How To Lower Alkalinity In Pool Without Affecting pH

Acid options to lower pool water alkalinity

Alkalinity is a way to measure the amount of bicarbonate and carbonate in water. Another way to understand the alkalinity of water is to see it as the limestone content contained in the water.

How To Lower Alkalinity In Pool Without Affecting pH

Greater alkalinity means that more limestone is applied and this causes the pH of the substrate to increase. To compensate for the increase in pH, acid is injected into the irrigation water to neutralize part of the alkalinity and decrease the limestone content in the water.

Reducing alkalinity in water isn’t seen as necessary in our environment, since we only have access to water of low hardness and alkalinity.

However, there are cases in which the supply water is heavily carbonated and basic. This is the case of some groundwater, with high alkalinity and pH values.

Trying to lower the pH under these conditions is almost impossible. This is because the alkalinity shields the pH and strong acids are neutralized in their pH-reducing function.

Here, we teach you the common practice on how to lower alkalinity in pool without affecting pH level.

The traditional method of lowering alkalinity is summarized in these recipes.


  1. Inject sharply and as thoroughly as possible (through a tube for example) undiluted hydrochloric (muriatic) acid at nightfall with stopped treatment. The amount, of course, depends on the volume of water. As a rule, you should not try to lower the alkalinity more than 100 ppm at a time. We should, in that case, do it in two or more sessions with no less than 24 hours of interval between them. For example, if in the previous pool we wanted to lower the alkalinity to 70 ppm, we should lower a total of 185-70 = 115 ppm.
  2. At 24 hrs of the first session, we have to measure new pH and alkalinity. We can meet with the following situations.

(a) Alkalinity very close to the desired value and pH within the range of our disinfectant: <7.5 for chlorine, <7.8 bromine. Here, we have both parameters correct.

(b) Alkalinity still high and pH> 7.2: Here, we have to repeat another session to continue lowering the alkalinity, but if the pH is very close to 7.2, we should do sessions of very little reduction of alkalinity, for example, 10 in 10 ppm. This is because if in one of the sessions, we add too much acid and after that, we measure a pH <7, the process is finished. Although, we have not yet managed to reduce the alkalinity to the desired value. And it is not necessary to continue decreasing alkalinity with pH lower than 7, we will have to recover the pH up to 7.2.

how to lower alkalinity in pool without affecting pH

(c) High alkalinity and pH equal to or less than 7: Here,  we should not even try to reduce the alkalinity with pH values ​​lower than 7. We must first recover the pH to at least 7.2 How?

  1. Raise pH from 7 to 7.2 without increasing the alkalinity. With the traditional pH elevators (the strong bases of sodium carbonate or sodium hydroxide), or when in both cases, we will also increase alkalinity. The only known method is by deaeration; that is, subjecting the water to a physical process of atmospheric contact in such a way that it loses its dissolved gases, especially the one we are looking for: carbon dioxide (CO2). We are not going to enter into chemical disquisitions, but the reality is that extracting CO2 from water raises its pH, while dissolving CO2 increments.
  2. Once we recover the pH up to 7.2, we can make another session to reduce the low margin of ppm of alkalinity so that the pH does not drop below 7. These sessions of low alkalinity reduction can be made several times a day, even in a continuous way. For example, those who have a pH meter can adjust it to 7, inject acid continuously and the artificial source extracts CO2 continuously; that is, it reduces the alkalinity in continuous forms, after all the dissolved CO2 is the initial cause of the alkalinity through its conversion into carbonic acid.

How to aerate water

There are several practical ways to aerate the water. The most immediate is to place the impellers in a way that causes a greater whirlpool. However, the aeration here will be very small.

A very practical solution consists in threading a 50 mm PVC pipe into an impeller, then a 45° elbow, then another pipe.

In the end, a female plug is glued to where the drill has drilled multiple holes.

With the rest of the returns partially plugged to get a pressure (careful not to reach the red area), we will have a nice source that will raise pH without chemicals and with no increase in alkalinity and hardness.

What to take note of before and during the procedure

  1. Selection of an acid: The most common acids to decrease the alkalinity of water are sulfuric, nitric, phosphoric and citric acids. Citric acid does not provide any important level of elements, while the three other ones do.
  2. Tips for mixing acids: The best option is to inject the acid directly from a concentrated container in the irrigation pipe, to avoid handling. It is the safest option. The acid must be diluted in water before injecting it. Remember to add acid to water and not water to acid, as this can cause dangerous reactions. Although, these acids are liquid, they are heavier than water and will settle in the bottom. Keep in mind that the acid has a caustic effect on metal parts.
  3. Safety in the use of acid: Citric acid is the safest of the acids used, followed by sulfuric acid and phosphoric acid. Keep in mind that the more concentrated the acid (the higher the percentage of active ingredient), the more caustic and dangerous. With any of these acids, avoid contact with skin or eyes, as it can cause irritation or even minor burns. These acids can damage clothes causing holes after washing clothes. Nitric acid is more caustic and can burn the skin or eyes. It can also cause dangerous gases that are not safe when breathed in. When working with an acid, the best option is to wear safety glasses, a protective face mask, acid resistant glove and an apron.


We hope the tutorial was explanatory enough for you to understand and practice on your own. When dealing with acids, it’s necessary that you err on the side of caution.

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