On a weekly service route, pH is one of the most important — and most misunderstood — parameters you test. It affects sanitizer performance, water balance, equipment longevity, and swimmer comfort. Unlike chlorine, where you are often maintaining a residual, pH is something you are actively steering. Small shifts can have outsized effects.

Why We Test pH

pH controls how aggressive or scale-forming the water is, and it directly influences chlorine effectiveness. Low-pH water is corrosive — it can damage plaster, etch surfaces, and accelerate metal corrosion. High-pH water promotes scale formation, cloudy water, and reduced sanitizer efficiency.

Even when a pool “looks fine,” pH may be drifting. Aeration, temperature, bather load, and especially alkalinity all push pH upward over time. Without regular correction, most pools will trend high.

What We Are Measuring

pH reflects hydrogen ion activity (often approximated as concentration in pool testing) — expressed on a logarithmic scale from 0 to 14. Each whole number represents a tenfold change in acidity. In pool terms, the practical range is narrow: roughly 7.2 to 7.8.

This is not just an arbitrary range. Around pH 7.5, water is near physiological pH (~7.4) and comfortable for eyes and skin. More importantly, this range balances corrosion and scaling tendencies while supporting effective sanitation. pH also affects chlorine chemistry. As pH rises, chlorine shifts toward the less effective hypochlorite ion (OCl-); as pH drops, more exists as hypochlorous acid (HOCl), the stronger sanitizer. In stabilized pools, cyanuric acid significantly buffers chlorine activity, reducing the practical impact of pH on sanitizer strength compared to unstabilized water. However, pH still strongly influences overall water balance and should not be ignored.

Methods Of Testing

Test strips provide a quick estimate using color pads (typically using phenol red–type indicators). They are convenient but limited by resolution, lighting conditions, and timing. Best used for screening.

Liquid reagent (phenol red) tests are the industry standard. A few drops of phenol red indicator produce a color change that is compared to a chart, typically covering pH 6.88.2. This method is reliable when performed consistently.

Digital meters measure pH electronically and can provide precise readings. However, they require regular calibration, clean probes, and proper storage. Without maintenance, accuracy quickly degrades.

For most field service, a liquid phenol red test offers the best balance of accuracy, reliability, and practicality.

Testing Interferences

pH testing is generally stable, but several factors can distort results:

• High chlorine levels: At elevated free chlorine (typically above ~10 ppm unless neutralized), phenol red can oxidize and shift toward a purple color, falsely indicating high pH.

Response: allow chlorine to drop, or use a test kit with a chlorine neutralizer.

• Lighting and color perception: Color matching depends on neutral lighting.

Response: read tests outdoors or under consistent white light.

• Sample contamination: Residues in the vial can alter results.

Response: rinse thoroughly with pool water before testing.

• Reagent condition: Old or heatexposed reagents can fade or shift color.

Response: replace regularly and avoid storing kits in hot environments.

• Timing: Letting the sample sit too long can slightly alter perceived color.

Response: read promptly after mixing.

A pH reading is straightforward, but consistency in technique is critical. What To Do With The Result

For most residential pools, a practical target range is 7.4-7.8, with 7.5-7.6 as a common working setpoint.

To Lower pH (most common adjustment):

Use muriatic acid (31.45% hydrochloric acid).

Typical dosing guidelines:

• In 10,000 gallons, ~12–20 oz. of muriatic acid will lower pH by about 0.2-0.3, depending heavily on total alkalinity and starting pH.

The amount of acid required is strongly influenced by total alkalinity — higher alkalinity resists pH change and requires larger doses.

Add acid slowly in front of a return with the pump running. Recheck after circulation.

To Raise pH:

Use soda ash (sodium carbonate).

Typical dosing:

• In 10,000 gallons, ~6 oz. by weight raises pH by roughly 0.2, though the effect varies with total alkalinity.

Note: soda ash also raises total alkalinity and can cause temporary clouding in high-calcium water. If alkalinity is already high, aeration (running water features, pointing returns up) can raise pH without adding chemicals.

Operational Approach

Most pools trend upward in pH over time due primarily to carbon dioxide (CO2) outgassing, especially in water with higher total alkalinity or aeration.

In pools using borates, pH rise may be slower due to additional buffering. Acid additions are therefore routine. The key is consistency — small, regular corrections prevent large swings.

Most Common Mistake

Ignoring pH because chlorine is “in range.”

A pool can test perfectly for chlorine yet still be corrosive, scaleforming, or inefficient if pH is out of balance. Maintaining pH in range is not optional — it is central to water quality, equipment protection, and overall system stability.

Why pH Is Often Misunderstood

pH appears simple — just a number on a test block — but it represents a dynamic chemical system that is not immediately intuitive. Much of the confusion comes from how it is commonly taught and discussed in the field.

First, pH is logarithmic. A change from 7.5 to 7.8 may look minor, but it reflects a significant shift in hydrogen ion activity. Without understanding that scale, it is easy to underestimate how meaningful small changes can be.

Second, many assume chlorine — especially liquid chlorine (sodium hypochlorite) — is the primary driver of rising pH. While it raises pH when added, much of that effect is offset as chlorine is consumed.

Chlorine contributes to short-term movement, but it is not the main cause of long-term pH rise.

Third, most pools rise in pH due to carbon dioxide (CO2) outgassing. As dissolved CO2 leaves the water, acidity decreases and pH increases. This process is accelerated by higher total alkalinity and aeration, yet it is often overlooked because it is not directly visible.

Fourth, saltwater pools are often assumed to have inherently high pH due to chlorine generation. In reality, while the cell produces high pH locally, hydrogen gas generation at the cell increases aeration, accelerating CO2 outgassing, which is the primary driver of ongoing pH rise.

Finally, pH is often treated as a fixed target rather than a moving system.

In reality, it is constantly shifting based on alkalinity, aeration, and carbon dioxide balance. Without understanding those drivers, pH can feel unpredictable — even when it is behaving exactly as chemistry dictates.