Water Balance

pH..................................................    (7.2 - 8.0)

Total Alkalinity (TA).......................    (100-150) ppm

(Fiberglass, vinyl-lined and others)... (125-175) ppm

Calcium Harness (CH).................   (175-300) ppm

Total Dissolved Solids (TDS) .....   <2,000 ppm (max permissible range)

Langelier Saturation Index ........    ( 0 ) (+/- 0.5)

Water Balance is a figurative term representing the dynamic interaction between pH, total alkalinity, calcium hardness, dissolved solids, and temperature. The importance of water balance is oftentimes oversimplified. This oversimplification is due, in part, to a misunderstanding of each water balance parameter's influence on one another.

pH

Recommended Levels of pH in pools and spas

pH Levels should always be maintained between 7.2 to 8.0

Before pH can be convincingly discussed, one must first survey the fundamental properties of water.  We fill our pool with it; we wash our clothes with it; we even power generators with it; but many uninformed pool and spa owners/operators underestimate its solvency potential.

Water is composed of two (2) hydrogen atoms covalently bonded to one (1) oxygen atom. The result is water H2O. But there's much more than just water in water. Water dissociated similar to hypochlorous and hypobromous acid. This dissociation generates hydrogen and hydroxide ions. The hydrogen ion concentration determines the pH. The greater the hydrogen ion concentration, the lower the pH, and the more acidic is the water. Decreasing the hydrogen ion concentration increases the pH and reduces the water's acidity. A Neutral pH exist when the hydrogen ion (H+) concentration is equal to the hydroxide (OH-) ion concentration. A pH of 7 is termed neutral; below 7 is acidic; above 7 is basic.

pH is a logarithmic function permitting the relative acidity or basicity to be expressed in simple mathematical terms: 0-14. It is this simplification that also causes misunderstandings. Each unit division on the pH scale represents a ten-fold increase or decrease in acidity. For example, a pH of 5 is ten times more acidic than a pH of 6. A pH of 3 is one thousand times more acidic than a pH of 6. Each division is a multiple of ten. Frequent water test will enable the pool or spa owner/operator to monitor pH and make necessary corrections.

Total Alkalinity

Recommended Ranges for Total Alkalinity

Plaster and marsite pools and spas  ...........100-150ppm

Fiberglass, vinyl-lined and others  ..............125-175ppm

Note: A minimum level of 80 ppm is permissible in hard water (high calcium hardness) areas with hardness exceeding 500 ppm.

Total alkalinity (TA) is the quantitive measurement of alkaline components present in water to act as a buffer against rapid pH changes. Proper total alkalinity levels are important to ensure optimum chemical balance in pools, spas and other aquatic systems.

For the most part, total alkalinity is a contribution of carbonate (CO3-2), bicarbonate (HCO3-5), and hydroxide (OH-1), with borates, silicates and phosphates possibly making a minor contribution in certain geographic areas.

Low Total Alkalinity is a highly corrosive condition resulting in damage to copper heat sinks and heat exchangers, light rings, stainless steel ladders, and pool surfaces. Low total alkalinity is frequently coincidental with green water, plaster etching and accelerated corrosion rates.

High Total Alkalinity and its associated increases buffering capacity will make it difficult to adjust pH with the addition of acid or base (alkali). In addition, high total alkalinity is frequently concurrent with high pH and/or cloudy water.

Calcium Hardness (CH)

Recommended ranges of Calcium Hardness :

175-300ppm (parts per million)

10-18gpg (grains per gallon)

Calcium Hardness (CH) indicates the calcium content of water usually expressed as milligrams/liter (mg/l) or ppm. Not to be confused with Total Hardness, which is the sum of calcium and magnesium, calcium hardness is the primary parameter that dictates water balance.

1. Calcium hardness determines subsequent adjustment   of pH and total alkalinity. This is true because the reduction of calcium hardness is very difficult in pool and spa applications. Therefore, in geographical areas with high calcium hardness, pH and total alkalinity must be adjusted to compensate for these elevated CH levels.

2. Plaster and marsite surfaces frequently found in pool and spa systems are primarily composed of calcium carbonate, commonly known as calcite. Because of its reduced solubility (approximately 9.32 mg/l) over its magnesium counterpart (265 mg/l), calcium hardness dictates water balance.

Of the previously listed problems that may occur with improper calcium hardness, low calcium hardness levels are most critical. Low calcium hardness in conjunction with low pH and alkalinity will disrupt the solubility equilibrium of calcium carbonate and increase the aggressiveness of water. The result:

1. Surface etching and pitting. An aggressive condition dramatically increases the solubility of calcium carbonate, the principle building block of plaster and marsite surfaces associated with pools and spas. Etching can occur very quickly, resulting in expensive replastering.

2. Corrosion of metal components, especially heat exchangers and copper heat sinks. This occurs because the insoluble nature of calcium corbonate provides a thin protective film on the surface of copper heat exchangers, thereby reducing direct metal-water flow interaction, but not disturbing the heat exchange process. If this invisible protective film is destroyed because of improper water balance, heat exchangers and associated components will suffer premature deterioration. This unique protective feature of calcium carbonate is due to calcium carbonate's inverse solubility-temperature relationship. As the water temperature increases, the solubility of calcium carbonate decreases. It is one of the few inorganic compounds that displays this inverse relationship.

