The Science of Salt Generators and Phosphates

This commentary is in regard to the recently posted blog entitled Failing Salt Generators: The Phosphate Connection. I would like to begin by saying that I am in no way discouraging the use of a product that pool owners and operators believe to be beneficial or profitable. However, I do believe that the science behind the explanations for product recommendations must be correct.

I would specifically like to address the paragraph in the article entitled “The Phosphate Connection.” The article makes the statement “a product known as zinc orthophosphate is used in drinking water systems because it adheres to metal pipes and acts as an anti-corrosion agent.” Zinc orthophosphate is an insoluble compound. Because it’s insoluble, it could not be dissolved in water and applied for this purpose. 

Compounds such as zinc polyphosphate are commonly used. These products can eventually break down into orthophosphate over time and will increase the measured phosphate level in water, but zinc orthophosphate is not used for this purpose. This paragraph states “they attach to the cathode”. As a corrosion inhibitor, phosphate works at the anode not the cathode, as the anode is typically where galvanic corrosion takes place.  The paragraph also mentions that “orthophosphates attach to metals”. Orthophosphate will bind with soluble metal ions forming an insoluble complex, but it does not attach to solid metal surfaces. There must be a counter-ion (positively charged, such as calcium) to make an insoluble complex, then that insoluble complex can adhere to the metal surface.

I would like to provide a little more information around the science of calcium phosphate and its potential to form scale. Calcium phosphate is an insoluble compound that will precipitate in water. If this type of scale (or any type of scale) were to adhere to a chlorine generator cell plate, then chlorine production would be hindered. However, it is important to distinguish between precipitation (or insolubility) and actual scale formation. Scale is adherence to a surface. Just because something precipitates or is somewhat insoluble does not mean that it will adhere. It is also important to understand that the formation of different types of calcium scale is driven by different circumstances.

Calcium phosphate scale is not driven by high pH as phosphate is not part of the carbonate equilibrium. The formation of calcium phosphate crystals is somewhat more complicated than calcium carbonate as there are some intermediate steps that take place. It is important to note again, however, that the formation of a precipitate does not necessarily mean that it will adhere (form scale). Listed below are the possible locations where adherence could take place:

• Bulk Solution/Pool Surface – as mentioned, calcium phosphate is not very soluble and will begin to precipitate. With the filtration system running, this material will be captured on the filter bed and removed with routine cleaning. It will be trapped in the filter even easier if using alum as a clarifier (which many products on the market contain). So with proper filtration and filter cleaning, the precipitation in the bulk of the water can be managed.

• Heater – calcium phosphate will tend to deposit on warm surfaces. However, adherence is also affected by shear (or turbulence). Shear significantly reduces the chance for adherence. Because the most efficient heat transfer is with turbulent flow, the likelihood of calcium phosphate depositing on a heating element is very low due to the high level of shear inside the heater.

• Chlorine generator cell – as mentioned earlier, calcium phosphate scale is not caused by high pH. Therefore, a chlorine generator cell is not a preferred location for this type of scale to form. 

The majority of calcium phosphate will precipitate and be caught in the filter. While it is not impossible for calcium phosphate scale to adhere to some surfaces, it is much less likely.

The most common type of scale seen in any location in swimming pools is calcium carbonate. Another important aspect to consider is that there is always a much higher level of carbonate in pool water. Even in what would be considered a high phosphate pool (1,000 – 2,000ppb), it is still a very low level compared to carbonate. For example, 2,000ppb is only 2ppm. There is typically well over 100ppm of carbonate in the water. Because reactions are often “competitions” between two species, the one with the higher concentration tends to react faster.

We know from basic science that orthophosphate has no interaction with chlorine. A major cause of decreased chlorine production in a saltwater pool is adherence of a scale compound to the chlorine generator cell plate over time. Years of field research and data collection, along with basic scientific principles, has confirmed that calcium carbonate is the primary culprit in these situations and should be the focus of pool owners/operators as well as manufacturers.

Karen Rigsby is business support manager at BioLab, a Chemtura company.