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Pool Water Chemistry
The pH is one of the most important factors in pool
water balance. The water’s pH is a measure of its total acid –
alkalinity balance, the relative proportion of acids and alkalis in the
water. The pH range goes from 0 – 14. i.e. on the pH scale, 0-6 level
indicates acidity, 7 indicates a neutral state and 8-14 indicates
alkalinity.
pH is really a measure of the relative amount of free
hydrogen and hydroxyl ions in the water. Water that has more free
hydrogen ions is acidic, whereas water that has more free hydroxyl ions
is basic. Since pH can be affected by chemicals in the water, pH is an
important indicator of water that is changing chemically.
pH is reported in "logarithmic units," like the Richter
scale, which measures earthquakes. Each number represents a 10-fold
change in the acidity / basicness of the water. Water with a pH of 5 is
ten times more acidic than water having a pH of six.
pH = inverse log of the concentration (activity) of free
H + or
pH = -log [H +]
The ideal level for swimming pool water is between 7.2
and 7.8
Why is pH so important?
The pH value affects the amount of hypochlorus acid
(free available chlorine) that is formed, and therefore determines the
effectiveness of the chlorine as a killer of bugs.
At pH 6.5, 90% of the chlorine will be hypochlorous acid
At pH 7.5, 50% of the chlorine will be hypochlorous acid
At pH 8.0, 20% of the chlorine will be hypochlorous acid
1. Unfortunately you cannot run your pool at pH 6.5 - it would acidic
enough to corrode the metal fittings in your pool circulation system and
it is too far from the Human body's pH of 7.4 to be comfortable to bathe
in. The compromise is 7.2 to 7.6, preferably midpoint of 7.4. Remember,
if you let the pH drift out of this range, you will have to use more
chlorine to get adequate disinfection.
2. Bather comfort, at high pH, the water will make your eyes sting and
possibly give you a sore throat
3. At high pH (above 8) there are two dangers.
a. The danger of scale forming on your pool surfaces, pipe work and
fittings.
This is because at a pH of around 8.0, the calcium in the water combines
With carbonates in the water. Result? Calcium carbonate or scale.
b. Calcium carbonate can form into tiny particles and float around in
the
Water giving it a cloudy, turbid appearance.
4. A low pH (below 7) can corrode metals, eating away at copper fittings
and heat exchangers leaving metal oxides to stain pool surfaces. Under
certain conditions the precipitated (particulate) metals can tint your
hair,
A final note: Before adding any chemicals to
adjust your pH levels, you must first consider total alkalinity.
2. Total Alkalinity
Total alkalinity is a measure of the amount of alkaline
materials in the water. This alkalinity will usually be present as
bicarbonates, but with a very high pH carbonates and hydroxides can be
present as well.
In basic terms TA acts like a buffer for your pH level.
Total alkalinity is a measure of the pH-buffering capacity, or the
water's resistance to a change in pH. This ability to resist change in
pH is due primarily to the presence of the family of carbonate ions, but
certain other compounds also provide buffering. The carbonate ions have
a special role in water saturation. The operator must control both the
amount of carbonate alkalinity and the pH to provide enough calcium
carbonate to saturate the water without having so much that scale forms.
Alkalinity does not have to be tested for as often as pH.The good total
Alkalinity will make it much easier to maintain good pH. The appropriate
range for Total Alkalinity in pool water is between 75 and 120 ppm
(parts per million). High Total Alkalinity (above 120 PPM) will allow
your pH to slowly creep up and resist efforts to change.
Low Total Alkalinity (below 75 PPM) allows your pH to "bounce" from one
extreme to the other, making it very difficult to keep your pH in the
appropriate range. Ideal Total Alkalinity (between 75 and 120 PPM) can
be achieved by adding Alkalinity Increaser if the Total Alkalinity is
below 75 PPM and pH Reducer if Total Alkalinity is above 120 PPM now for
the third factor affecting your swimming pool water chemistry.
3. Total Dissolved Solids
By definition, TDS is absolutely everything dissolved in
your pool water, from metals to chlorine to alkalinity to sulfates and
salts. This apparent contradiction in terms refers to conductive
chemicals that can accumulate in the pool particularly when the water
evaporates, or when the pool is not 'diluted' with sufficient fresh
water.
You cannot see them because they are dissolved, but this doesn’t stop
those corroding metal parts (pumps, pipe work, filters) on account of
their conductivity. They are mostly made up of chlorides and sulphates.
Chlorides can accumulate with long-term use of sodium hypochlorite.
Regular addition of alum based clarifiers (aluminum sulphate) and dry
acid (sodium bisulphate) can increase sulphate levels. Periodic
backwashing and water replacement are the best ways of controlling
The acceptable range of TDS in a swimming pool is between 1,000 and
2,000 ppm.
4. Temperature
Temperature is the property that gives physical meaning
to the concept of heat. If an object is cold, we say it has a low
temperature. If it is hot, we say it has a high temperature. It can also
be observed that if a hot poker is plunged into cold water, the poker
becomes cooler and the water becomes warmer. This means that the hot
body gives up some of its heat to the cold body.
5. Turbidity
Turbidity refers to the concentration of undissolved,
suspended particles present in a liquid measured in Nephelometric
Turbidity Units (NTU).
By definition, turbidity is not a direct measure of these particles but
rather measures of how these particles scatter light.
A final note: In order to improve the turbidity of your pool water, try
using oxidizers. These include calcium hypochlorite, chlorinated
isocyanurates, and sodium hypochlorite. You can also add chemicals to
your pool water which will force these little particles together and
allow the filter to pick them up. Try organic polymer or alum.
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