Technical definition
Sulphates are a measurement of the concentration of dissolved sulphate ions (SO₄²⁻) present in the heating system water, expressed in milligrams per litre (mg/L).
Sulphates occur naturally in mains water as a result of minerals dissolving into the water supply, but they may also be introduced through treatment chemicals, contamination or changes in the water source.
Within a heating system, sulphate is primarily useful as an informational and diagnostic parameter. Fernox includes Sulphates within its laboratory water-testing capability and currently categorises the parameter as “For Information”, rather than applying a universal pass/fail concentration.
Sulphate is a dissolved ion and therefore contributes to the overall ionic content and conductivity of the system water. Under certain conditions, sulphates may also be involved in corrosion processes, microbiological activity or mineral deposition.
Simplified definition
Sulphates are naturally occurring dissolved salts that may be present in the water used to fill a heating system.
Testing sulphates helps build a picture of the chemical composition of the system water and can identify unusual changes, contamination or conditions that may contribute to corrosion, deposits or microbiological problems.
Sulphates are useful because their concentration provides additional information about the source and condition of the heating system water.
An abnormal or unexpected Sulphates result may indicate:
Naturally elevated sulphate in the original mains or filling water.
Introduction of a different water source.
Contamination from another chemical or fluid.
Changes in water chemistry following draining, flushing or refilling.
Residual treatment or cleaning chemicals, depending on their composition.
Increasing concentrations of dissolved salts within the system.
Conditions that could contribute to mineral deposition when suitable hardness-forming minerals are also present.
A potential relationship with microbiological activity in susceptible systems.
One important consideration is the relationship between sulphate and microbiologically influenced corrosion. Fernox specifically notes that sulphate-reducing bacteria can occur within heating systems and can be associated with the production of hydrogen sulphide, often recognised by its characteristic “rotten egg” smell.
Sulphate-reducing bacteria can use sulphate as part of their metabolism under low-oxygen conditions. Their activity can create highly corrosive local environments and may contribute to:
Microbiologically influenced corrosion (MIC).
Formation of hydrogen sulphide gas.
Unpleasant odours.
Localised corrosion of metallic components.
Black deposits and contamination.
Accelerated component deterioration in severe cases.
Sulphates can also interact with hardness-forming minerals. For example, where sufficient calcium is present, calcium sulphate compounds may contribute to mineral deposits or scale under suitable temperature and concentration conditions.
Elevated sulphate also contributes to the total concentration of dissolved ions within the system water and may therefore be associated with an increase in conductivity.
Testing Sulphates can consequently help:
Characterise the original filling water.
Compare circulating system water with the filling water.
Identify unexpected chemical changes.
Support investigation of abnormal conductivity.
Provide additional context when investigating deposits or scale.
Support investigation of suspected microbiological contamination.
Build a more complete chemical profile of the heating system water.
Sulphates should therefore be considered alongside parameters such as Conductivity, Calcium, Total Hardness, pH, Chloride, Suspended Solids and microbiological indicators when assessing the overall condition of the system.
Sulphates should not normally be assessed in isolation, and the presence of sulphate does not automatically indicate a problem with the heating system.
Fernox currently categorises Sulphates as “For Information”, meaning that there is not a single universal Fernox pass/fail concentration applicable to all heating systems.
Some sulphate may be naturally present in the original filling water, so a measurable concentration can be completely normal.
The result becomes more diagnostically useful when there is a significant difference between the filling-water and system-water concentrations, or where an unusual sulphate level occurs alongside other abnormal parameters such as high Conductivity, Calcium, Suspended Solids or evidence of microbiological activity.
It is also important to distinguish between Sulphate (SO₄²⁻) and Sulphide (S²⁻). They are chemically different substances. Sulphate can be reduced by certain bacteria under low-oxygen conditions to form sulphide compounds, including hydrogen sulphide, but a sulphate result alone does not demonstrate that this microbiological process is occurring. Fernox specifically associates hydrogen sulphide formation with sulphate-reducing bacteria in contaminated systems.
The most useful interpretation is therefore obtained by comparing Sulphates with the original filling-water result, previous water tests, system conditions and other relevant chemical parameters.
