Langelier Saturation Index
The Langelier Saturation Index (LI), a measure of a solution’s ability to dissolve
or deposit calcium carbonate, is often used as an indicator of the corrosivity of
water. The index is not related directly to corrosion, but is related to the
deposition of a calcium carbonate film or scale; this covering can insulate pipes,
boilers, and other components of a system from contact with water. When no
protective scale is formed, water is considered to be aggressive, and corrosion
can occur. Highly corrosive water can cause system failures or result in health
problems because of dissolved lead and other heavy metals. An excess of scale
can also damage water systems, necessitating repair or replacement.
In developing the LI, Langelier derived an equation for the pH at which water is
saturated with calcium carbonate (pHs). This equation is based on the
equilibrium expressions for calcium carbonate solubility and bicarbonate
dissociation. To approximate actual conditions more closely, pHs calculations
were modified to include the effects of temperature and ionic strength.
The Langelier Index is defined as the difference between actual pH (measured)
and calculated pHs. The magnitude and sign of the LI value show water’s
tendency to form or dissolve scale, and thus to inhibit or encourage corrosion.
Although information obtained from the LI is not quantitative, it can be useful in
estimating water treatment requirements for low pressure boilers, cooling
towers, and water treatment plants, as well as serving as a general indicator of
the corrosivity of water.
Parameter Measurement
The Langelier Saturation Index can be calculated easily by using Hach products
to determine the pH of calcium carbonate saturation (pH) and the actual pH of a
solution. Using a simple formula, the pH is derived from the values for calcium
hardness, total alkalinity at pH 4.5, temperature, and total filterable residue
(total dissolved solids). All of these procedures are included in this manual.
Temperature:
Temperature can be measured in degrees celsius with a laboratory thermometer
(Cat. No. 566-01). If only a Fahrenheit thermometer is available, conversion to
degrees Celsius will be necessary.
[°C = 5/9 x (°F – 32)].
Calculation
After the preceding parameters have been determined, calculate the pH from the
following formula:
Where:
Constant A takes into account the effect of temperature. It is found by selecting
the value from Table 1 on page 3 that corresponds to the measured temperature
in degrees Celsius.
Constant B is a correction for the ionic strength of the sample. It is determined
using Table 2 on page 4 by taking the value that corresponds to the measured
total filterable residue or the estimated total dissolved solids (TDS).
pHs = A B C – D + –
Value C is obtained from Table 3 on page 4 by reading the value corresponding to
the calcium hardness (in mg/L CaCO3) of the sample.
Value D is obtained from Table 3 by reading the measured value for total
alkalinity (in mg/L CaCO3) of the sample.
The Langelier Saturation Index is the difference between the actual pH of the
solution and pHs calculated above.
Interpretation
The LI is a gauge of whether a water will precipitate or dissolve calcium
carbonate. If the pHs is equal to the actual pH, the water is considered
“balanced”. This means that calcium carbonate will not be dissolved or
precipitated. If the pHs is less than the actual pH (the LI is a positive number),
the water will tend to deposit calcium carbonate and is scale-forming
(nonaggressive). If the pHs is greater than the actual pH (the LI is a negative
number), the water is not saturated and will dissolve calcium carbonate
(aggressive). In summary:
pHS = pHactual, water is balanced
pHS < pHactual, water is scale forming (nonaggressive)
pHS > pHactual, water is not scale forming (aggressive)
It is important to remember that the LI value is not a quantitative measure of
calcium carbonate saturation or corrosion.
Because the protective scale formation is dependent on pH, bicarbonate ion,
calcium carbonate, dissolved solids, and temperature; each may affect the
water’s corrosive tendencies independently. Soft, low-alkalinity waters with
either low or excessively high pH are corrosive, even though this may not be
predicted by the LI. This is because insufficient amounts of calcium carbonate
and alkalinity are available to form a protective scale.
Waters with high pH values and sufficient hardness and alkalinity may also be
corrosive, even if the LI predicts the opposite. This is the result of calcium and
magnesium complexes that cannot actively participate in the scale forming
process. Analytical procedures do not distinguish between these complexes and
available calcium and magnesium; therefore, the LI value is not accurate in such
situations.
Corrosive tendencies may also be exhibited by water containing high
concentrations of sulfate, chloride, and other ions which interfere with uniform
carbonate film formation.
As a result of these and other problems, the LI is useful only for determining the
corrosivity of waters containing more than 40 mg/L of alkalinity, sufficient
calcium ion concentration, and ranging between pH 6.5 and 9.5.
LI pHactual pHs = –
Aggressive Index
The Aggressive Index (AI), originally developed for monitoring water in
asbestos pipe, is sometimes substituted for the Langelier Index as an indicator of
the corrosivity of water. The AI is derived from the actual pH, calcium hardness,
and total alkalinity. (Use procedures contained in this handbook). Where it is
applicable, it is simpler and more convenient than the LI. Because the AI does
not include the effects of temperature or dissolved solids, it is less accurate as an
analytical tool than the LI.
Calculation
After obtaining the pH, total alkalinity, and calcium hardness, use the following
formula to calculate the AI:
Al = pHactual + C + D
Where:
Value C is obtained from Table 3 on page 4 by reading the value corresponding to
the calcium hardness (in mg/L CaCO3) of the sample.
Value D is obtained from Table 3 by reading the measured value for total
alkalinity (in mg/L CaCO3) of the sample.
Interpretation
As with LI, the AI is not a quantitative measure of corrosion, but is a general
indicator of the tendency for corrosion to occur, and as such, should be used
with proper reservation. An AI of 12 or above indicates nonaggressive (not
corrosive) water. AI values below 10 indicate extremely aggressive (corrosive)
conditions. Values of 10–11.9 suggest that the water is moderately aggressive.
Corrosivity characteristics of water as indicated by the LI and AI are compared
in Table 4.
* Factor C is the logarithm (base 10) of the calcium hardness expressed in mg/L
** Factor D is the logarithm (base 10) of the total alkalinity expressed in mg/L
Table 4
Corrosive Characteristics Langelier Index Aggressive Index
Highly aggressive < –2.0 < 10.0
Moderately aggressive –2.0 to 0.0 10.00 to 12.0
Nonaggressive >0.0 >12.0
READ ORIGINAL ARTICLE @ https://stpnq.com/wp-content/uploads/2014/08/Langelier-index.pdf
LSI Calculator: https://www.lenntech.com/calculators/langelier/index/langelier.htm
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