DonR6702

Richard, you can in fact strip dissolved carbon dioxide from water with aeration, as you can strip almost any dissolved gas from water. It is done all the time in industry. Carbon dioxide is different from some gases in that it reacts with water, forming an equilibrium between carbon dioxide and carbonic acid, but the stripping of carbon dioxide from water is not dependent on that equilibrium. It does, in fact, disrupt that equilibrium, which is quite weak at normal spa pH values. The great majority of carbon dioxide in water exists as the dissolved gas at pH 7.5. Stripping of gases via aeration is a purely mechanical process, so it can't be described by equilibrium equations. With vigorous enough aeration for a long enough time you can strip out practically all of the carbon dioxide, including that which is in the carbonic acid form - in this case, the equilibrium actually aids the stripping process because as carbon dioxide gas is purged from the water, carbonic acid dissociates to produce more carbon dioxide, which is subsequently stripped from the water. This is why the pH goes up when you aerate water. With moderately alkaline water having moderately high calcium hardness, it is possible to raise the pH high enough, with aeration only, to precipitate calcium carbonate. Your statement: is quite confusing. I guess it indicates that your experience is limited to pool and spa water starting from water with relatively low alkalinity. Water chemistry is much more complex than that. I just refilled my spa, and the tap water had pH 8.0 with 35 ppm alkalinity. This pH is higher than normal for my location, but the alkalinity is right in line. My tap water normally runs from pH 6.8 to 7.5, with alkalinity between 30 and 40. I have been testing and logging my tap water chemistry for years (for aquaria use as well as for my spa), and the alkalinity has never been below 30, while the average pH is about 7.2. There are some parts of the US where tap water alkalinity runs to 150 ppm or higher. There is no "normal" value of alkalinity in water, and your term "over-carbonated" is misleading. The carbonate species in spa water are in equilibrium by definition except when steps have recently been taken to make adjustments. In some parts of the US it is necessary to lower the alkalinity of tap water after adding it to a pool or spa. This water, even if the natural alkalinity approaches 200 ppm, is not "over carbonated" or "over-saturated" with carbon dioxide. It is in fact in equilibrium, and the amount of dissolved carbon dioxide is very low compared to the bicarbonate and carbonate species. If the spa water was "over-saturated" with carbon dioxide, you wouldn't need to aerate in order to purge the carbon dioxide, it would happen naturally. After you strip carbon dioxide, or any other gas present in the atmosphere, from water by vigorous aeration, those gases diffuses back into the water and dissolve according to Henry's Law and Graham's Law. In most cases this is a slow process, though it's faster for carbon dioxide than for many other gases (19 times faster than oxygen, for example). Carbon dioxide continues to diffuse into the water from the atmosphere and to dissolve until equilibrium is reached, and the pH drops. If you don't believe it, don't take my word for it, try it. Measure your spa's pH, then aerate vigorously for an hour or so. Measure the pH - it will have risen, of course. Then let the spa sit, with no chemical additions and normal water circulation without air injection for 24 hours or so. The pH will have fallen back towards the first value. How could it be any other way, when the alkalinity and temperature remain unchanged?

Good analysis, Richard. I would add one thing to it which you mentioned in one of its contexts but failed to mention in the other context - the equilibrium between dissolved carbon dioxide and carbon dioxide in the air. You mentioned that outgassing of carbon dioxide to the air is slow, which it is. But you didn't mention that dissolution of carbon dioxide from the air is also quite slow, and it is this dissolution that re-establishes the equilibrium after carbon dioxide has been stripped from the water with vigorous aeration. When you add acid to the spa, it reacts with bicarbonates (there are practically no carbonate species in aqueous solution below pH 8.5) to produce the salt of the acid used plus carbonic acid, as you noted. The alkalinity drops and the pH drops, and a new equilibrium between carbon dioxide, carbonic acid and bicarbonate is established quickly. By the time the added acid is mixed thoroughly with the water that reaction is finished. But the new, lower pH means that carbon dioxide is less soluble, so some of it escapes to the atmosphere via that slower mechanism you mentioned. Though slower than the initial reactions, that mechanism too will be complete within minutes, and will be accelerated by mixing and/or aeration. But what happens now if you turn on the pumps at high speed with air injection? Carbon dioxide is stripped from the water fairly quickly. The carbonic acid --> carbon dioxide equilibrium is pushed to the right with carbonic acid decomposing to produce more carbon dioxide, which is stripped from the water, etc., etc., and the pH rises. It is possible with vigorous aeration at pH in the mid 7's to drive most of the carbon dioxide and carbonic acid out of the water. But in this process the alkalinity remains unchanged, so the equilibrium is disrupted. After the vigorous aeration ceases, the equilibrium is re-established by the reaction of atmospheric carbon dioxide with water at the water surface. This reaction is relatively slow, depending on the water temperature and pH, and the ratio of water volume to surface area. It could easily take more than an hour for the pH to return to its new (lower) equilibrium value. In short, vigorous aeration will always raise the pH, but it doesn't change the alkalinity, so the pH will gradually return to its equilibrium value. Don

