The Difference Between pH and Alkalinity

One of the most common tasks involved in pool maintenance is keeping your water balanced. Typically this means maintaining effective chlorine levels and balancing pH. However, another measure, total alkalinity, is also critical to keeping your pool water balanced. Many pool owners are confused about the difference between pH and alkalinity. This is not surprising, because pH and alkalinity are closely related and affect each other. Another point of confusion is the term alkalinity. We know that the pH scale measure how acidic or alkaline (a.k.a basic) a substance is. Alkaline sounds a lot like alkalinity, so it’s no wonder people get them confused. However, knowing the difference between pH and alkalinity is vital to your pool’s health, so read on to learn this critical differentiation.

The Difference Between pH and Alkalinity [infographic]


What is pH?

pH is a measure of how acidic or how basic a substance is. It is measured on a scale from 1 to 14, with totally pure water right in the middle at 7. As the pH gets closer to 14, a solution is more basic. As the number gets closer to 0, the solution is more acidic. The ideal pH for a pool is between 7.2 and 7.4. This slightly basic pH level is perfect because it is as close as possible to the pH of the human body. With a pH of 7.2-7.4, pool water will have little effect on sensitive human parts like eyes, skin, and mucous membranes.

The term pH stands for “power of hydrogen” and is the measure of hydrogen ions in a solution. As the amount of hydrogen ions goes up, pH goes down, and vice versa. The pH scale is logarithmic, which means each step is an increase of ten times. A solution with a pH of 6 is ten times more acidic than pure water, with a pH of 7. A solution with a pH of 5 is ten times more acidic than a solution with a pH of 6, or 100 times more acidic than pure water.

When pH is high, the pool is too alkaline. That can be fixed by adding CO2 or muriatic acid. If the pH is low, the pool is too acidic. To correct the pH, add something alkaline, like soda ash or sodium bicarbonate.

Why pH Is Important in a Swimming Pool

Managing pH levels in your swimming pool should be one of your main concerns during pool maintenance. In fact, you should be checking your pH levels every few days, or at least weekly, and adjusting as necessary. pH has a strong influence on how effective your chlorine is. The lower the pH, the more effective chlorine is. But don’t imagine that you can just drop the pH of your pool as much as you want to maximize the effects of chlorine.

Low pH, in other words, high acidity, is not a good situation for your pool or its patrons. Highly acidic water can damage pool liners and etch plaster. It can also corrode metal components, such as rails and ladders. Your patrons will notice, too, since high acidity (low pH) will irritate or even damage eyes, skin, and mucous membranes (such as the inside of your nose). Low pH also tends to lower total alkalinity.

High pH is also a problem for your pool. When the pH is too high, your water is very basic. That can cause scaling on the walls and other components of your pool. It can stain metal components, make your water cloudy, and significantly reduce the effectiveness of your chlorine. Like high acidity, very basic water can damage and irritate human skin, eyes, and other sensitive body parts.

The type of chlorine you use can affect your pool’s pH. Some types of chlorine, like Trichlor (a stabilized chlorine popular for outdoor pools), have a very low pH. Calcium Hypochlorite (Cal Hypo) is on the opposite ends of the pH scale, with a pH of almost 12. It is crucial never to mix different types of chlorine. Mixing an acidic and a basic chlorine can cause the mixture to explode, potentially causing serious injuries to anyone in the way of the solution.

What is Total Alkalinity?

As we have discussed, pH is measured on a scale. Total alkalinity, on the other hand, is measured in parts per million (ppm). That means the total alkalinity is an absolute measure of the concentration of all alkaline substances in a solution. The most common alkaline substances on swimming pool water are carbonate, bicarbonate, and hydroxides. These alkaline substances are important because of their effect on pH. They act as buffers, keeping pH from rising or falling. They do this by neutralizing acids. In other words, total alkalinity is a measure of how strongly your pool water can resist changes to pH.

The ideal level for total alkalinity is 80-120ppm. Where you aim in that range depends on the type of chlorine you are using. If you are using a low pH chlorine like Trichlor, you should keep your total alkalinity closer to 120ppm to counteract the strong acidity of the chlorine. If you are using a high pH chlorine, you want to keep your total alkalinity lower, closer to 80-100ppm. Liquid chlorine has a very high pH, nearly 13, so you don’t need to worry about counteracting acidity levels.

It is important to note that total alkalinity is a measure of how much alkaline substance is in the water. It is not a measure of pH, or how alkaline the water is. This subtle difference in terminology is one of the reasons so many pool owners get confused.

If you want to raise alkalinity, you will need to add more alkaline substances. The most common “alkalinity up” products are sodium bicarbonate (baking soda) and sodium carbonate (soda ash). “Alkalinity down” products are acids. The most common is muriatic acid.

Raising Total Alkalinity

If you find that your pH is drifting and it’s hard to keep it balanced, your alkalinity is probably too low. Remember, total alkalinity is your pool’s ability to withstand changes to pH. Raising alkalinity is not hard. Adding baking soda or soda ash will increase the total alkalinity, and when it is diluted in water, it should have a minimal effect on pH.

Lowering Alkalinity Without Lowering pH

High alkalinity can make it hard to adjust pH, along with the other problems we mentioned above. To lower alkalinity, you will need to add an acid, most commonly muriatic acid. The problem with adding acid is that it is almost impossible to lower alkalinity without lowering pH. If you add enough muriatic acid to lower total alkalinity to an acceptable level, you may lower your pH to an unsafe level. However, there is a way around this.

Adding acids to reduce total alkalinity will inevitably lower pH. To lower total alkalinity without lowering pH, you will need to follow a two-step process. First, add the acid until the alkalinity is at the level you want. Don’t worry about the pH. Just make sure that the acid is diluted in water (such as in a bucket) before you pour, and that you walk it around the pool instead of pouring it all in one place. This will avoid lowering the pH too much in one spot, which can damage a pool lining and etch plaster.

Once the alkalinity is correct, you need to raise your pH without raising your alkalinity. This can be done by simply aerating the pool. The key is to give your pool water as much surface area as possible. One way to do this is to point your returns up toward the surface of the water and turn on your pump at full blast. This will disturb the surface of the water. For even more water disturbance, you can place use an air compressor with a long hose ending in a weighted air diffuser at the bottom of the deep end. You can also add a spa blower to your pool’s return line. However you do it, aerating your pool will raise pH without affecting alkalinity.

Easily Manage Alkalinity and pH

The easiest way to manage your pH and alkalinity is to bring in a professional. When you let the pros at The Pool Butler case for your pool, you can be confident that your pool water is well balanced. We take the worry and work out of pool maintenance, so all you have to do is enjoy your pool. Contact us today to learn more about all of the services we offer.

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