What am I missing? Everything seems good except CSI

[platinum903]

I would like to thank you in advance for your help. My new pool was finished in early October and things have been going good so far. I have learned a ton on this forum. One thing that always seems to be off for me is my CSI. Its seems to always hover between -.7 and -.45. It seems as though all my readings are coming out in the normal range - is there something I am missing that is preventing my CSI from being closer to O?

FC - 4
TC - 4
pH - 7.6
TA - 60
CYA - 40
CH - 270
Salt - 2800
Temp - 49 degrees
CSI - -0.66

Is there anything I should do to get my CSI back in check or should I just not worry about it? The water is crystal clear.

 

[duraleigh]

Thanks for allowing me another opportunity to get back up on my soapbox.

If you maintain the pertinent water parameters within the guidelines suggested on this forum (and you have), there is no need to even think about CSI.

Enjoy your crystal clear water and keep your levels just about where they are.

 

[Lana537]

Dave is right! However, if you are feeling haunted and compelled to pull your CSI closer to 0, go to the Pool Calculator and fiddle around with some numbers to see what you could do to alter your water balance.

I had a go at it. If your TA were raised to 80, and your Calcium Hardness were increased, your CSI would come inside that "-.6 scary number". Then, once winter melts away and the pool water heats up, the change in temperature alone will move the number even closer to zero.

Lana

 

[platinum903]

I have played with The Pool Calculator and that's what seems weird to me. Of my above numbers, the ones I could stand to change would be increasing TA, CYA, and maybe lowering pH a little, but that really barely helps at all with CSI. What kinds of numbers do people even have to get close to 0?

 

[MattM]

I'm sure someone already has done the same, but after playing around with the csi calculators out there -- I've forced myeself to remember the phrase "low temp and low ta don't mix". Both at same time kill csi, so choose whichever is easiest and acceptable for you to modify/control and increase it if you want to fix the csi. Likewise, in summer....high temp and high ta cause a csi nightmare in the other direction. I guess CH is also something you can modify, but since this is a number that is really hard to bring down...I don't consider it something to be controlled.

 

[Richard320]

If your pool is only 4 months old, CH is going to rise. Wait.... is it plaster or vinyl or fiberglass? Back to my original thought: CH will rise as it cures, which will raise the CSI all by itself. I wouldn't add anything until the water starts warming up a little.

 

[platinum903]

It is plaster.

 

[chem geek]

Colder water will naturally have a higher pH and that tends to compensate for the CSI. So if the water temperature is lower but you are keeping the pH at the same level as when it was warmer, then that is what is causing your CSI to be lower. I wouldn't worry about this when the water is colder since all reaction times are significantly slower, but as for how to get closer to a zero CSI when the water is warmer, the main parameter to change is the CH level. In an SWG pool that has higher salt levels, the CSI will be around 0.2 lower so is normally compensated with a higher CH level. The TA you usually need lower to have less carbon dioxide outgassing and the CYA level is usually higher which makes the carbonate alkalinity even lower. Though the pH target can be a little higher (say, 7.7 instead of 7.5), CH is really the only parameter you can freely play with to change the CSI without affecting anything else.

As Richard notes, with newer plaster that is curing, the CH and pH will both tend to rise. In order to help reduce scaling in the salt cell, we usually recommend targeting a somewhat negative CSI such as -0.2 or so. At a TA of 70 ppm, CYA of 80 ppm, CH of 300 ppm, 3000 ppm salt, temp of 84ºF, one has a CSI of -0.3 if the pH is 7.7 while raising the CH to 400 ppm gets one to around -0.2. These are all in the recommended ranges though you will note that the pH and CH are on the higher side. The higher pH target will tend to reduce the rate of pH rise while the higher pH and CH both help to raise the CSI.

If one is at the low CSI levels of the recommended range for bleach (non-SWG) pools with pH 7.5, TA 70, CH 250, CYA 50, < 500 ppm salt then at 84ºF the CSI is around -0.2. However, if one is at the low CSI levels of the recommended range for SWG plaster pools with pH 7.5, TA 60, CH 250, CYA 80, 3000 ppm salt then at 84ºF the CSI is between -0.6 and -0.7. Personally, I believe the recommended CH and low-end pH levels for SWG plaster (and perhaps fiberglass) pools should be raised to compensate for the higher salt, lower TA and higher CYA levels. A pH range of 7.6 to 7.8 and a CH range of 350-450 would still have the CSI be negative, which would be -0.4 at the low-end of the ranges but around -0.2 for the midpoint of the ranges. Note that the use of 50 ppm Borates lowers the CSI by around 0.1 as well. A discussion about this should be done after my post in The Deep End rather than in this thread.

