Why does scale seem to form so much more in a spa relative to a pool? Why does the pH rise so quickly in a hot tub compared to a pool?

The answers to these questions reside in the key differences between hot tubs and pools: heat and aeration.

That, and how the chemistry of calcium carbonate (of which scale is primarily composed) is affected by those differences.

This is going to get pretty technical – we’re about to dive into the deep end here.

Scale formation occurs more readily in a spa than in a pool, and the reasons are related to the spa’s jets and the increased temperature. It is also due to the tendency for the pH to drift upward, which itself is also related to the spa’s jets and increased temperature.

These factors change the concentrations of the various species involved in the carbonate portion of calcium carbonates equilibrium reactions.

In the pool and spa environment, many of the reactions that take place are equilibrium reactions, where reactants come together to form products, and then the products undo the reaction and reform the reactants. A key feature of equilibrium is that at a given temperature, once chemical equilibrium has been reached, the concentrations of these components become constant with time. This type of equilibrium is true for the carbonate.

The equilibrium reaction is as follows:

CO2 + H2O ↔ H2CO3 ↔ H++ HCO3- ↔ H+ + CO32­

In words, carbon dioxide reacts with water to form carbonic acid, which dissociates to form a hydrogen ion and bicarbonate, which further dissociates to form a hydrogen ion and carbonate.

This equilibrium takes place in both pools and spas, but compared to a pool, both the aeration from a spa’s jets as well as the higher temperatures will change the concentrations of each of carbonate’s components.

Jets play a role by increasing the rate of outgassing of carbon dioxide. Put simply, carbon dioxide escapes from a hot tub much faster than in a pool. The concept is the same as opening a can of soda and stirring it up. If air were blown through a straw into the soda it would cause the soda to lose its fizz.

Such is the effect of the jets in a hot tub. It causes the spa to lose its “fizz.”

Similarly, heating the hot tub also increases the rate of carbon

Fraction of carbonates at various pH levels.dioxide loss to the air, compared to a swimming pool.

That loss of carbon dioxide upsets the equilibrium, which must rebalance itself. Because there was a loss of carbon dioxide, there are no longer equilibrium concentrations of the constituents, and the carbonates components quickly combine with one another to form more carbon dioxide to satisfy equilibrium concentration.

As they do so, hydrogen, H+, gets used up, and the pH rises.

That is because pH is a measure of the H+ in a solution. Free H+ in solution lowers the pH, so the loss of that H+ causes the pH to go up.

When carbon dioxide outgases, or leaves the system, the equilibrium concentrations are upset, causing carbonic acid to unmake itself into carbon dioxide, to satisfy the requirement to maintain those equilibrium concentrations. This, in turn, causes the bicarbonate to make itself into carbonic acid, and when it does this, it consumes a hydrogen ion. It was that existing hydrogen that had given rise to the neutral pH in the first place, so the pH begins to rise.

This explains why the pH rises in a hot tub compared to an adjacent pool.

But what about a hot tub’s increased propensity to form scale? What is it about a little extra heat that causes scale to form readily on the spa’s tile lines, not to mention heaters?

Two components explain this. First, the pH rises because hydrogen is being consumed. That shifts the equilibrium toward producing more carbonate ion (CO32-). This effect can also be seen in the accompanying graphic which shows the relative abundance of the different carbonate species at different pH values.

Because there is more carbonate available, there is a greater probability that calcium can combine with it to form calcium carbonate, or scale, as shown below.

Ca2+ + CO32- → CaCO3

However, it’s important to remember that the carbonate ion is only half of the reason for scale. The other half is calcium.

Obviously, having high calcium content in the water will increase the likelihood of scale formation, but the question remains: why it is more likely to form more in the spa environment?

From a thermodynamic point of view, the reaction between calcium and carbonate is changed by changing the temperature.

In other words, increasing the temperature also assists the reaction between calcium and carbonate to form scale. That is because the precipitation of calcium carbonate is endothermic, meaning that it takes a certain amount of heat to produce scale.

This notion is fairly counterintuitive to many who are aware that a lot of substances tend to dissolve, rather than precipitate, with increased temperature. For example, if one added sodium chloride, or table salt to water, it would be expected to dissolve better with heat. Calcium carbonate, or scale, does the opposite. It forms more easily and in greater amounts with increased temperatures.

Temperature and aeration make a world of difference in water chemistry, affecting both pH and scale production.