Does chlorine lower the pH?
Not necessarily: it all depends on the type of chlorine. Trichlor tablets lower the pH and alkalinity (TAC) week after week. Dichloro shock chlorine barely lowers it and only briefly. Liquid chlorine raises it at the time of addition, then the pH drops. If your pH drops, check your tablets and your TAC.
2.8 to 3 pH of a dissolved trichlor tablet
6 to 7 pH of dissolved dichloro
12 to 13 pH of liquid chlorine
What each type of chlorine does
"Chlorine" covers several chemicals, whose pH levels are completely different. The chart places them on the same scale.
Slow-release tablets and pucks: trichlor
It is the only true acid in the family. CTX 370 slow-release chlorine is one of them. Used as the main chlorine, trichlor continuously lowers the TAC and keeps the pH low. Our article on tablet dosage also explains its effect on stabilizer.
Granulated shock chlorine: often dichloro
CTX shock chlorine is a dichloro. It is almost neutral. The pH drop upon addition remains low and temporary, and its effect on TAC remains minor. However, like trichlor, it adds stabilizer. Some shock chlorines are based on calcium hypochlorite: check the label.
Liquid chlorine: sodium hypochlorite
It is stored at a very high pH. The pH rises just after addition, hence the instruction on the CTX liquid chlorine data sheet: readjust the pH two hours after an initial treatment.
Why the immediate effect is misleading
When chlorine works, or when the sun breaks it down, it releases acid. With hypochlorite, this acid almost exactly compensates for the base added. Over time, liquid chlorine and calcium hypochlorite therefore have an almost neutral effect on the pH.
The pH you measure just after an addition therefore does not tell you much. Wait until the filtration has distributed the product. To judge a trend, compare several measurements taken at the same time over several days.
Trichlor, on the other hand, has no base to compensate for. Its acidity adds to that produced by the chlorine in operation. This is why its downward trend is seen over several weeks, not all at once.
Record your measurements and the products added in a notebook. After a few weeks, you will see if the pH follows your tablets, your water top-ups, or the weather, and you will correct the real cause.
The role of TAC
Alkalinity acts as a buffer: as long as it is sufficient, the pH moves very little. When it is too low, the pH becomes unstable and can drop. A pool chlorinated with tablets precisely consumes this reserve.
Our rule: mineralize before treating. Below a KH of 7, or about 125 mg/L of TAC, raise the alkalinity before correcting the rest. The CTX-21 Alka Plus data sheet states the same threshold. Without this, a dose of pH plus will not hold.
Your pH is dropping: what to do
Our target remains a pH between 7.0 and 7.4, without exceeding 7.6. The CTX pH plus data sheet indicates 150 g per 10 m³ to raise the pH by 0.2. Proceed in small doses, distribute over the surface, without swimmers, and measure the next day.
If the phenomenon returns, change the approach rather than increasing the doses. Depending on your installation, some of the chlorine can be switched to liquid chlorine or a one-off shock chlorine. Our chlorine maintenance guide details the order of adjustments.
Caution. Never mix chlorine with a pH corrector, neither in a bucket of water nor in the skimmer: the mixture can release chlorine gas. Use a clean and dry dosing device for each product, pour them separately, with the filtration running, and read the label.
Useful products
Start by measuring: without pH and TAC values, any correction remains a guess.
Chlorine and pH test kitTo monitor the pH and free chlorine at the same time, several times a week.View productCTX-21 Alka PlusIf the TAC is below 125 mg/L: 180 g per 10 m³ increases the TAC by 10 mg/L, according to the data sheet.View productCTX pH plus 5 kgFor a pH that is too low: 150 g per 10 m³ increases the pH by 0.2, according to the data sheet.View productCTX-15 pH Minus liquidIf the pH goes above 7.6: 3.25 L per 100 m³ reduces it by 0.2, according to the data sheet.View product
Your pH is not holding? Send us your volume, your type of chlorine, and your latest pH and TAC measurements.
Ask my questionFrequently asked questions
Does shock chlorine lower the pH?
It depends on its nature. Dichloro, like CTX shock chlorine, has a pH of 6 to 7 once dissolved: the drop remains slight and temporary. A shock treatment based on hypochlorite, on the contrary, raises the pH at the time of addition. Measure the next day.
Does liquid chlorine raise the pH?
Yes, at the time of addition, because it is very basic. The pH then drops as the chlorine is consumed, and the net effect over time is close to zero.
Why is the pH rising when I use tablets?
Tablets pull the pH down, but the water tends to raise it: carbon dioxide escapes, especially with agitated water, and limey filling water also weighs in. The result depends on this balance of forces. Only a measurement can tell.
Should I measure the pH before or after the chlorine?
Before. First adjust the pH, then add the chlorine. CTX data sheets require this, because a correct pH makes the treatment more effective. Never pour both at the same time.
Read also
To dose your tablets: How many chlorine tablets per m³? Tablet dosage according to your pool
READ THE ARTICLE
Read also
For the complete routine: Maintaining your pool with chlorine or salt without the hassle: the complete guide
READ THE ARTICLE
Read also
If the level is too high: Too much chlorine in the pool: 6 ways to lower the level, from the gentlest to the fastest
READ THE ARTICLESources and references
- Orenda: Does chlorine affect pH?
- Aqua Magazine: Key misunderstandings and problems in pool water chemistry, part II
- Anses: Pool and spa disinfectants, follow the precautions for use
- Poison Control Center (Belgium): Pool products
- Foudebassin product data sheets: CTX liquid chlorine, CTX shock chlorine, CTX-21 Alka Plus
Last updated: November 4, 2026.
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