HCL or Na2HSO4

Thanks energyconversion ,

Yes it as I thought.

There is a scale line at the sides of the pool.

Some people have added some random chemicals and I do not know what they are.

It is a sand filter and under powered/sized.

Sorry for mistake in NaHSO4 / Na2HSO4.

At least you know your stuff.

Cheers.

I am not sure that adding acid will solve your problem. Most likely the limestone is causing the low pH, so adding acid will only be a short term solution. A off the cuff estimate would be adding acid once a year for that much water, but it might be as often as once a month. For the amount that you need, HCl will be your cheapest option.

Be careful with how you add the acid. Many of them fume horribly, especially HCl. Also don't forget that the limestone will release CO2 as it is broken down by the acid. This can become concentrated in the low laying areas for the next several days after treatment.

"the limestone is causing the low pH" - you mean HIGH pH. The water is ALKALINE (pH > 7).

Why is adding acid only a "short term solution"?

Are you kidding?

No.

Adding HCl is the standard treatment for lowering alkalinity in swimming pools. Since the carbonate/bicarbonate/carbonic acid must equilibrate with the CO2 in the atmosphere (a huge stable reservoir). When you add HCl it drives the equilibruim towards off gassing the CO2 and reduces the alkalinity. As long as you have chloride ions present and a neutral (relatively more acidity) condition the calcium, magnesium and iron will have a higher solubility.

It would be nice to have more chemistry. I assume you are trying to lower pH for hypochlorination. Are you targeting a pH of 7.4-7.6 as ideal? What is the actual alkalinity, pH, temperature, Total hardness ((calcium + magnesium) as CaCO3), conductivity, and Langlier Index? In swimming pool hypochlorination (liquid or granular), it is common to reduce pH with HCl. Note that the pH of a pool using liquid or granular chlorination will gradually increase the pH so you must check the pH and adjust it frequently. Follow the rule of 25000 as explained here. You do not want to create calcium sulphate as it will make the pool cloudy. Lots of pools are filled with ground water from limestone aquifers. Most of these aquifers have high hardness (Ca and Mg) and high alkalinity. Lots of good books on chlorination of pools and do suggest you get one if this is an ongoing operation.

Portugalphilip,

I assume, from the wording of your post, that the pool is not contained within a limestone rock shell, but is filled with water that comes from a borehole or well in limestone. Your problem is very common for swimming pools. I agree with post #1 who pointed out that you don't want the calcium sulfate. Hydrochloric acid is usually used by pool maintenance contractors for lowering the pH. It can be poured slowly from the bottle directly into the pool at a location where incoming water will disperse it. You want goggles and gloves when doing this. I also suggest that you test your water for what is called "alkalinity" in the USA. This is a measure of its capacity to buffer the pH and make it more stable. The normal chemical used for increasing the "alkalinity" is Sodium bicarbonate (baking soda), or NaHCO₃. Its use raises the pH a little. The recommended range of its concentration is very wide, so you are very unlikely to add too much. Without doing a careful calculation, I believe an OK dosage is in the range of 1 kg per 15,000 to 20,000 liters of water.

--JMM

This is an interesting problem. It is in fact no problem but due to lack of knowledge it is creating words strung together.. If the hole is in limestone rock and surrounded by limestone there is no acid treatment that will work to lower pH until the entire "mountain" is dissolved down. Check the PH and the chemistry of the water and see what is causing the high PH. It is probably just the calcium carbonate. You have "hard" water. If so swim in it. If you think you have a swimming hole. 80 liters would be like a big bathtub. Is this a 3rd world comment. If you only think there is a hole maybe nothing of this situation exists. It may not even be limestone but simply people dumping their laundry water on the ground with a dip in it.. That will also give you high pH.

Wow, for such a aggressive tone, upon review it is surprisingly less than astute than would be expected. While at equilibrium this would be true, it is not necessarily true the way it was presented because you have not taken into account the contact surface and reaction kinetics. Acid falling through a large enough hole fast enough might not react fast enough to neutralize the acidity to a equilibrated pH with the limestone. You could actually get a lower pH if the rate of transport was fast enough, and the contact surface relative to the volumetric flow rate was small enough. (as an example of reaction kinetics, take a piece of limestone and drop it in a swimming pool balanced between 7.0 and 7.2 pH, which is slightly acidic of equilibrium and see how long it takes to dissolve, I bet you'd be surprised how long a solid rock can sit in a relatively acidic environment).

There is actually other reasons to balance the pH and alkalinity of swimming pools, one is to maintain the active disinfectant properties of hypochlorite, as well as other active agents. Additionally, pH effect deposition of solids through out the system, which can lead to issues of clogging in filtering systems as temperatures/pressures change. It can also cause valve problems and issues for some other moving parts. Alkalinity is a standard probelm with both potable water and swimming pools that must be controlled, typically by the introduction of 2-stage treatments like carbonates and carbonic acid, or 1-stage hydrochloric acid.

Right - the OP is not at all clear but it does sound like a pit or small quarry has been filled with water.

With the information given, I would have to say that hydrochloric acid is the way to go. However, it is not clear exactly what you are trying to do. If you are trying to reduce alkalinity as well as lower the pH, there are different procedures to follow than simply adding the acid. If you are also trying to lower the calcium levels, there are other procedures as well.

This site should give you a better feel about how pool chemistry works.

Thanks that is excellent information.