Chemical Composition of Sodium Percarbonate: An Inquiry-Based

Laboratory Experiment pubs.acs.org/jchemeduc

Chemical Composition of Sodium Percarbonate: An Inquiry-Based Laboratory Exercise Takeshi Wada and Nobuyoshi Koga* Department of Science Education, Graduate School of Education, Hiroshima University, 1-1-1 Kagamiyama, Higashi-Hiroshima 739-8524, Japan S Supporting Information *

ABSTRACT: An inquiry-based laboratory exercise is described to determine the chemical composition of sodium percarbonate (SPC), which is a major component of domestic oxygen bleach. The students’ knowledge of the chemical properties and reactions of sodium carbonate (SC) and hydrogen peroxide (HPO) is used to determine the chemical composition of SPC because SPC is the adduct of SC and HPO. If the hypothetical composition is assumed to be xNa2CO3·yH2O2, the composition can be determined by selecting one experimental method from the range of methods available. Synthetic SPC crystals or commercially available granular SPC can be used as the sample in the experiment. Because many choices involving different experimental methods are available, various learning programs can be designed to match the objectives and students’ knowledge stage. This inquiry-based laboratory exercise is suitable for a high school chemistry course or a college introductory chemistry course and requires three hours.

KEYWORDS: First-Year Undergraduate/General, High School/Introductory Chemistry, Analytical Chemistry, Inorganic Chemistry, Laboratory Instruction, Inquiry-Based/Discovery Learning, Acids/Bases, Oxidation/Reduction, Gravimetric Analysis, Titration/Volumetric Analysis

I

life experiences, is used in determining the chemical composition of SPC. In addition, assuming that the composition is xNa2CO3·yH2O2, the composition can be determined by selecting one experimental method from the various experimental methods that are generally used in high school chemistry courses. The sample can be selected from synthetic crystals, which have the ideal chemical composition, or commercially available granules from different manufacturers. Accordingly, inquiry programs with different emphases can be designed for different learning exercises and different learning stages. This is one of the advantages of the SPC inquiry exercise over the previously reported SSC inquiry exercise.7 We describe an inquiry exercise involving the determination of the chemical composition of SPC, based on experiences in laboratory classes in an advanced course at high schools and an introductory chemistry course at our university. The program is constructed in four sections: (i) introductory demonstrations by the instructor; (ii) student discussions and proposals about

nquiry-based laboratory exercises employing chemical substances and chemical phenomena relating to students’ daily lives are useful to link chemistry learning with daily life and to help students recognize the role of chemistry.1−14 We have reported a learning exercise in an inquiry laboratory for determining the chemical composition of sodium sesquicarbonate (SSC), which is one of the components of alkaline detergents.7 Here, we propose an alternative inquiry exercise that offers a wider applicability from a high school chemistry course to a college introductory chemistry course: the determination of the chemical composition of sodium percarbonate (SPC: Na2CO3·1.5H2O2), which is a major component of domestic oxygen bleach. The experimental approach for determining SPC composition in commercially available oxygen bleach has been reported as being suitable for a high school chemistry course by Bracken and Tietz.13 An alternative oxygen bleach, sodium peroxoborate, has been used as a sample for students to practice redox titrations.14 Because SPC is the adduct of hydrogen peroxide (HPO) and sodium carbonate (SC), the students’ knowledge of the chemical properties and reactions of SC and HPO, as well as their daily © XXXX American Chemical Society and Division of Chemical Education, Inc.

A

dx.doi.org/10.1021/ed400077q | J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Laboratory Experiment

the hypothetical composition of SPC and the possible experimental methods to determine the chemical composition; (iii) laboratory exercises to determine the composition; and (iv) postlab exercises and expanded learning. The entire exercise takes about three hours and can be divided into 1.5 h sessions on two days, where one experimental method is selected by each student group. This exercise is most effective for students in advanced high school chemistry, but it is also possible to apply it in a conventional high school chemistry course by reducing the student exercise and selecting the experimental technique for determining the composition of SPC.

