In the Classroom edited by
Resources for Student Assessment
John Alexander University of Cincinnati Cincinnati, OH 45221
Use of Chloroisocyanuarates for Disinfection of Water Application of Miscellaneous General Chemistry Topics Gabriel Pinto Departamento de Ingeniería Química Industrial y del Medio Ambiente, E.T.S.I. Industriales, Universidad Politécnica de Madrid, 28006 Madrid, Spain;
[email protected] Brian Rohrig Aurora High School, Aurora, OH 44202
Chemistry often remains abstract and alien to students and subsequently they are not motivated to study further. There is evidence that students will better appreciate chemistry concepts if these concepts are applied to compounds found in everyday lives. The idea of using a familiar context with chemistry is common but not usually found in textbooks. Thus, chemistry teachers are always looking for examples of how chemical phenomena affect our lives (1–5), and for novel problems that will stimulate students’ interest once they have mastered the more routine skills. This article describes a motivating and holistic approach to the study of water chlorination. The compounds used, sodium dichloroisocyanurate (anhydrous and dihydrate) and trichloroisocyanuric acid, seem mysterious to the students at first, yet studying water chlorination allows students to focus on the versatility of the general chemistry concepts. This paper represents a continuation of a research program intended to help university-level instructors include connections between students’ daily experience and chemical principles taught in the classroom (6–11). After an introduction to the subject, a series of questions is proposed to address various topics, including, formulation, molar mass, stoichiometry, chemical equations, balanced equations, oxidation states, acid–base reactions, tautomerism, resonance, chlorinating products, and use of chlorine as a disinfectant. Chlorine as a Disinfectant Industry and commercial settings use the term “chlorine” as a generic term for several different chlorine-containing compounds. However, it is important that we develop the habit of calling chemical compounds by their proper names, otherwise students may think, for example, that the copper in the blood stream is the same as metallic copper. Chlorine is a disinfectant that has been used in water treatment for a little more than a hundred years (12). Its use as a disinfectant for drinking water and swimming pools is widely known, as well as its use as a bleaching agent (13– 16). The purpose of water disinfection is to destroy pathogenic organisms, thus preventing waterborne diseases such as typhoid, cholera, dysentery, and hepatitis. Elemental chlorine dissolves slightly in water and disproportionates into hypochlorous acid (HClO) and hydrochloric acid (HCl)
Cl2 + H2O
HClO + H + + Cl−
(1)
Where the equilibrium constant (17) at 25 ⬚C is given by
HClO H + Cl − K = ≈ 4.5 × 10−4 Cl 2
(2)
HClO is a weak acid and it partially dissociates HClO
H + + ClO −
(3)
The corresponding dissociation constant (18), at 25 ⬚C is
H + ClO− Ki = = 2.9 × 10− 8 HClO
(4)
Hypochlorous acid is the principal water disinfectant. It is widely used to kill bacteria and algae and is an effective biocide since ClO⫺ is a strong oxidizing agent. Thus, chlorine as HClO is needed for sanitation and chlorine as ClO⫺ is needed for oxidization. The disinfecting action is due to both species, HClO and ClO⫺, but not directly to the Cl2. HClO, a weak acid, is not harmful to people. HCl, a strong acid also produced by hydrolysis of the dissolved chlorine gas (eq 1), is not harmful because of the low concentration produced. The pH of water determines how much HClO dissociates into H⫹ and ClO⫺, in accordance with eq 4, due to the relationship HClO = HClO + ClO −
1 ClO 1+ HClO −
=
1 K i (5) 1+ + H
Chlorine is most efficient at pH of approximately 7.4–7.6. In accordance with eq 5, at a high pH chlorine does not produce very much disinfectant, HClO; it is mostly in the form of hypochlorite (plenty of oxidizing agent and low disinfectant). For example, at pH 7.0, 78% of the chlorine exists in the active hypochlorous acid form, and increasing pH to 8.0 reduces the hypochlorous acid concentration to only 26%.
