SIMPLIFIED ANSWER
The variation of ORP with pH has a lot to do with the change in pH in addition to the HOCl vs. OCl
- concentration. The following shows ORP at various pH with 1 ppm FC and no CYA (using Chemtrol ORP) in the first section and 1 ppm HOCl in the second section and constant pH in the third section. I also show the theoretical ORP from the Nernst equation which no real ORP sensor seems to track since among other things the Nernst equation depends on chloride (salt) concentration whereas real ORP sensors apparently aren't affected by salt concentration (explanation for this is in
this later post). Also, the mV per doubling of real sensors if much higher than the Nernst equation predicts as if the real sensors are only measuring 0.6 to 0.8 electrons instead of the expected 2.
...
Chemtrol ................................
Nernst
pH ..
ORP ..
HOCl ....
OCl- ....
FC ....
ORP
9.0 .. 541 ... 0.029 ... 0.971 .. 1.00 .. 1097
8.0 .. 716 ... 0.229 ... 0.771 .. 1.00 .. 1154
7.5 .. 768 ... 0.484 ... 0.516 .. 1.00 .. 1179
7.0 .. 805 ... 0.748 ... 0.252 .. 1.00 .. 1199
6.0 .. 862 ... 0.967 ... 0.033 .. 1.00 .. 1227
9.0 .. 749 ... 1.000 .. 33.65 . 34.65 .. 1143
8.0 .. 772 ... 1.000 ... 3.365 . 4.365 . 1173
7.5 .. 791 ... 1.000 ... 1.064 . 2.064 . 1188
7.0 .. 813 ... 1.000 ... 0.336 . 1.336 . 1203
6.0 .. 862 ... 1.000 ... 0.034 . 1.034 . 1227
7.5 .. 693 ... 0.048 ... 0.052 .. 0.10 .. 1149
7.5 .. 768 ... 0.484 ... 0.516 .. 1.00 .. 1179
7.5 .. 842 ... 4.845 ... 5.155 . 10.0 ... 1209
[EDIT] Even if one subtracts 230 mV from the Nernst ORP due to the silver/silver chloride reference electrode potential, it's still too high and has a smaller mV per doubling of HOCl concentration. [END-EDIT]
So for ORP sensors, they do vary more from HOCl than from OCl
-, but are also affected independently by pH. The theoretical Nernst equation has a 10x increase in HOCl or a 1.0 drop in pH (10x in H
+) be 29.6 mV (30 mV shown above), but for Chemtrol it's about 74 mV (implying 0.8 electrons) for 10x HOCl changes but only 41 mV for a 1.0 drop in pH.
Thermodynamically, one can't really say that HOCl is a stronger oxidizer than OCl
-. They are in equilibrium and there is a pH dependence on the reduction potential (it gets reduced, but oxidizes another chemical). One really refers to the HOCl/OCl
- system relative to Cl
- and it is the chlorine atom that is going from an oxidation state of +1 to -1.
Disinfection is an entirely different thing than oxidation because disinfection DOES depend on the charge because the cell surfaces are negatively charged so tend to repel the OCl
- while HOCl being neutral and looking a lot like water is able to penetrate cells more readily. That is the primary reason that HOCl can be considered to be the main disinfectant.
For oxidation, some chemical reactions are with OCl
- and not with HOCl. The slow oxidation of CYA by chlorine occurs faster at higher pH and is due to OCl
-, not HOCl. Also, ORP and thermodynamics says what CAN happen and not what WILL happen. Reaction kinetics determine what will happen. Based on thermodynamics alone, your body should be getting oxidized by the oxygen in the air, but fortunately the kinetics for that at normal temperatures is very slow.
DETAILED EXPLANATION
The oxidation potential does not have to do with electric charge. It has to do with what you wrote about the propensity to take electrons. With both HOCl and OCl
- it is the chlorine atom that changes in oxidation state going from +1 to -1 (taking electrons to become more negative) as shown below where I also show the standard reduction potentials (higher numbers are stronger oxidizers -- they themselves get reduced hence higher reduction potentials while the chemicals they react with get oxidized). At a constant pH, most of the thermodynamic change has to do with this chlorine atom changing its oxidation state.
..
+1 .......................
-1
HOCl + H
+ + 2e
- ---> Cl
- + H
2O ..... E
0 = +1.482V
.