 

[onBalance]

I am not sure I agree with the idea that CH continues to rise in new plaster pools after one month has past by. The majority of the carbonation of the plaster surfaces occurs in less than one month. If some pool owners are seeing a continued rise in CH after one month has past, I wonder if that is because the plaster dust (calcium carbonate) formed during the first month and then trapped in the filter is getting dissolved as (perhaps aggressive) water passes through it (of course if the filter hasn't been cleaned).

I know that the pH may rise more than normal with new plaster pools during the entire first year, but that may be due to CO2 continuing to convert hydroxide into carbonate in the plaster surface, resulting in the increased loss of CO2 and raising the pH. That reaction would not affect the calcium in plaster and not cause an increase in the CH. Also, the above pool water readings are aggressive; would that not result in a CH increase in time? My experiments with aggressive water and subsequent calcium loss from plaster coupons (and calcium increase in the containment water), was with a temperature of 65 to 68 degrees.

 

[duraleigh]

This thread now qualifies to be in the deep end. :lol:

 

[stevodevo]

I was about to start my own thread in the deep end to raise the subject of how to wait before adjusting CH in a new plaster pool, but this thread is starting to answer that question.

I like the theory about new plaster being caught and dissolved in the filter. If that theory is sound, it may go toward explaining why opinions on this subject are so wildly different depending on who I ask. My builder and his own handover guy even had vastly differing opinions.

I opted to wait it out and keep an eye in it. The water in my pool has been in for just over 2 months and it appears to be settling down although we've just had some really heavy rain so that is throwing things out a bit at the moment. It did seem to steadily climb right up unil a couple of week ago. I'm now seriously considering giving the filter cartridge a clean out to see how that affect things.

 

[chem geek]

I am not sure I agree with the idea that CH continues to rise in new plaster pools after one month has past by.

As I wrote in this post the process of plaster curing has different possible phases and outcomes. Actual "curing" would be the following producing calcium hydroxide that would increase both CH and pH if the calcium hydroxide were to dissolve into the water:

2Ca₃SiO₅ + 7H₂O --> 3CaO•2SiO₂•4H₂O + 3Ca(OH)₂ + heat
Uncured Pool Plaster + Water --> Cured Pool Plaster + Calcium Hydroxide + heat

If one does a bicarb start-up, then this CH rise effect can be minimized as follows (and the amount of pH rise is cut down as well since the result is half as much hydroxyl ion):

Ca(OH)₂ + HCO₃^- ---> CaCO₃(s) + H₂O + OH^-
Calcium Hydroxide + Bicarbonate Ion ---> Calcium Carbonate solid + Water + Hydroxyl Ion

which would have the pH rise with no change in CH nor in TA (the TA decrease from loss of bicarbonate ion is offset by the TA increase from gain in hydroxyl ion). This is the same effect seen with carbon dioxide outgassing so perhaps that is why we may sometimes confuse the two in newer pools and that the above may occur after the first month regardless of start-up procedure.

So I agree with you that one could have the pH rise with no rise in CH (or TA) and have this due to additional hardening of the plaster as a calcium hydroxide to calcium carbonate process rather than true curing of the plaster itself. That is an interesting result since it says that we can't really distinguish between plaster hardening from bicarbonate ion becoming solid calcium carbonate vs. carbon dioxide outgassing. In both cases, carbonates are "lost" from the water, either by becoming solid or by outgassing, and the pH rises with no change in TA nor CH. Standard precipitation or scaling, on the other hand, decreases pH, TA and CH while dissolving of plaster increases pH, TA and CH.

 

[onBalance]

Chem geek, consider the following chemical reaction; calcium hydroxide and carbonic acid - Ca(OH)2 + H2CO3 after the first month in new plaster pools. How do you see that affecting outcomes?

