The Chemistry of Swimming Pool Maintenance - American Chemical ...

The Chemistry of Swimming Pool Maintenance - American Chemical ...

Chemistry for EveryoneProducts of ChemistryThe Chemistry of Swimming Pool Maintenanceedited byGeorge B. KauffmanCalifornia State UniversityFresno, CA 93740WCarl Salter* and David L. LanghusDepartment of Chemistry, Moravian College, Bethlehem, PA 18018; *csalter@chem.moravian.eduThe chemistry involved in swimming pool maintenanceprovides many potential applications in undergraduate chemicaleducation. For example, students can learn the importanceof pH in water chemistry because pH affects chlorine sanitationand calcium balance. Both chlorination and calcium solubilityprovide practical examples of chemical equilibrium atwork. The reactions of ammonia with chlorine introduce unusualredox chemistry. There are many obvious applicationsof pool chemistry in an analytical chemistry course, and theeffect of temperature on pool equilibria is a possible physicalchemistry topic. The purpose of this paper is to explain thebasic chemistry behind the common practices involved inswimming pool maintenance. We hope it stimulates the inclusionof pool chemistry throughout the curriculum.As a topic for discussion in general chemistry classes,swimming pool chemistry is both interesting and approachable.Swimming pools are enjoyed by almost everyone—swimmers and nonswimmers alike—and most people areaware that chlorine is usually added to pool water to keepswimming pools clean because they have experienced the effectsof chlorination on their eyes and nose. Pool owners knowthat, in addition to chlorine, they must measure and controla number of other factors that affect the quality of their poolwater: pH, water hardness, total alkalinity, and total dissolvedsolids. Pool distributors market a wide array of compoundsand test materials to help pool owners. According to datafrom the Association of Pool and Spa Professionals, there areroughly 8 million residential pools in the United States, andeach year pool owners spend about 4 billion dollars on theirmaintenance (1).Despite the fact that pool chemistry is an interestingapplication of chemical principles, there has been only onepaper in this Journal about pool maintenance, and its discussionis outdated (2). There are reports of laboratory experimentsdesigned around certain aspects of pool chemistry(3a–3d), and one recent discussion of chlorination (4), yetnone which focus on pool maintenance.At the center of pool chemistry is pH, which affects thebalance of two important processes: the equilibrium betweenhypochlorous acid and hypochlorite ion, and the equilibriumbetween calcium ions in solution and calcium carbonate inthe cement or plaster liner of many pools. Hypochlorous acidis the most effective antibacterial species formed by chlorine,and so, other things being equal, the pH of pool water shouldbe low enough to ensure that a good portion of the dissolvedhypochlorite is protonated.HOCl (aq) OCl − (aq) + H + (aq)hypochlorous hypochloriteacidConversely, too much acid can dissolve calcium carbonate,and you certainly don’t want your swimming pool to dissolve!2H + (aq) + CaCO 3H 2 O + CO 2+ Ca +2 (aq)To balance these processes, it’s critical to hold the pH of thepool water within strict limits.Measuring Pool pH without a pH MeterPhenol red is the indicator of choice. Pool test kits usuallyprovide phenol red in solution or impregnated into test strips,and provide a series of color references to read the pH. AtpH 7.5 a solution of phenol red will appear orange due tothe mixture of yellow (acid) and magenta (base) forms of theindicator.Figure 1. The absorption spectra of phenol red at various pH values.Spectra were obtained using an Ocean Optics USB2000 UV–vis spectrometer with 25 µm slits. The