ORP and pH Rise Over Hours in Sample

[rowdydoc]

Hi - I've been reading this Forum over the last few weeks and have got a lot food for thought from it.

However I have not been able to get my head around the behaviour of my pool water regards ORP and pH.

Firstly here is the current details etc:

pH is auto controlled to 7.4-7.6 via 32% HCl
CYA:26, TA:80-100, CH:200-250, Salt:4500, Temp:28-32degC(82-90degF),
Pool is a 55kL plaster finish, 24g/h SWC, pool blanket is on almost always except 2-3 h/day for swimming on good days.
The pool water looks clear - however I have had some "black spot" algae starting to show over the last few weeks.

I got excited a few weeks back and bought a Hanna Combo pH/ORP/Temp (I like the green colour!) and have been doing some testing to see if it is feasible to control my SWC on ORP.
After digesting the impact of the CYA on the effective HOCl (and realising that I needed higher FC levels than the pool shop tells me!) I started experimenting with the ORP vs FC (I took a pool sample at low FC and added diluted Cl and HCl progressively to map out ORP vs FC at constant pH). This went really well at first but I am now noticing that the ORP of samples is sometimes depressed significantly.

For example, with a FC of around 6-8ppm (DPD) in the pool around midday I got an ORP reading of 435mV at 30degC and 7.47pH. What is strange is that testing this sample on the bench inside shows that the ORP and pH progressively rise over a number of hours at about +50mV/h and 0.08pH/h. Eventually after about 6 hours the readings stabilised at 680mV and 7.95pH. Temperature of the sample dropped in the first hour to 27degC and stayed about there for the rest of the duration.

I initially blamed the Hanna instrument and sent it back but it has checked out OK and I get good stable readings of other things (like tap water and pH buffers) in 10 minutes max. For these readings I tended to leave the probe in the water for the duration so the readings should have been OK throughout.

Given the pH seems to rise along with the ORP - I am suspecting that this is a real chemistry going on rather than an instrumentation issue. However, I am struggling to understand what that chemistry may be.

Would love to hear from anyone who can pick what is going on!

PS While I am here - my pool blanket instructions say that the chlorine level should not be higher than 3ppm in order to not degrade the plastic. However the FC-CYA charts indicate that it would not be unusual with an SWC and 60-80ppm CYA to require > 3ppm as a matter of course. How should I interpret the manufacturer's statement? Should it be the equivalent of 3ppm FC at the associated CYA level?

 

[JasonLion]

Welcome to TFP!

ORP is complex, depending on a number of factors that aren't obvious at first and not all of which are well understood. Here are a couple of the factors you need to keep in mind.

ORP goes down by between 5 to 30 in direct sunlight, and by smaller amounts in indirect sunlight. After bringing a sample indoors from the pool it will take a little while to go back down (well under an hour). I don't understand the chemistry of this but I believe it has something to do with CYA.

ORP is lowered by dissolved hydrogen. The SWG cell produces hydrogen and some of that hydrogen will become dissolved in the water (the rest bubbles out). It takes from six hours to two days for the hydrogen to come out of solution. In many pools this effect is small, but occasionally it can be significant. I am not sure what effect hydrogen gas coming out of solution will have on the PH.

As disinfecting chlorine gets consumed, ie when the FC level goes down, the PH will go down. Bleach is basic, rising the PH. The consumption of disinfecting chlorine is acidic. The net effect is almost neutral, so if you are constantly adding bleach to maintain the FC level the PH won't shift very much. A sample left to sit out will slowly (faster in sunlight) lose chlorine and the PH will go down.

The pool blanket is sensitive to the disinfecting chlorine (HOCl) level. The concentration of HOCl varies with the CYA level. Chem Geek has a good discussion of HOCl vs CYA in this post. As the CYA level goes up the FC level the blanket can handle goes up because it takes a higher FC level to get the same HOCl level.

 

[Guest]

One word of caution on the Hanna combo...you get what you pay for. It is a relatively inexpensive combo meter and,IMHO, not the greatest when compared to more expensive controllers and meters. It is basically identical to the Eutech handlheld meters (I suspect it is a rebranded Eutech...they have a much bigger range of meters availabe)
The Hanna does seem to be a better meter than the LaMotte handhelds, however.

 

[chem geek]

Normal ORP sensors report a decrease in ORP as the pH rises. This is partly due to the lower hypochlorous acid concentration at higher pH (which isn't that much lower when CYA is present) and partly due to a pH dependence in the half-reaction with hypochlorous acid. However, some sensors show an opposite relationship, such as those from Sensorex and I've never figured out why. Different manufacturers not only have different absolute setpoints (bias), but even more strange is that they have different slopes -- mV ORP change with doubling of hypochlorous acid concentration. To me, this makes ORP very questionable as any sort of "absolute" standard and instead is really only useful for regulating FC levels at a fixed pH.

