Because pH determines how much of the chlorine present is in its active sanitising form - at high pH most of it is not.

Chlorine dissolved in water exists as hypochlorous acid and hypochlorite ion. Hypochlorous acid is the effective sanitiser; hypochlorite ion is far weaker.

The proportion between them is set by pH. Within the normal operating range the active fraction changes dramatically for what looks like a small pH movement.

So a pool running above its target pH can test at an adequate free chlorine level while actually sanitising at a fraction of the expected rate. Algae appears, water goes dull, and the natural response is to add more chlorine - which does not fix the cause.

pH is also what bathers feel. Outside the comfortable range water stings eyes and irritates skin regardless of the chlorine level.

Correct pH first, then assess whether chlorine is genuinely short.

It buffers pH, holding it steady against additions - which is why alkalinity is adjusted before pH, not after.

Total alkalinity measures the water's capacity to resist pH change. With adequate buffering, adding acid or chlorine moves pH a little; without it, pH swings dramatically with every dose.

Low alkalinity produces the pattern known as pH bounce: readings that are different every time and impossible to hold in range. It also makes the water aggressive to surfaces and metal.

High alkalinity has the opposite effect - pH becomes very difficult to move and tends to drift upward continuously, and scaling and cloudiness follow.

Correct alkalinity into range first with an increaser or with acid, allow the water to circulate and settle, then adjust pH.

Trying to hold pH in a pool with wrong alkalinity is a losing battle that consumes a great deal of chemical.

Sodium bicarbonate raises alkalinity with little pH effect; soda ash raises pH; acid lowers both, in different proportions depending on how it is added.

Sodium bicarbonate is the standard alkalinity increaser. It raises total alkalinity substantially and pH only slightly, which is what allows the two to be adjusted separately.

Sodium carbonate - soda ash - raises pH strongly and alkalinity somewhat, and is used when pH alone is low.

Acid, whether dry acid or liquid, lowers both. Adding it slowly in a broad stream lowers pH more than alkalinity; adding it in one place with slow circulation lowers alkalinity more. Technique matters, and product instructions describe it.

Dose with the circulation running and never add different chemicals at the same time or in the same place.

Retest after several hours. Corrections applied and retested immediately produce misleading numbers and a cycle of over-correction.

It is the dissolved calcium in the water. Too low and the water attacks surfaces and metal; too high and it deposits scale.

Water low in calcium is aggressive. It dissolves calcium from wherever it can find it - plaster, grout, tile adhesive, concrete - which etches surfaces and can cause real structural damage over time. It also corrodes metal components.

Water high in calcium deposits it as scale on surfaces, in pipework, on heat exchangers and in filters, reducing efficiency and eventually causing failures.

Hardness is raised with calcium chloride. It cannot easily be lowered except by dilution with softer water, which is why hard fill water is a long-term operating consideration rather than a one-off correction.

Calcium hypochlorite sanitiser adds hardness continuously, which matters in hard-water areas.

Vinyl-lined pools tolerate a wider range than plaster or tiled pools, where the surface itself is the source the aggressive water attacks.

It combines pH, alkalinity, calcium hardness and temperature into a single figure indicating whether water will scale or corrode - useful as a check, not as a substitute for the individual readings.

The index expresses whether the water is saturated with calcium carbonate. A positive value indicates a tendency to deposit scale; a negative value indicates aggressive water that will dissolve surfaces.

It is genuinely useful because it captures the interaction between parameters that individually look acceptable.

It is not a substitute for keeping each parameter in its own range. A pool can produce an acceptable index with pH too high and hardness too low, and that pool has a sanitising problem the index does not show.

Temperature is part of the calculation, which is why heated pools and spas behave differently from cold outdoor pools with identical chemistry.

Use it as a periodic check on the overall balance, alongside the routine tests.

Because aeration drives it up, and some sanitisers raise it - the two together mean most pools tend upward continuously.

Aeration removes carbon dioxide from the water, which raises pH. Any feature that agitates the surface - fountains, jets, waterfalls, spa blowers, splashing bathers - does this continuously.

Sodium hypochlorite and calcium hypochlorite sanitisers both raise pH with every dose.

High total alkalinity makes the upward drift worse and harder to correct.

The consequence is that most pools consume acid routinely rather than alkali, and automated systems generally dose acid for pH control.

Where pH is rising very fast, look at alkalinity first - bringing it down into range usually slows the drift substantially and reduces acid consumption.

Always add acid to water, never water to acid; store it separately from chlorine products; and use full eye and skin protection.

Pool acid, whether liquid or dry, is a hazardous chemical. Splash injuries to the eyes are the characteristic accident.

Adding water to concentrated acid generates heat rapidly and can spit the mixture. The rule is invariant: acid into water.

Acid and chlorine products must never be stored together or allowed to contact each other - the combination releases chlorine gas.

Dose into the pool at a point with good circulation, never into a skimmer where it will pass concentrated through the pump and heater.

Ventilated storage away from metal equipment is important, since acid vapour corrodes surrounding plant.

Follow the safety data sheet, and make sure eyewash and copious water are available where the product is handled.