Ok so from reading on PGN I understand the reasons behind the high alkalinity start ups but what then? Once the pool has stabilised what does the alkalinity do that the calcium hardness doesn't by way of protecting the surface. I don't get the buffering idea as bicarbonate of soda buffers at too higher pH so requires acid to drag down the pH which requires more bicarb in an endless cycle. Increasing the calcium carbonate hardness would protect the surface as it's pretty much the same as the plaster or grout so can anyone shed some proper chemical light on this please.
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A high alkalinity will more quickly and effectively "carbonate" (converting calcium hydroxide into calcium carbonate) a new plaster surface creating a more dense and harder surface. Once that is accomplished (generally 30 days), the alkalinity should be lowered to normal operating levels, and the plaster surface will be more resistant to aggressive water, and also balanced pool water.
As Richard points out, if the plaster has not been properly mixed and applied, it (a porous surface) can be eventually broken down by balanced pool water, and of course, aggressive water will do it much faster. So the quality and durability of a plaster finish plays a big part in the overall lasting ability of pool plaster.
Once plaster has cured, both calcium hardness and alkalinity are equally important to protect plaster, with, of course, the pH being the most dominant factor.
Thanks Kim, do you think maintaining the higher TA will also work long term on cured plaster by continuing to convert calcium hydroxide into calcium carbonate? For instance once the surface is brushed as part of the regular on going house keeping side of the pool.
Any thoughts on Richards suggestion on applying densifier (sodium silicate) to the surface?
Once the new plaster has cured for about 30 days, I don't see as much of an advantage to continually maintain a very high alkalinity. The surface is carbonated and the layer of calcium carbonate is very thin, but also very dense. So the carbonation process is mostly over, and a normal TA will still have the ability to continue carbonating when it comes into contact with calcium hydroxide.
Generally, a sodium silicate densifier is added to the plaster mix, not to the surface after the fact. My guess is that adding it to the plaster surface will cause a non-uniform discoloration. Whereas, a Bicarb start-up will do a good job of carbonating the surface without any discoloration occurring.
Thanks Kim, It's what is required as a TA level that is the question. We know the standard 80-120ppm but as Richard showed:
TA ..... pHeq
. 20 .... 7.75
. 40 .... 8.04
. 60 .... 8.21
. 80 .... 8.33
100 .... 8.42
120 .... 8.49
The TA is buffering too high to really regulate the pH level so acid is added to drag the pH down to our normal levels which is costly and potentially is beginning to soften the surface compared to 20-40ppm which would require minimal effort to maintain the pH.
Richard have you any figures for the TA/pH with the effects of CYA at say 20, 30, 40, 50 ppm as that will also be buffering the pH down a bit from the bicarbonate?
You are mixing up two different effects. Just because one uses acid to lower the pH below the carbon dioxide outgassing equilibrium, that does not mean it is softening the plaster surface. One usually has the Calcium Hardness and other parameters make the calcite saturation index near zero so that the plaster surface will not be affected at normal pool pH.
Also, though the pHeq numbers I show are when carbon dioxide outgassing would stop completely at various TA levels, in practice such outgassing slows down considerably before reaching that pH. It depends on the amount of aeration in the pool. The use of waterfalls, spillovers, fountains, etc. increases the rate of carbon dioxide outgassing and therefore the rate of rise in pH.
As for the effect of CYA or of borates that are both pH buffers, I show their buffer capacity (and that of the carbonates) in the thread pH Buffer Capacity. Note that increased pH buffering does not directly change the rate of carbon dioxide outgassing. It slows down the rate of pH rise, but it still takes the same amount of acid per time to maintain the pH. This is because the pH buffering works both ways. While it lessens the rise of pH from carbon dioxide outgassing, it also requires more acid to lower the pH by the same amount. Basically, the amount of acid needed is dependent on the amount of carbon dioxide outgassed, not on the amount the pH moved.
When there aren't other sources of rising pH such as the calcium hydroxide from plaster, then if one is using a hypochlorite source of chlorine (e.g. bleach, chlorinating liquid, Cal-Hypo, lithium hypochlorite) then a lower Total Alkalinity (TA) will lessen the rate of pH rise. This is because pool's are intentionally over-carbonated to (ironically) provide pH buffering and to protect plaster surfaces. However, this over-carbonation leads to carbon dioxide outgassing and that causes the pH to rise (with no change in TA).
Generally speaking, you wouldn't want the TA above 80 ppm but the real rule is to lower it to find a sweet spot where the pH doesn't rise too much. If your TA is on the low side, then you may need to raise the CH to keep the saturation index near 0 to protect plaster surfaces. Also, there is less carbon dioxide outgassing at higher pH so one can also have a higher pH target and not try and keep the pH at 7.5, but instead target something like 7.7 or so. This technique is needed even more in saltwater chlorine generator pools since they have increased aeration of the water that may increase carbon dioxide outgassing (they may also have undissolved chlorine gas outgassing, but that's another matter).