You shouldn't be checking the pH during or shortly after aerating. Aerating disturbs the equilibrium between carbonic acid, bicarbonate and bicarbonate, and the pH will go up, but when aeration ceases, the equilibrium will gradually be re-established. You should run the pumps for a few minutes after adding acid, but leave the air injection off. When you add acid (assuming you're adding enough to make a difference) you permanently destroy some of the alkalinity, and the pH **WILL** go down. The equilibrium shifts away from carbonate (high pH) towards bicarbonate (moderate pH) and carbonic acid/carbon dioxide (low pH). Aerating will always raise the pH temporarily, but the new balance will take effect after aeration ceases for a while. When I add acid to my spa, I don't aerate, but just run the pumps for a few minutes. The pH always goes down, and stays down unless I check it during or shortly after aerating. It is, after all, the equilibrium pH that you are interested in, since most of the time (I'm assuming here) you are not aerating your spa. It is that equilibrium pH that determines the effect that your water has on bathers and on the equipment. Try checking it first thing in the morning, and don't aerate after adding chemicals to adjust the pH. The carbon dioxide fluctuations that can cause rapid temporary pH changes can mislead you.

I would advise storing strips and kits at room temperature. Taylor has a generic recommendation for all of their strips and kits that they be stored at 36 to 85 degrees Fahrenheit. Both my LaMotte and BioGuard test strips specify on the bottle that they be stored at room temperature.

I don't use bromine, but I will try to answer. If you're adding bromide salts once a month or so to maintain a bromide bank, the MPS you add will be used up converting the bromide to bromine, as long as there is sufficient bromide reserve. So in that case it's important to maintain that bromide reserve. Unfortunately there's not a test kit available for bromide, so you have to follow the manufacturer's recommendations for how often to add bromide and how much to add. If you start seeing the bromine level fail to rise after adding MPS, your bromide level is probably low or depleted entirely. If you're using bromine tablets and just adding the MPS as a shock, try to keep the bromine dispenser full so (hopefully) the bromine level never drops below about 3 ppm, and only add MPS when indicated by water quality issues, when the bromine level can't keep up with the load, or when your experience tells you there's about to be a problem due to usage. I got into trouble by getting into a routine of adding MPS on a regular schedule when it wasn't really needed. Anyone else out there who uses bromine and MPS, feel free to correct or add to this. Don

