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Ph And Total Alk Explained Better


cporro

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relative noob here. bought a house 8 months ago with a spa. never wanted a spa, never thought i'd be into maintaining one. now i'm slightly obsessive. not that i add chemicals left and right. but i do log all my measurements.

here is where i am

refilled: 4/13/09

ph: 7.9

total alk: 90

dichlor: i dose after uses (5 days a week) 2 tsp gets me over 2ppm.

combined chlor: i have had some strange results for this lately. i think because i shocked with potassium peroxymonosulfate. it tested very high for a while, but no smell. now it tests less then .2 ppm.

usage: me and my wife about 5 days a week.

cyanuric acid: 30

hardness: 250

i'm lookign for a clear explanation of ph and total alk. i have read many online. none very clear. i took some chem in college so i know about ph. and total alk is a buffer for ph yes?

here is whats confusing. its hard for me to hit the right ph and total alk. (i'm using a taylor 2006)

after reading about ph and alk i had to figure out a model/analogy that made sense to me. here it is.

ph balance is like a teeter tauter. base on one side alk on the other. pure water is neutral. kinda like a teeter tauter with no one on it. although it is ph balanced it doesn't take much to unbalance it. like if one kid jumps on an empty teeter tauter. boom! them hit the ground.

so it sounds like total alk is like loading up the teeter taunter with kids. a few on one side, a few on the other. its still balanced, but if a new kid jumps on the acid side the thing doesn't abruptly hit the ground. imagine a teeter taunter with 100 kids on each side. now the introduction of a new kid to one side does nearly nothing.

alplied to my spa. its seems i have to introduce bicarb to the get the alk right...then my ph will be high (as it has been) then i need to ad dry acid...to balance ph...then check total alk again...then ph....

this seems a bit crazy to me. what i thought i could do was adjust the total alk and the ph would be correct. i'm trying to simplify my care as much as possible with accurate test kits, a firm understanding of principles, and only warrented addition of chemicals.

so, to recap. where is the good information on the relationship of ph to total alk?

thanks

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Regarding Combined Chlorine (CC), non-chlorine shock (MPS) will register as CC -- initially at high doses it may even measure as Free Chlorine (FC).

With Dichlor, for every 10 ppm Free Chlorine it adds, it will also increase Cyanuric Acid (CYA) by 9 ppm. So CYA will increase over time with continued use and this will lower the effectiveness of chlorine.

Total Alkalinity is mostly a measure of the bicarbonate in the water (it also measures a portion of CYA as well). Pools and spas are intentionally over-carbonated to provide a pH buffer and to saturate the water with calcium carbonate to protect plaster surfaces.

The piece of information you are missing is that the amount of carbonation in the water is much higher than the equilibrium amount that would result from exposure to air (i.e. there is more carbon dioxide in the water than would occur naturally from exposure to the carbon dioxide in air). So what happens is that carbon dioxide outgasses (just as with a carbonated beverage, though you don't see bubbles). This process is accelerated with aeration which is typical in spas. When carbon dioxide outgasses, it causes the pH to rise (because carbonic acid is removed from the water). So higher TA levels result in a higher or rising pH.

Usually when you are using Dichlor, this isn't as much of a problem because the addition AND consumption/usage of Dichlor is net acidic so lowers pH and TA so when combined with the carbon dioxide outgassing the net result is often a fairly consistent pH with a slowly dropping TA.

This concept of higher TA causing the pH to actually be less stable and cause it to rise when the TA is too high is one of the most counter-intuitive things in all of pool/spa water chemistry. It is because TA has TWO distinct effects. TA is a pH buffer that makes pH move less from OUTSIDE sources of acid or base, but on the other hand TA is itself a SOURCE or rising pH due to carbon dioxide outgassing. This latter effect outweighs the former at higher TA levels. So the rule to managing this is to adjust the TA level such that the pH is roughly stable over time. When using very acidic sources of chlorine such as Trichlor, a higher TA or 120-140 ppm is usually used in pools. With Dichlor in spas, the TA for a stable pH is somewhat lower but depends on the amount of aeration, usually 80-120 ppm. When using bleach or other hypochlorite source of chlorine, the TA should be even lower, 60-80 ppm.

