SW Pool with pH8 and TA 110

Problems relating to pH and total alkalinity.
Increase ph, increase TA. Reduce pH, reduce TA.
pH chemistry advice and techniques for the pool.
osmosis
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SW Pool with pH8 and TA 110

Postby osmosis » Wed 04 Oct, 2006 20:23

Hi

I have a salt water pool of about 45 000L. My pH remains at 8 despite religiously adding liquid acid. My TA is at 110. Should I be raising my TA and then adding acid (will this help lower my pH) or should I persevere by adding more HCl?

Thanks for any replies.


Demons1964
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Postby Demons1964 » Thu 05 Oct, 2006 18:23

Welcome to the world of increasing pH with the SWG, the only downside to using them.

First, can I ask if you're using a drop-based reagent kit or test strips? Test strips can be very unreliable, and pH in particular is very difficult to read.

Although you're adding acid religiously, it all depends on how much acid you're adding per day. I'm adding about 0.4L per day to my 60,000L pool and I bounce around between a pH of 7.5 and 7.7 depending on whether I miss a day or not. The amount of acid I add seems to be a fairly typical amount for a fastidious pool owner.

I've read theories about having a TA in the vicinity of 60ppm to reduce acid demand, and I'm currently trialling it at the moment.

As to your solution, add hydrochloric (muriatic) acid at the rate of no more than 1L per day to get your pH down to at least 7.6. Your TA will drop appropriately, and if you wish to maintain it at 110, then for every litre of acid you add you'll need to add about 1.1kg of sodium bicarbonate. I suggest you get your pH correct first before correcting the TA.
osmosis
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High pH and TA of 100 now

Postby osmosis » Thu 05 Oct, 2006 18:33

Thanks so much for the reply.

Yes, I use a drop based kit (Blue Devil).

I probably have not been adding as much acid as you have -- maybe I'm "under-acidifying". 400ml a day seems a lot to me. Are you goig through many of those 5L containers!

1. Does one have to dilute the liquid acid before dispensing?
2. Also, I have read that dispersing the acid around the pool while the filter is on is different to adding it into a still spot when the pool is off in terms of affecting TA. Is this true and if so why?
3. Does the hardness affect the balance at all? My water is a little on the soft side at the moment.

Many thanks again.
Demons1964
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Postby Demons1964 » Thu 05 Oct, 2006 19:20

1. Yes, highly recommend you dilute the acid. Hydrochloric acid has a specific gravity greater than water so if you pour it in the one spot you run the risk of it sinking to the bottom before dispersing evenly. I dilute 0.1L in 5L of water before broadcasting evenly around the pool with the pump running due to safety concerns.

2. I've also heard that, but I haven't seen any evidence to support it. Maybe you can experiment and become the official guinea pig. If you've got a vinyl pool do not attempt this under any circumstance, you'll bleach the liner in no time.

3. Water balance is a measure of a whole bunch of factors: pH, temperature, TA, calcium hardness. Calcium hardness is is particularly important with a plaster finish where calcium can leach out of the plaster into the water to acheive neutrality, but this doesn't have any bearing on pH.

Low calcium hardness is beneficial for a SWG, by reducing calcium buildup on the plates. For a self-cleaning SWG this is not so crucial but for a single polarity unit high hardness requires constant manual cleaning and less effective chlorine production while the plates are coated.
osmosis
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Postby osmosis » Thu 05 Oct, 2006 23:37

Isn't your experiment with low TA going to encourage leeching?
Demons1964
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Postby Demons1964 » Fri 06 Oct, 2006 17:00

All that TA provides is a buffering capacity to reduce pH fluctuations, so variations in TA have no effect on calcium reatained within the pool surrounds. Degradation of plaster might be encouraged by a pH below 7.2, not a problem you or I are ever going to face!! The key question is what is the ideal buffering capacity, maybe the recommended TA of 80-120ppm is too high running a SWG and contributes to high pH.

Besides I've got a vinyl pool so calcium hardness is not something I worry about :wink:, except to say that in my pool it's very low and hence trouble-free SWG operation.
osmosis
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Postby osmosis » Sat 07 Oct, 2006 16:18

Thanks again for the reply.

Do you think it is a good idea to increase my TA to say 150 before I lower my pH, based on the assumption that the TA will fall as I lower pH?

Also, is it better to buy buffer from the poolshop or just use regular baking soda from the supermarket.

Thanks.
Demons1964
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Postby Demons1964 » Sat 07 Oct, 2006 19:03

There will a small pH rise when adding sodium bicarbonate and given your pH is already so high (and hence you'll need elevated chlorine levels to compensate), I suggest you add acid first. Keep adding acid until you either achieve a pH of 7.4 or your TA hits 80ppm. If your TA bottoms out at 80 and your pH is still high then start adding sodium bicarbonate. Keep it between 80 and 120ppm while continuing to add acid and that should allow you to achieve your desired pH.

As to whether to use baking soda do a price comparison in your area. Firstly just check there's no other ingredients in the baking soda at the supermarket, apart from sodium bicarbonate. Then whatever is cheaper will do the job for you. Large department stores and/or hardware stores also carry pool chemicals at prices sometimes cheaper than pool stores.

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