swimming pool water buffer chemistry - … the chemistry and treatment of swimming pool and spa...

10 The Chemistry and Treatment of Swimming Pool and Spa Water

Swimming PoolWater Buffer Chemistry

John A. WojtowiczChemcon

Buffering is the resistance of water to pH change.Water itself has little buffering in the 5 to 9 pH range.Therefore, buffering agents are necessary to preventsevere pH fluctuations that would otherwise occurwhen acidic or basic sanitizers are added to poolwater. The presence of alkalinity in the form ofbicarbonate and cyanurate imparts buffering toswimming pool water. A buffer system consists of aweak acid and its anion (e.g., carbonic acid andbicarbonate or cyanuric acid and cyanurate). Abuffer system resists pH changes in either directionbecause it can react with either acidic or basicsubstances that are added to swimming pool water.However, because of the relatively low concentra-tions of buffering alkalinity (carbonate, cyanurate,or borate) in swimming pool water, the acid or baseneutralizing capacity without significant pH changeis limited. Although the buffers in swimming poolscan neutralize minor quantities of acidic or basicsubstances with only small pH changes, significantpH changes will occur if large quantities of sanitiz-ers (especially acidic) are added or if acid is added,eg, during pH adjustment. Maximum buffering oc-curs at a pH where the molar ratio of acid to anionis one. At 80°F and 1000 ppm TDS maximumbuffering occurs at pH 6.3 and 6.8 for the carbonicacid/bicarbonate and cyanuric acid/cyanurate sys-tems, respectively. By contrast, maximum bufferingin the boric acid/borate system occurs at pH 9.2. Ona molar basis, the cyanuric acid/cyanurate systemprovides more effective swimming pool water buffer-ing at pH 7.5, 80°F, and 1000 ppm TDS because itspH of maximum buffering is closer to pool pH.However, on a ppm basis, the buffer intensity of thecarbonic acid/bicarbonate and cyanuric acid/cya-nurate systems are roughly comparable over therecommended pH range (7.2-7.8) and greater thanthat of borate at pH < 7.8. At pH 7.8, the buffering ofthe three systems are roughly comparable on a ppm

BufferingBuffering is the resistance of water to pH

change. Water itself has virtually no buffering inthe 5 to 9 pH range because of the low concentra-tions hydrogen (H+) and hydroxyl ions (OH–). Thus,buffers are necessary to prevent severe pH fluctua-tions when acidic or alkaline substances are addedto pool water. For example, addition of only 3 ppmof gaseous chlorine to pure water at 25°C woulddecrease the pH from its normal value of 7.0 to 4.4due to the hydrogen ions formed by hydrolysis ofchlorine (Cl2 + H2O H+ + Cl– + HOCl). Bycontrast, if 100 ppm of bicarbonate alkalinity waspresent in the water at the same pH, the pH wouldhave decreased to only 6.95. This is due to the factthat bicarbonate reacts with hydrogen ions releasedby ionization of the hydrochloric acid (formed byhydrolysis of chlorine) minimizing the change inpH. A solution whose pH changes on addition of acidor base by much less than pure water is consideredto be buffered.

Buffer SystemsA buffer system typically consists of two spe-

cies, e.g., a weak acid and its anion (Wojtowicz1995b). The acid is capable of reacting with basichydroxyl ions and the anion with acidic hydrogenions, e.g., carbonic acid and bicarbonate ion. The pHof a buffer depends on the ratio of acid to anion. Thiscan be shown by mathematical analysis of the ion-ization of carbonic acid into hydrogen and bicarbon-ate ions.

H2CO3 H+ + HCO3–

The ionization equilibrium constant (Ka) is givenby:

basis. At and above pH 7.8, borate significantlysupplements swimming pool buffering.

Originally appeared in theJournal of the Swimming Pool and Spa IndustryVolume 3, Number 2, pages 34–41Copyright © 2001 by JSPSIAll rights of reproduction in any form reserved.

