Swimming Pools, Hot Rods, and QualitativeAnalysis Dale D. Clyde Trinity University, San Antonio, TX 78284 Since 1958, swimming pools have been treated with chlorine stabilizers such as cvanuric acid and its chloroderivatives ( 1 ) . Recently, nitrogen oxides in the exhaust from diesel engines have been eliminated with the aid of cyanuric arid (2).Thispaper desrrihessome reactions for the identification and anplication of cvanuric acid. The premise uf'this paper is that, because many teenagers ar; interested in swimming pools or hot rods, they may find this applied chemistry interesting. Cyanuric acid, first prepared by Scheele from the pyrolysis of uric acid in 1776 (3), was assigned the correct formula by Drechel in 1875 (41, but a complete understanding of its parent compound, 1,3,5-triazine, had to wait until 1954 (5). Thermally stable in a sealed tube a t 500 'C, the solid exists as a keto tautomer, but in a basic solution the en01 form is more stable. Derivatives of the two forms are commonly called cyanurates and isocyanurates, respectively (6). See Figure 1. Unlike triazene, which is very labile and undergoes ring cleavage with nucleophiks, ryilnuric acid reflects the chemistry of itssuhstituents. Only at high temperatures and pressures will cyanuric acid react withammonia to producemelamine. In aqueous solutions cyanuric acid does not esterify directly unless i t is first converted to a metal salt. Halogen derivatives result from the reaction of cyanuric acid with nhosnhorus chloride. haloacids or active haloeen (7). . . See kigu;e 2. An i m ~ o r t a nhaloaenated t triazine is cyanuric chloride. I t can be s$nthesizedfr& the action of chlorine on hydrocyanic acid under various conditions or from cyanoaen halides in the presence of catalysts (8).One industrial process is the trimerization of cyanogen chloride in the gas phase on activated charcoal (9). Cyanuric chloride is useful as a starting material for the preparation of the herbicides, fiher-reactive dyes, and optical brighteners (10). It hydrolyzes in 10%sodium hydroxide a t 125 OC to form cyanuric acid (11). See Figure 3. The halogenated triazines of interest in this paper are the chloroisocyannrates since they are used as disinfectants in swimming pools. They can be prepared by the controlled chlorination of sodium and potassium salts of cyanuric acid. Isocyanic acid, HNCO, is produced by the depolymerization of cyanuric acid a t temperatures above 330 "C and has been shown to react with noxious nitrogen oxides (2). Thus, cyanuric acid may have a useful role in the exhaust systems of our automobiles. In the experimental section below, qualitative tests are described for the identification of cyanuric acid and its decomposition product, isocyanic acid. This is followed by a discussion of the chemistry of cyanuric acid in swimming pools and diesel engines. ~
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1.3.5-Triazene Cyanurates lsocyanurates Figure 1. Fwmulas for triazene, cyanurates. and isocyanurates.
REACTIONS OF CYANURIC ACID
-
-0,~"\~~0 I
+ 2H10=ZHOCl
i
N
-0, Co N \C, O H f
I
11
N
Oc'
\CI
N
I
1 I
OH
0
Dichioro Cyanurate
Anion of ISO
Cyanuric Acid
Figure 3. Preparation of cyanuric acid.
listed below. A 0.2 M aqueous solution of potassium cyanate was used to check the silver ion and cupric-pyridine tests. Phenylisocyanate dissolved in acetonitrile was used to check the Kuhitz test.
Experimental
Silver Ion Test A few drops of cyanate are added to a 0.10 M AgNOg solution. A positive test results in the formation of an insoluble, yellow-white precipitate,AgOCN.