Na 2CO3 ·1.5H 2O2 (s) → Na 2CO3(s) + 1.5H 2O(g) + 0.75O2 (g)

(1)

SPC crystals dissolve in water easily to produce a mixed solution of SC(aq) and HPO(aq). HPO decomposes spontaneously under such basic conditions without requiring a catalyst. Granular SPC is commercially available as a chemical reagent (CAS: 15630-89-4) and as a domestic oxygen bleach (Figure 1B). Granular SPC usually includes some additives such as SC and sodium silicate, which are formed or added during the granulation process.19 In the simplest case, the granular SPC is coated with a surface layer of SC, making the n(H2O2)/ n(Na2CO3) ratio in the granule lower than 1.5. Granular SPC dissolves more slowly than SPC crystals in water, allowing the bleaching effect to be maintained for a long time. Granular SPC can be stored for a long time because the SPC crystals in the interior of granules are protected from atmospheric water vapor by the SC surface layer. When granular SPC is used, the sample has to be dried at about 363 K for 30 min before the students’ experiments to remove water from the hydrated surface layer (see the instructor information in the Supporting Information).

■

INFORMATION ON SODIUM PERCARBONATE SPC crystals can be synthesized easily by the instructor, following previously published reports,15−17 and used as one of the samples. The synthetic method is described in the instructor information in the Supporting Information. Figure 1A shows a scanning electron microscope (SEM) image of the

■

INTRODUCTORY DEMONSTRATIONS After being presented with granular SPC, as an oxygen bleach, by the instructor, the students discuss the uses and chemical properties of oxygen bleach on the basis of their experiences in daily life. A bleaching demonstration can be used for the introduction of SPC. Subsequently, several demonstrations are performed by the instructor to help the students focus on the chemical composition of SPC and to recall their knowledge of the chemical properties and reactivities of SC and HPO. At this stage, the objective of the inquiry exercise, which is the determination of the chemical composition of SPC, is introduced to the students. Demonstrations by the instructor help the students to deduce the chemical composition of SPC and propose experimental methods to determine its composition. The following demonstrations each take less than 5 min to perform. Details of the demonstrations are described in the instructor information in the Supporting Information. Students are given the first part of the student handout (parts 1−3).

Figure 1. Typical SEM images of (A) SPC crystals and (B) granular SPC.

SPC crystals, which are columnar with an axis length of ca. 20− 30 μm. Figure 2 shows typical thermogravimetry−differential

Flame Test

The flame test is performed on an aqueous solution of granular SPC to identify the metal ion it contains. The students confirm the existence of sodium ions from the yellow flame observed. Catalytic Decomposition of HPO

The evolution of gas is observed when manganese(IV) dioxide is added to an aqueous solution of granular SPC. The collected gas is subjected to the combustion test. The students deduce that HPO is a component of SPC from the catalytic decomposition and oxygen evolution.

Figure 2. Typical TG−DTA curves for SPC crystals (m0 = 5.0 mg) recorded at a heating rate, β, of 5 K min−1 in a flowing N2 atmosphere (80 cm3 min−1).

Reaction with Acid

Granular SPC is reacted with dilute HCl(aq) (ca. 1 M) in a Ytype test tube. The evolved gas is introduced into limewater through an induction pipe. From the cloudy limewater, students confirm the evolution of CO2 and deduce the possible existence of carbonate ions or bicarbonate ions in SPC.

thermal analysis (TG−DTA) curves for the SPC crystals (m0 = 5.0 mg) recorded at a heating rate, β, of 5 K min−1 in flowing N2 (80 cm3 min−1). The thermal decomposition of SPC crystals begins at around 375 K and proceeds in a single massloss process, accompanied by an exothermic DTA peak. The total mass loss during the course of the reaction, 32.32 ± 0.33%, is in good agreement with the value, 32.48%, calculated using the following reaction:18