Are you using MPS (non-chlorine) shock to activate your bromine? I had this problem with the K-2006 Chlorine FAS-DPD test. The conclusion reached by Taylor tech support was that the interference came from MPS (non-chlorine shock). It is well known in the analytical chemistry world that MPS reacts with ferrous ammonium sulfate (FAS, the titrant in the Taylor K-2006 and K-2106 kits), so if you perform the test while there is still significant active MPS in the spa the results will be high, whether you are analyzing free chlorine or bromine. The conventional wisdom is that if you wait "a few hours" after adding MPS, the interference will be gone, as the active component of MPS decomposes and no longer reacts with FAS. But the question is "How many hours are a few?". The answer is, it depends on the overall water chemistry, the water's oxygen demand, and the level of MPS added. Twelve hours after last adding MPS to my spa, I saw interference causing the free chlorine to read about 9.5 ppm too high. The interference diminished slowly over the course of a few days, but it was still present to the extent of about 1 ppm (high chlorine reading) after 10 days. It's still unclear to me why this interference persisted for so long - the Taylor tech support rep was reluctant to blame the reagents in my kit, and I concurred with that since I analyzed chlorine standards made with bleach and bottled water and those analyses worked fine. I can say that my spa water was about 3-1/2 months old and the spa had been used relatively heavily (for my situation at least, perhaps lightly for some of you), so I was preparing to change the water anyway. I understand that with bromine sanitation you may be adding MPS frequently to activate the bromine. In this case, I think it's important to determine just how much is needed to do the job and avoid adding large excesses. In my case, my spa began leaking around a fitting just before I was going to change the water, so it's now empty and awaiting a service rep. When I fill it again, I will either stop using MPS or at least use it much more sparingly. Don

No, it shouldn't make the TA reading unreliable - but it does lower TA as well as pH. Alkalinity and pH go hand-in-hand. Anything that lowers the pH will lower the alkalinity too, and vice versa. There's no need to wait around for things to correct themselves because they probably won't. If you're using non-buffered MPS, add TA increaser (sodium bicarbonate, or baking soda) to get the alkalinity back up, and your pH will come up to. I use non-buffered MPS and practically never need to add anything to raise the pH, only the alkalinity.

I use them occasionally because the odor of chlorine bothers my wife (personally, I like the odor of chlorine in low concentrations). I haven't noticed any problems due to the use of liquid spa fragrances except a slight increase in chlorine demand. The brand I use lists the ingredients on the bottle, and all ingredients are relatively innocuous organic liquids. Some of the liquid fragrance labels I have looked at claim that the products inside make the water "silky" or "smooth", or some other similar adjective. Those labels imply that the products contain oils, surfactants, softenes or some combination of these. I steer away from those products. Some of the crystalline spa fragrances, on the other hand, contain mineral salts like magnesium sulfate (epsom salt), which will increase the total dissolved solids, and in the case of magnesium salts, they will increase the hardness too. Depending on the hardness test method you use, magnesium may not show up as total hardness. It does show up as total hardness with the EDTA titration test employed in some of the Taylor kits. Hardness due to magnesium does not tend to form waterline deposits easily since magnesium carbonate is significantly more soluble in water than calcium carbonate, but the contribution of magnesium to buldup of scale around your heater will be quite significant at high concentrations. Don

Thanks for the help, no ingredients on the "Ultra-Spa" bottle, but I did find this in a post from Tom at Arctic Spas...."I received the MSDS from our supplier. Ultra-Spa is a proprietary blend of borates used as a spa water conditioner and buffer." The bottle says to use 6 capfuls at re-fill, then 2 caps per week. Our delivery guy told us "you can't use too much of this stuff". So, since it smells really good, we'd throw in a cap or two every couple of days or so for the first week or two..been using it much less since then though. So, from what I'm guessing, we should use this stuff a little more sparingly now that the water is clear from the clarifier, and also lower pH slightly as well and see it that helps. Without knowing exactly what's in it, it's impossible to recommend how much to use. Personally, if the manufacturer isn't willing to tell me what's in their product I'm not willing to put it in my spa. The use of borax, which is sodium borate, as a pH buffer in a spa is well documented, and there are products containing borax whose compositions are well known. The laundry aid "20 Mule Team Borax" for example is 99.5% sodium borate decahydrate. When added to a spa, it is necessary to add acid along with it to keep the pH in check, as borax forms an alkaline solution in water. Without the acid, borax is not a buffer, it is a base. Since your water apparently reached the calcium carbonate saturation point, I recommend draining a significant part of it if not all of it. Lowering the pH will help, but you will likely get back to that saturation point before long unless you replace a significant portion of the water. And again, if I were you, I wouldn't be using that product without knowing what's in it. If it's the fragrance you like, you can buy fragrances whose compositions are known (at least the list of components in order of relative concentration). I would recommend trying some of the liquid fragrances made specifically for spas, as they should have minimal effect on the water chemistry, unlike some of the crystalline fragrance products, which increase the hardness and TDS, pushing the water towards that dreaded saturation point. Regarding the filter, the MicroPure is a "depth filter", meaning that it excludes particles by virtue of not only its porosity but its thickness. The porosity is larger than the rated retention (1 micron in the case of the MicroPure), but particles of the rated size are unlikely to make their way all the way through the thick filter medium. Depth filters can actually increase in efficiency as trapped particles accumulate, and can handle heavy particle loads until one of two things happens - either the filter becomes clogged or the trapped particles begin to break through. In either case you have a "filter" that's worse than no filter at all. Since conventional pleated filters are completely satisfactory for normal spa use and can be cleaned and reused, the use of depth filters like the MicroPure is not justified IMHO. In any case, I think it's time to change your filter after cleaning up the preciptitate problem. Good luck, Don

