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Pool pH and chlorine: why you should measure both

Chlorine disinfects, but pH helps decide how well it does so. If you only measure one of the two, you're missing half the story. An explanation of the link, your type of system and your test method.

By AQELIO-redactie5 min read

Short answer: chlorine kills germs, but the pH of the water determines how much of that chlorine is present in its most effective form. At too high a pH the same chlorine level works less well; at too low a pH skin, eyes and equipment can suffer. That's why you always measure them together, correct the pH first and then assess the disinfection.

What are you actually measuring?

pH tells you how acidic or basic the water is, on a scale of 0 to 14. Water at pH 7 is neutral. For home pools and hot tubs, the US CDC gives a recommended range of 7.0 to 7.8.

Free chlorine is the chlorine still available to disinfect. It's the value you usually read as "free chlorine" on a test strip or in a test kit. There is also combined chlorine: chlorine that has already reacted with contaminants and barely disinfects any more. Some tests also show "total chlorine", the sum of both.

How pH and chlorine are linked

When chlorine dissolves in water, two forms appear: hypochlorous acid and hypochlorite. Together they make up free chlorine. Hypochlorous acid is by far the stronger disinfectant, and the pH sets the ratio between the two. The higher the pH, the smaller the share of the strong form. This is explained by, among others, the UK Pool Water Treatment Advisory Group (PWTAG) and the CDC.

Two practical consequences:

  • A chlorine reading doesn't tell the whole story. The common DPD test measures both forms together. At a high pH the chlorine value may look fine while disinfection is weaker than you think.

  • pH first, then chlorine. Adding chlorine while the pH is too high is trying to fix a problem with the wrong control.

Too low isn't good either: according to the CDC, a pH below the range can irritate skin and eyes and damage equipment.

SituationpHFree chlorine
Pool without stabiliser7.0 – 7.8at least 1 ppm
Pool with stabiliser (cyanuric acid)7.0 – 7.8at least 2 ppm
Hot tub / spa7.0 – 7.8at least 3 ppm

ppm means the same as mg/l. The CDC advises against using stabiliser or stabilised chlorine products in hot tubs. Always follow the instructions for your equipment and products too: they may give a more specific range for your situation.

Your type of system matters

How pH and chlorine behave also depends on how you disinfect:

  • Chlorine tablets or granules with stabiliser: the stabiliser protects chlorine from breakdown by sunlight, but it builds up in the water. As a result you need a higher free chlorine level.

  • Liquid chlorine or unstabilised products: without stabiliser, chlorine breaks down faster in the sun; test more often on sunny days.

  • Salt chlorination: the system makes chlorine from salt. You still measure pH and free chlorine, because production has to match demand.

  • Hot tub: high temperature and heavy use on a small volume of water can make demand rise quickly.

The composition of your product also determines its effect on pH. So always check the label of the product you use, rather than a general rule of thumb.

How to test reliably

  1. Follow your test's instructions. Dip time, reading time and order differ by brand.

  2. Test at a fixed time, not straight after adding something, unless the label asks you to.

  3. Know the limits of your method. According to the CDC, a DPD test kit is the most accurate method for home use; test strips are usable but less precise. Above 10 ppm chlorine a test can bleach out and show a reading that is too low.

  4. Measure pH and chlorine in the same round, so you can assess the values together.

Read more about reading results in how to use and read pool test strips.

Correcting in the right order

  1. Test pH and free chlorine.

  2. If the pH is out of range, correct it following the label and your water volume.

  3. Let the water circulate and test again.

  4. Then assess free chlorine and add disinfectant if needed, following the label.

  5. Test again after the waiting time stated on the label.

Never mix pH products and chlorine products: the CDC warns that chlorine and acid in particular can release toxic gases when combined. Add them separately, with time in between.

Handy for this job

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Frequently asked questions

Why does my pH keep rising?

There can be several causes, such as the type of disinfectant, your source water or the way your equipment works. Test at fixed times for a few days in a row; the pattern helps you or your pool professional find the cause.

Is more chlorine always safer?

No. Too much chlorine can cause irritation and disturb test results. Aim for the recommended range for your situation and follow the label dosage.

This article is general information, not personal advice. Always follow the label of your products and the instructions for your equipment; when in doubt, ask a pool professional.

Sources

  1. Home Pool and Hot Tub Water Treatment and Testing — U.S. Centers for Disease Control and Prevention (CDC)
  2. Understanding the importance of the active free chlorine content of the free chlorine test (TN60) — Pool Water Treatment Advisory Group (PWTAG)
  3. Guidelines for Keeping Your Pool Safe and Healthy — CDC

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