Disinfection with chlorine or chloramine involves a series of oxidation reactions (that is, electrons are removed). In the water chemistry literature, these are written as reductions (that is electrons are added); the difference is that the signs are reversed for the free energy change and for the equilibrium constant. See oxidation-reduction pdf.
There is some confusion in the literature about the values for these equilibrium constants, and it is important to use an internally-consistent set. Trogolo & Avery (2017) have used theoretical models to develop a consistent set of values for the free energy of formation of chlorine and chloramine compounds:
| Compound | ΔGmeas | ΔGmodel |
| Cl2(g) | 0 | 0 |
| Cl2(aq) | 6.95 | 6.95 |
| Cl- | -131.26 | -131.26 |
| ClO- | -36.27 | -32.48 |
| HOCl | -79.89 | -76.1 |
| H2O | -237.14 | -238.3 |
| OH- | -157.33 | -158.4 |
| NHCl2 | 176.6 | 179.3 |
| NH2Cl | 66.9 | 67.1 |
| NH3 | -26.6 | -24.3 |
| NH4+ | -79.36 | -76.8 |
| Mn2+ | -228.1 | |
| MnO2 | -456.62 | |
| MnO4- | -447.2 | |
| MnOH+ | -405 | |
| Mn(OH)3- | -744.2 | |
| Fe3+ | -4.6 | |
| FeOOH | -488.55 | |
| Fe2+ | -78.87 |
Notes: The error limits are generally +/- 6 kj/mole. Some values of N compounds have been adjusted away from the mean value but within this band to give diagrams that match more closely observed behavior.
These free energy values can then be used to calculate equilbrium constants for reactions of interest among these compounds, which in turn can be used to contruct Eh-pH diagrams.
Reaction (expressed as a reduction) |
Free Energy of Reaction ΔGr(kj) | Log K | Standard electrode potential Eo, volts | |
| 1 | Cl2(g) + 2e- = 2Cl- | -262.52 |
46.00 |
1.360 |
| 2 | Cl2(aq) + 2e- = 2Cl- | -281.36 |
49.30 |
1.458 |
| 3 | Cl2(g) = Cl2(aq) | 6.95 |
-1.22 |
na |
| 4 | 2HOCl +2H+ + 2e- = Cl2(g) + 2H2O | -324.40 |
56.84 |
1.681 |
| 5 | 2HOCl +2H+ + 2e- = Cl2(aq) + 2H2O | -310.50 |
54.41 |
1.609 |
| 6 | HOCl + H+ + 2e- = Cl- + H2O | -293.46 |
51.42 |
1.521 |
| 7 | ClO- + 2H+ + 2e- = Cl- + H2O | -337.08 |
59.06 |
1.747 |
| 8 | ClO- + H2O + 2e- = Cl- + 2OH- | -177.28 |
31.06 |
0.919 |
| 9 | HOCl + 2e- = Cl- + OH- | -213.56 |
37.42 |
1.107 |
| 10 | HOCl = ClO- + H+ | 43.62 |
-7.64 |
na |
| 11 | O2 + 4H+ + 4e- = 2H2O | -476.6 |
83.51 |
1.235 |
| 12 | 2H+ + 2e- = H2 | 0.00 |
0.00 |
0.00 |
| 13 | H2O = H+ + OH- | -79.90 |
14.00 |
na |
| 14 | NHCl2 + 3H+ + 4e- = NH4+ + 2Cl- | -518.62 |
90.87 |
1.344 |
| 15 | NH2Cl + 2H+ + 2e- = NH4+ + Cl- | -275.16 |
48.21 |
1.426 |
| 16 | NH2Cl + H+ +2e- = NH3(aq) + Cl- | -222.66 |
39.02 |
1.154 |
| 17 | NH2Cl + H2O +2e- = NH3(aq) + Cl- +OH- | -142.76 |
25.01 |
0.740 |
| 18 | NH4+ = NH3(aq) + H+ | 52.50 |
-9.2 |
na |
| 19 | NHCl2 + H+ +2e- = NH2Cl + Cl- | -243.46 |
42.66 |
1.262 |
| 20 | NH2Cl + H2O = NH3 + HOCl | 70.80 |
-12.41 |
na |
| 21 | NH3(aq) + HOCl = NH2Cl + H2O | -65.82 |
11.53 |
na |
| 22 | NH2Cl + HOCl = NHCl2 + H2O | -62.40 |
10.93 |
na |
| 23 |
Notes: Reaction 7, the acid form of the reaction, is often cited in the literature with an Eo of 0.9 volts, based on Glaze (1990). However this value is substantially in error. Instead, this value applies to the basic form, as written in reaction 8.
References to chlorine-chlormine thermodynamics
Copeland, A., & Lytle, D. A. (2014) Measuring the oxidation-reduction potential of important oxidants in drinking water. Journal: American Water Works Association, 106(1), E10-E20.
Glaze, W.H. (1990) Chemical Oxidation. In: Water Quality and Treatment: A Handbook of Community Water Supplies.4th ed. McGraw-Hill, New York.
Radepont, M. (2013) Understanding of chemical reactions involved in pigment discoloration, in particular in mercury sulfide (HgS) blackening (Order No. 3574018). Available from ProQuest Dissertations & Theses A&I; ProQuest Dissertations & Theses Global. (1450071109). Retrieved from https://search.proquest.com/docview/1450071109?accountid=2909
Radepont, M., Coquinot, Y., Janssens, K., Ezrati, J. J., de Nolf, W., & Cotte, M. (2015) Thermodynamic and experimental study of the degradation of the red pigment mercury sulfide. Journal of Analytical Atomic Spectrometry, 30(3), 599-612.
Rajasekharan, V. V., Clark, B. N., Boonsalee, S., & Switzer, J. A. (2007) Electrochemistry of free chlorine and monochloramine and its relevance to the presence of Pb in drinking water. Environmental science & technology, 41(12), 4252-4257.
Trogolo, D. and Arey, JS (2017) Equilibria and Speciation of Chloramines, Bromamines, and Bromochloramines in Water. Environmental Science & Technology 2017 51 (1), 128-140. DOI: 10.1021/acs.est.6b03219
Vasquez, Ferdinand (2005) "The Effect Of Free Chlorine And Chloramines On Lead Release In A Distribution System" Electronic Theses and Dissertations. 411. http://stars.library.ucf.edu/etd/411
Vasquez FA, Heaviside R, Tang Z, Taylor JS. (2006) Effect of free chlorine and chloramines on lead release in a distribution system. J Am Water Works Assoc. 98(2):144–154.
Xie, Y. (2010) Dissolution, formation, and transformation of the lead corrosion product lead dioxide: Rates and mechanisms of reactions that control lead release in drinking water distribution systems (Order No. 3418401). Available from ProQuest Dissertations & Theses A&I; ProQuest Dissertations & Theses Global. (750983357). Retrieved from https://search.proquest.com/docview/750983357?accountid=2909
Xie, Y., Wang, Y., & Giammar, D. E. (2010) Impact of chlorine disinfectants on dissolution of the lead corrosion product PbO2. Environmental science & technology, 44(18), 7082-7088.
.