Water Balance

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Overview

One of the most important parts of running a pool is keeping the water balanced. Unbalanced water can cause damage to concrete, grout, marcite (pool plaster) and metals. Water can corrode surfaces or deposit substances called scale.

Unbalanced or "aggressive" water can also destroy filters and copper heat exchangers. One way to identify corrosive water is by staining, which a deposit of colored metal salts of iron (brown) or copper (blue, green, grey or black). Scaling water does the opposite - it attempts to deposit or precipitate calcium carbonate out of the water, causing deposits on the surface and plugging of the filters and piping.

Parameter Ideal Range Method to Adjust
Increase Decrease
Alkalinity 80 to 120 ppm Add Sodium Bicarbonate (Baking Soda) Add Hydrochloric (Muriatic) Acid
pH 7.4 to 7.6 Add Soda Ash Add Hydrochloric (Muriatic) Acid
Calcium Hardness 150 to 400 ppm Add Calcium Chloride Dilute (add water)

As important as it is to adjust these factors to maintain water balance, the sequence is the most important:

  1. Total Alkalinity is always the first parameter to be corrected. Exceptions can be made when pH drops to a very low level or if metals are present in the water.
  2. Then, adjust pH (in our pool, this is managed automatically by the Chemtrol Pool Controller).
  3. Finally, adjust Calcium Hardness.

When lowering calcium hardness by diluting the water, pH and total alkalinity are adjusted after the dilution has been completed.

Total Alkalinity

Total alkalinity is the measure of the water's ability to "buffer" from wide pH swings. A buffer is a chemical system that resists change when acids or bases are added to the water. In water with no buffer ability, pH can shift dramatically. These rapid fluctuations of pH levels are called pH bounces, and the result is a highly unbalanced water condition.

Low Total Alkalinity High Total Alkalinity
Impact
  • Corrosive water
  • Pitting of concrete
  • Metals dissolving
  • Staining of walls
  • Scaling water
  • Plugged filters
  • Reduced circulation
  • Cloudy water
Non-Balance Problems pH bounce pH drifts upward

Testing

Total alkalinity, like calcium hardness, is tested by titration. At the endpoint the color changes from green to red. Make sure to follow the directions on the inside of the test kit thoroughly. Observe that some samples will turn from green to purple instead of green to red. This color change can also be used as a reliable endpoint. It is caused by the use of a polymeric biguanide (PHMB) in the pool.

False Readings

The most common problem in the total alkalinity test is caused by a high level of chlorine. When excessive chlorine is present in the sample, one of the two reagents used in the total alkalinity indicator will bleach out. When this happens, the endpoint changes from blue to yellow instead of green to red. To avoid this problem, repeat the test with an extra drop of the first reagent, thiosulfate. This should destroy the excess chlorine and solve the problem.

pH

pH is measured on a scale from 0 to 14:

  • A pH of 7 is neutral.
  • A pH below 7 is acidic.
  • A pH above 7 is basic (alkaline).

The pH scale is logarithmic, which means for every whole unit of change, it's ten times different than the prior value.

Examples:

  • A pH of 6 is 10 times more acidic than than a pH of 7.
  • A pH of 4 is is 100 times more acidic than a pH of 6.

The ideal range of pH (7.4 to 7.6) is very close to the pH of the human eye (7.5), which aides in swimmer comfort.

Testing

pH is a colorimetric measurement that uses a color indicator to identify pH ranges. Most tests use phenol red, with a test range of 6.8 to 8.4. When testing pH, it is important to carefully understand and follow the directions provided with test kit.

False Readings

Sometimes, high levels of chlorine will quickly convert phenol red into a new indicator that is dark purple when the pH is above 6.6. A common mistake would be to confuse the purple color for dark red and wrongly add acid to the pool.

Calcium Hardness

Water that has little calcium or magnesium in it is called soft. Hard water contains high levels of calcium and magnesium salts and consumes soap, making it hard to form suds. In pools, having too much calcium may cause the water to scale. Having little to no calcium can cause the water to become corrosive or "aggressive" and may eat away at concrete, marcite (pool plaster) or grout.

Low Calcium Hardness High Calcium Hardness
Impact
  • Corrosive water
  • Etching of marcite (pool plaster)
  • Pitting of concrete
  • Grout erosion
  • Pitting of the pool deck
  • Scaling water
  • Plugged filters
  • Reduced circulation
  • Cloudy water
  • Heater inefficiency

Testing

Calcium hardness is tested by titration with an endpoint that changes from red to blue. Be sure to follow the directions on the inside of the test kit thoroughly.

False Readings

The most common error in testing calcium hardness is called a fading endpoint. Instead of observing a well-defined color change and a permanent endpoint, some color change occurs, but the sample then reverts back to its original color. This usually happens due to small concentrations of iron or copper in the pool water, causing the the test to turn purple instead of blue. To remedy, add five or six drops of the titrant to the water sample first, then conduct the test as usual. When a complete color change is observed, add up the drops (including the five or six initially added) to obtain a total. The initial five or six drops act as a bonding agent, holding together the interfering metals so the color change is apparent and the test is conclusive.