dioxide loss to the air, ….
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.