solutions were buffered andcontained the same concentration of phenol red. Notice that theabsorptivity of the base form at its wavelength maximum is aboutthree times that of the acid form.Although the pK a2of phenol red is 7.96 (5), most of the colorchange takes place around pH 7.5. The visible absorptionspectra of phenol red at different pH values (Figure 1) showthat the absorptivity of the conjugate base is about three timesgreater than that of the acid form, which is the reason whythe color change is most pronounced at pH 7.5.1124 Journal of Chemical Education • Vol. 84 No. 7 July 2007 •

Chemistry for EveryoneAdjusting Pool pHTo decrease pool pH either hydrochloric acid or sodiumhydrogen sulfate (“dry acid”) can be added to the pool water.To increase the pH, sodium carbonate should be used; sodiumbicarbonate will increase total alkalinity but will notmove the pH efficiently.The Effect of pH on Chlorine DisinfectionThe pK a of hypochlorous acid is 7.53 (6). Although bothHOCl and OCl can kill bacteria, hypochlorous acid is morepotent (7); as a neutral species it will cross the cell membranefaster. Once inside the cell, hypochlorous acid inactivateskey enzymes by oxidizing side chains, especially thosecontaining sulfur (8).Above pH 7.5 most of the hypochlorous acid is ionizedto form hypochlorite ion, so the effectiveness of the chlorineprotection is reduced. On the other hand, hypochlorous acidis more easily lost from pool water by outgassing and sunlightdegradation, so it is not good to have all of the activechlorine as the acid. For this and other reasons the pH ofpool water is kept close to the pK a of hypochlorous acid.Sources of Chlorine for PoolsThere are a variety of chemicals that can be added topool water to generate the desired hypochlorous acid. Pooldistributors will usually tell you that there are three kinds ofchlorine: solid, liquid, and gas. Of course, chlorine, Cl 2 , is agas at room temperature and pressure, not a liquid or a solid.Gaseous chlorine is sometimes used for large-scale chlorination,such as at a municipal pool. Private pool owners usuallyuse either “liquid chlorine”, NaOCl in aqueous solution,or “solid chlorine”, which can either be cakes or tablets ofCa(OCl) 2 or chloroisocyanurates marketed under thetradenames Dichlor and Trichlor. Calcium hypochloritealso adds calcium to pool water, which can be an advantagewhen the water available to fill the pool is soft. Dichlor andTrichlor are called “stabilized chlorine”; they react with waterto form cyanuric acid and hypochlorous acid. Their advantageis that cyanuric acid protects the hypochlorous acid fromsunlight degradation as discussed below.Aqueous Chlorine ChemistryChlorine gas at 1 atmosphere and 25 C will form a saturatedaqueous solution of 0.73% (wtwt) or 0.092 M Cl 2 .In a saturated solution roughly one-third of the dissolvedchlorine reacts with water to form hypochlorous acid, leadingto final concentrations of [Cl 2 ] 0.062 M and [HOCl] [Cl ] 0.030 M (9); the equilibrium actually favors chlorineand water, as can be seen from the Gibbs free energies(10) of the speciesCl 2+ H 2 O H + (aq) + Cl − (aq) + HOCl∆ f G/(kJ/mol): 0 −237 0 −167 −66H +Cl − HOCl−= 4 × 10 4Cl 2∆ r G = +4 kJ/molor the equilibrium expression. Of course, pool water is notsaturated with chlorine. For dilute solutions of chlorine abovepH 4, the equilibrium is displaced strongly to the right, andvery little free chlorine exists in solution (7). The reaction ofchlorine with water is both a hydrolysis reaction and a disproportionationreaction. Because the reaction releases hydrochloricacid, the addition of chlorine to pool water will lowerthe pH.The solubility of chlorine can be vastly increased by dissolvingthe chlorine in strong NaOH solution; most householdbleaches are solutions of 5% NaOCl.Cl 2+ 