Technical definition
P-Alkalinity is a measurement of the portion of the water’s alkalinity associated primarily with hydroxide (OH⁻) and carbonate (CO₃²⁻) ions, expressed as milligrams per litre of calcium carbonate (CaCO₃).
The “P” refers to the traditional phenolphthalein endpoint, which occurs at approximately pH 8.3 during an alkalinity titration. At this endpoint, hydroxide alkalinity and approximately half of the carbonate alkalinity have been neutralised.
Unlike M-Alkalinity, which represents the broader total alkalinity of the water, P-Alkalinity is more specifically associated with the higher-pH components of the alkalinity present.
In heating system water, P-Alkalinity can therefore provide additional information about the balance of carbonate and hydroxide chemistry, particularly when considered alongside M-Alkalinity and pH.
Simplified definition
P-Alkalinity measures the part of the water’s alkalinity associated with carbonate and strongly alkaline substances.
It helps show how the system water is chemically balanced, particularly at higher pH levels. Comparing P-Alkalinity with total alkalinity can provide useful information about the type of alkaline substances present in the water.
P-Alkalinity is useful because it helps provide a more detailed understanding of the alkalinity and pH chemistry within the heating system.
An abnormal or unexpected P-Alkalinity result may indicate:
Significant levels of carbonate alkalinity within the system water.
The presence of hydroxide alkalinity, particularly where the pH is relatively high.
Changes in water chemistry caused by chemical treatment products.
Differences between the original filling water and the current system water.
Conditions that may increase the potential for calcium carbonate scale formation when calcium is also present.
Water chemistry that has become excessively alkaline.
Changes caused by contamination, system treatment or repeated filling and topping-up.
P-Alkalinity is particularly useful when assessed together with M-Alkalinity. The relationship between the two measurements can help indicate whether the alkalinity is predominantly derived from bicarbonate, carbonate or hydroxide species.
This is important because these different forms of alkalinity can behave differently within a heating system and can influence both pH stability and scale-forming potential.
Where calcium is present, elevated carbonate concentrations can encourage the formation of calcium carbonate scale, particularly on high-temperature surfaces such as boiler heat exchangers.
Very high alkalinity and pH can also be undesirable for certain system materials, meaning the result can help identify water chemistry that may require further investigation.
P-Alkalinity should therefore be considered alongside parameters such as M-Alkalinity, pH, calcium, conductivity and inhibitor concentration when assessing the overall chemical condition of the heating system water.
P-Alkalinity should not normally be assessed in isolation, and a low or zero P-Alkalinity result is not necessarily a problem.
In water where the alkalinity is predominantly present as bicarbonate, P-Alkalinity may be zero even though the water has a measurable and perfectly normal total M-Alkalinity.
The relationship between P-Alkalinity and M-Alkalinity is therefore generally more informative than the P-Alkalinity value alone. Together, the two measurements can help indicate the dominant forms of alkalinity present in the system water.
The result should also be interpreted alongside pH, calcium concentration, treatment regime, system materials and applicable water-quality guidelines before determining whether corrective action is required.
