There have been some ridiculous practices in swimming pool water chemistry over the past couple of years,, and one of the worst offenders is in the pseudoscience of allowing pH to seek its level. Often attempting to back up this tomfoolery with references to Henry’s Law, a principle that describes the relationship between a concentration of a gas in a liquid and the pressure of that same gas above the liquid, it seems as though the goal is to baffle the reader with bullshit rather than dazzle with brilliance. The ‘law,’ named for the English chemist William Henry who first proposed it in 1803, is especially important in chemistry, and is used to understand the behavior of gases in solution. However, claims that it can apply to swimming pool water chemistry are far-fetched. The truth of the matter is Henry’s Law fails with pool pH.
Bad Chemistry
According to Henry’s Law, the concentration of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid. In other words, as the pressure of the gas increases, the gas concentration in the liquid also increases. This is not being disputed and mathematically can be expressed as:
C = kP
Here C represents the concentration of the gas in the liquid, P is the partial pressure of the gas above the liquid, and k is a constant of proportionality known as Henry’s law constant.
The problem is this relationship between concentration and pressure does not hold true for all gases in all solutions. In fact, it is known that when the gas in question also reacts with the solution it is in Henry’s law does not apply. This is the scenario we experience with swimming pool water where carbon dioxide (CO2) reacts with water (H2O), forming carbonic acid (H2CO3). So, the practice of allowing pH to seek its own level is nothing short of bad chemistry.
When CO2 dissolves in water, it reacts with the water molecules to form carbonic acid (H2CO3). This equilibrium reaction can be expressed as:
CO2 + H2O ⇌ H2CO3
With additional side reactions as H2CO3 dissociates to H+ and HCO–3
Gas reactions
The formation of carbonic acid affects the concentration of CO2 in the water and, therefore, violates Henry’s Law. This is because the concentration of carbon dioxide in the water is no longer solely determined by the partial pressure of carbon dioxide above the water. Instead, it is also affected by the concentration of carbonic acid in the water.
As a result, the relationship between the concentration of CO2 in water and the partial pressure of carbon dioxide above the water is no longer linear, as predicted by Henry’s Law. Instead, it becomes more complex, and the equilibrium between the gas (carbon dioxide) and carbonic acid must be considered.
Limitations of Henry’s Law: This law is only applicable when the molecules of the system are in a state of equilibrium. Henry’s law does not hold true when gases are placed under extremely high pressure. The law is not applicable when the gas and the solution participate in chemical reactions with each other.
In addition to affecting the concentration of CO2 in water, the formation of carbonic acid also affects the pH and TA (Total Alkalinity) of the water. Carbonic acid is a weak acid that can donate a proton to water molecules to form bicarbonate ions (HCO–3) and hydrogen ions (H+). The hydrogen ions contribute to the measurement of hydrogen ion activity in the solution lowering the pH, whereas the bicarbonate ions contribute to the Total Alkalinity. It is important to understand the complex interplay between CO2, carbonic acid, pH , and TA in order to comprehend why the principle does not apply.
The theory of the ‘pH ceiling’ in swimming pools is flawed.
Applicability of Henry’s Law: Henry’s law only works if the molecules are at equilibrium. Henry’s law does not work for gases at high pressures… Henry’s law does not work if there is a chemical reaction between the solute and solvent…
The pH level of water plays a crucial role in aspects beyond LSI and water balance. It has a significant impact on the growth of algae, the efficacy of chlorine as a disinfectant, chlorine loss, and the potential for chemical reactions that can alter the overall chemistry of the water.
High pH promotes algae growth
We know alga prefers a more alkaline environment with a pH on the higher side of the acceptable range. This is because the nutrients that algae need to grow, such as phosphorus and nitrogen, are more readily available at those pH levels above 7.6. The higher the pH of the water the more ideal ideaitions for algae to growth and colonization.
It is also understood that the effectiveness of chlorine is pH dependent. The higher the level, the lower the concentration of hypochlorous acid which is the form of chlorine that is most effective at killing bacteria and other microorganisms in water.
High pH promotes chlorine loss
The lower the concentration of hypochlorous acid, the more significant the concentration of hypochlorite ions. That’s a problem. Hypochlorite ions do not share hypochlorous acid’s attraction to CyA (cyanuric acid). The higher pH has left the lion’s share of chlorine in the water subject to solar degradation. As a result, the amount of chlorine necessary to maintain even a minimal level will increase exponentially.
Remember that the saturation index only pertains to the protection of the vessel, whether the water is corrosive or scale-forming. The pool service pro needs to consider the big picture and every way a target pH can affect the water quality. The same holds true for CH (calcium hardness) which can easily be an entire article on its own.