John A. Wojtowicz – Chapter 1.2 11

Acid

carbonic, H2CO3

cyanuric, H3(NCO)3

boric, B(OH)3

Table 1 – Buffer Systems Important in Swimming Pools

Anion

bicarbonate, HCO3–

cyanurate, H2(NCO)3–

orthoborate, B(OH)4–

pH of Maximum Bufferingat 80°F and 1000 ppm TDS

6.36.89.2

Ka = [H+][HCO3– ] / H2CO3

The following equation for the pH of a buffer isobtained by taking logs and rearranging.

pH = pKa + log [HCO3– ] / [H2CO3]

Maximum buffering occurs at a pH where the molarratio of acid to anion is one. This pH is also equal tothe pKa of the acid which is the negative logarithmof the ionization constant. Below this pH, the baseneutralizing capacity exceeds the acid neutralizingcapacity, whereas, above this pH, the reverse istrue. The practical pH range of a buffer is one pHunit above or below the optimum pH. Examples ofbuffer systems important in swimming pools areshown in Table 1. Table 1 shows that cyanuric acidis the better buffer because its pH of maximumbuffering is closer to the swimming pool pH range.

Since bicarbonate ion has a replaceable hydro-gen, it forms a second buffer system in combinationwith carbonate ion, which exhibits maximum buff-ering at pH 10.1. By contrast with carbonic acid,cyanuric and boric acids have three ionizable hydro-gens and thus can potentially form three buffersystems (see previously published ionization dia-grams for carbonic and cyanuric acids, Wojtowicz1995a). However, only the first ionizations are ofpractical importance in pools.

The pH of human blood is maintained at 7.4 by

a mixture of bicarbonate and phosphate buffers. Atfirst glance the phosphate system, consisting ofmonophosphate (H2PO4

–) and diphosphate (HPO42–)

ions, appears potentially attractive for pools be-cause it exhibits maximum buffering at pH 7.2.However, closer examination reveals that phos-phate promotes the growth of algae and can alsocause cloudy water because of precipitation of cal-cium phosphate which can occur at relatively lowphosphate concentrations (~5 ppm). Therefore, phos-phates are not recommended for swimming pooluse.

Hypochlorous acid and hypochlorite ion alsoform a buffer system, with maximum buffering atpH 7.5. However, the concentration is too low toexert significant buffering. Mixtures of bases andtheir salts can also serve as buffers, e.g., ammoniaand ammonium ion. However, this would not bepractical for use in pools.

Strength of Buffering (Buffer Intensity)The strength of buffering is measured by the

buffer intensity (β) which is the incremental changein alkalinity required to change pH by one unit, i.e.,β = ∆Alk/∆pH. For a diprotic acid such as carbonicacid and its salts it is expressed numerically by:

β = 2.3{CT[α1(α0 + α2) + 4α0α2] + OH–

+ H+}•50•103 ppm CaCO3

at pH 7.530.412.94.6

Buffer

cyanuratebicarbonate

borate

Concentration, M

0.0020.0020.002

Maximum*57.557.659.3

Buffer Intensity (ppm CaCO3)

*At pH of maximum buffering shown in Table 1.Table 2 – Buffer Intensity of Swimming Pool Buffers


where: CT is the total molar concentration of car-bonic acid, bicarbonate, and carbonate and α0, α1,and α2 are the respective ionization fractions. Al-though buffer intensity is usually expressed asequivilents per liter, it is expressed here and else-where as ppm CaCO3, similar to alkalinity. For amonoprotic acid and its salt it is given by thefollowing equation:

β = 2.3(CTα0 α1 + OH– + H+)•50•103 ppm CaCO3

Although cyanuric acid is a triprotic acid (i.e., it hasthree ionizable hydrogen atoms), the ionization ofonly the first hydrogen is important at pool pH.Thus, the above equation can be used to calculate itsbuffering effect; where: CT is the sum of the concen-trations of cyanuric acid and cyanurate ion and α0and α1 are the respective ionization fractions. Thisequation can also be used for boric acid. In the pHrange important to swimming pools, the contribu-tion of hydrogen and hydroxyl ions to the bufferintensity can be neglected because of the low con-centration of these ions. For example, the bufferintensity of water itself at pH 7.5 is equivalent toonly 0.04 ppm CaCO3.

Buffers for laboratory use typically have con-

centrations in the 0.02–0.05 molar range. By con-trast, the concentration of buffering agents in swim-ming pools is much lower, consequently the buffer-ing effect is correspondingly less; e.g., 100 ppm ofbicarbonate alkalinity is equivalent to only 0.002molar. Calculated buffer intensities for 0.002 Mbicarbonate, borate, and cyanurate buffers at pH7.5, 80°F, and 1000 ppm TDS are tabulated inTable 2.