Coloration of SPC Solution

After dropping phenolphthalein (an acid−base indicator) into separate aqueous solutions of SPC, SC, and sodium hydrogen B




separated from the first-half part of the instruction that has been provided before the laboratory exercise. The instructor plays an important role to lead students from their experimental design to the standard experimental procedures without negating the benefit of the student inquiry. A student exercise of calculating the required quantity of reagents for a given sample quantity and the expected experimental values are useful to link their general idea to the experimental part of the exercise. For reference, the chemical compositions of SPC using the five methods listed above, determined by teaching assistants, are listed in Table 1. The chemical composition of

carbonate, the colorations of the solutions are compared. Because the coloration of the SPC solution is comparable with that of the SC solution, students confirm that the origin of the CO2 evolved in the previous demonstration for the reaction with acid is the carbonate ion rather than the bicarbonate ion. Thermal Decomposition

Granular SPC is heated in a test tube over a burner flame. The evolved O2 is collected and subjected to the combustion test. At the same time, droplets that are deposited near the lip of the test tube are confirmed as being H2O using cobalt(II) chloride paper. The thermal decomposition demonstration supports the students’ assumption that HPO is a component of SPC.

Table 1. Typical Results of SPC Chemical Compositions Determined Using Different Experimental Methods by Teaching Assistants

Redox Reaction

Granular SPC is added to a H2SO4 acidic solution of potassium iodide, and the color of the solution changes from colorless to yellowish brown (i.e., I2 molecules are formed). The liberation of I2 molecules by the redox reaction gives further evidence for the existence of HPO.

y/x in xNa2CO3·yH2O2 Experimental Method

■

(1) Neutralization titration (2) Gravimetric analysis (3) Redox titration (4) Catalytic decomposition (5) Thermal decomposition

STUDENT DISCUSSIONS From the results of the above demonstrations and their knowledge gained, the students easily deduce that SC and HPO are components of SPC. Through further discussion among student groups and by the class as a whole, the hypothetical chemical composition of SPC, with unknown coefficients x and y, xNa2CO3·yH2O2, is derived by the students. Subsequently, the discussion turns to the possible methods that can be used to determine the unknown coefficients, x and y, and various experimental methods are devised by the students. Following are typical examples for determining the SPC composition proposed by the students. (1) Neutralization titration of the carbonate ion (2) Gravimetric analysis of deposited barium carbonate (3) Redox titration of H2O2 (iodometry) (4) Measurement of the oxygen volume evolved from the catalytic decomposition of H2O2 (5) Mass-loss measurement for the thermal decomposition The students discuss the detailed experimental procedure for each method. They derive the mathematical formula that relates each experimental result to the unknown coefficients, x and y, by using the reaction formula for each quantitative method. Through this discussion, the students become aware that the coefficients x and y can be determined by selecting any of the quantitative analysis methods listed above. A discussion lasting about 30 min is adequate to prepare for the subsequent experiments. When the laboratory class is divided over two days, the derivation of formula for determining the ratio y/x can be assigned to students as homework. If necessary, guidance on the derivation of the formula can be given by the instructor. An instruction guideline is described in the instructor information in the Supporting Information.

SPC Crystalsa

Granular SPCb

Household SPCc

1.525 ± 0.006

1.313 ± 0.005

1.574 ± 0.003

1.511 ± 0.019

1.359 ± 0.018

1.646 ± 0.007

1.501 ± 0.018 1.47 ± 0.02

1.302 ± 0.022 1.28 ± 0.03

1.297 ± 0.001 1.27 ± 0.03

1.519 ± 0.010

1.312 ± 0.016

1.337 ± 0.006

a

Synthesized in our laboratory. bChemical reagent (Sigma-Aldrich, Japan). cDomestic oxygen bleach (Nihon Garlic Co., Japan).

