The Langelier IndexThe Langelier index indicates the corrosivity of water the LSI (Langelier Saturation index). If its value is lower than - 0.5, then water is corrosive, if it's higher than + 0.5 then the water has a high scaling potential, and it can form deposits in the piping. How is the Langelier Index calculated?The Langelier Index is calculated using the following variables: TDS = Total dissolved solids (mg/l) T = Temperature in K C = Calcium hardness (concentration of CaCO3 mg/l) A = Alkalinity (concentration of CaCO3 mg/l) pH = pH of the water
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| The Langelier formulapHs = (9.3 + a + b) – (c + d)
met: a = (log( TDS ) – 1) / 10
b = -13.12 * log (T) + 34.55
c = log(C) – 0.4
d = log(A)
LSI = pH – pHs
SI = 2* pHs - pH
Go to the Langelier calculator SI stands for Stability index, this tells if the water is stable. If the SI is lower than 6 the water is potentially scalant. If the SI is higher than 8 the water is corrosive. Between 6 and 8 the water is in equilibrium. |
The theory behind the Langelier IndexAll the above-mentioned variables have influence on the corrosivity and/or scaling (precipitate). An extensive explanation of each variable follows below: TDS
As TDS increases water becomes more corrosive, this in evident for instance in compating tap water and marine water. (Salt)see water gives faster rust attack than tap water. This happens also concerning the eletric conductivity of water, because salts dissolves in water splitting up in positive and negative ions. The more salt there is in water the easiest is for the water to carry electrons. In the corrosion process electrons are exchanged. Temperature
The second variable is the temperature. When the temperature increases, the solubility Ca(OH)2, Mg(OH)2 and CaCO3 decreases. These substances will precipitate forming deposits. Alkalinity and hardness
Calcium hardness and alkalinity makes water less
corrosive, but if their concentration is too high deposits will form. Calcium can precipitate when the temperature increases. Alkaninity is formed by all the substances which increase the acidity of the water even if insmall quantities. This type of substances can form scaling. In the process of corrosion electrons are exchanged.
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pH
The pH is given by the proportion H3O+ and OH-. If the pH is high (above 7) in proportion more ions OH- are present, if the pH is low (under 7) more H3O+ are present. The solubility of red ochre depends on the pH: at low pH the red ochre is more soluble then at high pH. Low pH water is more corrosive than high pH water, and high pH cause higher scalling. For many units it is therefore important to have the correct proportion between the described variables.
TDS determinationUse the following table to convert the conductivity to TDS if the TDS is unknown. Conductivity (microohm/cm) | TDS (mg/l) | 1.0 | 0.42 | 106.0 | 42.5 | 212.0 | 85.0 | 812.0 | 340.0 | 1008.0 | 425.0 | Conversion table conductance/TDS |
Determination of alkalinityMany are the substances which contributes to the alkalinity of the water. It is common to determine this value using H2SO4 by use of filtration by H2SO4 the alkalinity is detrmined, as indication the word “broomkresolgroen” is used. Links to related subjects/ inside Lenntech website[ Corrosion treatment ]
[ Water softener ]
[ Water hardness ]
[ Fresh water conditioning ] Bron: Chemisty for environmantal engineering; C.N. Sawyer, P.L. McCarty, G.F. Parkin; 4th edition; 1994; chapter 17
Bron: http://www.corrosion-doctors.org | 
