table 6-11. Test Fiber specimens by Solubility – observed data

Solvent Q 1 2 3 4 5 6 7 8 9 10 11 12
acetic acid (cold)                          
acetic acid (hot)                          
acetone                          
hydrochloric acid, 6 M                          
hydrochloric acid, 12 M                          
sodium hydroxide, 5%                          
sodium hydroxide, 50%                          
sodium hypochlorite                          
sulfuric acid, 60%                          
sulfuric acid, 70%                          
zinc chloride                          

Review Questions

Q1: At room temperature, a questioned fiber specimen dissolves in 6M hydrochloric acid. What type of fiber do you suspect?

Q2: At room temperature, a questioned fiber specimen dissolves in acetone, but not in glacial acetic acid. What type of fiber do you suspect?

Q3: A questioned fiber specimen proves to be insoluble in all of your room-temperature reagents. What fiber type do you suspect, and what reagent would you use to confirm your suspicion?

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