In the Laboratory
New Analytical Method for the Determination of Detergent Concentration in Water by Fabric Dyeing
W
Set Seng and Masakazu Kita* Department of Chemistry, Faculty of Education, Okayama University, Okayama 700-8530, Japan; *
[email protected] Reiko Sugihara Department of Home Economies, Faculty of Education, Okayama University, Okayama 700-8530, Japan
Several analytical methods have been used in determining the quantity of detergent in water. The most common analytical method in Japan uses methylene blue (MB, a cationic dye) in association with an anionic detergent in which the neutral compound produced is extracted into chloroform (1–2), according to the Japanese Industrial Standard (JIS MB method). Similar methods have also been published in this Journal in which copper–ethylenediamine ion is used instead of methylene blue (3–4). The Ponal Kit Abs method (5) is another technique. It uses a purple dye, a cobalt(III) complex cation, to produce a neutral compound with anionic detergent; benzene is the extraction solvent. However, all the above methods require organic solvents and expensive lab equipment for analysis, such as an atomic absorption spectrometer, an anodic stripping voltammeter, and a spectrophotometer, and are not suitable for high school experiments. In the present article, a simple method of analyzing detergent concentration in water was introduced to upper-level high school students. The method used an acrylic fabric in dye solutions containing different detergent concentrations. The color depth of the dyed fabric was determined using a handmade reflection photometer. Thirty-four students were divided into 10 groups of three or four members that conducted the experiment. Two periods of 100 minutes were required to complete all the activities, such as lesson introduction, students’ experiment, discussion group of experimental results, and lesson conclusion.
Apparatus Setup and Reagent Preparation The body of the reflection photometer (Figure 1) is an L-shaped PVC tube usually used for tap water. The light source is a light-emitting diode (LED) placed in a hole in a PVC cap connected to two 1.5 V dry cells. Light reflected from the dyed fabric is detected by a CdS device attached to another PVC cap, which shows low resistance in the light and high resistance in the dark. The CdS resistance value is measured with a multimeter. White acrylic fabric is cut into pieces of ca. 4 cm × 4 cm. Five aqueous detergent solutions of sodium dodecyl sulfate (SDS) are prepared in concentrations of 2 ppm, 4 ppm, 6 ppm, 8 ppm, and 10 ppm, respectively, to serve as the standards. An aqueous solution of 5 ppm SDS is also prepared and set as the unknown detergent concentration. Dye solutions of 0.005% w兾v methyl violet (cationic dye) and 0.020% w兾v brilliant blue (anionic dye) are prepared. Introduction to Student Before the experimental procedures were explained, students were given some background information about the nature of the SDS detergent, methyl violet dye, and brilliant blue dye to teach them about the chemical interactions that would occur during the fabric dyeing. In this step, the chemical structures of detergent and dyes showed the types of ionic
Figure 1. Hand-made reflection photometer: (A) materials and (B) assembly for measuring color depth of dyed fabric.
www.JCE.DivCHED.org
•
Vol. 84 No. 11 November 2007
•
Journal of Chemical Education
1803
In the Laboratory
charges on each component, that is, detergent has negative charge, methyl violet dye has positive charge, and brilliant blue dye has negative charge on their ionic structures. The students were then able to give their predictions on what would happen when detergent is mixed with each dye. Experimental Procedure
Figure 2. Experimental increase in the color depth of the acrylic fabric dyed in methyl violet dye.
Dyeing in Methyl Violet Pure water, 10 mL, which served as the blank, and each SDS solution including the unknown concentration were put into separate 100 mL beakers. Methyl violet solution, 2 mL, was added to each beaker and stirred well. The solutions containing SDS turned a purple color. Pieces of acrylic fabric were dipped into each solution for 15 minutes maintained at 25 ⬚C in a water bath. The pieces of dyed fabric were removed from the solutions with tweezers and air dried with an electric dryer. The dried fabric was taped to a wooden board and the color depth of each fabric was measured as the resistance value on the multimeter by the handmade reflection photometer (Figure 1B). Because the dyed fabric color was purple, a yellow LED light, which is assumed to be its complementary color, was used in the measurement. Dyeing in Brilliant Blue The experimental procedures for brilliant blue (anionic dye) were similar to dyeing in methyl violet. However, the duration of dyeing was 5 minutes. The orange LED light, which was assumed to be the complementary color of the blue dyed fabric, was used in the photometer. Hazards
Figure 3. Experimental decrease in the color depth of the acrylic fabric dyed in brilliant blue dye.