The pH of a water sample may rise if it outgasses carbon dioxide, but usually you won't see this happen quickly unless the TA level is very high (or the pH starts out low). As Jason points out, if the chlorine gets used up through breakdown from sunlight or oxidation of organics/urea/ammonia, then the pH will fall. If the chlorine outgasses as hypochlorous acid (more common in hot spas, uncommon at the lower temperature of pools), then the pH will rise. I suspect that for whatever reason, your sensor is taking an unusually long time to equilibrate in "real" pool water.

As for the pool blanket, who knows what CYA level the manufacturer was thinking about when they gave their 3 ppm FC limit. I wouldn't worry about the FC level and just target the recommended FC/CYA ratio per the charts and the degradation rate of your pool cover will be about as low as it can be while still keeping away algae. You already have an SWG so the FC/CYA ratio you need to maintain is already lower (FC 4.5% of CYA) than for non-SWG pools (FC 7.5% of CYA). If for some reason you find that the cover degrades faster than you would like, then you could use a supplemental algaecide, such as PolyQuat 60 on a weekly basis, and lower your chlorine level even further (say, to 2.5-3%), but that costs more and I really doubt it will be necessary.

As for the black algae, that is an indication that your chlorine level was probably too low because it's somewhat rare to see in well-maintained residential pools. Regular brushing will help, especially quite frequent brushing while exposing the algae to higher chlorine levels while you are trying to get rid of it.

Richard

 

[rowdydoc]

Thanks for the very prompt replies and useful feedback.

You've hit the nail on the head in terms of the use of the ORP - I just want to regulate the SWC on-off time to try to maintain consistent chlorine levels. I was hoping to use relative changes in ORP to do this, rather than try to use it as an absolute calibration of FC (or HOCl). One of my main motivations is that I want to reduce energy consumed by the pump by fitting a VVVF drive for the pump to run it for a longer time at lower flow. This approach fits micely with the on-off control of the SWC.

I see fairly consistent stabilisation of ORP readings in various other (non-pool) samples within 10-20 minutes. Also the pH tends to be a stable reading within 5 minutes. The fact that I get both the ORP and pH rising together over such a long period leads me to suspect that it may be the hydrogen gassing off from the water as JasonLion has suggested. If release of the hydrogen gas would increase the pH that is - are any of you able to confirm this is correct from the chemistry? So, the obvious questions for me to ask are whether:
1. the depressed ORP value corresponds to a depressed HOCl concentration, or
2. the hydrogen in solution lowers the ORP due to a process unrelated to HOCl concentration, or
3. there is no real ORP lowering and it is the hydrogen in solution casuing the ORP probe to read incorrectly.

Am I correct in understanding that the ORP reading should be a measure of the oxidising power of the solution? If this is so and the ORP reading is real, does this not mean that the oxidising process in my pool is compromised (despite the high FC reading)?

The other thing I just tried was stirring the pool water sample vigorously while measuring ORP and pH and it was quite dramatic! The slow drift of the ORP and pH upwards was accelerated - I could get a jump of 20-100mV and 0.05-0.2pH within 30 seconds depending on how vigorously I stirred. Once the stirring stopped the ORP held at the new value and continued the slow drift upwards again. Certainly the ORP seems to eventually stabilise at a value that seems roughly in line with my original "calibration" experiment for FC vs ORP.

Some thoughts: I guess I could confirm the Hydrogen gas theory by taking a sample, covering it, shaking it up for a few minutes and then lighting a match...I wonder if the solution of hydrogen in the water is related to my obsession with keeping the pool blanket on...

So if I can't make ORP work consistently - is there any other way to automate SWC operation on feedback?

Also, the recommended FC vs ORP in the sticky has FC's much higher than my DPD test kit. Is it reasonable to just dilute the sample for the DPD test and then calculate the real FC (eg 1 part pool water to 3 parts distilled water and then multiple the reading by 4)?

 

[JasonLion]

ORP is suggestive of the ability to keep the pool sanitary, but it does not correspond directly. For swimming pools the FC level appears to be more closely correlated with the ability to keep the pool sanitary, though it isn't perfect either. There has been a lot of debate about this over the years. As best I can tell ORP is a more accurate predictor of sanitation that FC in some interesting cases, but that over the complete range of conditions that may occur in a swimming pool ORP can occasionally have significant errors while FC remains reasonably well correlated. For example, dissolved hydrogen gas has no significant effect on pool sanitation yet it does affect the ORP reading significantly.