2NaOH(aq)NaCl(aq) + NaOCl(aq) + H 2 OThe addition of hypochlorite to pool water increases the pHby releasing hydroxide ions.OCl − (aq) + H 2 OHOCl + OH − (aq)In addition, if bleach is the source of the hypochlorite, itsexcess sodium hydroxide will also raise the pH.Outdoor pools can lose chlorine protection during thedaytime: it is estimated that direct summertime sunlight candestroy 90% of free chlorine in two hours (11). Sunlightdestroys HOCl, presumably by a sequence of reactions thatultimately leads to oxygen gas (11, 12), a reaction that is exoergicat standard conditions.∆ f G/(kJ/mol):2HOCl2HCl(aq) + O 2−66 −167 0∆ r G = −202 kJ/molThe reactions are probably initiated by the photolysis reactionHOCl hυ → HO Cl.Cyanuric acid stabilizes chlorine by preventing sunlightdecomposition of HOCl, as it appears to sequester chlorinein a form that is impervious to photolysis. Cyanuric acid canbe added directly to pool water, or it can be added with chlorineusing Dichlor or Trichlor. The recommended concentrationof cyanuric acid in pool water is 30–50 ppm. Pinto andRohrig have discussed the chemistry of • Vol. 84 No. 7 July 2007 • Journal of Chemical Education 1125

Chemistry for Everyonein this Journal (4), and developed study questions for the topic.Clyde has discussed the chemistry of cyanuric acid and itsderivatives in this Journal (3c). Donnelly has described severalgeneral chemistry experiments designed to investigate theprotection of hypochlorous acid by cyanuric acid (3d).called a Würster dye (14, p 1174):Free Chlorine and Combined ChlorineThat “chlorine pool” smell is really NH 2 Cl, monochloramine,which is produced by the reaction of hypochlorousacid and ammonia.HOCl + NH 3NH 2 Cl + H 2 O(Dangerous amounts of chloramines can be produced by mixinghousehold bleach with ammonia-based cleaners. Manybleaches and cleaning solutions bear warnings against thispractice.) In pool water the most likely sources of ammoniaare urine and sweat. Monochloramine is a weaker disinfectantthan HOCl. At low concentrations it can be a useful disinfectantin municipal water supplies because it is long-lasting(7), but its irritating smell makes it undesirable in pool water.The sum of chlorine present as Cl 2 , HOCl, and OCl iscalled free chlorine. Chlorine present as the chloraminesNH 2 Cl and NHCl 2 is called combined chlorine. The sumof free chlorine and combined chlorine is total chlorine. Foreffective sanitation, total chlorine should be maintained between1–3 ppm with the bulk of this as free chlorine. Combinedchlorine concentrations in excess of 0.2 ppm impart adiscernable “chlorine” odor to the pool water and cause aburning sensation of the eyes. This condition is routinely toleratedin heavily-used public facilities, yet it can be avoidedaltogether simply by keeping combined chlorine values undercontrol.Pool ShockingWhen the combined chlorine rises above an acceptablevalue, it is time to “shock” your pool. Shocking, also calledsuperchlorination or “breakpoint chlorination” (7, 13), requiresa dramatic increase in chlorine, up to 10 ppm, whichcauses further oxidation of monochloramine, leading toNHCl 2 and then NCl 3 which decomposes to N 2 . The overallprocess of breakpoint chlorination converts ammonia togaseous nitrogen:2NH 3+ 3Cl 2N 2+ 6HClOther oxidizers such as potassium peroxymonosulfate can beused for pool shocking. Many pool instructions describeshocking as “burning the ammonia” out of the water, and inthe sense that it is an oxidation of nitrogen in ammonia, themetaphor is not misplaced.Measuring ChlorineTest kits for pool chlorine should be able to measure bothfree and total chlorine residual. N,N–diethylphenylenediamine,DPD, which forms a colored product due to oxidation,can be used for both measurements. DPD is oxidizeddirectly by free chlorine to form a colored