And back to Platinum903, while others may disagree, but as Chemgeek suggested, to make your pool water less aggressive and raise the CSI, add some calcium chloride to raise the CH about 100 ppm to 350 or so, and maintain pH between 7.6 and 7.8. That I what I would do so that the water is not so aggressive. Or heat and raise your water temp. Just kidding.

 

[chem geek]

Chem geek, consider the following chemical reaction; calcium hydroxide and carbonic acid - Ca(OH)2 + H2CO3 after the first month in new plaster pools. How do you see that affecting outcomes?

There is no difference whether it is carbonic acid or bicarbonate ion that reacts with the calcium hydroxide to form calcium carbonate because the carbonates are in equilibrium with each other. The reaction with bicarbonate is the dominant one, but since the pH rises then technically there is some shift from carbonic acid (and therefore dissolved carbon dioxide) to bicarbonate ion and from bicarbonate ion to carbonate ion.

Ca(OH)₂ + H₂CO₃ --> CaCO₃(s) + 2H₂O
Calcium Hydroxide + Carbonic Acid --> Calcium Carbonate solid + Water

However, when carbonic acid is removed from water at typical pool pH, bicarbonate ion produces more of it as follows and that makes the pH rise:

HCO₃^- + H₂O --> H₂CO₃ + OH^-

so the pH rises as I had shown before. Multiple equations are all occurring at the same time with multiple equilibria being achieved. I was only giving the simplified and dominant equation in the discussion. I've added these other processes, such as bicarb startup and plaster hardening (calcium hydroxide to calcium carbonate) to my PoolEquations spreadsheet. The following table shows the effect from the process indicated assuming that 10 ppm calcium carbonate is affected (assumptions are a starting pH of 7.5, TA of 80 ppm and the CYA is 30 ppm).

Process ............................... pH ....... TA ..... CH
Adding CaCl₂ ....................... 7.50 ....... 0 ... +10 .... increasing CH levels by adding calcium chloride
Dissolving CaCO₃ ................ 7.99 ... +10 ... +10 .... plaster etching and degradation from low CSI
Dissolving CaO from CO₂ .... 8.51 ... +10 ... +10 .... speculative and may not occur in practice
Curing Plaster (traditional) ... 8.51 ... +10 .. +10 .... assumes calcium hydroxide dissolves in water
Curing Plaster (bicarb) ......... 8.00 ....... 0 ....... 0 .... assumes calcium hydroxide combines with carbonates to form calcium carbonate
Hardening Plaster ................ 8.00 ....... 0 ....... 0 .... assumes calcium hydroxide combines with carbonates to form calcium carbonate

Though I refer to calcium hydroxide combining with bicarbonate to form calcium carbonate, it can combine with carbonic acid or carbonate with the same result because of the equilibrium between the carbonates that subsequently occurs. Note that the curing of plaster with the bicarb procedure has the same result as subsequent hardening of plaster because the primary process in both cases is the conversion of calcium hydroxide to calcium carbonate via the carbonates in the water.

 

[onBalance]

Yes, there are a lot of reactions going on.
I notice that in the reaction of calcium hydroxide with bicarbonate, there is a hydroxyl ion left over, but with carbonic acid, there isn't a hydroxyl ion, and that seems to be a difference.
And I thought when carbonic acid is removed in the reaction with calcium hydroxide, that reduces the amount of aqueous carbon dioxide in the water, which would increase the pH in that manner.

 

[chem geek]

It is true that when carbonic acid is reduced, that carbon dioxide is reduced as well, but if that were all that was going on then there would be no change in pH whatsoever. That is, with the following equilibrium

H₂CO₃ <--> CO₂(aq) + H₂O

there is no change in pH if you were to remove both carbonic acid and carbon dioxide in equally proportional amounts IF there were no other carbonate equilibria involved. In practice, if carbonic acid is lowered, then bicarbonate ion creates some more carbonic acid having the pH rise. And yes, there is some dissolved carbon dioxide to produce carbonic acid as well but that, by itself, has no effect on pH. It is only the carbonic acid <--> bicarbonate ion <--> carbonate ion equilibria that affects the pH.