Thanks Richard, yes from your other information we exchanged some time ago I have been experimenting lowering the alkalinity levels. I used an analogy with a customer of treating alkalinity like a rubber band on a scale of 0 to 14 with the rubber band placed at 8.2. The more alkalinity you add the thicker the rubber band is so more pull is required to get it to 7.2 Using a much lower alkalinity is like using a much thinner rubber band so it's easier to move and keep at 7.2.
I expected to run into trouble with my alkalinity at around 38 (pH 7.3) as I had read a paper describing a cliff and if you go off the edge then the pH would plummet but that would require a chain reaction to release that much hydrogen. On a customers pool they got chemical additions wrong and with soft water ended up at alkalinity of 22 but still bad things didn't happen.
My understanding is
When added to water, an acid releases hydrogen ions. Hydrogen ions in water combine with the water molecules to form hydronium ions, thereby decreasing the pH and increasing the acidity of water.
Water exposed to air is mildly acidic because it absorbs small quantities of CO2 from the atmosphere. The CO2 breaks down to carbonic acid that then dissociates to release hydrogen ions into the water, decreasing the pH. So at some point it maybe possible to get the wrong side of the dissolved carbonic acid equation and actually have the pH fall.
It seems to me that the levels suggested for pools have come from ponds where it's generally accepted that you wish to encourage life and all sorts of plants in the ecosystem. By contrast we really want pools of zero life so all nutrient levels should be reduced to a minimum where possible.
No, your understanding about this is wrong. Though it is true that there is an equilibrium between the carbon dioxide in the air and the water and that carbon dioxide in the water forms carbonic acid that then further dissociates into bicarbonate ion and carbonate ion, most pool water is OVER-CARBONATED. What I mean by that is that there is MORE carbon dioxide in the water than the equilibrium amount based on the amount of carbon dioxide in the air. That means that unless there are other acid/base additions to the water, the water will tend to RISE in pH. Technically, the pH won't stop until equilibrium is reached which is at the pH listed below for each carbonate alkalinity level (TA if no CYA or borates are present):
TA ..... pHeq
. 20 .... 7.75
. 40 .... 8.04
. 60 .... 8.21
. 80 .... 8.33
100 .... 8.42
120 .... 8.49
Now in practice, the pH slows down in its rise so much that it appears to settle in below the pH noted above. Also, there are always some acid or base additions to the pool. Even hypochlorite sources of chlorine contain a small amount of base -- excess lye in sodium hypochlorite chlorinating liquid or bleach, for example, or some calcium hydroxide in Cal-Hypo.
Why in the world are you trying to keep the pH at 7.3? That is too low. The rate of carbon dioxide outgassing is higher at higher TA or at lower pH or with more aeration. See the following chart that shows how much the water is over-carbonated at various TA and pH:
http://richardfalk.home.comcast.net/~richardfalk/pool/CO2.htm
So in general, you do NOT have carbon dioxide entering the water from air to lower its pH. Usually it's the other way around with carbon dioxide outgassing causing the pH to rise.
Finally, you shouldn't normally have your TA as low as you have, most especially if you have plaster surfaces that need to be protected by saturating the water with calcium carbonate. I usually say not to go below 50 ppm, but as you have found so long as you aren't adding any acid or net acidic chemicals accounting for chlorine usage/consumption (e.g. Trichlor or Dichlor or MPS) then the TA does not "crash" and the pH doesn't drop precipitously.
Thank you Richard, I haven't got it wrong, I do understand but I think I placed a red herring in my previous post mentioning the air acidifying the water, please forget that part. I know from your chart that we have more carbon dioxide in the pool hence off gasing.
The table is very useful and still shows why one of the pools (vinyl) can still behave perfectly with TA at 22ppm. My own, again vinyl at 38ppm is still slightly climbing in pH requiring a cup of acid or two over a season
My pH is happy at 7.3-7.4 and requires very little acid additions. In reality I have noticed the cheap NaClO has contained a bit more sodium hydroxide this year as the pH did rise marginally more than last season. Also chlorine is more potent at the lower pH and
Separately why does bicarbonate do more to protect plaster than calcium, as calcium is the bulk of the plaster/grout?
You are right that I honed in on that air acidifying water statement so glad that wasn't your core understanding.
Plaster/grout doesn't have just calcium. The actual solid is calcium carbonate in between the calcium silicate hydrate (CSH). The calcium in the CSH is strongly bound and won't generally dissolve in water, but that is not true for the calcium carbonate. Basically what happens is that uncured plaster is mostly tricalcium silicate and when it cures it absorbs water and converts the tricalcium silicate into calcium silicate hydrate while releasing calcium hydroxide. In a bicarbonate startup, the calcium hydroxide which is soluble in water is instead converted to calcium carbonate in place (i.e. in the plaster matrix) at least near the surface of the plaster.
The calcium carbonate is susceptible to dissolving in water, especially if the pH is low, and saturating the water with calcium carbonate and maintaining a reasonable pH help prevent this dissolving from occurring. If you don't do this, then the calcium carbonate can dissolve into the water producing pitting and if it is severe enough it can have the CSH break apart as well.
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