SPC crystals closely corresponds to the theoretical value, y/x = 1.5; however, lower values, approximately y/x = 1.3, are estimated for granular SPC because of the SC covering the granule surface. The results of the quantitative analyses of carbonate ions tend to be overestimated for some household granular SPC products, which is likely caused by binders or stabilizers added during the granulation process. The students’ experiments are outlined below. Neutralization Titration of the Carbonate Ion (Required Time: 45 min)

An accurately measured quantity (approximately 1.5 g) of solid SPC is dissolved in 50 mL of distilled water in a beaker, and then the solution is warmed in a water bath at approximately 50 °C to decompose H2O2 in order to avoid the indicator, which is added later, from being bleached by H2O2. After cooling the solution to room temperature, the SPC solution is diluted with distilled water to 100 mL to give an approximately 0.1 M Na2CO3 solution. Next, 10 mL of the SPC solution is titrated with a standard solution of HCl (0.1 M) using a methyl orange indicator to determine the end point. The neutralization titration is repeated several times. The y/x ratios determined by the students over the last year were 1.548 ± 0.031 (number of data N = 4) and 1.670 ± 0.027 (N = 6) for the synthetic SPC crystals and the household granular SPC, respectively. The overestimation of the y/x ratio for the household granular SPC can also be seen in the students’ results.

■

LABORATORY EXERCISE According to the discussions detailed above, each student group assesses the chemical composition of SPC by selecting one or several experiments, depending on the time allowed. As an option, the compositions of SPC crystals and granular SPC can also be compared using one experimental technique. The details of the experimental procedures (the second-half part of the instructions for the students in the Supporting Information, part 4) are provided by the instructor at this step, which is

Gravimetric Analysis of Deposited Barium Carbonate (Required Time: 60 min)

An accurately measured quantity (approximately 4.7 g) of solid SPC is dissolved in distilled water and diluted to 100 mL to give an approximately 0.3 M SPC solution by using a measuring flask. Then, 50 mL of barium chloride solution (ca. 0.1 M) is C



■


HAZARDS Students are required to wear safety goggles and protective gloves throughout the experimental work. The aqueous solutions that are used, HCl, H2SO4, and Na2S2O3, are hazardous in the case of skin contact, eye contact, or ingestion. SPC may cause irritation to the skin, eyes, and respiratory tract, and may be harmful if swallowed or inhaled.

added to exactly 10 mL of the SPC solution in an Erlenmeyer flask. The resulting precipitate, BaCO3, is filtered using a glass filter funnel, and the glass funnel containing the precipitate is dried at 393 K in an electric oven for about 30 min. Several replicates of the gravimetric analysis are carried out simultaneously. The y/x ratio is determined from the gravimetric analysis of the BaCO3 precipitate. Only one student group conducted this experiment for the household granular SPC last year, and they provided a satisfactory result with respect to the value listed in Table 1. To avoid the use of the barium ions, which are toxic, gravimetric analysis of deposited CaCO3 can be used as an alternative when the experiment is carried out by students who are inexperienced in the laboratory. The results of the gravimetric analysis of deposited CaCO3 are comparable with those of deposited BaCO3.

■

POSTLAB LEARNING EXERCISE Students calculate the value of y/x ratio from the experimental data. The chemical composition of SPC estimated from the results of the different experimental methods used by the different groups is shared with the whole class. Through the discussion about the characteristics of each experimental technique and the reliability of the results, students conclude that y/x = 1.5 for the synthetic crystals and is less than 1.5 for the granules. The discussion turns to the chemical function of SPC and how that relates to its use as oxygen bleach, using the SPC chemical composition determined by the students as a basis. The students recognize that an aqueous solution of SPC is a basic HPO solution. A simple demonstration of the decomposition rates of HPO in different pH solutions allows the role of SC in SPC to be discussed. The students reach the conclusion that the decomposition of HPO occurs without catalysis in the basic SC solution. At this stage, the role of the hydroxyl and hydroperoxyl radicals produced during HPO decomposition in the bleaching phenomenon is introduced by the instructor. If the study is designed to also allow the comparison of the compositions of the synthetic SPC crystals and granular SPC, the discussion is extended to the structure of granular SPC and its surface SC layer. The instructor leads the discussion to the function of the surface SC layer in protecting the interior SPC crystals from atmospheric water vapor and the hydration and dehydration of the surface SC layer. An extended inquiry can be designed to demonstrate the role of the surface SC layer in granular SPC using comparative experiments to trace the decomposition rate of SPC crystals and granular SPC at a high water vapor pressure by monitoring the change in y/x ratio with time. Details of the extended inquiries will be reported separately.

