PH bounce and alkalinity

PH bounce, what exactly does this mean?  Should I expect to see some kind of rebound or is this another pool phrase.

I am talking specifically about vinyl liner pools and the need for the alkalinity buffer to create a stable pH.  We are used to adding bicarbonate of soda to the water to create a buffer at levels of 80-200ppm but bicarbonate of soda actually buffers at pH8.1+, too high for pool use really so we have to add acid to maintain the pH at the correct level and generally the pH climbs (drifts)  slightly through splashing or aeration or remnants of sodium hydroxide in the chlorine dosing liquid.

As I am talking about vinyl pools not concrete/plaster/tiled to prevent/lessen the drift upwards in pH I have been lowering the alkalinity steadily over the last few years and currently my pool is at 38ppm alkalinity and I have customers who also have vinyl pools running at 45ppm and 22ppm without any issues and I am still waiting to hear the pH has either climbed through to roof or dropped off the edge and heading downwards.  The thing is for each addition of chlorine we can reasonably predict the pH rise and for any acid addition the fall. At no point does this create a chain reaction where hydrogen ions are lost or formed causing a catastrophic “bounce”.  What it has meant is barely any acid has been used to correct the pH and there has been no need for soda ash to raise the pH it just stays reasonably stable and why not have the pH move easily without dragging the buffer solution down, so where is the bounce?

I realise that cyanuric acid stabiliser (CYA) is also a pH buffer to prevent the pH from rising but in my own pool I have been experimenting without CYA as well and the dosing system is more than capable of keeping the pH adjusted with very minor amounts of acid, if any at all.

I am trying to minimise chemical usage but also as bicarbonate of soda is carbon and oxygen which is required for almost all life/growth of bacteria etc so restricting unnecessary extra food supplies is where I am heading. I wouldn’t use CYA either as that is also food for bacteria by virtue of it’s nitrogen source.  Obviously the use of CYA is currently needed to protect the chlorine from photo degradation but as we know some bacteria can use this and leave ammonia behind for us to deal with, there does seem to be a possible link to CYA and strains of E coli.

oligotrophic (low nutrient) water tends to have lower alkalinity while eutrophic (high nutrient) water tends to have higher alkalinity.  So I am trying to work with the low nutrient water, it’s not a pond, it’s a pool and if I could just have H2O.  I have been trying to throw my pool water out of the prescribed ranges waiting for bad things to happen but they don’t I just have perfect clear bacteria free 0.5-1 NTU water.

What are others thought on alkalinity in vinyl pools and pool chemistry?

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  • Sorry to not respond to this earlier (it slipped by for some reason).

    There is nothing wrong with you having a lower TA level to help reduce the rate of pH rise.  Just note that a higher pH target also helps as shown in the following table that shows how over-carbonated pool water is compared to the amount of carbon dioxide in air.

    As for the relative buffer capacity between carbonates, CYA, and borates, see the following post I wrote:

    As for hypochlorite sources of chlorine, though they raise the pH when added, the pH drops back down when the chlorine is used/consumed because that is an acidic process.  The only net pH rise from this is the small amount of excess lye in chlorinating liquid or bleach.  Any additional pH rise is due to carbon dioxide outgassing which you are minimizing by keeping the TA lower.  See the following post for chemistry on the pH of chlorine sources:

    As for algae growth and TA, that's where you are off and not quite right.  You'd have to get the TA a LOT lower to starve algae.  It's far easier to lower the phosphate level with a phosphate remover if that is your intention, but remember that you can prevent algae growth regardless of algae nutrient (phosphate, nitrate) level by maintaining a sufficient FC/CYA ratio.

    • Thanks Richard, many of the links you have posted were the initial spark for the experiment of using low TA in vinyl pools.  Like other areas of chemistry you tackled earlier on the TA prevents pH bounce was something I couldn't get to grips with. The pH doesn't bounce, it responds to a certain amount of pH+ or minus in an orderly fashion.

      The pH doesn't suddenly drop of a cliff as I once read in an article, for that to happen it would need a chain reaction.

      I keep the pH at the lower end because of one of your articles where it last longer and of course is more potent a bactericide.

      I realise from your chart it would be necessary to go very low with TA to try and starve algae but every little helps together with low phosphates.  I forget how many seasons have passed now with my outdoor pool running very low chlorine (0.2ppm) and no CYA but using titanium dioxide nano particle as a sun screen for the chlorine and photocatalytic  oxidation to improve the redox potential. but the results are good.

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