Have you ever asked "what's the difference between pH and alkalinity? Many of us in aquatics confuse total alkalinity and pH. It’s understandable, given how blurred the line is between words like “alkaline” and “alkalinity.” Indeed, alkalinity and pH in water chemistry are closely related, but they are not the same. This article will distinguish between them.
Listen to this topic on our Podcast:
What is pH?
pH stands for potenz Hydrogen, more commonly referred to as the "power of Hydrogen". It is the negative logarithm of Hydrogen ion concentration in a solution. The lower the pH, the exponentially more Hydrogen.
pH = -log(H+)
We measure how acidic or basic (also called alkaline) substances are by using the pH scale. The pH scale goes from 0 to 14, and is relative to pure water, which has a perfectly neutral pH of 7. When the concentration of hydrogen ions goes up, the pH goes down (and vice versa). The scale is logarithmic, which means each whole number on the 1-14 pH scale is 10 times more or less than the numbers around it. For example, something with a pH of 6.0 would be 10 times more acidic than 7.0 pH distilled water. Something with a pH of 5.0 is 10x more acidic than 6.0, and 100x more acidic than 7.0 (10x10=100).
To put the pH scale in perspective, let’s do some quick math. Trichlor, a popular type of stabilized chlorine used primarily in outdoor pools, has a pH of about 3. For this math, let’s just round it to exactly 3.0. How acidic is 3.0pH Trichlor compared to 7.0pH water? Let’s figure it out.
(7.0pH - 3.0pH) = 4.0
Logarithmic scale means 104, or 10x10x10x10 = 10,000
Trichlor is about 10,000 times more acidic than pure water.
Don’t worry about the math, it’s just to explain the pH scale itself. From a water chemistry perspective, the textbooks tell us the ideal pH for a swimming pool or spa is between 7.4-7.6. But realistically, pH rises over time in most pools, so the normal swimming pool range is more like 7.4 to 8.2.
The roles of pH in water chemistry
Chlorine strength (%HOCl)
Managing pH is a critical component of maintaining healthy and balanced water chemistry. It is so important because pH is a driving factor in the Langelier Saturation Index (LSI), and also because pH determines how effective chlorine will be in non-stabilized pools. So in pools with no cyanuric acid (CYA), the lower the pH, the stronger chlorine will be, because pH controls the dissociation equilibria between strong and weak chlorine:
HOCl ⇌ H+ + OCl-
Hypochlorous Acid dissociates into Hydrogen and Hypochlorite Ion
Again, pH controls the strength of chlorine (%HOCl) because the lower the pH, the higher the concentration of Hydrogen. That means a higher % of strong HOCl vs. weak OCl-. This is not as relevant in pools with CYA, because isocyanurates fundamentally change pool chemistry.
Calcium carbonate solubility
While the LSI determines when water is oversaturated with calcium carbonate (CaCO3), pH is a major factor on the LSI. And since LSI violations occur locally, not universally throughout a pool, pH matters a lot. For instance, there's a chart below that shows the equilibria of alkalinity types. When the pH gets to 8.2 or 8.3, it becomes very difficult for calcium to stay in solution, because carbonate alkalinity (CO3- -) binds really well with calcium (Ca++). So when your pH gets too high, calcium tends to precipitate out of solution even if the overall LSI of your pool is balanced. We see this phenomenon cloud up pools, like when someone adds soda ash too quickly.
CO2 determines pH
The amount of carbon dioxide (CO2) in your water largely determines the pH. The more CO2, the lower the pH, and vice versa. This explains why water features, vanishing edges and spa jets raise the pH faster than in still water; aeration accelerates the loss of CO2 to the atmosphere.
Low pH lowers the LSI, and can cause damage to surfaces and equipment, corrosion of metal components in and around the pool, skin and eye irritation, as well as general patron discomfort. On the other hand, high pH raises the LSI, and leads to carbonate scale formation, cloudy water, and weaker chlorine (in non-stabilized pools). The textbooks say to stay within the 7.4-7.6 range to avoid those problems, but an LSI-based strategy will avoid the problems better than simply staying in that range.
What is Total Alkalinity?
Total alkalinity is a measurement of the concentration of all alkaline substances dissolved in the water that can both attract and release Hydrogen ions. This interference with Hydrogen is why alkalinity buffers against change in pH. Total alkalinity is primarily bicarbonate, carbonate, and hydroxide, along with a few others like cyanurate alkalinity. When acid is added, these alkali have the ability to neutralize some of the acid. In simpler terms, total alkalinity is a measurement of the water’s ability to resist change in pH. In particular, alkalinity slows the reduction of pH. Too much alkalinity is actually a source of rising pH. The more alkalinity you have, the more acid it takes to reduce pH.