radical speciesChloramines do not oxidize DPD. To measure total chlorine,I is added to the test solution. I is oxidized to I 3by chloramines,then I 3oxidizes DPD just as free chlorine does. Notethat there is no direct measure of combined chlorine.Maintaining Good BalanceSoft water or acidic water could leach minerals frommetal pipes or calcium-based pool liners. Such water is called“aggressive” or “corrosive”. Water high in calcium could precipitatecalcium carbonate, leading to cloudy pool water orclogged filters. Such water is called “scaling”. Water hardnessand alkalinity must be adjusted so that a close balance is maintainedbetween aggressive water and scale-forming water.Measuring Calcium HardnessWater hardness is primarily a measure of the calcium andmagnesium ion concentration in a water sample; since calciumis usually the predominant ion, hardness is expressed asa concentration of CaCO 3 . Calcium hardness (CH) is typicallymeasured by titrating a fixed volume of pool water withEDTA at a moderately basic pH using a metallochromic indicator,such as Murexide. A poolside procedure that doesn’trequire special-purpose apparatus is conveniently built arounda drop-count titration. A multiplier for converting the numberof drops to ppm CaCO 3 is readily determined by titrating aCaCO 3 standard. The CH of pool water should be between100–500 ppm CaCO 3 . CH can be increased by adding CaCl 2to the pool water; decreasing CH requires partial draining andrefilling with water of lower Ca concentration.Measuring Total AlkalinityTotal alkalinity (TA) is a measure of the buffer capacityof a water sample. The primary buffering component presentin swimming pool water is carbonate, so TA is primarily ameasure of dissolved carbonate. Note, however, that at neutralpH the predominant carbonate-containing species will be theHCO 3 ion. Like CH, TA is expressed as a concentration ofCaCO 3 . TA can be measured by a drop-count titration inwhich a fixed volume of pool water is titrated with standardacid (usually dilute H 2 SO 4 ) to the bromocresol green endpoint.An appropriate multiplier for converting the drop countto ppm CaCO 3 can be obtained by titrating a CaCO 3 orNaHCO 3 standard. (The CH in ppm CaCO 3 need not matchor bear any discernable relationship to TA in ppm CaCO 3 ;the two titrations are independent. In pool water calcium isusually present in higher molar concentration than carbonatebecause chloride is the predominate counter ion.) Pool watershould have a TA between 80–120 ppm CaCO 3 . Increase TAby adding sodium bicarbonate; decrease it by adding acid.1126 Journal of Chemical Education • Vol. 84 No. 7 July 2007 •

Chemistry for EveryoneMeasuring Total Dissolved SolidsTotal dissolved solids (TDS) is a measure of the ionicstrength of a water sample. While all the ions present in thepool water contribute to its TDS, primary contributors areusually Na and Cl , so TDS is ordinarily expressed as ppmof NaCl. Compared to CH and TA, TDS exerts a very weakeffect on calcium carbonate solubility; however, it can varywidely during the swimming season. For example, at the beginningof summer an outdoor pool filled with fresh well watermight exhibit a TDS value of a few hundred ppm NaCl. Ifsodium hypochlorite is used for disinfection, then over thecourse of several months the TDS could rise to 2000 ppmNaCl. The only way to lower TDS would be to drain the waterfrom the pool and start over with fresh well water. In generalit is necessary to make do with the TDS value one has.TDS is obtained by measuring the electrical conductivityof a water sample. The most convenient poolside method formaking this measurement is a commercially available, battery-poweredconductivity meter calibrated in ppm NaCl.Using Test StripsMany pool supply stores carry test strips that can