Maintaining proper calcium hardness is very critical in the care of you pool & spa!

Total Dissolved Solids (TDS)

Total dissolved solids is an electrochemical measurement of water's ability to conduct an electrical current. The electrical conductivity increases with increasing TDS or the presence of dissolved ("charged") chemical species in the water.

To better understand TDS, let's consider "pure" water; water that is free of any chemical or microbiological impurities. "distilled" or "deionized" water is considered "pure" water and its quality is determined by measuring its electrical conductance. The lower the conductance, the purer the water.

A similar concept has been adopted for pool and spa water, but it is expressed as Total Dissolved Solids. TDS is a semiquantitative measurement of a water's dissolved ion content. The conductance (measured in micromhos; a micromho is the practical unit of conductance equal to the reciprocal of the ohm) increases with increasing salt or dissolved ion content. It is important to remember that TDS is a measurement of "charged" chemical species (ions) whether they carry a net positive (+) or negative (-) charge; "uncharged" or neutral chemical species are not measured as TDS.

Virtually all chemicals added to a pool or spa will have a positive influence on TDS. TDS of typical fill water ranges from 100 to 400 ppm, but exceptions do exist in various geographical areas depending on the water supply system.

High TDS (above 2,000 ppm) can: (1) enhance algae growth rates. In addition to supplying essential nutrients, high TDS may offer ideal harboring sited for algae spores to gain a foothold in the pool or spa; (2) increase "natural" (galvanic) corrosion. Each reader is certainly familiar with metal deterioration (corrosion) that occurs near the seashore or to automobiles exposed to "salted" roadways during the winter. These accelerated corrosion rated are associated with high salt content or increased TDS. The resulting increase in electrical conductance promotes "galvanic" corrosion or electrolysis. This is of course followed by premature heater failure, corroded light rings or damaged circulation systems. A good example is heated seawater pools where heater manufacturers use copper/nickel heater exchangers and heat sinks in the place of pure copper. The reason: Copper/nickel is much more resistant to corrosion; (3) accelerate staining and scaling of pool and spa surfaces.

In conclusion, TDS is a general indicator of pool or spa water acceptability. If too high, the pool should be partially or completely drained to reestablish acceptable levels. Many pool/spa codes have adopted a policy of daily water replacement based on the average daily bather use. In addition to maintaining proper levels, a daily water replacement policy optimizes the potential for good water quality resulting in a safer, more enjoyable swimming or soaking experience.

Temperature and the Langelier Saturation Index

The effect of temperature on water balance cannot be fully understood without discussing Langelier's Saturation Index.

Langelier's Saturation Index is frequently employed as a method of determining whether a water will support or deposit calcium carbonate. It was primarily developed for the boiler industry, but other direct applications soon followed to include pools and spas. The correctness of the Langelier Index has frequently been questioned, but to date, it has withstood criticism and remains the principle water balance index because of its ease and convenience.

According to the Langelier Saturation Index, five (5) primary factors effect water balance and are therefore incorporated into the saturation index formula. They include: pH, calcium hardness, total alkalinity, temperature, and total dissolved solids.

Of these water balance parameters, calcium hardness, total alkalinity, and temperature are expressed as factors: CF, AF, and TF, respectively. The resulting Langelier Saturation Index formula is as follows:

Saturation Index = pH + CF + AF + TF - 12.1

The waters pH is substituted directly into the formula. An average factor of -12.1 is the representative constant for the total dissolved solids when less than 1,000 ppm.

With this information and the appropriate factors listed in Table 9-4, the pool or spa owner/operator can quickly calculate the saturation index for any given system.

Table 9-4

Calcium Hardness Total  Alkalinity Temperature

ppm      =           CF ppm          =            AF 'F         =        TF

5                    0.3                5                          0.7                    32                  0.0

25                    1.0               25                         1.4                    37                   0.1

50                    1.3               50                         1.7                    46                   0.2

75                    1.5               75                         1.9                    53                   0.3

100                    1.6              100                        2.0                    60                  0.4

150                    1.8              150                        2.2                     66                  0.5

200                    1.9              200                        2.3                     76                  0.6

300                    2.1              300                        2.5                     84                  0.7

400                    2.2              400                        2.6                     94                  0.8

800                    2.5              800                        2.9                    105                 0.9

1000                   2.6             1000                       3.0                    128                 1.0

*If the Saturation Index is 0, The water is perfectly balanced.

*If the Saturation Index is a negative value, corrosion tendencies are indicated.

*If the Saturation Index is a positive value, scale forming tendencies are predicted.

Values for the Saturation Index are considered acceptable if they fall between +0.5 and -0.5. A positive value is considered more acceptable than a negative index in pool or spa applications.

Example:  A 20,000 gallon pool has a temperature of 84F, pH of 7.4, total alkalinity of 150 ppm, and calcium hardness of 200 ppm. What is the Saturation indexof this water? Does it exhibit corrosive or scaling tendencies? Are adjustments necessary?Saturation Index = 7.4 + 1.9 + 2.2 + 0.7 - 12.1 = +0.1With an index of +0.1, this pool falls slightly on the scaling side of the index no water balance adjustments are necessary.