We are on all Arctic Pure brand chemicals right now that came with the startup kit: bromine with floater set on 2 or 3, with ozone. Bromine levels are generally between 3-5ppm. We use a full capful of Arctic Pure Refresh MPS shock after every use, which is every one or two night or so. Also have a "Zorbie", if that helps at all. pH is generally around 7.5-7.8, TA around 80-100, total hardness is bang on according to test strips. No buildup on shell surface or anything like that. I even gave it a good super shock of 4 capfuls of Refresh a couple of days ago to see if that would help, it did nothing. When you look close at the water, it looks like billions of little white particles floating around...from a distance, just looks hazy. From what I've read, we are using more than enough sanitizer...most people on ozone use 0.5-1 ppm bromine and don't shock all the time. We are at 3-5ppm and always shock. How could this be microbial? Anyway...cloudiness was not always there...water was clear for 2 weeks until we started to notice it gradually get cloudier. The cloudiness is all the time, then gets worse when the jets are on. Goes back to the original cloudiness after a couple of minutes...I guess some of this might be from air bubbles. Anyway, now after the Arctic Pure Easy Clear clarifier, the water is perfect again. Fyi, we also use Arctic Pure Ultra-Spa conditioner, as recommended by the dealer...it is supposed to "condition and clarify the water" and "maximize effectiveness of other chemicals". It's a blue, flour texture stuff...not sure if we even need it, but it came with the startup kit. Maybe it's part to blame, who knows. Well, it sounds like your sanitizer level is just fine, so it doesn't seem to be a microbial problem. Your pH is on the high side though. If the Ultra-Spa Conditioner is a borate salt solution, and your hardness is on the high side of the OK range (200 to 300 ppm), with your pH on the high side too, you're probably experiencing calcium precipitation, which would definitely cloud the water and could be hard to filter out without a clarifier. Does the Ultra-Spa Conditioner package list the ingredients? If it is borates, it could easily raise your saturation index to the point of precipitating calcium carbonate, considering where you're starting from. Your description of the "billiions of little white particles" fits this explanation. If that's what happened, you might want to drain 1/4 to 1/2 of the water and refill, then forego using the conditioner. It may be necessary to change the filter too. Then try to keep the pH below 7.6, and you shouldn't have the problem again. Don

What sanitizer are you using? Cloudy water is likely due to either bacterial growth, due to inadequate sanitizer, or preciptiation of minerals, which shouldn't occur unless there is a drastic change in the chemical composition. What happens when you leave the pumps off for a few hours? Does the cloudiness settle out, disappear or remain? FYI, dissolved solids don't cause cloudiness, suspended solids do. And filters can't remove dissolved solids, they can only remove suspended solids. The Micropure filter won't filter out bacteria even though they are suspended because they are too small to be caught by the filter, unless you flocculate them, which is what your clarifier does. Some suspended minerals are also too small to be filtered by a 1 micron filter like the Micropure MF-170 unless flocculated, so you could be dealing with mineral or a microbial haze, but the microbial haze is more likely if it occured suddenly with no drastic change in water chemistry. Don

