Saturation indices

The logarithm of the quotient of the ion activity product (IAP) and solubility product constant (KSP) is called the saturation index (SI). The IAP is calculated from activities that are calculated from analytically determined concentrations by considering the ionic strength, the temperature, and complex formation. The solubility product is derived in a similar manner as the IAP but using equilibrium solubility data corrected to the appropriate water temperature. 

The saturation index SI indicates, if a solution is in equilibrium with a solid phase or if under-saturated and super-saturated in relation to a sohd phase respectively. A value of 1 signifies a ten-fold supersaturation, a value of -2 a hundred-fold undersaturation in relation to a certain mineral phase. In practice, equilibrium can be assumed for a range of -0.2 to 0.2. If the determined SI value is below -0.2 the solution is understood to be undersaturated in relation to the corresponding mineral, if SI exceeds +0.2 the water is assumed to be supersaturated with respect to this mineral. 

Some elements in aquatic systems exist only at low concentrations (pg/L range) in spite of readily soluble minerals. This phenomenon is not always caused by a generally small distribution of the concerned element in the earth crust mineral as for instance with uranium. Possible limiting factors are the formation of new minerals, co-precipitation, incongruent solutions, and the formation of solid-solution minerals (i.e. mixed minerals). 

For elements like radium, arsenic, beryllium, thallium, molybdenum and many others, not only the low solubility of the related minerals but also the coprecipitation or adsorption with other minerals, plays an important role. For instance radium is co-precipitated with iron hydroxides and barium sulfate. 

The mobility of radium is determined by redox-sensitive iron, which readily forms iron oxyhydroxides under oxidizing conditions and thus limits the concentrations of iron and radium because radium is effectively sorbed on iron oxyhydroxide. Redox-sensitive elements are elements that change their oxidation state by electron transfer depending on the relative oxidizing or reducing conditions of the aquatic environment (chapter 1.1.5.2.4 and 0). Thus radium behaves like a redox-sensitive element, even though it only occurs in the divalent form. 

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Calcium Carbonate Protective Scale. The LangeHer saturation index (LSI) is a useful tool for predicting the tendency of a water to deposit or dissolve calcium carbonate. Work pubHshed in 1936 deals with the conditions at which a water is in equiHbrium with calcium carbonate. An equation developed by LangeHer makes it possible to predict the tendency of calcium carbonate either to precipitate or to dissolve under varying conditions. The equation expresses the relationship of pH, calcium, total alkalinity, dissolved soHds, and temperature as they relate to the solubiHty of calcium carbonate in waters with a pH of 6.5—9.5 ... 

The difference between the actual pH (pH of a sample of water and the pH called the LangeHer saturation index. This index is a... 

The LSI measures only the directional tendency or driving force for calcium carbonate to precipitate or dissolve. It caimot be used as a quantitative measure. Two different waters, one of low hardness (corrosive) and the other of high hardness (scale-forrning), can have the same saturation index. 

The stabihty index developed by Ry2ner makes it possible to distinguish between two such waters. This index is based on a study of actual operating results with waters having various saturation indexes. 

Water Quality Maintenance. In addition to controlling algae and microorganisms such as bacteria, proper swimming pool maintenance requires control of free and combined available chlorine, pH, alkalinity, hardness, and saturation index. Ranges for various swimming pool parameters (Table 2) are recommended by The National Spa and Pool Institute (14). 

Sa.tura.tion Index. Materials of constmction used in pools are subject to the corrosive effects of water, eg, iron and copper equipment can corrode whereas concrete and plaster can undergo dissolution, ie, etching. The corrosion rate of metallic surfaces has been shown to be a function of the concentrations of Cl ,, dissolved O2, alkalinity, and Ca hardness as well as buffer intensity, time, and the calcium carbonate saturation index (35). 

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—Ideally, maintain between -0.5 and +0.5 A negative LSI indicates corrosion tendencies. A positive LSI indicates CaCOs scaling tendencies. 

LSI (Langelier Saturation Index) an indication of the corrosive (negative) or scale-forming (positive) tendencies of the water. Hardness the total dissolved calcium and magnesium salts in water. Compounds of these two elements are responsible for most scale deposits. Units are mg/l as CaCOs. 

Fig. 2.15 Langelier saturation index chart (after the American Chemical Society).

Saturation Index an index which shows if a water of given composition and pH is at equilibrium, supersaturated or unsaturated with respect to calcium carbonate (or to magnesium hydroxide). 

Domestic heating coil internal corrosion. Where naturally soft or lean city water is supplied and the Langelier Saturation Index (LSI) is below -1.0, acid corrosion takes place as a result of the acidic nature of the water. This water often has a high dissolved gas content, which additionally leads to pinhole corrosion. Where water velocities are too high (say, over 6 ft/s 1.8 m/s) the protective oxide layer is stripped off and erosion corrosion takes place. 

The dependency of rate of precipitation of barite from aqueous solution on time at room temperature studied by Nielsen (1958) suggests that the precipitation of barite from solutions of high levels of supersaturation (i.e., more than 30 as saturation index (S.I.) which is defined as the ratio, where A sp is the solubility... 

Water hardness in milligrams per liter of total Ca and Mg is considered soft at 0 to 17 mg/L, moderately hard at 60 to 120 mg/L, and very hard at > 180 mg/L. The Langelier saturation index, a measure of CaCOs solubility, should be < 0 in the retentate to prevent precipitation Pure Water Handbook, Osmonics, 1997). 

Larson, T.E. and Buswell, A.M., Calcium carbonation saturation index and alkalinity interpretations, J. Am. Water Works Assoc., 34, 1667-1684, 1942. 

Fig. 6-4. The dissolution rate as a function of the saturation index, omega-1, where omega = co3s cas/csat is the ion activity product divided by the saturation value.

Fig. 6-5. The evolution of the lagoon s waters in response to oscillations in biological productivity. The results show the adjustment of the system from an initial composition equal to that of seawater. This figure shows isotope ratios, calcium concentration, the saturation index, and productivity.

Figure 8-2 shows the depth profiles of the saturation index omegadel), the solution rate, and the respiration rate. At the shallowest depths, the saturation index changes rapidly from its supersaturated value at the sediment-water interface, corresponding to seawater values of total dissolved carbon and alkalinity, to undersaturation in the top layer of sediment. Corresponding to this change in the saturation index is a rapid and unresolved variation in the dissolution rate. Calcium carbonate is precipitating... 

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