.Chemical water balance
The chemistry of water balance water and the ability to manage it well, is one of the most important but least understood aspects of the Pool Management. There is a wrong perception among pool owners that as long as they have the correct amount of chlorine required.
(I.e.1 – 4 ppm) they are OK. This is far from the truth. Water balance has nothing to do with the chlorine level or the chlorine concentration in the water; in fact it is quite the opposite. The disinfection capability of your chlorine depends on the pH stability which in turn depends on the stability (Balance) of your water.
Let take a simple approach: The term “Chemical water balance” is a technical term used for the ratio of the amount of chemicals and mineral salts that are dissolved. And are present in the pool water at any time. This ratio will determine the quality, condition and strength of the water. This raises the question as to whether the water will be corrosive or scaling, pleasant, smelly or cloudy. None of these are overly good for us so the ideal situation we wish to attain is a “Neutral” point at which none of these conditions occur.
How to achieve this
To achieve this “Neutral” point we use common chemicals such as Sodium Bicarbonate (Bicarb) and Calcium Chloride (Hardness). The Bicarb is used as a pH buffer and is the chemical that provides our water strength (also known as Total Alkalinity). It is used to increase the Total Alkalinity (TA) and thus the higher the TA the higher the pH. It is used to ensure that the water pH level does not fall below 7.2 points and become exceedingly corrosive.
Calcium Chloride on the other hand gives our water the balancing strength (also known as Total Hardness). It is also used to ensure that the water pH will not exceed 8.3 points and become exceedingly scaling. On its own it will not prevent pH falling but it will assist it in speeding up the process.
By adding these two chemicals to our water in appropriate amounts, we have limited the range of water pH level to between 7.2 and 8.3 points. What should be understood here is that each of these chemicals on its own does nothing for the balance of our water but together they hold our pH level steady.
This brings the question to mind;
What does the above have to do with the amount of chlorine in our pool and the levels at which our chlorine readings register?
Well, the higher the Alkalinity the higher the pH. The higher the pH the less the chlorine percentage is available as free active chlorine in the pool, which means that we have to put more chlorine in the pool to achieve a particular level of chlorine. The opposite of this is also true.
As an example, look at pH – chlorine relationship table below. Now At pH 7, chlorine will be 75% active (free active chlorine available) and 25% (inactive chlorine). At pH 7.5% this ratio is close to 50/50% and the pH level 8 only about 20% of added chlorine in the water turns into active chlorine (known as Hypochlorous acid)
At pH level of Active chlorine (HOCL)% Inactive chlorine (OCL-)%
4.0 100% 0.0%
7.0 75% 25%
7.5 48% 52%
8.0 22% 78%
11.0 0.03% 99.97%
You should now understand why there is a sudden “burst” of chlorine in the water when you add acid to it and lower the pH from say. 8.0 to 7.5 without adding any extra chlorine.
By adding acid to lower your pH level, you are in fact lowering your Total Alkalinity level as well. A mistake in the amount of acid added can be a costly exercise. The reason being is the fact that the pH becomes volatile and if your TA falls below 70ppm. Then problems arise in achieving stability of the water and thus requiring additional doses of chlorine. – To be continued
Steve Johnson is the Managing Director of “WET” Water Engineered Technologies (Thailand) and offers a broad range of “Commercial & Domestic” water filtration solutions, potable water, plus swimming pool design, construction, equipment and consultation.
Steve can be contacted on Tel +66 (0) 848 428 317 or email us here