All reagents were analytical grade except the commercial products. Cyanuric acid (Aldrich Chemical Co.) was labeled as 98% pure. The commercial chlorine stabilizer (Aqua Chcm) was 96% sodium dichlorocyanurato. Several different test reagents are reported for the identification of cyanate (12). The preparation of tent reagents are
Cupric-Pyridine Test A few drops of 0.250 M eopper(I1)nitrate are added to a test tube. Two or three drops of pyridine are added, followed by two mL of chloroform. Two or three drops of the eyanate test solution are added. A positive test is confirmed by the formation of a blue precipitate that is soluble in chloroform after brisk shaking. Volume 65
Number 10 October 1988
911
Kubitz Test (13) The reagent reported in the literature is prepared from n-hutylamine. Aniline was substituted. (Warning: Aniline is rapidly absorbed through skin. Gloves must be warn duringthe preparationof this reagent.) Ten milliliters of saturated solution of malachite green oxalate in acetonitrile are pipeted into a flask. Thirty drops of aniline are added and swirled. The solution turns from a bright green to a dull yellow-green. The solution is centrifuged and decanted from an insoluble white precipitate, probably an o d i c acid salt. At this point, if solution is not yellow enough, add more aniline or any nonaqueous amine. For example, two drops of hexamethyldisilazane changes the reagent to a dull yellow. Since KOCN is not soluble in acetonitrile, the reagent is tested with phenylisocyanate. Apositive test is indicated by a change in color from yellow to green and with time a formation of a white preeipitete. Procedure for the Test of Cyanlc Acid, HOCN Gaseous HOCN is produced by the reaction of several milliliters of 6.0 M HCI and a concentrated solution of potassium cyanate in a large test tube. Because the reaction is vigorous, the acid must he added dropwise. The gas formed is bubbled through the test reagent. The reaction vessel consists of a two-holed rubber stopper fitted to a large test tube. A glass, U-shaped tube is inverted and inserted through one of the holes in the rubber stopper. A micropipet with bulb is filled with 6.0 M HCl and inserted through the other hole. The large test tube is filled with 2 or 3 mL of concentrated KOCN. The two-hole stopper with U-tube and pipet is inserted into the top of the large test tube. One end of the inverted U-tube is submerged into a solution of the test reagent in the small test tube. The acid is introduced dropwise into the KOCN solution in the large test tube. The liberated gas passes from the large test tube, through the U-tube, and bubbles through the test solution in the small test tube. Procedure for the Qualitative Test of Cyanuric Acid by the Formation of a Precipitate Approximately 0.1 g of sample is dissolved in 10-20 mL of hot water. To the solution are added 1.0 mL of 1.0 M calcium acetate, 1.0 mL of 1.0 M ammonium chloride, and 1.0 mL of 0.020 M copper(I1) chloride. The beaker is placed on a hot plate, and the contents are evaporated to near dryness. Residue is redissolved in a minimum amount of water, and 2 to 3 drops of concentrated ammonia are added with caution. Too much ammonia will prevent precipitation. Solution is again evaporated to near dryness. Two milliliters of 1.7 M ammonium acetate are added to the warm residue and allowed to cool. In approximately 30 min a purple precipitate, (C3H2N3Oa)zCu.2NHs,forms. A simpler procedure is usually suitable. To a hot solution of cvanuric acid add 1.0 mL of 0.02 M eoooerfII) .. . . chloride and several droos of concentrated ammonia. Pumle ~reeioiteteis formed ~~~~-unon cding. This prorrdure gives a positive test for chlonrderivativesg r f cyanurrc arid, also. Caution: Chlorine isliberated upon the additam of ammonia to chlorinated commercial blends; thus, tests for Aqua Chem must be done in the hood. ~
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Procedure for the Decomposition of Cyanuric Acid A 50-mL Erlenmeyer flask is fitted with one-hole rubber stopper. Approximately 1-2 g of cyanuric acid are added to the flask so that the bottom is evenlv covered. A bent elass tube is inserted into the stopper to deliver the gaseous isocyakc acid into the Kuhitz test solution. The flask is stoppered and placed on a hot plate. The end of the delivery tube is dipped into the test reagent in a test tube. With the hot plate set at full beat, the solid is heated at least 30miu before any evidence of isocyanic acid is obvious. Warning: Do not try this experiment with ehloroderivatives of cyanuric acid. The chloro compounds may he explosive. Dlscusslon The identification of cyanuric acid is based upon the formation of a slightly soluble, purple copper(I1) compound, [C3H2N3O3I2Cu.2NH3 (14). This test is also positive for alkyl and chloro derivatives. The optimum precipitation conditions cited in a gravimetric analysis are a cyanuric acid:. ammonia molar ratio of 1:6 for a 0.025 M solution of the acid (15). An acetate buffer has been recommended in another procedure (16). Before the decomposition of cyanuric acid could be dem-
