Langelier Index

The Langelier Index was developed by Dr. Wilfred Langelier at the beginning of the 20th century. His firm intention was to find out the driving force behind the flocculation and precipitation of salts in water, or conversely to find out when these salts were dissolved. The study shed light on the functioning of corrosion and sedimentation due to solutes in water.

The equation finally needed 6 variables other than water to calculate what is now known as the Langelier Saturation Index (ISL):

• pH
  The pH is the numerical representation of the amount of hydrogen ions in a solution and plays a very active role in the solution or precipitation of the elements dissolved in water. Hydrogen is a very reactive element that can modify the compounds present in water, making them more or less soluble.

The Langelier Saturation Index summarises the risk of clogging or corrosion in one value.

• Temperature
  Temperature affects the ability of a compound to dissolve in water or to precipitate instead. Different salts act differently. Some become more soluble as temperature increases. Others become more soluble as the temperature decreases. There are also salts that maintain their solubility regardless of temperature.

• Calcium hardness
  Calcium hardness is represented by ppm or mg/l of calcium carbonate (CaCO3). It indicates the concentration of calcium and magnesium present in the water. These are the elements most prone to the formation of blockages in pipes. Their solubility depends on the factors covered by the Langelier Index.
  It is calculated with the following equation:
  Hardness (mg/l CaCO₃) = 2,50 x [Ca concentration⁺⁺ in mg/l] + 4,116 x [Mg concentration⁺⁺in mg/l]. 

• Alkalinity 
  Alkalinity is sometimes confused with the concept of alkaline pH but is not so directly related. Alkalinity is defined as the amount of dissolved carbonates, bicarbonates and hydroxides in water. Waters with these solutes generally have a basic or alkaline pH and provide a very stable pH. Water with low alkalinity facilitates corrosion of materials through which the water flows, such as metals or rocks, and results in a pH with low stability. High alkalinity increases the risk of scaling.

• Total dissolved solids (TDS)
  This measure represents all solutes, flocs or any other type of material that will remain in residual form when the water is evaporated.

The danger of clogging is a silent evil because it slowly clogs the pipe until it can ruin an entire irrigation network.

The Langelier Saturation Index equation is calculated based on factors related to each of the variables. The intermediate values are obtained through a simple linear interpolation.

(pH)+(Temperature factor)+(Hardness factor)+(Alkalinity factor)-(TDS factor) = ISL

The Langelier Index can be positive, negative or neutral.

• ISL is negative, the water will be corrosive. This type of water has the potential to dissolve materials in the watercourse.
• ISL close to zero. It is a balanced water that will respect the physico-chemical form of the materials of its bed.
• ISL is positive, the water is incrusting. This type of water is saturated with elements and during its flow it will try to reduce its concentration by leaving the materials it has in saturation in the course of its flow.

Corrosion is particularly affected in sections with load losses such as elbows.

The desired values of the Langelier saturation index are close to zero, between -0.3 a +0.3 and through acid inputs in pumping is a good way to keep the dangers of corrosion or clogging of the irrigation network under control.

We can help you with the irrigation of your field through analysis and interpretation of irrigation water. And together with our water and soil experts we produce detailed reports in which we add value with explanations and guidelines.