Alkalinity plays a key role when it comes to pool chemistry, and that’s why it’s necessary for anyone who maintains a swimming pool to have some knowledge about it.
So in this article, I'll talk about pool alkalinity from all angles, including what it is, how it works, why you need it, how you can lower and increased it when not appropriate.
What is Pool Alkalinity?
Alkalinity refers to total alkalinity (TA), which is total concentration of dissolved carbonates, bicarbonates, hydroxides and cyanurates present in the water.
These are all alkaline substances, which just means they’re substances that measure higher than a 7 on the pH scale — putting them on the alkaline side of the spectrum.
Alkalinity can be measured by most swimming pool testing kits in parts per million (ppm), which is equal to one milligram per liter of water.
Finally, the term “pool alkalinity” is another way to say the same thing, meaning it also refers to total alkalinity. The only difference being that it relates specifically to the water in a swimming pool.
Alkalinity and pH
There’s often confusion around pH and alkalinity when it comes to pool chemistry, and despite popular belief, these are actually not the same thing.
When you measure pH, you’re looking at how acidic or alkaline the water is, which is influenced by the substances in the water itself. The scale goes from 1 (very acidic) to 14 (very alkaline), with 7 being neutral.
To put that into perspective, consider that: Lemon juice has a pH level of 2 (very acidic), Urine has a pH level of 6 (acidic), Water has a pH level of 7 (neutral), Sea water has a pH level of 8 (alkaline), Bleach has a pH level of 13 (very alkaline).
Alkalinity (or total alkalinity), on the other hand, is not measured on the pH scale but in parts per million (ppm).
Now, despite the fact that alkalinity and pH are independent of one another, there’s still a relationship between the two. Your pool’s pH level will usually (but not always) correlate with your alkalinity level, meaning: If your pH is low, your alkalinity is also likely going to be low and if your pH is high, your alkalinity is also likely going to be high.
To fully understand how this works, you need to understand the role that alkalinity plays in your water, so let’s cover that next.
How Important Is Your Pool Alkalinity?
The ideal pH level for your pool water is slightly alkaline at 7.4 to 7.6 (to clarify, I’m talking about alkaline on the pH scale, not total alkalinity. This lean towards alkaline is intentional because it’s where chlorine works best, as well as being easy on your skin and your pool equipment. There is some wiggle room here, but not all that much.
Unfortunately, pH is extremely sensitive and can quickly be thrown out of balance by temperature changes in the water, not to mention foreign substances such as rain, debris and even bodily waste.
That’s where good alkalinity comes in. The solution to maintaining your pH level is to add a pH buffer, which is exactly what alkalinity increasers are. You can almost think of alkalinity as adding a layer of armor to your pH, absorbing fluctuations and keeping your pool chemistry stable.
The Right Amount of Alkalinity.
The correct amount of alkalinity (total alkalinity) to have in your swimming pool is between 80 and 120 parts per million, depending on the type of chlorine being used.
Anything outside of this will not only reduce the effectiveness of your chlorine, but it can also lead to other undesirable side-effects.
Low Pool Alkalinity
This occurs when your pool falls below 80 parts per million (ppm) alkalinity which will eventually result in your water being too acidic, which can cause: Corrosion of pool surfaces and equipment, Etching and staining of pool surfaces and equipment, burning or itching of the eyes and skin, pool water to turn a shade of green, wild fluctuations in pH levels.
Causes Of Low Pool Alkalinity
If your pool’s pH level is on the way down, this will eventually begin to influence—in this case, lower—your alkalinity along with it.
A decrease in pH can be caused by excessive rainwater entering the pool and diluting the water, acid rain which can directly drive both your pH and alkalinity levels down, and even bodily fluids from swimmers such as sweat and urine (oh yeah, it happens).
And while pool shock can raise pH as explained above, commonly used chlorine tablets have a very low pH. In other words, if you let too much dissolve into dissolve in your pool water, it will lower your pH level and alkalinity along with it.
Raising the Low Pool Alkalinity
pH and alkalinity are closely related and that means the same methods of increasing or decreasing pH also affect alkalinity.