Because the pH of maximum buffering of thetypical buffering agents found in swimming poolslies outside the normal pH range of pools, thebuffering strength is less than optimum. On a molarbasis, cyanurate has the higher buffer intensity ofthe three buffers listed at pool pH.

Variation of Buffer Intensity with pHBuffer intensity increases not only with the

concentration of carbonate, cyanurate, and boratealkalinity but it also varies with pH because theratio of acid to anion changes with pH. This is showngraphically in Figure 1 where buffer intensity isplotted as a function of pH. By contrast to water, thebuffer intensity of weak acid–derived alkalinity(i.e., carbonate, cyanurate, and borate) is signifi-cant in the 5–9 pH range. The graph shows that the

Figure 1 – Variation of Buffer Intensity with pH

0

5

10

15

20

25

30

7 7.2 7.4 7.6 7.8 8 8.2

pH

Buf

fer I

nten

sity

CARBONATE CYANURATE BORATE


pH7.07.27.47.67.88.08.2

Table 3 – Variation of Buffer Intensity with pH80°F, 1000 ppm TDS, acid + anion concentration 100 ppm

Borate1.21.93.04.77.311.016.3

Cyanurate21.218.214.310.67.45.13.4

Carbonate25.018.012.58.76.24.94.5

Table 5 – pH Changes due to Alkaline Sanitizers in Typical Pool WaterpH 7.5, 80°F, Carb. Alk. 100 ppm, Cyanuric Acid 100 ppm, TDS 1000 ppm

HypochloriteSodiumSodiumCalciumCalcium

∆pH/week+0.02+0.04+0.01+0.02

oz/week56.0106.510.520.0

Treatmentmaintenance

shockmaintenance

shock

HypochloriteSodiumSodiumCalciumCalcium

∆pH/week+1.5+1.7+1.2+1.4

oz/week56.0106.510.520.0

Treatmentmaintenance

shockmaintenance

shock

Table 4 – pH Changes due to Alkaline Sanitizers in Unbuffered WaterpH 7.0, 80°F

buffer intensity of carbonate and cyanurate alkalin-ity decrease with increasing pool pH. By contrast,the buffering intensity of borate alkalinity increaseswith increasing pool pH. Both carbonate and cyanu-rate systems provide roughly comparable bufferingover the entire pool pH range (see Table 3). Com-pared to either carbonate or cyanurate alkalinity,the buffering effect of borate is significantly lessbelow pH 7.8, but greater above pH 7.8. Thus,borate significantly supplements the buffering ofpool water at pH ≥ 7.8.

Buffer ChemistryAcid or Base Neutralizing Capacity of

Buffers – Because of the relatively low concentra-

tions of buffering alkalinity in swimming pool wa-ter, the acid or base neutralizing capacity withoutsignificant pH change is limited. The carbonic acid/bicarbonate system has a greater capacity for neu-tralizing acid than base because the ratio of bicar-bonate ion to carbonic acid is about 17 at pH 7.5,80°F, and 1000 ppm TDS. Although the buffers inswimming pools can neutralize minor quantities ofacidic or basic substances with only small pHchanges, significant pH changes will occur if largequantities of sanitizers (especially acidic) are addedor if sufficient acid is added, e.g., during pH adjust-ment.

Effect of Basic Species – The carbonic acid/bicarbonate system is one of the simplest buffers. Ifa strong base (e.g., NaOH present in small amounts


in liquid bleach or Ca(OH)2 present in smallamounts in calcium hypochlorite) is added to watercontaining carbonic acid and bicarbonate ion, theOH– ions (formed by ionization of NaOH or Ca(OH)2)will be neutralized by carbonic acid according to thefollowing reaction:

OH– + H2CO3 H2O + HCO3–

Since the equilibrium for this reaction lies far to theright, the added hydroxyl ions are completely re-moved. There is a small decrease in the carbonicacid concentration and a small increase in thebicarbonate ion concentration, consequently thereis a small increase in pH. Cyanuric acid functionssimilar to carbonic acid:

