Calcium carbonate Langelier Saturation

Calcium—In general, calcium (as CaCOs) below 800 ppm should not result in calcium sulfate scale. In arid climates, however, the critical level may be much lower. For calcium carbonate scaling tendencies, calculate the Langelier Saturation Index or the Ryznar Stability Index. 

Langelier Saturation Index (LSI) is used to determine the scaling potential of calcium carbonate. (Note that LSI is used up to about 4,000 ppm TDS higher concentrations rely on the Stiff-Davis Saturation Index.) The LSI is calculated using the following formulas... 

Solution The saturation pH is given as pHj = 10. The actual pH is 8.7, so the system is not saturated with calcium carbonate and no carbonate will precipitate the precipitation potential is therefore zero. At equilibrium at the Langelier saturation pH, the calcium ion concentration will remain the same at [Ca ] = 10 gmol/L. Ans... 

In recarbonation, the available calcium ion in solution is prevented from precipitation. Therefore, it remains to determine at what pH will the equilibrium condition be, given this calcium concentration. This determination is, in fact, the basis of the Langelier saturation pH. Adding carbonic acid will increase the acidity of the solution after it has neutralized any existing alkalinity. 

LANGELIER SATURATION INDEX - An index (SI) based upon the pH of saturation of calcium carbonate used to determine the tendencies of a water supply toward corrosion or scaling. A positive index indicates scaling tendencies a negative one means corrosion tendencies. (Langlier Index = pH - pHs, where pH = actual pH of water and pHs = pH at which water having the same alkalinity and calcium content is just saturated with calcium carbonate. 

Langelier Saturation Index (LSI) It is a means of expressing the degree of saturation of water as related to calcium carbonate solubihty. The difference between the actual pH and the pH at which the water would be saturated with calcium carbonate, pHj. It quantifies CaCOs scaling potential. If LSI is positive. 

The solubility of elements in freshwater is limited and the solubility of calcium and magnesium carbonates are of particular importance in freshwaters. The solubility of carbonates is inversely proportional to the temperature of the water. In other words, as the water temperature increases, calcium and magnesium carbonates become less soluble. If the solubility decreases sufficiently, carbonates will precipitate and form a scale on the surfaces of the system. This scale can provide a protective barrier to prevent corrosion of the metallic elements in a system. Excessive scale deposits can interfere with water flow and heat transfer. The quality of the scale is dependent on the quantity of calcium that can precipitate as well as water flow and the chloride and sulfate content of the water. The tendency of water to precipitate a carbonate scale is estimated from corrosion indices such as the Langelier Saturation Index (LSI) and Caldwell-Lawrence calculations [6-8] which use calcium, alkalinity, total dissolved solids, temperature and pH properties of the water. Other indices, such as the Ryznar Index... 

Equilibrium curves can be determined and, on the basis of a given water composition, used to establish whether a water is scaling (not corrosive) or non-scaling (corrosive). Many different methods have been proposed for prediction of the formation of CaCOa. Commonly used equations or indices are the Langelier Saturation Index, LSI (Langelier, 1936) and the Stability Index, RSI, a modification of the LSI proposed by Ryznar (1944). In the German literature the LSI is known simply as the Saturation Index, Is (Morbe et al., 1987). These indices describe the algebraic difference between the actual pH of the water and the calculated pH at which it would be saturated with calcium carbonate ... 

The calcium carbonate group includes calcite saturation level, the Langelier saturation index, the Stiff-Davis index, and the Oddo-Tomson 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. 

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