simultaneouslymeasure pH, free and total chlorine, TA, and CH.Test strips are excellent for diagnosing pool problems, but areprobably not sufficiently accurate to prepare a chemical prescriptionfor correcting pool chemistry. Tests strips could beused to develop interesting laboratory experiments on simulatedpool water. The analytical chemistry and manufacturingtechnology behind these test strips deserve a separate paper.The Langelier Saturation IndexTo achieve balanced calcium carbonate solubility in yourpool water, you need the right mix of H , HCO 3 , and Ca 2 .The balance is judged using the Langelier saturation index(LI), which is the difference between the actual pH and thepH where CaCO 3 would neither dissolve nor precipitate.LI pH pH sThe pH of balanced solubility, pH s , is computed using amethod published by Langelier in 1936 for estimating thecorrosive or scale-forming potential of water in steam pipes(14, p 1194; 15, 16). The pH s is a strong function of TA andCH, while temperature and TDS affect pH s to a lesser extent.If the pool water’s LI is positive, the water is “scaling” andtends to precipitate calcium carbonate; if the LI is negative, thewater is “aggressive” and tends to dissolve calcium carbonate.Pool water should be balanced or slightly scale forming.Deriving the Langelier Saturation Index EquationThe following derivation explains the equation and tablesfor the calculation of pH s that appear in ref 14, p 1194.HCO 3−CaCO 3H + + CO 3−2Ca +2 + CO 3−2pK a2= 10.33pK sp= 8.35Table 1. Measurements and Values for Calculatingthe Langelier Saturation Index of Pool WaterParameterMeasureda See ref 14.Solve the first equilibrium for [H ] (since we seek pH s ). Solvethe second equilibrium for [CO 3 2 ] and substitute in the firstequation.+Hs=KKa 2sp−HCO 3+Ca 2CH and TA are expressed in ppm CaCO 3 ; the molar massof CaCO 3 is 100 g mol 1 . Substituting [Ca 2 ] 1 10 5CH and [HCO 3 ] 2 10 5 TA yields+HsK a2−10= 2 × 10 × TA × CHK sppHs = pK a 2− pK sp + 970 .− log TA − log CH10 10The effect of temperature is rolled into the combinedvalue of pK a2 pK sp in Table 1 of ref 14. The effect of ionicstrength is rolled into Table 2 of ref 14, which also containsthe combined conversion factors for CH and TA to molarityof calcium and bicarbonate (the constant 9.70 in the formulaabove). Table 3 of ref 14 contains logarithmic values for theconcentrations of TA and CH when they are expressed in ppm.Calculating the Langelier Saturation IndexLet’s assume that we have made measurements on poolwater and wish to compute its LI. Table 1 shows the assumedmeasurements and the corresponding values read from thetables in the Hach Handbook (14).For this sample of pool water, the pH s 2.00 9.83 1.70 1.76 8.37. If pH of the pool is 7.62, which isabout right for proper chlorination, then the LI 0.75, andthe pool water is moderately aggressive.To confirm that CaCO 3 should dissolve in this pool water,we must show thatCa +2Ca +2−2CO 3==MeasurementValues< Ksp Ksp = 3.3 × 1050 mg CaCO 3/L100 mg/mmolmmol−40.5 = 5 × 10 MLContributionto pHofBalanced SolubilitTemperature25°C 2.00TotalDissolved Solids200 ppm9.83Calcium Hardness 050 ppm1.70TotalAlkalinity 060 ppm1.76−9syaTA essentially measures [HCO 3 ], which is double the concentrationof the “CaCO 3 ” alkalinity. [HCO 3 ] 12 10 4 • Vol. 84 No. 7 July 2007 • Journal of Chemical Education 1127

