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 wrap their head around it.
So in this article, we’ll talk about pool alkalinity from all angles, including what it is, how it works, why you need it, and all the boring (but still important) nuances surrounding it.
Need a quick answer? Alkalinity (or total alkalinity) is a measurement of dissolved alkaline substances in water. It’s not the same as alkaline on the pH scale, but it does correlate. Aim to maintain an alkalinity level of 80 to 120 parts per million (ppm) or risk side effects that can be time-consuming or complicated to solve.
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. (More on that in the next section)
Alkalinity can be measured by most swimming pool testing kits in parts per million (ppm), which is equal to one milligram per litre 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.
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.
- 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.
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.
Spoiler alert: That’s where good ol’ 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 armour to your pH, absorbing fluctuations and keeping your pool chemistry stable.
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.
Falling below 80 parts per million (ppm) alkalinity 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
Rising above 120 parts per million (ppm) alkalinity 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
There are a number of reasons for changes in the alkalinity level of your pool water, whether it be from natural causes or chemical influence.
Some of these changes can be responsible for an increase in pool alkalinity, while other changes will lead to a decrease.
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.
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.
If your pool’s pH level is on the way down, this will eventually begin to influence—in this case, decrease—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.
As I’ve said throughout this article, pH and alkalinity are closely related and that means the same methods of increasing or decreasing pH also affect alkalinity.
Of course, 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.
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.
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. This is what we recommend you use.
- Alternatively, you can use dry acid (sodium bisulfate). This comes in granular form and is safer to handle, but it’s more expensive. Dry acid will also add sulfates to your water which can eventually build up and damage your pool surfaces.
Pool chemistry is a tough subject to master, but it’s an essential one for anyone who owns or services a swimming pool.
Whether you’re getting to grips with balancing pH and alkalinity, the causes of falling or rising alkalinity, or figuring out how to adjust alkalinity levels in your pool, this is one you’ll want to get right.