+1 .........................
-1
OCl
- + H
2O + 2e
- ---> Cl
- + 2OH
- ..... E
0 = +0.81V
So at first glance one might look at the above and think that HOCl is a stronger oxidizer than OCl
- but what you need to understand is that the potentials are "standard" meaning each chemical species listed is at 1 mole/liter (M) concentration except for water that is its standard 55.5 M but since it doesn't change it is left out of equilibrium and Nernst equations. So the above is misleading because the higher reduction potential is actually not due solely to HOCl but rather to the low pH implied by the 1 M H
+ concentration which means a pH of 0. Likewise, the lower reduction potential in the second equation is not due to OCl
- but rather to the high pH implied by the 1 M OH
- concentration which means a pH of 14.
Let's calculate what the reduction potential for each equation would be at a pH near 7.5 where the concentrations of HOCl and OCl
- were equal and for simplicity I'll assume they are each 1 M even though that's much higher than found in pools (it doesn't change the argument).
E = E
0 + RT/zF ln(a
Ox/a
Red)
where R = 8.314 472(15) J K
−1 mol
−1, T = 298.15 (at 25ºC), F = 9.648 533 99(24)×10
4 C mol
−1 and z is the number of electrons transferred in the reaction so using these and converting to base 10 logarithm (so multiplying by ln(10) = 2.3026) we have
E = E
0 + 0.05916/z log
10(a
Ox/a
Red)
So for the first and second equations we have (using concentrations instead of activities for simplicity) at pH 7.5 (so [H
+] = 10
-7.5 and [OH
-] = 10
-6.5):
E = 1.482 + 0.05916/2 log
10([HOCl]*[H
+]/[Cl
-]) = 1.482 + 0.05916/2 log
10(1*10
-7.5/1) = 1.482 + (0.05916/2)*(-7.5) = +1.260V
E = 0.81 + 0.05916/2 log
10([OCl
-]/([Cl
-]*[OH
-]
2)) = 0.81 + 0.05916/2 log
10(1/(1*10
-6.5*2)) = 0.81 + (0.05916/2)*13 = +1.195V
You can see how close these two values are and this is because if one accounts for the fact that HOCl and OCl
- are never found in isolation because they are in equilibrium with each other then one finds that the reduction potential of the two species separately doesn't make any sense. In other words, from a thermodynamic sense, the reduction potential of the HOCl/OCl
- system relative to Cl
- varies as a function of pH where it is higher at lower pH but attributing that solely to HOCl rather than to H
+ isn't correct. In fact, at equilibrium, these potentials should be equal to each other and should give us the equilibrium constant for HOCl relative to OCl
- and H
+.
1.482 + 0.05916/2 log
10([HOCl]*[H
+]/[Cl
-]) = 0.81 + 0.05916/2 log
10([OCl
-]/([Cl
-]*[OH
-]
2))
22.718 = log
10([OCl
-*[Cl
-]/([Cl
-]*[OH
-]
2*[HOCl]*[H
+]) = log
10([OCl
-]/([OH
-]*[HOCl]*10
-14)
[OCl
-/([OH
-]*[HOCl]) = 10
22.718-14 = 10
8.718
[OCl
-]*[H
+]/[HOCl] = 10
8.718-14 = 10
5.282
That equilibrium constant is not correct since it should be closer to 10
7.5. If the reduction potential for the OCl
- equation were +0.8756V instead of +0.81V, then the potentials would be equal at a pH of 7.5 as expected for this system.
Another way to look at this is if one uses the Nernst equation to recalculate the reduction potential of the OCl
- reduction potential equation at a pH of 0 (i.e. same as the HOCl reduction potential equation), it would be +0.4141 higher so would be +1.22412. This is closer to +1.482V showing that most of the difference is really attributable to the different pH used for these two reactions. The rest of the difference is then related to the conversion between HOCl and OCl
-.
While the thermodynamic reduction potential is interesting, it doesn't really tell you whether a chemical reaction WILL occur but rather whether it CAN occur. For some chemical reactions it is HOCl that is dominant while for others it is OCl
-. So saying that HOCl is the stronger oxidizer only means that there are more chemicals that HOCl COULD oxidize compared to OCl
-, but not that they WILL. For example, the slow oxidation of Cyanuric Acid occurs faster at higher pH and involves OCl
- and not HOCl.
