There is one chemical which must be present in every swimming pool in order for all other chemicals to work. Do you know what it is?
You may think the most vital chemical for a swimming pool is chlorine. Because chlorine acts as a sanitizer or disinfectant to ensure that the water is safe, clean and clear. Or possibly it is a chemical that affects the water balance such as acid or sodium bicarbonate. In order for chlorine to be effective the pH must be balanced in a good range between 7.4 to 7.6. These may seem to be reasonable answers however they are not the answer we are looking for.
In fact the answer might surprise you, but the most vital chemical in a swimming pool is water. That's right water is a chemical with a chemical symbol of H2O. Water has three atoms-two hydrogen atoms and one oxygen atom. It is the way that the hydrogen and oxygen combine to form the water molecule that gives water its special properties. The way in which hydrogen and oxygen connect by sharing electrons gives it a very tight chemical bond. Water is also defined as a "polar" molecule due to the two hydrogen atoms having a positive charge and the oxygen atom a negative charge.
Water has three characteristics that make it the most vital chemical for pools.
- Water is known as the "universal solvent". This means that water has the ability to dissolve anything from salt to cement. It is the polarity of the water molecule that gives it this distinctive property. When salt is dissolved in water ions are formed that are positively and negatively charged like many tiny electical plates. Water keeps sodium chloride in solution by pointing the positive hydrogens toward the negative chloride ions and the negative oxygen points towards the positive sodium ions. The sodium and chloride ions are kept seperate by a layer of water molecules that surrounds them. One important note on water as the most vital pool chemical is the fact that all forms of chlorine originate from sodium chloride and many of the other chemicals we use are comprised of salts, such as soda ash and bi-carb which are sodium carbonate and sodium bi-carbonate.
- Water is amphoteric. This means that water always naturally seeks to be balanced. Water has the ability to form either acids or bases in an attempt to balance itself. Pure H2O has a pH of 7.0 which is considered neutral on the pH scale. Pure water has a characteristic known as autoionization which is the transfer of a proton from one water molecule to another to produce a hydronium ion (acid) and a hydroxide ion (base). In this scenario one water molecule will act as an acid by furnishing a proton. Another water molecule will act as a base by accepting a proton. When the base water molecule accepts the proton it becomes the conjugate acid (hydronium ion). The water molecule that donates a proton becomes the conjugate base (hydroxide ion). Pure water creates the acid and base in equillibrium to give it a balance of the neutral pH 7.0. Source water that goes into pools can be either soft (having very little mineral) or hard (containing high amounts of minerals). Based on the make up of the source water if left untreated in a newly filled swimming pool water will seek its own balance. In the case of soft water it will take minerals from the plaster or any cemetitious surface in order to be balanced. Hard water will disperse scale mostly in the form of calcium carbonate which leads to hard white, sharp, cyrstaline deposits. This is one of the most important reasons to test source water and understand the chemical make up of the water before filling the pool.
- Water reacts with compounds through a process of hydrolysis. This is a chemcial reaction where molecules are broken down by reacting with water. Water acts like a chemical knife cutting apart molecules into smaller molecules. Water reacts with molecules of compounds by splitting the chemical bond and adding a hydrogen cation (H+) and hydroxide anion (OH-). This process is what makes water the most vital for swimming pools specifically when it comes to keeping it sanitized or disinfected. The cutting reaction of water to chlorine when it is added to a pool is what produces the most powerful germ killing agent.
When chlorine is added to water the hydrolysis reaction immediately kicks in as follows:
Cl2 +H2O →HOCl+HCl
Chlorine + water → hypochlorous acid + hydrchloric acid
The most important compound in this reaction is the hypochlorous acid (HOCl). It is the killing form of the chlorine. HOCl is the form of chlorine in the water that kills bacteria, algae, mold, mildew, and other disease causing organisms.
The hydrolysis reaction takes place almost instantaneously. Next another reaction takes place known as the dissociation reaction. This reaction causes the the hypochlorous acid to come apart into its ions as follows:
HOCl ↔ H+ + OCl-
Hypochlorous acid dissociates into hydrogen ion and hypochlorite ion.
This is a reversable reaction as is indicated by the arrow pointing in opposite directions. This reaction is not only reversable but it is also in equillibrium. This means that a change on one side is immediately compensated for by a change on the other side of the reaction. When HOCl gets used up killing germs and algae it is immediately replaced by some of the hypochlorite ion (OCl-) converting back to HOCl. As more HOCl is used up, more OCl- converts back to HOCl to do the killing of harmful germs and algae.
Water is the most vital and efficient chemical in our pools. Through its amazing properties it ensures that the other chemicals we add are converted into the most effective products.
The next time you are going to add water into the pool make sure you understand the make up of this vital chemical that is being added. Like any other chemical it is paramount that you test and know what your source water make up is. What is the pH, total alkalinity, calcium hardness and total dissolved solids (TDS) of the source water? What about metals and phosphates or nitrates. Before you fill test the source water because that is the most vital chemical you are adding to the pool.