Alkalinity, Hardness and LSI (Langelier Saturation Index)

Alkalinity, Hardness and LSI (Langelier Saturation Index)

Before explaining about these material, let's see the figure below:

figure 1. the carbonate cycle - surface water

Alkalinity is water's capacity to resist acidic changes in pH, essentially alkalinity is water's ability to neutralize acid. Alkalinity refers to the total amount of bases in water expressed in mg/l of equivalent calcium carbonate This ability is referred to as a buffering capacity. A water body with a high level of alkalinity (which is different than an alkaline water body) has higher levels of calcium carbonate, CaCO3, which can decrease the water's acidity. Therefore, alkalinity measures how much acid can be added to a water body before a large pH change occurs. So, while pH measures the strength of an acid (base), alkalinity measures the ability to neutralize acid (base). Mathematically, alkalinity can be determined by:

Alkalinity provides a buffering capacity to aqueous system. The higher alkalinity is, the higher the buffering capacity against pH changes. These 3 ions all react with H+ ions to reduce acidity, increasing alkalinity and pH. The following is those ion :

Hardness, the sum of the calcium and magnesium concentrations in water. Calcium (Ca) typically represents 2/3 total hardness; Magnesium ( Mg) typically represent about 1/3 total hardness. Alkalinity and water hardness are fairly similar--essentially they both come from sources in nature. Water moves through rocks (and picks up minerals as it does so) on its way to rivers and lakes. When limestone and dolomite dissolve in water, one half of the molecule is calcium or magnesium (the "hardness") and the other half is the carbonate (the "alkalinity"). This means that the level of water hardness and alkalinity in a place will be very similar. However, they are very separate measurements, and have very different importance.

Waters are often categorized according to degrees of hardness as follows:

0-75 mg/l - soft

75-150 mg/l - moderately hard

150-300 mg/l - hard

over 300 mg/l - very hard

The capacity of natural water to neutralize acidity or its alkalinity resides primarily in the amounts of bicarbonate and carbonate dissolved in it. Water with pH below 4.5 does not contain bicarbonate and has a source of acidity stronger than carbon dioxide. This is the reason the color change of methyl orange indicator from yellow to orange at pH 4.5 can be used to signal the end point of alkalinity titration. 

Such a low pH in water usually is from sulfuric acid that originates from the oxidation of naturally occurring iron sulfite in geological material or from acidic rain. Water with pH of 4.5-8.3 contains both bicarbonate (alkalinity) and carbon dioxide (acidity). Water with a pH above 8.3 contains carbonate and bicarbonate, but no measurable carbon dioxide.

figure 2. Relationships of carbon dioxide, bicarbonate and carbonate to pH

For example, kindly please to follow the calculation below :

Now is time to discuss about LSI (Langelier Saturation Index). The LSI is basically a way to determine if water is corrosive (negative LSI) or scale-forming (positive LSI). LSI between -0.30 and +0.30 is the widely accepted range, while 0.00 is perfect equilibrium ( with calcium carbonate). Water wants to be in equilibrium, and will find a way to get there. Under-saturation is corrosive, and over-saturation is scale-forming. Water can only hold so much calcium in solution. If water is in LSI equilibrium, neither etching nor scaling will happen. The LSI tells us how saturated the water is with calcium. The Langelier Index is one of several tools used by water operators for stabilizing water to control both internal corrosion and the deposition of scale. Water supply operators can optimize their water supply systems and identify leakage potentials with the Langelier Index. Leakage is a common problem in Newfoundland and Labrador due to the acidic nature of provincial natural waters.

Experience has shown that Langelier Index in the range of -1 to +1 has a relatively low corrosion impact on metallic components of the distribution system. Langelier Index values outside this range may result in laundry stains or leaks. The following is the LSI range as a benchmark:

figure 3. LSI range

How to calculate LSI?

The formula of LSI is . . . ...

figure 4. pH vs Alkalinity

To consider the value of pHsat,can be used graphic or formula as following example :

figure 5. LSI calculation by nomograph