Chemistry for EveryoneTable 2. Recommended Chemical Parametersfor Pool Water MaintenanceChemicalParameteraDimensionless, not presented in ppm.Using the carbonatebicarbonate equilibrium and thepH we can solve for the carbonate concentrationHCO 3−CO 3Ca +2DesiredRange, ppmpH7.4 –7. 6−2H + +−2CO 3K a2HCO −3= K a2+H−2CO 3aTo IncreaseParameter, Add:= 4.7 × 10 −11−= 22 . × 10 6−= 5 × 10 4 22 . × 10= 12 . ×10 − 9To DecreaseParameter, Add:Na2 CO3 HClFreeCl 1–3 OCl ; Dichlor—CalciumHardnessTotalAlkalinityTotalDissolvedSolidsCyanuricAcid100–500CaCl2 H 2 O80–120NaHCO3 HCl< 2000— H 2 O30–50Cyanuric acid;Dichlor< K sp−6Since the ion product is just below K sp , this water is notsaturated and will tend to dissolve calcium carbonate. Athigher pH there would be a higher carbonate concentration,and the water would be less corrosive. If the pH of the poolis already correct, then the appropriate action is to increasethe calcium concentration by adding CaCl 2 . Bieron et al. havedesigned experiments that lead students to compute the LIof pool water (3a).Conclusion: The Pool Doctor Says …There are certainly aspects of pool maintenance that havenot been covered in this paper. Calcium carbonate solubilityis not a serious issue for private pools constructed with vinylliners; however, vinyl liners have their own set of maintenanceconcerns such as puncturing, splitting, staining, and loss ofplasticizer. There are alternatives to chlorine disinfection—bromine can be used, and often is used in hot water spas becauseit is less likely to outgas. There are also some expensivedisinfection procedures based on silver and copper ions. Algaecan be a problem, and pool product suppliers sell a varietyof algaecides that maintain the attractiveness of pool water.But for most large swimming pools, maintaining chlorine andcalcium carbonate balance are the key concerns. We hope this—paper has provided a chemically informed view of the challengeof pool maintenance. To that end, we offer Table 2 asa summary of maintenance procedures.AcknowledgmentsWe thank David J. Wilson for his guidance in the preparationof the manuscript. We thank Bob Buettner and BobLowry for helpful advice, and we thank the anonymous refereesfor this Journal who suggested many improvements inthe manuscript. We also acknowledge the help of Leslie’sSwimming Pool Supplies of Phoenix, Arizona.W Supplemental MaterialAn annotated bibliography of swimming pool chemistryresources is available in this issue of JCE Online.Literature Cited1. Economic Impact of the Pool and Spa Industry. Associationof Pool and Spa Professionals: Alexandria, VA, 2005; (accessed May 2007).2. DeWindt, E. A. J. Chem. Educ. 1936, 13, 576.3. (a) Bieron, Joseph F.; McCarthy, Paul J.; Kermis, Thomas W.J. Chem. Educ. 1996, 73, 1021–1022. (b) Hardee, J. R.; Long,J.; Otts, J. J. Chem. Educ. 2002, 79, 633–634. (c) Clyde, D.D. J. Chem. Educ. 1988, 65, 911. (d) Donnelly, S. J. SwimmingPool Chemistry Lab. (accessed May 2007).4. Pinto, G.; Rohrig, B. J. Chem. Educ. 2003, 80, 41–44.5. Freiser, H.; Fernando, Q. Ionic Equilibria in Analytical Chemistry;John Wiley and Sons: New York, 1963; p 325.6. Harris, D. C. Quantitative Chemical Analysis; W. H. Freemanand Co: New York, 2002.7. Sawyer, C. N.; McCarty, P. L.; Parkin, G. F. Chemistry for EnvironmentalEngineering and Science, 5th ed.; McGraw-Hill:New York, 2003; Chapter 20, pp 571–586.8. Talaro, K. P.; Talaro, A. Foundations in Microbiology, 4th ed.;McGraw-Hill: New York, 2002; p 336.9. The Merck Index, 13th ed.; Merck & Co, Inc.: Rahway, NJ,2001; p 361.10. Standard thermochemical values for gas phase HOCl from CRCHandbook of Chemistry and Physics, 73rd ed.; CRC Press: BocaRaton, FL, 1992; p 5-12.11. Lowry, R. W. Pool Chlorination Facts; RJ Communications:New York, 2003; Chapter 18, pp 151–156.12. O’Leary, D. Chlorine. (accessed May 2007).13. Chemistry of Breakpoint Chlorination, Margerum Research LabWeb Page. May 2007).14. Water Analysis Handbook, 4th ed., 2nd rev.; Hach Company:Loveland, CO, 2003. (accessed May 2007).15. Langelier, W. F. Journal of the American Water Works Association1936, 28, 1500.16. Langelier Saturation Index Web Site. 2007).1128 Journal of Chemical Education • Vol. 84 No. 7 July 2007 •

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