Richard

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Regarding Combined Chlorine (CC), non-chlorine shock (MPS) will register as CC -- initially at high doses it may even measure as Free Chlorine (FC).

yes, my CC was way high for 2 days. over 2 ppm. and i put in the rec amount of mps for my vol...2 tbsp. my free c was also high. up the road i'm going to try and get away from MPS. i here it doesn't work well on CC, plus it smelled funny. i'd prefer to use chlorine as its own shock.

With Dichlor, for every 10 ppm Free Chlorine it adds, it will also increase Cyanuric Acid (CYA) by 9 ppm. So CYA will increase over time with continued use and this will lower the effectiveness of chlorine.

i didn't know CYA lowered the effectiveness of chlorine. i did know about the cya, but i figure with my low use and refilling it wouldn't be an issue. one thing i have thought about it trying an unstabalized chlorine. clorox, is cheap and as i understand it no residue. i know it degrades in storage, but if i were to buy every few weeks i should be ok right? then there is the ph thing. 11-13. alkaline. and i don't want to be adding acid all the time or have swings.

Total Alkalinity is mostly a measure of the bicarbonate in the water (it also measures a portion of CYA as well).

yeah. i was looking at the calculation to adjust for cya and TA. the cya, when factored in, yields a lower TA looks like.

Pools and spas are intentionally over-carbonated to provide a pH buffer and to saturate the water with calcium carbonate to protect plaster surfaces.

interesting. but i have no plaster. seems i could run at lower TA

The piece of information you are missing is that the amount of carbonation in the water is much higher than the equilibrium amount that would result from exposure to air (i.e. there is more carbon dioxide in the water than would occur naturally from exposure to the carbon dioxide in air). So what happens is that carbon dioxide outgasses (just as with a carbonated beverage, though you don't see bubbles). This process is accelerated with aeration which is typical in spas. When carbon dioxide outgasses, it causes the pH to rise (because carbonic acid is removed from the water). So higher TA levels result in a higher or rising pH.

Usually when you are using Dichlor, this isn't as much of a problem because the addition AND consumption/usage of Dichlor is net acidic so lowers pH and TA so when combined with the carbon dioxide outgassing the net result is often a fairly consistent pH with a slowly dropping TA.

This concept of higher TA causing the pH to actually be less stable and cause it to rise when the TA is too high is one of the most counter-intuitive things in all of pool/spa water chemistry. It is because TA has TWO distinct effects. TA is a pH buffer that makes pH move less from OUTSIDE sources of acid or base, but on the other hand TA is itself a SOURCE or rising pH due to carbon dioxide outgassing. This latter effect outweighs the former at higher TA levels. So the rule to managing this is to adjust the TA level such that the pH is roughly stable over time. When using very acidic sources of chlorine such as Trichlor, a higher TA or 120-140 ppm is usually used in pools. With Dichlor in spas, the TA for a stable pH is somewhat lower but depends on the amount of aeration, usually 80-120 ppm. When using bleach or other hypochlorite source of chlorine, the TA should be even lower, 60-80 ppm.

that's good stuff. my issue is i can't seem to hit the sweet spots for TA and ph at the same time. i first brought my TA to 120 but my ph was +8. it seems that you are saying adjust my TA to where my ph is good...say 7.4 and then keep an eye on my ph to see if its stable there. my guess is my TA will be more in the 80 range. i would like to run my ph a little low for the chlorine. i'm guessing at +8 my chlor has lost much effectiveness.

thanks richard.

Richard

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Cporro,

I have a few comments.

MPS does work quite well at lowering CC. It works even better at keeping CC low if used before (during) heavy bather loads. It will help the Chlorine oxidize waste.

If you do use unstablized Chlorine (i.e. Bleach), make sure you first use some Dichlor to raise your CYA to 20-30 ppm. Otherwise the Bleach will be too strong. Also, if you keep your TA low around 50-60 ppm, you won't have issues with pH rising. Even though the pH of bleach is higher, as FC is used up it will actually lower pH. The net effect is close to being balanced.

To give you an idea. My current water is 5 months old. My CYA is 30 ppm, and I only use Bleach. I keep my TA at 55 ppm, and my pH is rock solid between 7.6-7.8. Since my initial balance (TA was 60) when I filled the tub, I have only added acid once (~1 oz). I also have 50 ppm Borates which act as a pH buffer that greatly reduces pH rise.

I highly recommend trying the Dichlor/Bleach method. I think you'll find it works great, and the water lasts a long time (about twice as long as using only Dichlor). Check out my link below for more information about this method.