OH– + H3(NCO)3 H2O + H2(NCO)3–

Table 4 shows pH changes due to maintenanceand shock treatment doses of calcium and sodiumhypochlorite in unbuffered water at an initial pH of7.0 and 80°F. The calculated data show very large

pH changes in the absence of buffering.Table 5 shows calculated pH changes in buff-

ered pool water due to maintenance and shocktreatment doses of calcium and sodium hypochlo-rite at an initial pH of 7.5, 80°F, 100 ppm carbonatealkalinity, 100 ppm total cyanurate, and 1000 ppmTDS. The pH changes are only a fraction (i.e., 1–2%)of those in unbuffered water.

Effect of Alkaline Species – Although thelow concentration of strong base present in hy-pochlorite sanitizers has a small effect on swim-ming pool pH, the hypochlorite ion provided bythese sanitizers exerts a much greater effect on pHbecause of its higher concentration. Hypochloriteion will increase pH by consuming hydrogen ions asshown below.

ClO– + H+ HOCl

At normal av. Cl levels this has a small effecton pH, e.g., ∆pH = 0.04 at pHi = 7.5, 80°F, av. Cl dose3 ppm, 100 ppm carbonate alkalinity, 100 ppmcyanuric acid, and 1000 ppm TDS. However, atshock treatment doses (i.e., 10 ppm av. Cl), the pH

Figure 2 – pH Change Due To Hypochlorite IonDuring Shock Treatment

0

0.1

0.2

0.3

0.4

0.5

0.6

40 60 80 100 120

Carbonate Alkalinity (ppm)

pH C

hang

e

CA 0 ppm CA 80 ppm CA 120 ppm


Carb. Alk., ppm4080

120

CA =100 ppm0.170.140.11

CA = 50 ppm0.260.190.15

CA = 0 ppm0.520.300.21

*Includes small contribution from Ca(OH)2 and CaCO3.

∆pH*

Table 6 – pH Changes During Shock Treatment With Ca(ClO)2pHi 7.5, 80°F, Av. Cl dose 10 ppm, 1000 ppm TDS

Sanitizerchlorine gas

TrichlorDichlor

oz/week6.87

10.5

∆pH/week–0.22–0.14–0.09

Table 8 – Calculated pH Changes Due to Acidic SanitizerspH 7.5, 80°F, Carb. Alk. 100 ppm, Cyanuric Acid 100 ppm, TDS 1000 ppm

Carbonate Alkalinity, ppm05050100100

Total Cyanurate, ppm00500

100

∆pH/week–3.7–0.55–0.38–0.33–0.22

Table 7 – Calculated pH Changes for Maintenance Doses of ChlorinepH 7.5, 80°F, Cl2 6.8 oz/week, TDS 1000 ppm

change would be significantly higher (i.e., ∆pH =0.12) under similar conditions. However, this wouldonly be a temporary pH change because decomposi-tion would convert all of the original hypochloriteion to chloride ion which has no effect on pH. Table6 shows calculated pH changes due to a shocktreatment dose of hypochlorite under different con-ditions of buffering. The data are also shown graphi-cally in Figure 2.

When sodium tetraborate (i.e., borax Na2B4O7)is added to water it ionizes to sodium and tetraborateions:

N a 2B4O7 → 2Na+ + B4O72 –

The tetraborate ion is a polynuclear complex thathydrolyzes to an equilibrium mixture of boric acidand orthoborate ion.

B4O72 –

+ 7H2O 2B(OH)3 + 2B(OH)4–

At low concentrations (≤ 0.025 M, ≤ 5035 ppmNa2B4O7), the equilibrium lies completely to theright (Cotton 1988). Thus, tetraborate will raisepool pH because one of its hydrolysis productsneutralizes hydrogen ions via the following reac-tion:

B(OH)4– + H+ B(OH)3 + H2O

The equilibrium for this reaction lies far to theright at pool pH. The pH change will depend on theconcentration of tetraborate added. Addition of largequantities can have a large effect on pH. For ex-ample, addition of 100 ppm of Na2B4O7•5H2O to pool


Carbonate Alkalinity, ppm404040808080120120120

Cyanuric Acid, ppm050100050100050100

∆pH/week+0.17+0.07+0.05+0.44+0.24+0.16+0.68+0.41+0.30

Table 9 – pH Changes due to Loss of Carbon DioxidepHi 7.5, 80°F, 1000 ppm TDS

water at pH 7.5 and 80°F with 100 ppm carbonatealkalinity, 100 ppm cyanuric acid, and 1000 ppmTDS would raise the pH to 8.7.