Hi Kammie, I can't be sure just by looking at the photo, but that looks like it could be "soap scum", which is produced in a spa that has low hardness, pH and/or alkalinity. Is that residue sort of soft and flaky or gooey, or is it hard and brittle? I assume you're using chlorine as a primary sanitizer, is that right? Any issues with water clarity? Don

That's a nice feature. My spreadsheet isn't intended for novice users and people aren't usually switching back and forth between U.S. and Metric -- they usually pick one and stick with it. Though I know how to attach a macro to a button, I'm not even sure how to attach one to a pull-down menu selection (in Excel) and it would take a macro to "update" a type-in field (as opposed to a calculated field that could simply look at the pull-down menu setting). I probably won't be changing this. Hi Richard, FYI Excel does have the ability to run macros when cell values change. Here's a web page describing the ability: http://www.ozgrid.com/VBA/run-macros-change.htm Don

Ah, OK, thanks - it works as advertised. I fill my gas tank with gallons, bake bread with cups, teaspoons and tablespoons and complain about the weather in degrees Fahrenheit. But due to my background I like to work metrically when dealing with chemistry. Now I know how to do it with your spreadsheet. I continue to be impressed as I use more features of it, and haven't found any errors yet in the formulas. Don

I'll take a crack at this one. Peroxysan is not particularly new. It, or similar formulations, have been around for over 20 years and are used successfully as an antimicrobial agent in several applications. The active ingredients are peroxyacetic acid, also called peracetic acid, and hydrogen peroxide, and the other major components are water and acetic acid. Peroxyacetic acid is always sold in solutions containing hydrogen peroxide and acetic acid, which stabilize it and prevent potentially violent decomposition. What may be new is the attempt to market Peroxysan as a primary disinfectant for spas. Peroxysan is approved in the US for disinfection of hard surfaces (i.e., in food processing plants) and is used for treatment of recirculating water in cooling towers. It is not, however, approved as a disinfectant for swimming pools, spas or public water supplies in any country as far as I know. In the US the federal government cannot yet force homeowners to adhere to particular regimes for pool and spa maintenance, but does enforce standards for public pools, spas and water supplies. The homeowner is well advised to steer clear of products that are not approved for municipal and commercial use, or at least proven effective by substantial, relevant data, IMHO. There are plenty of people out there who will take your money in exchange for worthless products. I'm not saying Peroxysan is not effective as a spa disinfectant - all I am saying is that it's not approved and not proven effective for spa use. There seems to be very little information available related to actual, real world experience with using Peroxysan in spas, and the information I was able to find on the internet unanymously concludes that its effective application is difficult at best. There is also very little information on the Peroxysan web site at http://www.peroxysan.com, other than marketing hype. That site talks about the hydrogen peroxide in Peroxysan, probably because use of hydrogen peroxide in spas is well established, though not as a primary disinfectant. They don't even mention the primary active component, peroxyacetic acid (at least I can't find it mentioned), probably because anyone who looked it up (try http://en.wikipedia.org/wiki/Peracetic_acid) would be leery of using it at home. Safe use of peroxyacetic acid would require the applicator (that's you) to wear heavy rubber gloves and full face goggles at least, and preferably a full length heavy neoprene apron, to avoid potentially dangerous burns from skin contact due to splashing and spills. Don

The buildup at the water line is probably saponified oils from bather load, body lotions, etc. What are your hardness and alkalinity levels? Getting them and the pH balanced (hardness around 150 - 200 ppm, alkalinity around 100 ppm, pH around 7.4) will fix that problem. Your low pH is part of the problem, and if your alkalinity and hardness are low too, which is very likely given your low pH, that accounts for the buildup. The EcoSmarte system is similar to the Nature 2 and Spa Frog systems in that it uses metal ions (copper in this case) as a santizer. It is different in that it generates oxygen to help with breakdown of organics, and in that they advise it is not necessary to use any additional sanitizer. I think that advice is questionable. It looks like you are using bromine (on the advice of someone?) which is fine, but you need to get that free bromine concentration up to an effective level or you are courting trouble. You can't use chlorine with the EcoSmarte system, so adding more liquid shock and perhaps adjusting the bromine floater if it's adjustable are in order. The idea that you can operate a spa successfully without adding any chemicals is pretty shaky IMHO. If you don't get your hardness, alkalinity and pH up you will have other problems down the road in addition to the waterline buildup. Anyway, the EcoSmarte system DOES add chemicals to the water - copper and oxygen, and perhaps carbon dioxide if you have the optional "Automatic CO2 Non-Chemical Feed". They just choose not to call those chemicals they add "chemicals". You should not be concerned about adding calcium and bicarbonate to get the water balanced - think of them as minerals, which they are. If the Mineral Magnet actually works it would be doing the wrong thing for your water (softening it), unless the water is already too hard (again, not likely due to your low pH). There is however a lot of debate over whether they do anything at all to the water. I am of the opinion that they probably don't. Don