Related: What is Alkalinity?
Total alkalinity is measured by its concentration in parts-per-million (ppm), and the ideal range is from 80-120 ppm, depending on the type of chlorine you use. For example, Trichlor has a low pH of 2.8, so given how acidic Trichlor is, a trichlor pool needs higher alkalinity. Another reason for more alkalinity in a trichlor pool is because the CYA in trichlor offsets alkalinity when doing LSI corrections.
Non-stabilized chlorines, like liquid chlorine (bleach) or cal hypo, each have a high pH. So you can have lower alkalinity, like 80-100 ppm, or even lower than 80 ppm if your LSI allows for it. Salt chlorine pools will often struggle with scale buildup that flakes off the salt cell and blows into the pool. Salt pools, in season, can usually benefit from lower levels of alkalinity, like 60-70 ppm, provided the LSI remains balanced. The key is to measure acid and dilute it before pouring to avoid overcorrections. Too much acid with lower alkalinity will cause bigger problems, so be careful.
Did you notice that total alkalinity is not measured on the pH scale?
This is an important distinction to make, because this is usually how pH and alkalinity get confused. Total alkalinity is measured by the amount, or concentration (ppm) in the water, not by how alkaline (pH) the water is. 'Alkalinity up' products are prevalent in the pool industry. If you are in the market for an alkalinity increaser, the two most common are sodium bicarbonate (NaHCO3) and sodium carbonate (Na2CO3), more commonly known as soda ash. 'Alkalinity down' products will be an acid like muriatic acid or sodium bisulfate.
The role of Total Alkalinity in water chemistry
If you’re a pool operator, you probably already know that pH can fluctuate up and down. And when it does, the pool is constantly fighting against you. Having the right level of total alkalinity is a good thing, because it helps to keep the pH stabilized. A simple way to think about alkalinity is that it insulates pH from changing too easily, especially going down.
Technically, alkalinity buffers pH by either releasing or absorbing a Hydrogen ion (H+) as needed. So when acid is added, carbonate ions can absorb Hydrogen to create bicarbonate ions. This is an equilibrium, just like pH, so it can go the other way too. The equilibrium looks like this:
HCO3- + H+ ↔ H2CO3
Since CO2 controls the pH of the water, you can either add CO2 directly to lower pH, or you can use acid to convert bicarbonate alkalinity into carbonic acid (H2CO3), which is just dissolved CO2: (H2O + CO2 → H2CO3).
You could add several pounds of 8.3pH Sodium Bicarb and make the same pH impact if you added a much smaller amount of 11.6 pH soda ash. But the total alkalinity would be higher for the Sodium Bicarb, because there’s simply more of it in the water.
How to increase Total Alkalinity and pH
If you want to increase total alkalinity in a swimming pool, you need to know the gallonage of the pool, and your pH. It helps to have an accurate dosing calculator. Raising alkalinity is as simple as adding sodium bicarbonate to the water.
Related: How to raise alkalinity and pH
How to lower total alkalinity and pH
You can lower alkalinity using acid. Some experts recommend a "acid column pour" to lower alkalinity, but we do not. In our experience–and indeed, scientific experiments back this up–column pouring is more harmful than dilution. We recommend diluting the acid in a bucket of water, and evenly pouring it around the pool. Besides, column pouring can severely lower the pH in a concentrated location (at the bottom of your pool!), which can throw the LSI off, and in turn, etch the surface. Lowering pH without affecting total alkalinity can be accomplished by injecting carbon dioxide. But that's a topic for another time.
Related: How to lower pH and Alkalinity
Maintain your pool within LSI balance, first and foremost. Then, try to keep your alkalinity in a range that is most advantageous for your primary chlorine type. Use an alkalinity increaser like sodium bicarbonate to raise total alkalinity, or an acid to lower it. When using high-pH chlorines like salt chlorine generators, liquid chlorine or cal hypo, you will need less alkalinity, like 60-80 ppm. For low pH chlorines like Trichlor, 100-120 ppm might be a better range for you. Consult your test kit or the dosing calculator on the Orenda App for specific numbers.
As an aside, never mix different types of chlorine. They can explode. Literally.
People confuse pH vs. alkalinity and we can understand why. Just remember that pH is an equilibrium that tells us how acidic or how basic a solution is. Total alkalinity measures the amount or concentration (in parts per million) of dissolved alkali that can buffer the change of pH by releasing or attracting Hydrogen. We hope this helps!