The Langelier Index

Although the Langelier index is probably the most frequently quoted measure of a water s corrosivity, it is at best a not very reliable guide. All that the index can do, and all that its author claimed for it is to provide an indication of a water s thermodynamic tendency to precipitate calcium carbonate. It cannot indicate if sufficient material will be deposited to completely cover all exposed metal surfaces consequently a very soft water can have a strongly positive index but still be corrosive. Similarly the index cannot take into account if the precipitate will be in the appropriate physical form, i.e. a semi-amorphous egg-shell like deposit that spreads uniformly over all the exposed surfaces rather than forming isolated crystals at a limited number of nucleation sites. The egg-shell type of deposit has been shown to be associated with the presence of organic material which affects the growth mechanism of the calcium carbonate crystals . Where a substantial and stable deposit is produced on a metal surface, this is an effective anticorrosion barrier and forms the basis of a chemical treatment to protect water pipes . However, the conditions required for such a process are not likely to arise with any natural waters. 

The effect of pH on the corrosion of zinc has already been mentioned (p. 4.170). In the range of pH values from 5 -5 to 12, zinc is quite stable, and since most natural waters come within this range little difficulty is encountered in respect of pH. The pH does, however, affect the scale-forming properties of hard water (see Section 2.3 for a discussion of the Langelier index). If the pH is below the value at which the water is in equilibrium with calcium carbonate, the calcium carbonate will tend to dissolve rather than form a scale. The same effect is produced in the presence of considerable amounts of carbon dioxide, which also favours the dissolution of calcium carbonate. In addition, it is important to note that small amounts of metallic impurities (particularly copper) in the water can cause quite severe corrosion, and as little as 0-05 p.p.m. of copper in a domestic water system can be a source of considerable trouble with galvanised tanks and pipes. 

A positive value of the Langelier index indicates that the water is supersaturated and will deposit CaCOs, whereas a negative value indicates that the water is nndersatnrated and will dissolve any CaCOs that happens to exist at the particnlar moment. 

If the actual pH is below pH, the result is a negative index and CaCO will dissolve in the water. It is also generally assumed that this will indicate the water to be corrosive towards steel in the presence of oxygen. On the other hand if the Langelier index is positive (i.e. pH > pH,) and the water is saturated with CaCO scale formation is likely to occur. 

Kemmer [1988] states that the Langelier index is most useful in predicting likely events for low flow situations, e.g. in storage tanks, and the empirical Ryznar index is applicable only to flowing systems, where conditions at the wall are quite different from the conditions in the quiescent bulk. 

The Langelier index (see Chapter 14) suggests that the water is near equilibrium but it could have a slight tendency to either scale formation or to corrosion depending on local conditions. 

The evaluation of aggressivity according to the Langelier index of saturation is not ideal. The calculated pHg values are usually higher than the actual ones. The aggressivity of waters with low mineralization is normally overestimated, and that of the waters with high mineralization is underestimated. 

The Langelier Index The problem of determining whether or not a water is in equilibrium with CaCOgts) can be approached by determining whether the so-called Langelier Index (L.I.) (or Saturation Index, S.I.) is positive or negative. The L.I. is defined as the difference between the actual (or measured) pH of a water and the hypothetical pH the water would have if it were in equilibrium with CaC03(s). ... 

Calculate the Langelier Index of a water with the following characteristics total alkalinity = 8 x 10" eqAiter, [Ca +] = 3 X 10" M, pH = 9.6, and total dissolved solids = 250 mgAiter. Include ionic strength effects the temperature = 25"C. 

For calculating pHj in order to solve the equation, there are tables for values of pK - pK ) at various dissolved solids contents and various temperatures. The calcium content and alkalinity can be easily determined by the plant chemist. If the Langelier index (pH-pH,) is zero, the water is just saturated with CaCOj. If the index is positive, the water is supersaturated with and will tend to deposit CaC03 as well as be more or less scale forming and noncorrosive. If the index is negative, the water tends to be corrosive. 

The letterp refers to the negative logarithm of all these quantities. The saturation index, also known as the Langelier index, is defined as the difference between the measured pH of a water and the equilibrium pH, for CaC03, or... 

The primary factor affecting long-term corrosion is whether a water does or does not form scales, which in turn depends on hardness and pH. This property is determined by calculating the Langelier index or the Ryzner stability index. The simpler form of Langelier s equation is ... 

If the reaction proceeds from L -> R, CaCOs scales would deposit and if it proceeds from R - L, CaCOs scales would dissolve. The Langelier index is defined as... 

Stiff-Davis index. The Stiff-Davis index attempts to overcome the shortcomings of the Langelier index with respect to waters with high total dissolved solids and the impact of common ion effects on the driving force for scale formation. Like the LSI, the Stiff-Davis index has its basis in the concept of saturation level. The solubility product used to predict the pH at saturation (pHs) for a water is empirically modified in the Stiff-Davis index. The Stiff-Davis index will predict that a water is less scale forming than the LSI calculated for the same water chemistry and conditions. The deviation between the indices increases with ionic strength. Interpretation of the index is by the same scale as for the Langelier saturation index. 

The protective carbonate scale is not just a ftinction of the concentration of calcium and magnesium salts. It is also affected by the alkalinity of the water and concentrations of other salts. Saturation indexes have been developed for monitoring such concentrations. A popular saturation index is the Langelier index, which provides a simple method for determining the conditions and concentrations under which water will form this kind of protective film. Section 2.2 in Chap. 2, Environments, describes in detail the Langelier index and a few other indexes and methods to monitor scaling tendencies of waters. 

---