How to adjust pH without damaging the water

The triangle: alkalinity, pH, and chlorine — why order matters

Balancing pool water chemistry isn't just about pH. Three key factors — total alkalinity (TA), pH, and chlorine — form a triangle that, if not managed correctly, can lead to corrosion, scale, and reduced chlorine effectiveness. Total alkalinity acts as a buffer, keeping pH stable against outside changes. pH, in turn, determines how effective chlorine is at disinfecting the water. And chlorine, as the main disinfecting agent, needs the right pH to work properly.

The order in which you adjust things matters a lot. If you try to adjust pH first without balancing alkalinity, you may find the pH becomes unstable and needs constant re-adjusting. For example, if your pH is very high and you lower it with acid without first adjusting alkalinity, the pH could climb right back up, leading to an ineffective adjustment cycle. Alkalinity should be adjusted first because it acts as a buffer, protecting the pH from sharp swings.

Total alkalinity should be between 80 and 120 ppm for most pools. Below 80 ppm, the water is very sensitive to pH swings. Above 120 ppm, it can cause corrosion on pool surfaces. Keep in mind that alkalinity and pH are interrelated, and adjusting one affects the other. That's why you should always adjust alkalinity before worrying about pH.

Correct total alkalinity (TA) first: how and with what

To correct total alkalinity, use specific products such as sodium hydroxide (caustic soda) or sodium carbonate to raise it, or muriatic acid (hydrochloric acid) or sodium bisulfate to lower it. Which product to use depends on whether you need to raise or lower alkalinity.

If you want to raise alkalinity, sodium carbonate is a popular choice. Every 100 liters of water needs roughly 1.4 grams of sodium carbonate to raise alkalinity by 10 ppm. For example, if your alkalinity is at 70 ppm and you want to reach 100 ppm, you'll need to add 3.5 grams of sodium carbonate per 100 liters of water. Use an alkalinity test kit to verify the results.

If you need to lower alkalinity, sodium bisulfate is the safer option. Every 100 liters of water needs roughly 1.4 grams of sodium bisulfate to lower alkalinity by 10 ppm. For example, if your alkalinity is at 130 ppm and you want to reach 100 ppm, you'll need to add 4.2 grams of sodium bisulfate per 100 liters of water. Muriatic acid can also be used, but it's harsher and can damage pool surfaces if not handled carefully.

It's best to make alkalinity adjustments on a sunny day, since heat speeds up how quickly the chemicals dissolve. Also, always wait at least 24 hours after adjusting alkalinity before testing pH, since pH can shift as a result of the alkalinity adjustment.

Lowering pH: muriatic acid vs. sodium bisulfate

To lower pH, there are two main options: muriatic acid and sodium bisulfate. Both are effective, but they differ in important ways when it comes to their impact on the water and on safety.

Muriatic acid is strong and effective. Every 100 liters of water needs roughly 1.4 grams of muriatic acid to lower pH by 0.1 units. For example, if your pH is at 7.8 and you want to reach 7.2, you'll need to add 7.0 grams of muriatic acid per 100 liters of water. However, muriatic acid can damage pool surfaces if not used correctly, and handling it requires extra precautions, such as gloves and eye protection.

Sodium bisulfate is a safer, gentler alternative. Every 100 liters of water needs roughly 1.4 grams of sodium bisulfate to lower pH by 0.1 units. For example, if your pH is at 7.8 and you want to reach 7.2, you'll need to add 7.0 grams of sodium bisulfate per 100 liters of water. Unlike muriatic acid, sodium bisulfate doesn't damage pool surfaces and is easier to handle, making it ideal for those looking for a safer option.

Keep in mind that pH shouldn't drop below 7.2, since that can cause corrosion on pool surfaces. If your pH is very high and you need to lower it, it's best to adjust alkalinity first to avoid sharp swings.

Raising pH: sodium hydroxide and sodium carbonate

To raise pH, the most common products are sodium hydroxide (caustic soda) and sodium carbonate. Both are effective, but they differ in their impact on alkalinity and on safety.

Sodium hydroxide is strong and effective. Every 100 liters of water needs roughly 1.4 grams of sodium hydroxide to raise pH by 0.1 units. For example, if your pH is at 7.2 and you want to reach 7.6, you'll need to add 4.2 grams of sodium hydroxide per 100 liters of water. However, sodium hydroxide is harsher and can damage pool surfaces if not handled carefully. Be sure to use gloves and eye protection when handling this product.

Sodium carbonate is a safer, gentler alternative. Every 100 liters of water needs roughly 1.4 grams of sodium carbonate to raise pH by 0.1 units. For example, if your pH is at 7.2 and you want to reach 7.6, you'll need to add 4.2 grams of sodium carbonate per 100 liters of water. Unlike sodium hydroxide, sodium carbonate doesn't damage pool surfaces and is easier to handle, making it ideal for those looking for a safer option.

Keep in mind that pH shouldn't rise above 7.8, since that can cause scale to form and reduce chlorine's effectiveness. If your pH is very low and you need to raise it, it's best to adjust alkalinity first to avoid sharp swings.

How much product to add: calculating by water volume

Calculating water volume is essential for determining how much chemical product to add. The basic formula is: amount of product = (target − current) × water volume × conversion factor. The conversion factor varies depending on the product and its effect on pH or alkalinity.

For example, if you want to raise pH from 7.2 to 7.6 in a 50,000-liter pool, you'll need to calculate the required amount of sodium carbonate. Every 100 liters of water needs 1.4 grams of sodium carbonate to raise pH by 0.1 units. So for 50,000 liters, you'd need 50,000 × 1.4 × 4 = 280,000 grams of sodium carbonate, which is 280 kg. Use an accurate scale to measure the product.

If you're adjusting alkalinity, the calculation is similar. For example, if you want to raise alkalinity from 80 ppm to 100 ppm in a 50,000-liter pool, you'll need to add 50,000 × 1.4 × 2 = 140,000 grams of sodium carbonate, which is 140 kg. Be sure to follow the manufacturer's instructions and use an alkalinity test kit to verify the results.

Also, always wait at least 24 hours after adding a chemical product before testing pH or alkalinity, since the water needs time to balance out. It's best to make adjustments on a sunny day, since heat speeds up how quickly the products dissolve.

Classic mistakes and why pH keeps drifting out of range

The most common mistakes when adjusting pH include adjusting pH before balancing alkalinity, using the wrong products, not waiting long enough for the products to take effect, and not testing pH and alkalinity regularly. These mistakes can lead to pH swings and a constantly unbalanced pool.

A common mistake is adjusting pH without first balancing alkalinity. If pH is adjusted before alkalinity is stabilized, it can quickly drift out of range again. For example, if your pH is very high and you lower it with acid without first adjusting alkalinity, the pH could climb right back up, leading to an ineffective adjustment cycle.

Another mistake is using the wrong products. For example, using muriatic acid to lower pH can damage pool surfaces if not handled carefully. Likewise, using sodium hydroxide to raise pH can cause scale to form if alkalinity isn't properly balanced.

Not waiting long enough for products to take effect is another common mistake. pH doesn't balance out instantly, so it's important to wait at least 24 hours after adding a chemical product before testing. If you don't wait long enough, the results can be inaccurate and lead to unnecessary adjustments.

Finally, not testing pH and alkalinity regularly can lead to a constantly unbalanced pool. It's best to test pH and alkalinity at least once a week, and adjust if you spot deviations. Unstable pH can cause corrosion, scale, and reduced chlorine effectiveness.