Another way of looking at it is that if carbonic acid is removed, then one is removing a weak acid from the water and that would make the pH rise. Not all of the carbonic acid can be made up by having carbon dioxide create more. In the bicarb start-up or hardening plaster case that I showed above, the equilibrium amounts of various chemical species is given below in moles/liter.

pH 7.5, TA 80, CYA 30, Temp 80ºF, CH 300, TDS 525
[CO₂(aq)] .. 8.1794x10^-5
[H₂CO₃] .... 1.2603x10^-7
[HCO₃^-] .... 1.3467x10^-3
[CO₃^2-] ..... 2.8576x10^-6
[H⁺] .......... 3.4761x10^-8

pH 8.00, TA 80, CYA 30, Temp 80ºF, CH 300, TDS 519 (reduced from removal of some carbonates into calcium carbonate)
[CO₂(aq)] .. 2.4741x10^-5
[H₂CO₃] .... 3.8122x10^-8
[HCO₃^-] .... 1.2872x10^-3
[CO₃^2-] ..... 8.6293x10^-6
[H⁺] .......... 1.0998x10^-8

The following shows the delta change which shows that bicarbonate ion changed the most in absolute terms though aqueous carbon dioxide was a close second while the ratios (including hydrogen ion) have remained constant since they are based on the equilibrium constants.

[CO₂(aq)] .. -5.7053x10^-5
[H₂CO₃] .... -8.7908x10^-8
[HCO₃^-] .... -5.95x10^-5
[CO₃^2-] ..... +5.7717x10^-6
Total Carbonates . -1.109x10^-4
[H⁺] .......... -2.3763x10^-8

[CO₂(aq)] / [H₂CO₃] = 650
[H⁺] * [HCO₃^-] / [H₂CO₃] = 3.714x10^-4 ... pKa = 3.43
[H⁺] * [CO₃^2-] / [HCO₃^-] = 7.373x10^-11 ... pKa = 10.13

The pKa for the dissociation of carbonic acid to bicarbonate ion is usually calculated using the sum of aqueous carbon dioxide and carbonic acid since they are in a separate equilibrium and can be treated as a combined species so doing that with the above would give a pKa of 6.24. Note that the hydrogen ion concentrations above are all adjusted by the activity coefficient for hydrogen ion to account for ionic strength (and the other species concentrations are also concentrations, not activities). Measured pH measures the hydrogen ion activity, not its actual concentration though these are close. Also, equilibrium constants are ratios of activities, not concentrations, but the activity (ionic strength) did not change much between the two scenarios (TDS dropped only a little), so explains why these pKa are not exactly the same as the actual pKa from equilibrium constants.

So the dominant reactions from the above changes, in order, are the following with the percentages relative to total calcium carbonate exchange (not net) and with percentages relative to pH change (hydroxyl ion):

.............................................................................. Carb. ....... pH
Ca(OH)₂ + HCO₃^- ---> CaCO₃(s) + H₂O + OH^- ... 48.61% ... 83.77% ... increases pH
Ca(OH)₂ + CO₂(aq) ---> CaCO₃(s) + H₂O ............ 46.61% ..... 0.00% ... pH neutral
CaCO₃(s) + 2OH^- ---> Ca(OH)₂ + CO₃^2- ............... 4.71% ... 16.23% ... decreases pH
Ca(OH)₂ + H₂CO₃ ---> CaCO₃(s) + 2H₂O .............. 0.07% ..... 0.00% ... pH neutral

So you can see that the carbon dioxide reaction (net effect) reacts about half of the time and is pH neutral while bicarbonate ion is roughly the other half causing a rise in pH. Carbonate ion is also produced and this lowers the pH, but is a relatively minor reaction while carbonic acid itself can be essentially ignored. Note that the third reaction is an explanation of rising carbonate ion presuming a calcium carbonate source but that in reality the rise in pH had carbonate ion increase by conversion of bicarbonate ion. Basically, if you combine the first and third reactions, you get the following net reaction:

HCO₃^- + OH^- ---> CO₃^2- + H₂O

which is equivalent to the following:

HCO₃^- ---> CO₃^2- + H⁺

 

[JamesW]

It has been my experience that if the CSI is kept close to 0, then there will be very little to no calcium rise after the first two weeks.

ph-rise-in-new-plaster-t29483.html

 

[onBalance]

JamesW has brought this discussion back to the original issues of this post, as it should be. Does the CH continue to rise after a new plaster pool is a month old? Some believe that the ongoing “curing” of plaster increases the CH of the water “naturally” and cannot be prevented. JamesW states that his experience indicates that CH does not increase when the water is CSI balanced. I concur with him.

