Attempts at prognosticating the properties of water based on the data on its composition were first made quite a long time ago. The most widely known and properly elaborated is the Langelier method of estimating the corrosive and scale-forming properties of water. This method of qualitative estimation of water properties was devised by the chemist Langelier in the 1930s.

The researchers studying the properties of solutions noticed long time ago that the properties of the substance dissolved, as well as the properties of the solvent, depended on concentration of solutes. Concentration (usually symbolized by C) is interpreted as the mass of substance dissolved in a unit of volume of the solvent (or, rarely, a unit of mass). An attempt at taking into account the changes of the properties of the solution depending on its concentration was made in 1907 by the physico-chemist G. N. Lewis. He introduced the concept of the activity of the substance in a solution, that is, the observable result of the substance being present in the solution. The activity is usually symbolized as a. The relation between activity and concentration is defined by the activity coefficient:

f = a/C (1)

In highly diluted solutions, i.e., in solutions with very low content of dissolved substances, the activity is equal to the concentration. The activity coefficient  f , in this case, approaches unity. An ion-selective electrode measures the activity of the substance, that is, a manifestation of a certain amount of the substance being present in the solution as one of the components of equilibrium activities of all the components of said solution. Concentration, that is, the actual content of the dissolved substance, can be determined by chemical analysis. The solution theory and the ion equilibrium theory make it possible to calculate, from the data of chemical analysis of the solution, the equilibrium value of the hydrogen index pH_calc. It was the very difference between the calculated pH_calc and the measured pH_msr (activity of the solution components) that Langelier suggested should be considered as a numerical index of the corrosive and scale-forming water properties.

LSI = pH_msr - pH_calc (2)

When the LSI is lower than 0 (negative value), the water causes corrosion of steel. When the LSI = 0, the water is neutral and stable and does not cause corrosion or scaling. Since the Langelier saturation index is rather a qualitative than a quantitative characteristic, its being equal to zero should not be taken too literally. In addition, there are always instrumental errors and errors of approximation (calculations by conventional equations). Therefore, the value of the Langelier saturation index in the range of -0,4 to +0.4 should be taken as “zero". When LSI is > 0 (i.e., positive), the water tends to cause scaling on the surfaces of pipelines, heat exchangers, and other technological equipment.

Thus, when calculating the Langelier saturation index by the Eq. (2) it is necessary to know the value of pH_msr (as measured by a pH meter). However, calculating the pH_calc from the data of chemical analysis presents some difficulty. This is done by the equation:

pH_calc = lgSPCaCO₃ - lg K'' - (lgC_Ca2+  + lgC_HCO3-) - (lgf₁ + lgf₂) (3)

where lg = log₁₀ is the denary algorithm; SPCaCO₃ is the calcium carbonate solubility product; lg K'' is the negative algorithm of the second dissociation constant of the carbonic acid (that is, with formation of the CO₃^2- ions); lgC_Ca2+ and lgC_HCO3- are the algorithm concentrations of calcium ions and hydrocarbonate ions, respectively, as determined by chemical analysis; and (lgf₁ + lgf₂) is the sum of logarithms of the activity coefficients of divalent and monovalent ions.   

Further Reading

• Corrosion Doctors' Web site. Index of Natural Waters.

• Langelier Saturation Index. [1].

• Prisyazhniuk, V.A., 2005. The Langelier saturation index: Further development. PowerPlant Chemistry, 7(7):435 – 442.