ORP is defined in an interesting way: "the degree of completion of a chemical reaction by detecting the ratio of ions in the reduced form to those in the oxidized form as a variation in electrical potential measured by an ORP electrode assembly." At some point an incorrect definition was substituted: "measurement of a body of water's ability to oxidize contaminants." It would be wonderful if there was a sensor that measured the second quantity, but there isn't.

The most common system, percentage based control (setting the percentage of the time that the pump is running during which the SWG cell is active), is a very reasonable approach to SWG automation for outdoor residential pools with low bather loads. Since sunlight consumes a percentage of the available chlorine, a fairly wide range of SWG percentages will all result in plausible FC levels. Once properly started up, manual intervention is required to adjust the percentage run time only every month or two, which is easy enough to do.

There exist sensors which measure the FC level directly. However they are very expensive, somewhere around $3000, so only large commercial pools tend to use them. It is also possible to create constrained situations where ORP sensors can operate with reasonable reliability, though again this tends to get expensive. For example the water can be maintained at it's hydrogen gas saturation level, at which point operation of the SWG no longer affects the amount of dissolved hydrogen gas. But that requires hydrogen injectors, hardly a common consumer item.

The DPD test can be used with dilution, so long as the water used for dilution does not contain any chlorine (tap water usually does). This will significantly reduce the precision of the measurement, as the reading error is multiplied by the same factor, but in some situations that is acceptable. A much better approach is to get a FAS-DPD chlorine test, which works to far higher chlorine levels and is inherently more precise to begin with. The FAS-DPD test is available from TF Test Kits and from Taylor.

 

[Guest]

ORP is a guide, not the gospel. the 650 mv level that is often quoted as the reading indicating sanitized water was really pretty much arbitrarily determined when ORP control first came into use. Keeping your pool cover on all the time will certainly have an impact on your readings. IMHO, you must leave the pool uncovered if you want to depend on ORP for atuomatic control.

JasonLion hit the nail on the head when he listed the definition of ORP...remember it stands for oxidation-reduction potential and has nothing inherently to do with chlorine levels or sanitizing ability. Interestingly enough ORP is often used in aquariums and is a general indicator of water quality but since sanitization does NOT play a part in aquarium maintenance (I am ignoring UV light here, which only used in certain systems) it it a more valid use. Interestingly enough, in a marine aquarium the ORP of the water has a direct relationship to the type of algae that is growing with the lowest ORP readings corresponding to red and bluegreen algae, then higher readings to brown and finally the higest ORP to green algae, IF you know anything about marine aquariums you know that red and bluegreen algae indicate problems in the tank while green algae is desirable.
Ozone and hydrogen peroxide (in very small doses) are the two most common ways to increase ORP in an aquarium so this really falls in line with the first definition that JasonLion listed.
In effect, low ORP readings are just indicating an abundance of free radicals (reducing compounds) in the water while higher ORP readings indicate a higher percentage of anti-oxidants (oxidizing compounds) in the water. The relationship between ORP and sanitizing ability was established by purely empirical evidence which is why, IMHO, ORP is NOT a reliable indicator of water quality for recreational water. Remember that this industry has many "things that are known to be true" that really aren't. As an example I give you 'slugging acid to lower TA' which is widely accepted in the industry but is NOT backed up by scientific study and is, in fact, false!

Bottom line is this.....automation will not eliminate the need to test your water on a regular basis. Once you get your SWG adjusted properly it really should not need much tweaking over the course of the year...perhaps an adjustment to pump run time as temperatures change. Using ORP to control generator output is, IMHO, a bit of overkill...sort of like cutting butter with a chainsaw!

The fact that you have developed black spot algae indicates that your FC levels have not been high enough for an extended period of time! Black algae takes a long time to develop and only develops when the FC has been too low during that period. I question why your CYA is so low with a SWG. You might want to read this stickyto see why the higher CYA levels are recommened with SWGs. Black algae takes high chlorine levels for an extended period of time and a LOT OF BRUSHING WITH A WIRE ALGAE BRUSH to kill! It can take as long as a month to completely eliminate but then again it probably took at least that long for it to start!

Also, you did not indicate but I assume your pool is outdoors. If so, remember that UV light is part of the equation in breaking down CC along with allowing gases to leave the waer and enter the atmosphere. Keeping the pool covered all the time is counterproductive to this. If you are keeping the pool covered to maintain heat then cover it at night to prevent heat loss by evaporation and uncover it during the day to allow the sun to heat the water to the fullest. This is the most effective way to maximize the heat retention in the pool.