Redox Titration of H2O2 (Iodometry) (Required Time: 45 min)

About 1.0 g of potassium iodide is dissolved in 20 mL of 1 M H2SO4(aq) in an Erlenmeyer flask. An accurately measured quantity (approximately 25 mg) of solid SPC is added to the solution. Liberated iodine is titrated with a standard solution of Na2S2O3 (0.1 M) using a starch solution indicator to determine the end point. The redox titration is repeated several times. The y/x ratios determined by the students over the last year were 1.478 ± 0.017 (N = 4) and 1.300 ± 0.041 (N = 16) for the synthetic SPC crystals and household granular SPC, respectively. Measurement of the Oxygen Volume Evolved from the Catalytic Decomposition of H2O2 (Required Time: 45 min)

An accurately measured quantity (approximately 0.25 g) of solid SPC is dissolved in 5 mL of distilled water containing a small quantity of manganese(IV) dioxide. The evolved O2 is collected in a 50 mL measuring cylinder by the water replacement method, and the total volume of evolved O2 is measured. The volume measurement is repeated several times. The experimental y/x ratio for the household granular SPC determined by the students was 1.37 ± 0.11 (N = 4). The accuracy and reproducibility of this experiment can be improved if a gas buret was used for measuring the volume of evolved gas. In that case, it is desirable to calculate the molar quantity of evolved O2 by considering the actual atmospheric pressure, temperature, and saturated water vapor pressure.

■

CONCLUSIONS The determination of the chemical composition of SPC is a suitable inquiry-based laboratory exercise for a chemistry course at high school or an introductory course at university. For the inquiry exercise, students are required to use their knowledge of the chemical properties and reactions of SC and HPO. Because the chemical composition of SPC can be determined using one experiment selected from several possible simple experiments, various learning exercises can be encompassed depending on the learning objectives and the students’ stage of learning.

Mass-Loss Measurement for the Thermal Decomposition (Required Time: 45 min)

Approximately 0.25 g of solid SPC is weighed in a ceramic crucible, and the accurate sample mass is recorded. The crucible is covered with a ceramic lid, and the accurate total mass of the sample and crucible is recorded. The crucible is heated over a flame for a couple of minutes. After the crucible is cooled to room temperature, the accurate total mass is recorded. The mass-loss measurement is repeated several times, and the y/x ratio is determined. An excess mass loss is sometimes observed when using household granular SPC, which is possibly caused by the thermal dehydration of the hydrated surface layer and the decomposition of impurities. Although a limited number of student results were available for this experiment over the last year, the reported results were nearly in agreement with those listed in Table 1. The applicability of this experiment has also been confirmed in our previous article on the compositional determination for SSC.7

■

ASSOCIATED CONTENT

S Supporting Information *

Instructions for the students and instructor information. This material is available via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. D




Notes

(19) Zhubrikov, A. V.; E Legurova, E. A.; Gutkin, V.; Uvarov, V.; Khitrov, N. V.; Lev, O.; Tripol’skaya, T. A.; Prikhodchenko, P. V. XPS Characterization of Sodium Percarbonate Granulated with Sodium Silicate. Russ. J. Inorg. Chem. 2009, 54, 1455−1458.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The present work was supported partially by a grant-in-aid for scientific research (A)(25242015), (B)(22300373), and challenging exploratory research (23650511) from Japan Society for the Promotion of Science.

