edited bv
JAMES0.SCHRECK
filtrates a residues Plastic Density Determination by Titration Raymond ~ r u z a n ' Lanphier High School Springfield, IL 62702
Univenity of Nonhem Colorado Greeley. CO 80639
Sample of Experimental Data and Calculated Results Trial #I
Trial #2
1. Volume of ethanol
15.00 mL
15.00 mL
Douglas Baker SpringfieldPlastics, Inc. Auburn. IL 6261 5
2. Mass of ethanol
11.80g
11.81 g
The concept of distinguishing between low-density and high-density polyethylene presented by Kolb(1) led to the development of a density lab involving volumetric analysis. Often called the flotation method, this procedure may be used to determine the density of a sohd that is insoluhle in two miscible liauids of different densities (2). The plastic bar& of a Bic Round Stic2 Pen serves a s the solid. I t is com~osedof ~ o l v o m ~ v l e and n e contains the Diement titanikdioxide;'I'ib;z. l%e density of the plastic, as reported by the Bic Corporation, i s 0.903g/mL and must fall between the densities of the two liquids. Thus, ethanol and water serve as suitable liquids. Using titration, the end point i s reached when a solution of the two liauids has a densitv eauivalent to the ulastic of the barrel. At this point, the a a s i i c j u s t becomes buoyant. can be calculated since the The solution's mass and d e n s ~ t v solution volume and volumesof both liquids a t the end point of titration a r e known. The following procedure is used to determine density of the two liquids. Density of the plastic barrel is then determined utilizing titration and ethanol displacement. Sample data and calculated results are provided i n the table.
4. Volume of water
15.00 mL
15.00 m l
5. Mass of water
14.93 g
14.88 g
Procedure A Density of Ethanol and Water 1. Fill two 50-mL burets, one with distilled water and one with ethanol. 2. Label two 100-mLbeakem, #1 and #2. 3. Mass beaker #1 and run 15.00 mL of ethanol into it from the buret.
'Author to whom correspondenceshould be addressed. 'Bit Round Stic is a trademark of the Bic Corporation. Milford. CT 06460.
Procedure A
3. Calculated density of ethanol [2 + 1 ]
6. Calculated densly of water15 + 41
0.9953 g/mL
0.9920 g/mL
Procedure B 7. Volume of water used in
12.20 mL
12.50 ml
8. Volume of solution at end point
26.0 mL
26.0 mL
9. Calculated mass of solution [(7x 6) + 21
23.94 g
24.21 g
titration
10. Calculated solution density and plastic density [ 9 + 81
0.921 g/mL
0.931 g/mL
11. Calculated volume reduction [(I + 7) - 81 + [(I + 7)]x 100
4.4%
5.5%
12. Mass of pen barrel
1.77g
1.829
13. Volume of pen barrel
1.97mL
2.00 mL
14. Calculated density of barrel by displacement [I2 + 31
0.898 g/mL
0.910 glmL
15. Percent error by titration
1.99%
3.10%
16. Percent error by disolacement
0.55%
0.78%
Procedure C
Volume 70 Number 5 May 1993
397
4. Mass the beaker and ethanol. Save this for Procedure B. 5. Repeat the above procedure using water and beaker #2.
Save this for Pmcedure B.
Procedure B Density of Plastic by Tiration 1. Remove and discard from the barrel the inside ink cartridge and the end caps of a Bie Raund Stic pen. 2. Use a sharp blade with caution to cut off two 3-4 mm pieces of the barrel. Save the remainder of the barrel for Procedure C. 3. Drop one piece into each of the two beakers containing liquids from Procedure A. 4. Observe evidence that the plastic's density is greater than ethanol but less than water (See the figure). 5. Place beaker #1,containing ethanol and plastic, under the huret with water. While stirring vigorously, titrate with water until the plastic just becomes buovant (See the fig. we). 6. Record the volume ofwater rwuired to reach the end ooint. 7. Hrmovr the plnstw rnrrfully fmm the aolutmn and use a ~ ~ a d u a uryhnder d to determlnr the solution's volumr. Procedure C Density of Plastic by Displacement 1. Determine the mass of the remainii Bic Round Stic pen barrel saved from Procedure H 2. Record the volume ufethanol remaining in the buret. 3 I)mo rho hamel mtn the ethanol buwr and record the new Calculations 1. Calculate the density of the two liquids fmm the Procedure A data. 2. Calculate the mass of both liquids (M= dV) in the titration using the densities obtained in Procedure A. Add the masses of water and ethanol to determine the solution's mass at the end paint. 3. Calculate the density of the plastic by using the solution's mass and volume. At the end uoint. the solution's densitv is equivalent to that of the p&tic. ' 4. Calculate the percentage of volume reduction of the titrated solution. 5. Calculate the plastic density again, this time using the barrel mass and volume displacement of ethanol obtained in Procedure C. 6. Calculate oercentaee of error for the densities of water and ethanol. ~ a l c u l a ~ t oercentaee he of error for the densitv of plastic determined i y both &ration and ethanol displacement. Discussion This exercise has been useful in advanced high school and introductory wllege chemistry classes. As a new twist to a n old concept, it provides a vehicle for the early introduction of volumetric methods. Students apply the density
398
Journal of Chemical Education
Left to right. Bic Round Stic pen plastic in water, ethanol, and at titration end point. Bic Round Stic barrel is in the foreground. (Photo by James Elledge.)
data for the two liquids to determine the mass of the titrated solution and become aware that the total volume of the titrated solution is not the sum of the volumes dispensed from the two burets. After calculating the approximate 4% volume reduction, students should attempt to explain why the solution's volume is less than the sum of the volumes of the two liquids. Percentage of error analysis is made on the densities determined for water, ethanol, and the plastic pen barrel using both titration and ethanol displacement. Percent error for the plastic ranges from 0.00% to 6.0% by both methods, with ethanol displacement usually providing bett e r r e s u l t s . S t u d e n t s a r e encouraged t o suggest weaknesses of both methods that might account for errors. The Bic Round Stic pen was chosen because i t is readily available, fits in the 50-mL buret. and is a common obiect in the lives of many students. plastic containers impriked with recycling code 4, low-density polyethylene (LDPE), and 5, polypropylene (PP), provide other sources of plastics whose density may he determined by titration using water and ethanol. Additional related studies can explore the separation and identification of various plastics with solvents of different densities as described by Kolb (3).
Lab Safety No o p e n flames i n the lab a r e a s h o u l d be permitted. Wear protective e y e w e a r and aprons. Literature Cited 1. Ko1b.K E.;Kolb, D. K i l Cham. Edue. 1986.63, 417.
2.Craig,R.E.R. J. Cham Educ 1989.66.599400. 3. K o k K E.:Kolb, D. K J. Cham. Edue. 1991,68, 348.