JChemEd.chem.wisc.edu • Vol. 80 No. 1 January 2003 • Journal of Chemical Education
41
In the Classroom
The hypochlorite ion is reduced to inactive chloride as it functions as a biocide, following the reduction half-reaction ClO − + 2 H + + 2 e−
Cl − + H2O
(6)
Chlorine is available in a number of forms for water disinfecting effects (19–21), mainly: chlorine gas (delivered as a liquid in pressurized containers), chloroisocyanurates (sodium dichloroisocyanurate or trichloroisocyanuric acid), calcium hypochlorite, lithium hypochlorite, and sodium hypochlorite. These compounds provide free available chlorine, that is, chlorine available in the forms of hypochlorous acid and hypochlorite ions (22). Free available chlorine, known by the acronym FAC in the water treatment industry, is a measure of the oxidizing or biocidal power of the active chlorine in a compound expressed in terms of elemental chlorine, usually referred to as percent by weight (20). Despite the fact that the terms “free” and “available” mean almost the same thing, the redundant term “free available chlorine” is widely used in industry. Combined available chlorine is chlorine existing in water in chemical combination with ammonia or organic nitrogen compounds such as amines and proteins. These combined forms are known as chloramines, such as NH2Cl, NHCl2, or NCl3. The chloramines do not exhibit any substantial sanitizing power and are actually the cause of some unpleasant problems such as eye and skin irritation and strong odor in pools (23). Chloramines are also known as combined residual chlorine and should be kept to a minimum. Table 1 gives the formula of chloroisocyanurates and the corresponding FAC as found in several references (18, 20) and in different labels of chlorinating products found currently in supermarkets. Chlorine products are required by law to display on the label their chemical name and concentration. Thus the label of these products can be a source of questions for students. Advantages and disadvantages of each type of chlorine product for use as pool or spa water disinfectant or oxidant can be evaluated by considering the properties, cost, and safety (21). Ultraviolet light degrades chlorine by a photochemical reaction 2Cl2 + 2H2O
UV
(7)
4HCl + O2
In the absence of isocyanuric acid (1,3,5-triazine-2,4,6 (1H,3H,5H )-trione, see Figure 1) as a stabilizer, on a bright sunny day 90% of the active chlorine can be destroyed by sunlight in just 2 h. HClO and ClO⫺ are thought to closely attach to one of the three bonding sites of the isocyanuric
acid molecule. As long as HClO and ClO⫺ remain attached, they are not degraded by sunlight. Cyanuric acid stores chlorine in the form of HClO and releases it to do its work on bacteria and algae. This compound and its chlorinated derivatives form a complex ionic and hydrolytic equilibrium system, consisting of at least ten isocyanurate species (20). Sodium dichloroisocyanurate (sodium 1,3-dichloro1,3,5-triazine-2,4-dione-6-oxide, see Figure 1), NaDCC, is the only popular chlorine product that does not require the addition of either a neutralizing chemical or isocyanuric acid, because NaDCC solutions have a pH close to 7 and it supplies the proper ratio of both chlorine and stabilizer. Apart from pools, the use of chlorine release tablets formulated using NaDCC to make up disinfectant solutions is well established throughout the world as a disinfectant for baby bottles and other nursery equipment (24, 25), and for the daily disinfecting of soft hydrophilic contact lenses. NaDCC, an oxidizing compound, dissociates into HClO (free chlorine) and isocyanuric acid (stabilizer) in water, which helps prevent sunlight from degrading the chlorine. The NaDCC is highly efficient and yields a good performance with no harm to humans. It is available in granular form or as a tablet, sometimes in an effervescent base. Trichloroisocyanuric acid (trichloro-1,3,5-triazinetrione), TCC, is used mainly for pool water. It is highly acidic and will corrode equipment and pool tile if improperly used. It is necessary to add about 350 g of soda ash (the trade name for sodium carbonate) for each kg of trichlor used. The addition of isocyanuric acid is not required. Questions The following questions cover various topics of general chemistry that are related to the use of chloroisocyanurates for chlorination of water. These questions are merely a starting point; the reader is encouraged to develop additional questions that will encourage students to apply chemical concepts to the chlorination of water.
