Ideal ORP readings

[Retep]

In http://www.ncbi.nlm.nih.gov/pmc/article ... 6-0108.pdf, their conclusion was that

In short, it can not be claimed that a high redox potential could function as an absolutely sure yard stick of the disinfecting power of swimming-poolwater.

You forgot to post the next sentence after the one you quoted :

In short, it cannot be claimed that a high redox potential could function as an
absolutely sure yardstick of the disinfecting power of swimming-pool water. But
according to the results of this investigation, the error is less than that experienced
when using free available chlorine as a standard.

 

[chem geek]

(I was just writing this as you posted)
To be fair, fuller quotes are needed:

In short, it cannot be claimed that a high redox potential could function as an absolutely sure yardstick of the disinfecting power of swimming-pool water. But according to the results of this investigation, the error is less than that experienced when using free available chlorine as a standard.

and

Values from different analyses on the same sample showed relatively good correlation between the redox potential and the free available chlorine. In bath 3 both the redox potential and the available chlorine were weakly correlated to the bacterial count, but in bath 4 there was no such correlation. No other factors were well correlated with the bacterial count either.

and

The main reason for performing this investigation was to test whether the redox potential could be used instead of measurement of the available chlorine to estimate the bacteriological quality of swimming-pool water. Available chlorine is obviously not reliable in all waters, and in some cases not even a high amount of free available chlorine necessarily guarantees a germ-free water (bath 3). Considering that in most swimming pools probably only the total available chlorine is being measured (Black et al. 1970), the reliability may be very low if repeated bacteriological examinations are not performed. The proposed method of using certain minimum values of the redox potential of the water as guarantee for hygienic safety, however, seems to involve a lesser degree of uncertainty than the available chlorine does. Neither does a high redox potential necessarily mean a germ-free water in all cases, however, according to the present investigation.

This is, of course, consistent with what we have been saying since some of the water they used could have had CYA-like substances (and I am assuming that CYA itself was not used in any of these pools/"baths"), though also note that they did not measure HOCl directly. If they instead had any CYA in the outdoor pools (especially pool #3), then this paper was completely irresponsible to ignore that, but since this was done in Sweden I would presume that there was no CYA in these pools.

 

[Retep]

Richard,
You posted ( at 4:06 pm )- I edited at the same time, so I guess you did not read what I added to my posting:
Here it is what I added ->
(from the World Health Organisation) ->

5.12.4 Oxidation–reduction potential
Continuous control of the oxidation–reduction potential (ORP) is considered useful by many
experts. Together with pH and free chlorine, the ORP value gives an indication of the
disinfection efficiency (Carlson et al., 1968; Jentsch, 1973). In chlorinated pools, ORPs of 750
mV (for pH 6.5–7.3) and 770 mV (for pH 7.3–7.8), measured against a silver/silver chloride
reference electrode with potassium chloride electrolyte (Denecke and Althaus, 1986), are
required to guarantee the safe inactivation of microorganisms (Deutsches Institut für Normung,
1997a).

Here another link ( some people here will argue that this article was written by someone who also works for a company
which manufactures controllers. Just don't click on it. . . lol)
http://www.sbcontrol.com/ppmorp.pdf

 

[chem geek]

I did read that since that's where I got the "770 mV standard" I referred to. I have also read the other link you gave and referred to it in my ORP post you apparently didn't read. Jacques M. Steininger, who is the board chairman at Chemtrol talked about comparing ppm FC against ORP in commercial spa water that contained Cyanuric Acid (CYA) in it, but, he could have readily calculated the actual hypochlorous acid (HOCl) concentration given that they actually measured the CYA levels of the water. I did that and it showed that calculated HOCl was almost as good as ORP, but even more surprising was that a sort of the FC values showed FC to be an even better predictor than either HOCl or ORP. The reason is that bacteria are fairly easy to kill so it takes a fairly low chlorine level when taking measurements with bathers out of the water for such tests so having sufficient FC to not "run out locally" seems to be a more relevant factor, most especially in a spa that is high bather-load.

A far more comprehensive and extensive study was the Pinellas County, FL pool study of 486 pools. I analyzed those as well and found that the amount of chlorine needed to kill bacteria was so low that it was virtually impossible to distinguish between FC vs. calculated HOCl as the determining factor for water quality. Unfortunately, that study did not measure ORP. Nevertheless, the presence of pools with high FC and one with 5 ppm FC and no CYA having very high bacterial counts does show that in some pools there is more than just CYA that can affect chlorine effectiveness. It is for those situations where ORP would be better, but I'm saying that an HOCl sensor would be even better still.

Again, I am not saying that ORP doesn't have any value (especially in a relative, not absolute, sense), but that an HOCl sensor would be a more direct measurement of what is really desired and that in most cases a calculated HOCl value does reasonably well. This is why this works for determining levels for algae prevention since that requires a far higher HOCl level than needed to kill most bacteria faster than they can reproduce.

Also, and yes I'm a broken record on this point, even the various studies and references you give can't agree on a specific ORP level to use as a minimum -- it is 650 mV, is it 700 mV, is it 750 mV, is it 790 mV? Part of the reason they can't agree is that the studies used different ORP sensors and had water under different conditions. Look at my ORP post where even "Clifford White’s Handbook of Chlorination" has inconsistent ORP values (and even more distrubing -- slopes of ORP mV per doubling of FC) in different tables and charts in his book. So again, setting an absolute standard based on ORP seems a bit ridiculous except for an order-of-magnitude "you might check other factors" sort of indicator. Whoa is the pool operator who has a saltwater chlorine generator artificially lowering his ORP values by the hydrogen gas bubbles over-saturating the water (with respect to air) even though the HOCl is still high enough for fast disinfection. If that operator just went by ORP alone, then the pool might end up with 2 or 5 ppm FC with no CYA and be over 10 times higher in HOCl than necessary. Whoa is the pool operator who uses non-chlorine shock artificially raising the ORP but not increasing disinfection rates as significantly. If that operator just went by ORP alone, then the pool might end up with 0.05 ppm FC with no CYA and be 6 times lower in HOCl than desired.

Then there is the physics/chemistry: ORP measures a thermodynamic quantity -- oxidation potential. It does not measure reaction rates which can be completely different. Thermodynamics predicts that your body will decompose and get oxidized by air right before our very eyes -- it effectively doesn't because the reaction rates are too slow without catalysts or higher temperature to get around the high activation energy for the reaction. ORP does not measure kill times either. With HOCl, one at least has decent data on both kill times and oxidation rates for many pathogens and chemical substances. So having a direct measurement of HOCl makes a whole lot more sense than ORP.

 

[chem geek]

See this post for a user with practical experience with many ORP sensors that is consistent with what Jason and I are saying (also note Jason's excellent post right above it).