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Cporro,

I have a few comments.

MPS does work quite well at lowering CC. It works even better at keeping CC low if used before (during) heavy bather loads. It will help the Chlorine oxidize waste.

ah the confusion. i was reading in the taylor booklet that came with the 2006 that MPS doesn't do much for CC. i log everything, but i don't have data to support either position yet. all i can say is that MPS seemed to cause my CC and FC to test falsely high for about 2 days.

If you do use unstablized Chlorine (i.e. Bleach), make sure you first use some Dichlor to raise your CYA to 20-30 ppm. Otherwise the Bleach will be too strong. Also, if you keep your TA low around 50-60 ppm, you won't have issues with pH rising. Even though the pH of bleach is higher, as FC is used up it will actually lower pH. The net effect is close to being balanced.

so maybe start off with dichlor and then switch to bleach? my cya is 30 at the moment. i'm in this experimental mode...when we got the house there were bromine tabs. i tried those for a few months but my gut told me there was a better way.

To give you an idea. My current water is 5 months old. My CYA is 30 ppm, and I only use Bleach. I keep my TA at 55 ppm, and my pH is rock solid between 7.6-7.8. Since my initial balance (TA was 60) when I filled the tub, I have only added acid once (~1 oz). I also have 50 ppm Borates which act as a pH buffer that greatly reduces pH rise.

borates. thats an area i haven't looked at yet.

a ph of 7.6-7.8 is on the high side for chlorine effectiveness, yes?

I highly recommend trying the Dichlor/Bleach method. I think you'll find it works great, and the water lasts a long time (about twice as long as using only Dichlor). Check out my link below for more information about this method.

much thanks, i will check it out.

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ah the confusion. i was reading in the taylor booklet that came with the 2006 that MPS doesn't do much for CC. i log everything, but i don't have data to support either position yet. all i can say is that MPS seemed to cause my CC and FC to test falsely high for about 2 days.

Yes, it will give false CC readings, however the MPS should be gone in a day or two. Check out my link about Chlorine Demand (CD). There is a link between CC and CD, meaning if you lower CD, your CC should also lower. The best (only) way to lower it is to shock.

Also, keep in mind MPS is acidic and will lower pH/TA. If you use it once a week as a shock, it will help reduce pH rise. You just have to keep an eye on TA. If TA gets below 50 ppm, your pH may stay too low. Then you'll have to add a little Baking Soda to raise it.

so maybe start off with dichlor and then switch to bleach? my cya is 30 at the moment. i'm in this experimental mode...when we got the house there were bromine tabs. i tried those for a few months but my gut told me there was a better way.

Yes, add about 35 ppm FC (~30 ppm CYA) using Dichlor first, then switch to bleach. If your CYA is 30 ppm now, you can start using bleach right now.

borates. thats an area i haven't looked at yet.

There's a product called Gentle Spa. It will had borates easily. The cheaper way is to use 20 Mule Team Borax and acid.

a ph of 7.6-7.8 is on the high side for chlorine effectiveness, yes?

Actually, with CYA (FC buffer) at 20-30 ppm the chlorine will be effecive in a much wider pH range. Chem Geek can give you the details.

much thanks, i will check it out.

No problem. Good luck, and keep us updated.

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a ph of 7.6-7.8 is on the high side for chlorine effectiveness, yes?

Actually, with CYA (FC buffer) at 20-30 ppm the chlorine will be effecive in a much wider pH range. Chem Geek can give you the details.

You probably don't want the details, but they are shown in graphs and described here.

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thanks you guys.

i think my big error was adjusting my TA after filling to the ideal (?) range of 120. after this the pH was always high. next time i'll adjust TA to get ph in the right range...then monitor to see how stable it is.

also, i'm going to try this di-chlor primer method...getting cya to 35ppm. i'll read it again next time i refill.

i'm excited because of the extended pH range this method gives chlorine.

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i'm excited because of the extended pH range this method gives chlorine.

Dichlor-only and Dichlor-then-bleach both give an extended pH range of effectiveness for the chlorine because Cyanuric Acid (CYA) is present in both situations. The difference is that Dichlor-only has the sanitation level drop over time as the CYA rises whereas Dichlor-then-bleach maintains a more consistent (and higher, on average, after the first weeks) disinfection level.

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