Langelier Index (LI)

Definition - What does Langelier Index (LI) mean?

The Langelier index (LI) is the difference between the actual (measured) pH and the calculated pH of water. It gives an approximate measure of the degree of saturation of calcium carbonate in water.

The sign and magnitude of the Langelier index show the water’s tendency to form or dissolve scale and thus to inhibit or encourage corrosion. It is one of the measurements used when stabilizing water in order to control deposition of scale and internal corrosion in water pipes.

Corrosionpedia explains Langelier Index (LI)

The Langelier index is calculated using:

  • Alkalinity
  • pH
  • Calcium concentration
  • Total dissolved solids
  • Water temperature

The calculation gives an indication on whether the water's calcium carbonate saturation:

  • Positive LI - Water is over saturated with calcium carbonate and has a tendency of depositing calcium carbonate, forming scales in the pipes or the distribution system.

  • Negative LI - Water is under saturated and has a tendency of being corrosive in pipes or distribution networks.

  • Near-zero LI - Water is saturated with calcium carbonate; neither strongly scale forming or corrosive.

The Langelier index is only an approximation of the tendency, but not a direct measure of the corrosive nature of the water. However, it is a very useful tool for showing this tendency. The LI value may vary and is dependent on factors such as the temperature and ionic strength.

The LI is very important in determining appropriate control measures. In an over-saturated condition, excess calcium carbonate drops out of the solution and forms scale, which coats the pipe’s interiors, thus reducing water-carrying capacity or causing damage. In an under-saturated condition, the relationship between calcium carbonate and alkalinity may increase the corrosiveness of water. Water operators must take the necessary actions to produce water which is in equilibrium and is neither corrosive nor scale-forming.

Typical affected areas include:

  • Low-pressure boilers
  • Water treatment plants
  • Cooling towers

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