912
Journal of Chemical Education
onstrated, a procedure had to he developed for the identification of the decom~ositionoroduct. isocvanic acid. The reaction of hydrochloric acid and potassium cyanate was used as a source for isocyanic acid. Actually, cyanic acid is probably initially produced, but all evidence in the literature indicates that the only species present in the vapor phase is isocyanic acid (17). The majority of the isocyanic acid, once formed, readily decomposes into carbon dioxide and arnmonia, and very little of it remains intact (12); therefore, it is essential that the qualitative test he a very sensitive one. Aqueous solutions of silver ion or cupric-pyridiue are reagents normally used to test for cyauate ion (12). These reagents were tried for the detection of isocvanic acid. It was ass;med that if the gaseous acid were bubked long enough throueh the test solutions. o e r h a ~ si t would dissolve and ioniz&o give sufficient cyanate f o i a positive test. If cyanate did indeed form, it was never observed. The only available test in the literature for isocyanates is one developed for organic copounds by Kuhitz (13). The Kubitz reagent cannot tolerate more than 5% water. Therefore, the procedure was first checked to see that water was not transferred from the reaction vessel to the test reagent. Since oxygen and carbon dioxide do not interfere with the test, the test apparatus was checked against carbon dioxide produced by the reaction of 6.0 M HC1 and Na2C03. A color change did not occur from bubbling COz through the test reagent for 15 min. However, for the reaction of KOCN and 6.0 M HCI. a ~ o s i t i v etest for easeous isocvanic acid did occur after10 k i n of continuous bubhling. The method is based on the formation of a colorless derivative made from the reaction of malachite green and a primary or secondary amine. The addition of isocyanate returns the derivative to a colored form. The procedure is very sensitive and capable of detecting 0.005 mmol per gram of sample. Although the addition of cyanuric acid to swimming pools affects the photodecomposition of the chloro species, the chemistry is not completely understood (I). The probable equilibria are illustrated in Figure 4. Concentrated solutions
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Cyanogen Cyanuric Cyanurlc Chloride Chloride Acid Figure 4. Equilibria of cyanuric acid undei swimming pool conditions.
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of trichloroisocyanurate, approximately 0.5% available chlorine, decompose to NC13, a lacrimator and dangerous explosive. This can be orevented bv keeoine the concentration level of the chloroisocyanuratk belbw-1000 ppm. At this concentration level a t a p H range of 3 to 8.5, the ring decomposition occurs slowly to produce NC13 and molecular nitrogen. At p H 8.5 to 14, only nitrogen is observed as a product (1). ,-, A photolysis study of NCO radicals and nitric oxide led to an investigation of the effects of isocyanic acid on the concentration levels of nitrogen oxides in diesel engine emissions (2). The experiment was designed to pass exhaust gases over cyanuric acid a t 625 O F , a temperature sufficiently high enoueh to decornoose cvanuric acid to isocvanic acid. The series of reactions occur with a final, overall stoichiometric molar ratio of 1:l between HNCO and YO. The ~ n ) d u c tare s molecular nitrogen, carbon dioxide, carbon monoxide, and water. The initiation chemistry is uncertain, but several models have been developed based on the formation of radi-
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cal species. See Figure 5. An advantage of the process is that particulates, or soot, can be controlled.
Overall Reaction HNCO +NO,-
N2+CO+C02+H 2 0
Suggested Mechanism HNCO 9 NHtCO NHtNO
---
OHtCO
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NH2 t CO
H+C02
NH2+ NO
-
The author wishes to thank the Faculty Development Committee and the University for an academic leave which allowed time for this project.
HtN20
HtHNCO
N2H
Acknowledgment
N2H+OH N,+H20
N2t H
Figure 5. Reaction mechanism for ipocyanic acid and nivwen oxides (2).
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1. Dreschel, E. J. Prokt Chem. 1875,lJ. 269.
5. Grundmann,C.;Kreufrbe~r,A.J. Am. Chrm.Soc. 1954,76,632-633. A. J.; MeKillop, 6. Quirke, J. M. E. In ComprehemiuoHrterocyelir Ch~misfry:Boulton, A , Ed..; Pergsmon: New York. 19% Vol.3, p 459. 7. Smolin. E. M.:Repoport. L. The Chsrnistryof Hsteroeyelic Campounds: InVracionm: Newyo&, ,?%'."'."