No chemicals or procedures used by students present any significant hazards. The dye solutions used in this experiment can stain the clothes and thus require careful handling. Results and Discussion
A
B
Figure 4. The association between cationic dye and anionic detergent and the extraction of resulting neutral compounds: (A) JIS MB method and (B) fabric dyeing.
1804
Journal of Chemical Education
•
Student results from this experiment are shown in Figures 2 and 3. The students were surprised to see the different color depth trends in the two dyes. The color depth of the fabric dyed in methyl violet increased with increasing SDS concentration, whereas it decreased in brilliant blue. Both dye results gave linear calibration lines. Students were able to use the calibration lines to determine the unknown concentration of SDS effectively by matching the color depth of dyed fabric as resistance in the resulting graphs. The experimental results show that the resistances of the color depth in the fabrics dyed in the SDS solution of unknown concentration were 273 kΩ and 975 kΩ in the methyl violet and the brilliant blue, respectively. Therefore both results gave the unknown concentration of SDS solution as about 5 ppm. The increase of color depth of the dyed fabric with the increase of SDS concentration in acrylic fabric with methyl violet as a cationic dye (Figure 2) indicates that the acrylic fabric acts similarly to an organic solvent, such as chloroform and benzene, to extract the neutral compound that is formed by the anionic SDS and the cationic methyl violet as illustrated in Figure 4. The increase of detergent produced an increase of the neutral compound that was adsorbed onto the
Vol. 84 No. 11 November 2007
•
www.JCE.DivCHED.org
In the Laboratory
fabric. Therefore, the color depth on the dyed fabric corresponded to the concentration of detergent. In contrast, the color depth decreased with increasing concentration of SDS when the acrylic fabric was dyed in an anionic dye, brilliant blue (Figure 3). In this case, the interaction between the acrylic fabric and the anionic detergent is considered to be stronger than that between the acrylic fabric and the anionic brilliant blue. Also the repulsion force between the same charges of detergent and brilliant blue ions can prevent acrylic fabric from being dyed by anionic brilliant blue (Figure 5).
Figure 5. Chemical interaction when acrylic fabric dyed in anion dye.
Student Discussion of Experimental Results
Acknowledgments
The students were able to examine the different color depths of the dyed fabrics in the different detergent concentrations. In the discussion, we asked some groups of the students to plot the graphs of the results on the blackboard, and then asked them to discuss and answer the questions in the experimental worksheets (see the Supplemental MaterialW ). By using the resulting graphs, the students were able to determine the unknown concentration of SDS solution. The diagrams in Figures 4B and 5 were used in the discussion to help the students interpret the chemical interactions occurred during the fabric dyeing. Generally, the experiment was successfully conducted by the students. Thus, the students could obtain good results and could correctly interpret the experimental results.
We wish to express our thanks David Ford, of The Royal University of Phnom Penh, Scott Gardner and Raphael Semel, of Okayama University, for helpful correction of our English and discussion of the present manuscript.
Conclusion This experiment allowed students (i) to determine detergent concentration in water; (ii) to recognize that the role of the acrylic fabric is similar to an organic solvent by adsorbing the neutral compound; and (iii) to understand the chemical interactions between differently charged ions and between ions with the same charge.
www.JCE.DivCHED.org
•
W
Supplemental Material
A handout for the students and a detailed discussion of the method development are available in this issue of JCE Online. Literature Cited 1. Japanese Industrial Standard, JIS K0101; Japanese Standards Association: Tokyo, 1991; p 68. 2. Longwell, J.; Manicie, W. D. Analyst 1955, 80, 167. 3. John, Richard; Lord, Daniel. J. Chem. Educ. 1999, 76, 1256–1258. 4. Crisp, P. T.; Eckert, J. M.; Gibson, N. A. J. Chem. Educ. 1983, 60, 236–238. 5. Dojindo Chemistry Research Institute Co., Ltd.; Ponal Kit-Abs, Operation manual for the simplified method for the measurement of anion surface-active agent. http://www.dojindo.co.jp/ (assessed Jul 2007).
Vol. 84 No. 11 November 2007
•
Journal of Chemical Education
1805