First, it is very common to use the word “curing” to describe the hardening of cement, but just want to make the distinction that “curing” of plaster describes the process and conditions, such as; humidity and temperature, that cement/concrete is initially hardened under, and the actual hardening of plaster is known as “hydration” with which cement with water react together.

The majority of cement hydration occurs within the first month. And during this first month, the top of the plaster surface forms a somewhat dense layer of calcium carbonate (with the conversion of calcium hydroxide into calcium carbonate). While it is true that there is continued hydration occurring after one month, and that more calcium hydroxide is being formed (but would be a minor amount), it is more difficult for the movement of calcium hydroxide to travel to the surface due to the increased density that exists by that time.

Chemgeek’s chemical equations show that the bicarbonate/hydroxide is the dominant reaction. I concur, and I suggest that after one month has past, this reaction more likely takes place within the cement matrix and does not allow the calcium hydroxide to be dissolved into the pool water. Therefore, after one month, with the surface primary consisting of calcium carbonate, in order for the calcium hardness to increase, the water would have to be aggressive according to the CSI. If some pools have experienced the CH increasing after one month, it probably is due to other reasons, one of which may be that the water was aggressive during that time period. We should not suggest to “allow” the CH to increase “naturally.” If the CH is low, then raise it sufficiently to balance CSI before it does so by itself. That is not doing the surface good.

The original question on this post was what to do with pool water that has a CSI of -0.66, even though all of the water parameters are within the recommended range. This brings us to points that Chemgeek has cited earlier and in other posts. For salt water pools, the water parameters recommended by TFP should be adjusted to account for the increased aggressive nature of high salt/TDS levels, and possibly lower TA. And as experiments have shown, an average CSI of -0.7 can dissolve pool plaster. Yes, it may be very slight, and take years to manifest itself, the fact is that it would be beneficial to maintain the CSI as close to zero as possible, and at least, within the minimum and maximum range established by TFP.

On a side note Chemgeek, in a day or two, I plan to start another post to continue our “chemical reaction” discussion.

 

[onBalance]

Instead of starting another thread, I just want to do a summary of what is written above.

As Chemgeek's chemical equations show, the hardening (hydration) of cement occurs when when water and cement react together and forms (the very hard) calcium silicates etc., and the (slightly soluble) calcium hydroxide. It is some of the calcium hydroxide that usually dissolves into the water to create "plaster dust" (calcium carbonate) during the first month of a new plaster. Also, some of the calcium hydroxide gets converted into calcium carbonate without being dissolved first into the water and create a thin layer on top of the surface, and also creating a somewhat denser surface.

Because the surface becomes mostly calcium carbonate (as opposed to some slightly soluble calcium hydroxide at surface), it is harder for the water to dissolve the calcium hydroxide within (below) the surface, and remove it. Instead, it is more likely that after one month (once the carbonation process is mostly over), the calcium hydroxide stays in the plaster and is not lost from the plaster finish. Therefore, I suggest that if the pool water is experiencing an increase in CH, then something else is making that happen. For instance, if plaster dust was caught in the filter, then as water passes through, it is possible to re-dissolve that plaster dust which would result in an increase in CH of the water. Also, another likely senario, is that the water is too aggressive for the new plaster. That means that the water needs an adjustment to raise the CSI. Any loss of calcium carbonate (or calcium hydroxide) from the plaster means that damage in being done, creating a more porous and rougher surface in which can stain, discolor, and age sooner. There is no reason to let that happen.

Also, whether it is bicarbonate or carbonic acid reacting with the (in-surface) calcium hydroxide (after the first month), the result is that the pH increases (for slightly different reasons), but not the CH. When carbonic acid is removed in the reaction with calcium hydroxide, some of the aqueous carbon dioxide converts into more carbonic acid (thus maintainaing the same equilibrium between the two) which then lowers the carbon dioxide content, and raises the pH. (Correct me if I am wrong).

So while some have noticed the pH increasing more than normal during the first few months, there should not be an increase in the calcium hardness unless other issues are at play. I strongly recommend a balanced (zero) CSI, especially for the first six months of a new plaster pool.