BIO302: Alkalinity Demand

1. BIO302: ALKALINITY DEMAND
Pool & Spa University

2. QUICK REVIEW
➤ There are five main components to proper water balance:
➤ TDS
➤ Saturation Index (SI)
➤ pH
➤ Total Alkalinity (TA)
➤ Calcium Hardness (CH)
➤ pH is a measure of how acidic or basic the water is, with 0 being acidic, 14 being basic, and 7 being neutral.
➤ pH and adjustments made to it are based on the concentration of hydrogen (H+) ions and hydroxide
(OH-) ions. At a pH of 7, the ratio of H+ ions to OH- ions is roughly 1:1.
➤ As the balance shifts toward more H+ ions, the water’s pH decreases; as it shifts toward more OH-
ions, the water’s pH increases.
➤ Total alkalinity is the measure of the buffering capacity of the water.
➤ Buffers are weak acids and bases that are designed to resist rapid changes to pH; they contribute H+
ions (or absorb OH- ions) when a base is added and absorb H+ ions when an acid is added.
➤ Major contributors to total alkalinity can stand on their own to protect pH, while minor contributors
cannot. (They will still contribute to the overall total alkalinity reading.) Major contributors include
carbonate, bicarbonate, and hydroxide ions.

3. LOW ALKALINITY
➤ When alkalinity decreases, what is actually happening is that
major contributors like carbonate and bicarbonate are
absorbing hydrogen ions (H+).
H+ + HCO3- H2CO3- H2O + CO2
In the above example, bicarbonate ions absorb hydrogen ions, forming carbonic acid,
which then forms water and carbon dioxide.
➤ The buffer—the bicarbonate ion—is lost, decreasing alkalinity
while maintaining pH or sharply reducing its change.
➤ As alkalinity falls, its ability to manage pH decreases; as the pH
falls, the balance of the water shifts toward an excess of hydrogen
ions, which are in turn absorbed by carbonate buffers, and so on.

4. ALKALINITY DEMAND
➤ Once the pH of the water falls below a certain threshold—for
pools, this is generally 5 or below—the buffers are entirely
consumed and additional applications of bicarbonate buffers
will be ineffectual.
➤ At this point, the relative quantity of hydrogen ions to buffers
is so in favor of hydrogen ions that alkalinity can be said to be
negative.
➤ Note that testing cannot reveal a negative alkalinity, since it
is not possible to actually have a negative quantity of the
major contributor ions.
➤ When alkalinity readings reach 0 or less, a pool is said to have
an alkalinity demand.

5. TREATING ALKALINITY DEMAND
➤ Generally, the process of balancing requires adjusting alkalinity
before pH. When treating alkalinity demand, this process is (sort
of) reversed.
➤ pH should first be adjusted to 5 or greater using Balance Pak® 200.
➤ Split the recommended dose into thirds and predissolve each
dose. Separate by 4-6 hours.
➤ Once pH is confirmed to be above 5, adjust up alkalinity with
Balance Pak® 100.
➤ Split the recommended dose into at least thirds and predissolve
each dose. Separate by 4-6 hours.
➤ Finish balancing pH.

6. PREVENTING ALKALINITY DEMAND
➤ Never assume a clear pool is a balanced pool. Regular testing is
required to ensure a properly maintained pool.
➤ Check pH weekly, adjusting as necessary.
➤ Check alkalinity at least monthly, adjusting as necessary.
➤ Dispel the notion that muriatic acid is a cure-all for cloudy pools.
➤ Muriatic acid is a pH decreaser; if the problem truly is high
pH, it may correct it. However, it will not fix sanitizer
problems or circulation/filtration problems.
➤ Heavy rain and snow can both rapidly decrease alkalinity, as they
generally have zero buffers. Pay careful attention to alkalinity and
pH after significant precipitation.

7. CONGRATULATIONS!
You have completed the activity!