Sodium carbonate (Na2CO3), more commonly known as Soda Ash, is an additive to raise the pH and alkalinity in swimming pools. Soda Ash has a high pH, somewhere between 11.3-11.7. Those of you reading this who have used Soda Ash may have had it cloud up a pool. This article will explain why that happens, but first let’s explain how Soda Ash impacts pH and alkalinity in water.
How does Soda Ash work?
To raise the pH of water, a substance must take away a hydrogen (H+). Soda Ash works by stealing a hydrogen atom to become bicarbonate. When Soda Ash snags a H+, the reaction looks like this:
Na2CO3 + H+ ↔ 2Na+ + HCO3-
Soda Ash + Hydrogen yields Sodium and Bicarbonate
This all ties into the broader discussion about alkalinity and pH. We know in the pool business that alkalinity is a pH buffer, but do we know why? In a less-than-scientific explanation, alkalinity buffers pH by either donating a Hydrogen or absorbing one. Alkalinity makes it more difficult for pH to fluctuate quickly.
Specifically, carbonate ions like to become bicarbonate, and vice versa. When you add Soda Ash to the pool, the carbonate anion (CO32-) wants to become bicarbonate (HCO3-). To do so, it needs to get a Hydrogen (H+) from somewhere. When carbonate takes its Hydrogen atom, the pH goes up. This pH and alkalinity chemistry is about equilibrium, so the opposite is also true. When you add acid to the pool, a Hydrogen is released and the pH goes down. It’s all about the hydrogen ions.
Now that we know what Soda Ash is and how it reacts in water, let’s talk about why it sometimes clouds up a pool.
Soda ash can cloud up a pool because it changes the pool chemistry around it
The high pH of Soda Ash itself can create a high-LSI violation in its immediate vicinity. This explains why the cloudiness does not happen all at once, rather the process creates a cloudy plume that slowly expands across the pool. This cloudiness is just calcium carbonate precipitating out of solution.
The high-LSI violation in a small area is similar to how plaster dust is formed. You know, where calcium hydroxide is stolen by aggressive water from the cement in the surface, but it’s super high 12.6 pH suddenly changes the LSI in the immediate area near the surface, and calcium carbonate dust (plaster dust) falls out of solution. Go back and try to read that entire sentence in one breath. We believe in you.
There is another way to look at this phenomena of why Soda Ash sometimes clouds up pools. Beyond the LSI violation aspect, the high pH converts bicarbonate ions into carbonate ions. Remember our article explaining alkalinity? It had an alkalinity graph showing how pH determines the type of alkalinity you have. Here is the graph as a reminder:
The pH determines the % and type of alkalinity present. It is an equilibrium, and can fluctuate.
As you can see, the higher the pH, more and more bicarbonate ions convert to carbonate ions. This matters, because calcium has a strong attraction to carbonate ions, which creates calcium carbonate. Once again, calcium carbonate precipitates out of solution and you get cloudiness.
Can carbonate clouding be prevented?
Eventually the cloudiness will fall as calcium dust or get filtered out. You can mitigate and possibly even prevent the annoying Soda Ash cloud problem by chelating the calcium in your pool with a product like SC-1000. If calcium is chelated, it is unable to bind to carbonate as easily, and therefore the cloudiness may not even occur. Of course, there are exceptions, like if your calcium levels and/or LSI are really high.
We hope this article helps explain why calcium carbonate clouding sometimes occurs when adding Soda Ash to a swimming pool. If you have had this problem and want more help, contact us directly.