Hi Richard, That's a very interesting article by O'Brien on the chlorine/cyanurate chemistry. I didn't realize that cyanurates could have such a large impact on available chlorine. After I drain and refill (this weekend if the weather allows) I am going to switch to the bleach regime you have described, after getting the CyA up to around 30 ppm with dichlor. Your spreadsheet is amazing. It will take quite some time to go over everything in it. I did find a little glitch though. I entered 80 for alkalinity and 100 for the target alkalinity, and the spreadsheet calculated 1.86 oz of bicarbonate needed, which is about right. Then I switched the units to metric, and the bicarbonate needed changed to 13.2 grams. Now 1.86 oz is about 52.7 grams, or about 4 times what the spreadsheet calculates. I didn't try to trace down the error but it looks like a simple error in a conversion factor. Regards, Don

Hi Nitro, Yes, that's what I do with the TA and pH. I find that I need to add some bicarbonate (maybe 2 oz or so) every month as the TA and pH creep down. I raise the alkalinity, and the pH follows. I practically never need to use a stronger base to get the pH up. I'm about to switch to the dichlor/bleach method describe on this forum for maintaining sanitizer, and to stop using MPS except perhaps rarely, so I will probably need to add some acid occasionally to bring the pH down. I have a bottle of concentrated potassium bisulfate solution, which should work to lower the pH/alkanity, and some sodium carbonate if I need to go the other way. Regards, Don

Hi Richard, Ah, it looks like The Deep End is another good resource, thanks for sharing it. I don't have a PhD either, just a BS, but I did work as an analytical chemist for several years. I have sort of stayed in the field, going to work for an analytical instrument manufacturer many years ago, and I still work there but now I develop software. Wow, that spreadsheet has a lot of info! I'll be glad to have a look at it. Thanks, Don

I'm really not familiar with the Nature 2 system. It's still important though to keep a chlorine residual in the water, and it seems to me that if you're trying to keep it low, around 0.5 ppm as the manufacturer suggests, you may need to add it more than once a week. How much do you add when you shock?

What sanitizer are you using? If you're using biguanide (e.g., BaquaSpa, SoftSwim) this a fairly normal phenomenom and nothing to be concerned about. BaquaSpa sanitizer (and probably the other biguanide spa and pool sanitizers) seem to contain a little surfactant (i.e., detergent or related compound) that tends to make things feel a little slippery. The surfactant is there to help minimize the buildup of scummy biguanide deposits at the waterline. It also accounts for the tendency of BaquaSpa-treated spas to turn milky white when the pumps are on high with air injection. This is due to millions of micro-bubbles produced by the injected air and facilitated by the surfactant. A lot of people really like the silky feel of biguanide-treated spa water. I used BaquaSpafor a couple of years but finally switched to chlorine. Don

Hi Richard, Actually, this doesn't surprise me at all - it's what I suspected all along, though it doesn't account for the production of red color when the DPD is added. In fact, the Dupont Oxone technical published by DuPont, which you can find on the internet, states that you can quantify Oxone in aqueous solutions by neutralizing it with a known quantity of ferrous ammonium sulfate (in excess), then back-titrating with potassium permanganate. This method is described as suitable for concentrations of Oxone higher than about 20 ppm, and indicates that Oxone reacts stoichiometrically with FAS at those concentrations. As far as the red color developed with DPD, I suspect that MPS either reacts directly to form the red dye (not very likely according to the info I can find on DPD) or reacts with other substances in the water to produce compounds that form the red dye with DPD. MPS does in fact have the ability to oxidize chloride to free chlorine, though it's a fairly weak reaction. We know of course that MPS readily oxidizes iodide and bromide, so if any of those have crept into your spa that would account for the red color. Thanks for the info. I just found this forum a few days ago and all I can say is "WOW!". There's a large amount of very valuable information here, keep up the great work! Thanks, Don