Effect of Acidic Species – When chlorine isadded to pool water, it hydrolyzes to a mixture ofhydrochloric and hypochlorous acids. Decomposi-tion of hypochlorous acid produces more hydrochlo-ric acid. Thus, all of the added chlorine is eventuallyconverted to hydrochloric acid. Ionization of hydro-chloric acid produces hydrogen ions which react

with bicarbonate ion to form carbonic acid as shownbelow.

H+ + HCO3– H2CO3

The equilibrium for this reaction lies far to theright. Thus, all of the hydrogen ions introduced bythe hydrochloric acid are neutralized. A decrease in

Figure 3 – Effect of Alkalinity and CA on pH Drift Due to CO2 Loss

00.1

0.20.30.4

0.50.6

0.70.8

0 20 40 60 80 100 120

Cyanuric Acid (ppm)

pH D

rift/w

eek

ALK 40 ppm ALK 80 ppm ALK 120 ppm


pH and alkalinity will occur, the extent dependingon how much HCl is formed, a shock treatment dosecausing a larger change than a maintenance dose.

Other acid forming sanitizers are Dichlor andTrichlor. On complete decomposition they form amixture of hydrochloric and cyanuric acids(Wojtowicz 1995b). Since cyanuric acid does notionize completely as does hydrochloric acid, it hassomewhat less effect on pH. Table 7 shows theweekly change in pH for maintenance doses ofchlorine (6.8 oz/week) with and without buffering at80°F, initial pH 7.5, and 1000 ppm TDS. The calcu-lated data show that the pH change decreases withincreasing concentration of carbonate and cyanu-rate alkalinity.

Table 8 shows the weekly change in pH causedby acidic sanitizers. The calculated data show thaton a daily basis, that the pH changes are quite smalldue to the buffering action of pool water. However,a single shock treatment dose (13 oz) of chlorinewould result in a pH change of –0.37, a greaterchange than a weeks worth of maintenance doses.

Effect of Carbon Dioxide Loss – Anotherfactor that affects pH is loss of carbon dioxide(Wojtowicz 1995c). Loss of carbon dioxide raises pHbecause it decreases the ratio of carbonic acid tobicarbonate ion. The calculated pH drift, based ontest data for an outdoor pool without bathers for a20,000–gal pool (6 feet average depth) at pH 7.5,80°F, and 1000 ppm TDS, and a turnover rate of0.84 (8–hr pump duty cycle and 35 gal/min pumpingrate) are given in Table 9. The pH in all of these

examples will tend to level off at pH 8.3. The calcu-lated data illustrate the buffering effect of cyanuricacid with a decreasing pH rise with increasingcyanuric acid concentration at constant carbonatealkalinity (see Figure 3). By contrast, the pH riseincreases with increasing carbonate alkalinity atconstant cyanuric acid concentration. This is due tothe fact that the CO2 loss rate varies with the squareof the carbonate alkalinity, whereas, buffering var-ies with the first power of carbonate alkalinity.

References

Cotton, F. A. and G. Wilkinson. Advanced InorganicChemistry, 6th ed. New York, John Wiley andSons, Inc., 1988.

Wojtowicz, John A. “Swimming Pool Water Balance– Part 1: The Effect of Cyanuric Acid and OtherInterferences on Carbonate AlkalinityMeasurement”. Journal of the Swimming Pooland Spa Industry 1:1(1995a):7–13.

Wojtowicz, John A. “Swimming Pool Water Balance– Part 2: Factors Affecting the CalciumCarbonate Saturation Index”. Journal of theSwimming Pool and Spa Industry 1:2(1995b):9–15.

Wojtowicz, John A. “Swimming Pool Water Balance– Part 3: Factors Affecting Loss of CarbonDioxide”. Journal of the Swimming Pool andSpa Industry 1:3(1995c):19–26.

swimming pool water buffer chemistry - … the chemistry and treatment of swimming pool and spa...

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