However, in some cases, you’ll want to increase or decrease alkalinity without affecting your pool’s pH level, and this is also possible with the right products to hand.
If your alkalinity level goes below the recommended range, you’ll need to add a substance to your pool water to help bring it back up.
Most pool experts will tell you to use a product branded as “alkalinity up” or “alkalinity increaser”, but these all contain baking soda (sodium bicarbonate) which is readily available as a cheaper, standalone product. The latter works just fine.
Sodium bicarbonate usually is packaged in powder form. It is safe to add to the pool this way, but try to avoid doing this in the wind as it can be carried by the air. Spread the amount of sodium bicarbonate into the pool.
Pour in light, arcing motions to prevent too much entering the water in one place. Baking soda dissolves quickly in water, so you should see the substance disappear in a few minutes.
Let the pool sit. Wait at least six hours before retesting, but try to wait no longer than 24 hours.
The other option is to use soda ash (sodium carbonate). This tends to be cheaper than baking soda but is less effective at raising alkalinity, and more effective at raising pH. It can also cloud up your pool water, so you’ll need to leave your filter system running after use.
High Pool Alkalinity
High pool alkalinity happens when the alkalinity of the pool rises above 120 parts per million (ppm). This will eventually result in your water being too alkaline, which can cause: Scaling of pool surfaces and equipment, burning or itching of the eyes and skin, pool water to turn cloudy, a high pH level that’s difficult to lower.
Causes of High Pool Alkalinity
If your pool’s pH level is on the rise, this will eventually begin to influence—in this case, increase—your alkalinity along with it.
An increase in pH is tends to come from body lotions or sweat washing off into the pool, as well as potentially using a high alkalinity water source to fill up your water.
Tip: Don't Over Shock
If you go overboard with the pool shock you can cause yourself issues. Chlorine-based pool shock is a high-alkaline substance, it will also naturally raise your pool alkalinity. If you're looking for advice for the best pool shock,
It’s also not uncommon for pool owners to go a bit overboard when shocking their pool, and since chlorine-based pool shock is a high-alkaline substance, it will also naturally raise your pool alkalinity.
Lowering the High Pool Alkalinity.
If your alkalinity level goes above the recommended range, you’ll need to add a substance to your pool water to help bring it back down.
For this, you have two options:
- The most common method is using muriatic acid (hydrochloric acid). It comes in liquid form, is available in a range of concentrations and requires careful handling.
- You can either dilute the muriatic acid, or pour the acid in directly to the water. Make sure to do this in the deep end of you have a diving pool, or pour a portion in several spots if you have a shallow or sport pool. Keep the stream tight, as you want the liquid to flow towards the bottom of the pool.
- Leave the pool alone for about one hour, and then turn the pool pump back on.
- Test the total alkalinity between six and 24 hours after adding the chemical to give the pool a chance to adjust to the muriatic acid you have added.
- Repeat if necessary. You may need to add more muriatic acid if the level has not dropped enough, but wait two to three more days before adding any more, as total alkalinity sometimes takes several days to re-balance.
- Alternatively, you can use dry acid (sodium bisulfate). This comes in granular form and is safer to handle, but it’s more expensive. Dilute the dry acid in a bucket of water. Make sure the substance is completely dissolved.
- At the deep end of the pool, pour the diluted dry acid into the water, making sure to keep the stream as narrow as possible. You want the stream to penetrate the upper area of water and get as deep as possible. This helps to avoid disrupting the pH level in the water.
- Leave the pool to stand still for one hour. After one hour turn the pool pump back on. Retest the total alkalinity level after six hours, but before 24 hours after you have added the dry acid mixture.
- Dilute and add more dry acid if you need to bring your level down a little more, but wait at two to three days before adding more, as the total alkalinity may still drop somewhat.
Dry acid will also add sulfates to your water which can eventually build up and damage your pool surfaces. So you’ll have to be careful in the amount you are applying to your pool.
Keeping total alkalinity in balance is one of the important three water balancing steps. You need to know how to test your total alkalinity, and how to lower the level if it is too high. You also need to know which chemicals are available to lower total alkalinity, and how much to add based on your pool size and how much you are over.