■

REFERENCES

(1) Logan, J. L.; Rumbaugh, C. E. The Chemistry of Perfume: A Laboratory Course for Nonscience Majors. J. Chem. Educ. 2012, 89 (5), 613−619. (2) Panzarasa, G.; Sparnacci, K. Glowing Teacup Demonstration: Trautz−Schorigin Reaction of Natural Polyphenols. J. Chem. Educ. 2012, 89 (10), 1297−1300. (3) Korolija, J. N.; Plavsic, J. V.; Marinkovic, D.; Mandic, L. Beer as a Teaching Aid in the Classroom and Laboratory. J. Chem. Educ. 2012, 89 (5), 605−609. (4) Gandia-Herrero, F.; Simon-Carrillo, A.; Escribano, J.; GarciaCarmona, F. Determination of Beet Root Betanin in Dairy Products by High-Performance Liquid Chromatography (HPLC). J. Chem. Educ. 2012, 89 (5), 660−664. (5) Fakayode, S. O.; King, A. G.; Yakubu, M.; Mohammed, A. K.; Pollard, D. A. Determination of Fe Content of Some Food Items by Flame Atomic Absorption Spectroscopy (FAAS): A Guided-Inquiry Learning Experience in Instrumental Analysis Laboratory. J. Chem. Educ. 2012, 89 (1), 109−113. (6) Egts, H.; Durben, D. J.; Dixson, J. A.; Zehfus, M. H. A Multicomponent UV Analysis of α- and β-Acids in Hops. J. Chem. Educ. 2012, 89 (1), 117−120. (7) Koga, N.; Kimura, T.; Shigedomi, K. Laboratory Inquiry for Determining the Chemical Composition of a Component in a Daily Use Detergent: Sodium Sesquicarbonate. J. Chem. Educ. 2011, 88 (9), 1309−1313. (8) Edionwe, E.; Villarreal, J. R.; Smith, K. C. How Much Cranberry Juice Is in Cranberry−Apple Juice? A General Chemistry Spectrophotometric Experiment. J. Chem. Educ. 2011, 88 (10), 1410−1412. (9) Smieja, J. A. Household Water Treatments in Developing Countries. J. Chem. Educ. 2011, 88 (5), 549−553. (10) Jones, C. D. The Kitchen Is Your Laboratory: A Research-Based Term-Paper Assignment in a Science Writing Course. J. Chem. Educ. 2011, 88 (8), 1062−1068. (11) Leacock, R. E.; Stankus, J. J.; Davis, J. M. Simultaneous Determination of Caffeine and Vitamin B6 in Energy Drinks by HighPerformance Liquid Chromatography (HPLC). J. Chem. Educ. 2011, 88 (2), 232−234. (12) Shaver, L. A.; Leung, S. H.; Pudebaugh, A.; Angel, S. A. Two Methods of Determining Total Phenolic Content of Foods and Juices in a General, Organic, and Biological (GOB) Chemistry Lab. J. Chem. Educ. 2011, 88 (4), 492−495. (13) Bracken, J. D.; Tietz, D. Analysis of OxiClean: An Interesting Comparison of Percarbonate Stain Removers. J. Chem. Educ. 2005, 82 (5), 762−764. (14) Copper, C. L.; Koubek, E. Analysis of an Oxygen Bleach: A Redox Titration Lab. J. Chem. Educ. 2001, 78 (5), 652. (15) Pritchard, R. G.; Islam, E. Sodium Percarbonate between 293 and 100 K. Acta Crystallogr., Sect. B 2003, 59, 596−605. (16) Nagaishi, T.; Kaneda, T.; Kotsubo, A.; Matsumoto, M.; Yoshinaga, S. Thermal Decomposition of the Addition Compound of Sodium Carbonate with Hydrogen Peroxide (Na2CO3·1.5H2O2). Kogyo Kayaku 1976, 37, 84−90. (17) Galwey, A. K.; Hood, W. J. Thermal Decomposition of Sodium Carbonate Perhydrate on the Solid State. J. Phys. Chem. 1979, 83 (14), 1810−1815. (18) Wada, T.; Koga, N. Kinetics and Mechanism of the Thermal Decomposition of Sodium Percarbonate: Role of the Surface Product Layer. J. Phys. Chem. A 2013, 117 (9), 1880−1889. E


Chemical Composition of Sodium Percarbonate: An Inquiry-Based

Recommend Documents