Question 1 Search the Internet, chemistry handbooks, organic chemistry books, or catalogs of chemicals to find the chemical structure of NaDCC, TCC, and isocyanuric acid. Question 2 When dissolved in water, NaDCC reacts to form hypochlorous acid and isocyanuric acid. Write the corresponding balanced reaction. +
−
O Table 1. Formulas and Free Available Chlorine, FAC, of Common Chloroisocyanurates Substance Chlorine
Chemical Formula Cl2
100
Sodium dichloroisocyanurate (NaDCC) Anhydrous
C3Cl2N3O3Na
62–63
Dihydrate
C3Cl2N3O3Na·2H2O
55–56
C3Cl3N3O3
89–91
Trichloroisocyanuric acid (TCC)
42
Cl
Cl N
N
FAC / wt. %
O
O
Na
N
O
O
O Cl
N
N N
O
H
O
H N
N N
Cl
Cl
H
a
b
c
O
Figure 1. Chemical structures of (a) sodium dichloroisocyanurate, NaDCC; (b) trichloroisocyanuric acid, TCC; and (c) isocyanuric acid .
Journal of Chemical Education • Vol. 80 No. 1 January 2003 • JChemEd.chem.wisc.edu
In the Classroom
Question 3 If the reaction of the previous question is a reduction– oxidation reaction, calculate the oxidation numbers of the different atoms and identify the oxidizing and reducing agents. Question 4 Repeat questions 2 and 3 for TCC. Question 5 Explain which species of the previous questions have resonance structures or the possibility of tautomerism. Question 6 Explain why the dissolution of NaDCC results in a almost neutral solution and the dissolution of TCC results in an acidic solution. Question 7 Deduce by stoichiometry the values of FAC for the compounds shown in Table 1. Answers to Questions
Answer 1 The chemical structure of these compounds can be found using several different sources (19, 20, 26). They are shown in Figure 1. Answer 2 C3H3N3O3(aq) + Na+(aq) C3Cl2N3O3Na(s) + 2H2O(l)
+ ClO −(aq) + HClO(aq)
(8)
Answer 3 To calculate the oxidation numbers of atoms in the NaDCC, we can use the method as described by Halkides (27). We can also consider the difference between valence electrons of isolated atoms and the valence electrons of bonded atoms as assigned by their electronegativity (28, 29) shown in Figure 2. The oxidation state or number is the charge on an atom in a molecule (or polyatomic ion) calculated on the assumption that bonding electrons between atoms are assigned to the more electronegative atom. Although calculation is normally done by a set of rules that assumes certain oxidation states for certain atoms, such as ᎑2 for O, −
−2 +4
O −2
0
N
N
C
C N Cl
C3Cl3N3O3(s) + 3H2O(l)
3HClO(aq) + C3H3N3O3(aq)
(9)
The three atoms of N are reduced from ᎑2 to ᎑3, acting as the oxidizing agent, and the three atoms of Cl act as reducing agent because they are oxidized from 0 to +1.
Answer 5 For isocyanuric acid and TCC, resonance has little importance because the other resonance structures, different from those drawn in Figure 1, have more formal charges not equal to zero, and thus have a smaller contribution to the resonance hybrid. However, the dichloroisocyanurate anion that forms NaDCC exhibits resonance, as exhibited in Figure 3. This question can be used for clarifying the distinction between formal charge and oxidation state (considered in question 3). Formal charge is the charge of an atom in a compound calculated on the assumption that the electrons in each bond are shared equally between the two atoms involved, and thereby underestimates electron transfer. Oxidation state formalism, where electron transfer is overestimated, is a convenient tool for dealing with redox reactions. Formal charge formalism can be used as a tool to evaluate and select the best Lewis structure (29). It is important to realize that the actual charge of the atoms in a compound lies somewhere between these two descriptions. As can be noted with the examples shown in this paper, oxidation numbers do not depend on the resonance form. It is also important to note, for checking the correct assignment of charges, that the sum of the oxidation states of all the atoms within a molecule must be equal to zero. In a polyatomic ion, the sum of the charges of all atoms must equal the net charge of the ion. The same rule applies to formal charges. Both structures shown in Figure 3 have the same contribution to the resonance hybrid. Other resonance structures have more nonzero formal charges, and thus they have a smaller contribution to the resonance hybrid.
