Question on Epsom Salts ... preventing scaling

bdare · Mar 10, 2015

My experience in water chemistry comes from maintaining salt water reef tanks. In reef aquariums we maintain magnesium levels of 1300-1400 ppm. One of the primary reasons is that Mg helps Ca and Alk from precipitating. Could it be that by adding Mg to swimming pools we could help prevent scale or calcium build up on pumps or salt cells?

Split by moderator from HERE. jblizzle

tim5055 · Mar 10, 2015

Re: Can I add Epsom Salt to my pool? (not a SWG pool)

Remember, it's not really a salt and can be good for the plants around your pool

JasonLion · Mar 10, 2015

Magnesium doesn't prevent scaling, but it does make scaling far easier to remove from SWG cells. There are some commercial products that are primarily magnesium and sold for this purpose. However, unless you want magnesium for some other reason, it is not the most cost effective solution. It is better all around to adjust your chemistry so that scaling does not occur in the first place.

bdare · Mar 10, 2015

Is scaling the same as precipitation or is that something else?

I agree maintaining correct water parameters to prevent scaling is ideal, but it's nice to have a little backup

chem geek · Mar 10, 2015

Scaling is precipitation on a surface of the pool or on an SWG plate.

Where there is more magnesium in the water, then the scale may incorporate some of the magnesium into it so instead of forming calcium carbonate "calcite", it forms some calcium magnesium carbonate "dolomite". Magnesium carbonate itself is roughly 10 times more soluble than calcium carbonate which is why we usually only concern ourselves with calcium carbonate with regard to scale.

bdare · Mar 10, 2015

chem geek said:
Scaling is precipitation on a surface of the pool or on an SWG plate.

Where there is more magnesium in the water, then the scale may incorporate some of the magnesium into it so instead of forming calcium carbonate "calcite", it forms some calcium magnesium carbonate "dolomite". Magnesium carbonate itself is roughly 10 times more soluble than calcium carbonate which is why we usually only concern ourselves with calcium carbonate with regard to scale.

If Mg can help prevent precipitation they why don't more people add it to their pools? What's the downside?

I understand I'm mixing 2 different areas here (nature and swimming pools) but I would think some of the same concepts apply.

Here is an excerpt from an article written by Randy Holmes Farley, a well known chemist in the reef world:
http://www.advancedaquarist.com/2003/10/chemistry

How does magnesium interfere with precipitation of CaCO3? The primary way involves magnesium poisoning the surface of growing CaCO3 crystals, slowing the precipitation. It can, in fact, be slowed to the point where it simply does not happen at rates problematic to an aquarist. In the following discussion it is important to remember that, other things being equal, alkalinity is a good indicator of the concentration of carbonate. So higher alkalinity equates to higher carbonate.

In short, while magnesium carbonate is not supersaturated in seawater (or in typical reef aquaria), and will not precipitate on its own, magnesium is attracted to calcium carbonate surfaces where the carbonate ions are already held in place by the calcium ions. With the carbonate ions held in place, magnesium finds this an attractive place to bind.

After a short time in seawater, a virgin calcium carbonate surface quickly attains a thin coating of Mg/CaCO3 (magnesian calcite) as magnesium pushes its way into and onto the crystal surface. Eventually, the surface contains a substantial amount of magnesium. The extent to which this happens depends on the underlying mineral, and is apparently much more extensive on calcite than aragonite. It also depends upon the relative amounts of calcium and magnesium in the water. Regardless, a new type of material is formed that contains both calcium and magnesium.

JasonLion · Mar 10, 2015

No doubt it is not commonly used because it does not actually prevent precipitation in a swimming pool context. As I mentioned previously magnesium is used in swimming pools to make cleaning of SWG cells plates simpler in high CH areas of the country. With magnesium in the water, the cell still gets coated with minerals just as quickly as before. The only difference is that the minerals can be removed with a high intensity water spray, rather than requiring an acid soak. There is also the issue that you need fairly large quantities, which end up costing hundreds of dollars. While the alternative involves several gallons of muriatic acid for $30 to $50 dollars.

bdare · Mar 10, 2015

JasonLion said:
No doubt it is not commonly used because it does not actually prevent precipitation in a swimming pool context. As I mentioned previously magnesium is used in swimming pools to make cleaning of SWG cells plates simpler in high CH areas of the country. With magnesium in the water, the cell still gets coated with minerals just as quickly as before. The only difference is that the minerals can be removed with a high intensity water spray, rather than requiring an acid soak. There is also the issue that you need fairly large quantities, which end up costing hundreds of dollars. While the alternative involves several gallons of muriatic acid for $30 to $50 dollars.

AHHH... that makes sense.

I suppose I was more worried about the nightmare of having to scrub it off the sides of my pool. I don't currently have any issues, but as a newbie it's something that keeps me up at night.

Thanks again guys!

chem geek · Mar 10, 2015

We generally find that in pools scaling on pool walls doesn't occur until the saturation index gets pretty high, at least +0.7 or so. In hot spas we've seen scaling occur at around +0.3 or so saturation index. Scaling in SWG cells occurs at even 0.0 or slightly lower saturation index levels because the pH at the hydrogen gas generation plate is much higher. This is why using 50 ppm Borates helps prevent such scaling because it cuts down the amount of that pH rise roughly in half.

While magnesium may slow down the rate of calcium carbonate scaling in water over-saturated with calcium carbonate, it's usually better to get at the root problem and maintain a saturation index level that does not promote scaling regardless of magnesium level. Increasing the magnesium level would be one last-resort approach of preventing scaling in water with very high CH. However, there are other scale inhibitor chemicals (e.g. sodium hexametaphosphate, organophosponates such as PBTC and HEDP, polyacrylic acids, zinc) that are much more efficient for this purpose, but again these would be last-resort methods.