Borates are found throughout nature, and are refined for a variety of diverse applications. They have a very low acute mammalian toxicity and a strong buffering capacity, and are known to be cost-effective, especially for use in swimming pools.

What do borates do?

Arguably the most important function of borates in pool water is to serve as a buffer, and many of the other attributes stem from this one. Borates are excellent at holding pH; this is the reason why it is so difficult to adjust the pH to neutral with some borate products.

Borates in solution form an equilibrium between the acid (undissociated) and the borate ion (dissociated and negatively charged). Between the two, some borate chains are thought to also occur. It is the interaction of this balance (equilibrium), and its ability to adsorb acid (protons) and base (hydroxyl groups) without significantly changing pH, that allow borates to perform so well as buffers. 

Borates perform as a weak Lewis acid. Most acids donate a proton, but Lewis acids accept a hydroxyl group (OH-), usually from water:

B(OH)3 + H2O    B(OH)4-  +  H+

In a situation with borate in a pool at neutral pH, most of the borate is present in its undissociated trivalent form, and some is present as the tetrahydroxy borate anion. When you add acid (H+ or protons), some of the tetrahydroxy borate anions lose a hydroxyl group (OH-) to form water with the protons. When you add base (hydroxyl group or OH-), some of the undissociated tri-coordinated borate becomes dissociated and tetra-coordinated, absorbing the base. Thus, the perfect balance is maintained.

Some technicians swear by borates and will use them as a complete replacement for alkalinity; after all, with the carbonate system it is continuously being lost from the water as CO2. However, borates also work well with carbonate and cyanurate to form three layers of buffering.

Stopping algae

The fact that borates work well with carbonate disproves the common misconception that they control algae by removing CO2. You are adding soda ash or bicarb to a pool on a regular basis to maintain alkalinity, and relying on the carbonic acid carbonate equilibrium for buffering and to control pool pH. Obviously, the borates did not “remove” any carbon dioxide; and if even if they did, the next time you added carbonate, you would be adding more CO2. Also, there is always enough CO2 dissolving into the water from the atmosphere to support algal growth.

So how do borates control algae, then? In the simplest explanation, borates stop the algal cell from both producing food (photosynthesis) and then eating it (metabolism). They do this by locking up special molecules in the cell called nucleotide co-enzymes; these include both NAD+ (nicotinamide adenine dinucleotide) and NADP+ (nicotinamide adenine dinucleotide phosphate). The NAD+ is needed and used in metabolism, and the NADP+ is needed and used in photosynthesis. But they are almost identical in chemical structure.

Similarly to the way borates can accept a hydroxyl group from water by behaving as a Lewis acid, they can also accept hydroxyl groups from organic molecules to form a complex (a bit like a chelate). This occurs preferentially with sugar alcohols with adjacent –OH groups due to the enhanced stability of such complexes.

The molecules NAD+ and NADP+ have the cis-adjacent hydroxyl groups (side-by-side OH groups) required for complex formation on a ribose sugar within the molecule. But in addition to this, the positive (cationic) charge of the nitrogen (N) within the nicotinamide moiety of the nucleotide allows for electrostatic stabilization of the complex with the borate negative (anionic) charge. Providing the most stable complex in biological systems basically means that this is where the borate will be most of the time.

It has been concluded that the same mechanism is likely occurring in all organisms, including algae; however, with algae these nucleotide co-enzymes are equally important in photosynthesis as well as supporting metabolism in the mitochondrion. So in essence, algae get hit twice. 

Controlling corrosion

Borates have been used for many years as corrosion inhibitors, primarily on steel and zinc. Borates act as non-oxidizing anodic inhibitors, having insufficient oxidizing power of their own to affect passivation; but in the presence of oxygen, iron-containing metals show passivation.

The presence of cloudiness and scale in pools is a major aesthetic issue. Cloudiness can be caused by a number of factors, only one of which is water hardness caused by high calcium content, which can also lead to scale formation. Scale is formed when the precipitation of salts, such as calcium carbonate or calcium sulfate, crystallize from solution as scale on the pool sides, typically at the water surface where evaporation is taking place and the solution is locally super-saturated. Both problems are also exacerbated by high pH.

Borates help this problem in two ways. First, they are good buffers, so they generally reduce the tendency for pH to creep upwards. Second, they have mild sequestration ability due to their divalent cation binding property, so they lock up calcium almost like a chelate. A 1:1 complex of borate and calcium or magnesium ions is believed to form:

Ca2+  +  B(OH)4-     CaB(OH)4+

This effect may also contribute to the water clarity observed in a borate pool, as well as the softer feeling water, which is gentler on the skin — and which is why borates are also added to washing detergents and bath salts.

With the recognition of all the benefits borates provide for pool water, and the technological advances that have overcome the previously perceived disadvantages of borates (such as low solubility and high pH), these products are becoming ever more popular as part of a pool treatment arsenal. With the growing emphasis on the use of natural products, the protection of resources and the importance of environmental impact, many customers are coming to prefer these products as well.