+4
C
Cl
Answer 4
−2
O
0
NaDCC is a special case as electronegativities of N and Cl are the same; the electrons in each bond N–Cl must been shared equally between the two atoms involved. The Pauling electronegativities of O, N, Cl, C, and H, according to ref 30 are 3.5, 3.0, 3.0, 2.5, and 2.1, respectively. Following the same procedure for the molecule of isocyanuric acid, C3H3N3O3, we conclude that in eq 8 two atoms of N are reduced from ᎑2 to ᎑3, and two atoms of Cl are oxidized from 0 to +1 (in the form of HClO or ClO⫺). Thus, two N atoms act as oxidizing agent or oxidant and the two Cl atoms act as reducing agent or reductant.
−
−3 +4
O −2
O
Na
+
+1
−2
Figure 2. Oxidation numbers for sodium dichloroisocyanurate.
O
Cl
O
Cl
N
N N Cl
O
O
N
N N
O
−
Cl
Figure 3. Resonant structures of the dichloroisocyanurate anion.
JChemEd.chem.wisc.edu • Vol. 80 No. 1 January 2003 • Journal of Chemical Education
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In the Classroom
Student Assessment column. One of us (G.P.) dedicates this work to his children (Lucía and Elisa), because, thanks to them, disinfecting their baby bottles gave him the opportunity of knowing about the studied chlorinating compounds.
H O
O H
H N
N
N
N
H O
N
O
O
N
O
H
Literature Cited
H
a
b
Figure 4. Tautomeric forms of (a) isocyanuric acid and (b) cyanuric acid.
Isocyanuric acid tautomerizes with cyanuric acid, as illustrated in Figure 4. Lone pairs have been placed in the structures in Figures 1–4, in order to facilitate the counting of electrons.
Answer 6 As demonstrated in eq 8, the dissolution of NaDCC generates a weak acid and a buffer formed by HClO and ClO⫺, but the dissolution of TCC, as shown in eq 9, generates two acids. Answer 7 First it is necessary to calculate the molecular weights of the chlorine-containing compounds: anhydrous NaDCC (220.0 g兾mol), NaDCC dihydrate (256.0 g兾mol), TCC (232.5 g兾mol), and isocyanuric acid (129.1 g兾mol). Examining the reaction of anhydrous NaDCC dissolution as described in eq 8 and the equivalence between chlorine and hypochlorous expressed in eq 1, the FAC of 100.0 g of NaDCC anhydrous is
(
2 mol ClO− or HClO mol NaDCC × 100.0 g NaDCC × mol NaDCC 220.0 g NaDCC mol Cl2 × mol ClO− or HClO
(
)
)
70.9 g Cl 2 = 64.5 g Cl 2 × mol Cl 2
As seen in Table 1, the tabulated value of available chlorine for this compound is 62–63%, that is, slightly lower than the obtained value. Proceeding in a similar way, the value of FAC for 100.0 g of NaDCC dihydrate is 55.5 g and for 100.0 g of TCC the value is 91.6. In both cases these values are also close to the tabulated values. For chlorine, FAC is 100% by definition. Acknowledgments The authors would like to gratefully recognize the financial support provided by the Fundación Española para la Ciencia y la Tecnología (Spanish Foundation for Science and Technology) under the Project “Teaching/Learning of Chemistry and Everyday Life” (Grant No. 17502/57-1F). Also, we would like to thank the helpful comments of the reviewers and especially John J. Alexander, editor of Resources for
44
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Journal of Chemical Education • Vol. 80 No. 1 January 2003 • JChemEd.chem.wisc.edu