Depending on your tap water's composition you may find that balancing the alkalinity will bring the pH into balance, so I like to adjust the alkalinity first. Raising the hardness with calcium will have little effect on the other parameters so you can do that at any time during startup. I always add the chlorine last, at about twice the normal dose, but that small amount of chlorine won't have much effect on the other parameters either, so you could do it at any time - but if you're going to be running the pumps to mix all the chemicals for an hour or so, you might as well conserve your chlorine by not adding it until the last step.

Well, I have some resolution on this. Taylor called back today and the tech support rep said that he was able to duplicate the problem by adding MPS to a water sample. He said it took "quite a bit" of MPS to bleed through into the free chlorine test - I asked how much is "quite a bit" and he said around 20 ppm as potassium monopersulfate. That's about how much you get when you add two tablespoons of non-chlorine shock containing 45% MPS (the normal formulation) to a 400 gallon spa. He agreed that the MPS seems to interfere with the DPD-FAS method in two different ways - by producing a faint pink color with the DPD reagent and then by reacting with the FAS titrant, causing more drops to be needed before the pink color is discharged. Taylor's solution is to use their deox reagent, R-0867, before titrating with FAS. The tech support rep said he was able to eliminate the inteference this way. How much of the deox reagent you need depends on how much MPS is in the water, of course, and he said a 20 ppm dose could dissipate in hours or could take days, depending on the water's oxygen demand. I guess the moral is, if you use the DPD-FAS method (and to a lesser extent the DPD color matching method), you shouldn't add MPS unless you really need it, and then no more than you really need. Or switch to shocking with chlorine. I'm happy with that answer, and I will continue to use the Taylor K-2006 kit. I will buy a small bottle of the R-0867 deox reagent, but likely will switch from MPS to chlorine for shocking in the near future.

Hi Richard, Yes, it will be interesting to see what Taylor says. I'm not sure what I expect them to say or do about this, but I will find out. I bought the Taylor kit after using test strips and cheap kits with the o-toluidine chlorine drop test successfully for several years. I like to keep a close eye on the chemical balance, and I was never satisfied that I was getting good results with the methods I used, especially for alkalinity and hardness. I found out about the Taylor K-2006 kit, which does indeed use very good alkalinity and hardness methods, an easily readable pH comparator, and as an extra added benefit the ability to get accurate combined chlorine results - or so I thought. I'm very happy with the kit except for the chlorine issues, which apparently have to do with MPS. To be fair about it, I was pleased at first with the chlorine test too, and that was soon after I had drained and refilled the spa. After a couple of months I started seeing some questionable results on free chlorine, and I called Taylor and basically got nowhere, but didn't really have time to pursue it. Then I drained and refilled the spa again and all went well for a while, until recently. So I think that there may be issues in addition to the recent use of MPS, perhaps related to total dissolved solids, buildup of cyanurates, sulfates, etc. Water chemistry is a pretty complex subject, and spa chemistry is closer to wastewater chemistry, an even more complex subject, especially if it's been a while since you've drained and refilled. So we'll see ... on the brighter side, it's about time to be draining and refilling again, so perhaps I'll get back to a more predicatable regime soon. Your figures on chlorine usage are right on according to my experience. With my spa, it's usually just my wife and I, and more often than not just me, for 15 minutes on most nights when the weather allows. A teaspoon of dichlor does the trick on the nights when it's just me for 15 minutes. My water has always been crystal clear since I switched from biguanide to chlorine. Right now though I'm willing to sacrifice water quality for the benefit of finding the answer to this issue, since it's almost time to change the water anyway. I'll let it go without adding chlorine until the Taylor kit says to add it, or until I know why not. Thanks, Don