Langelier Saturation Index Calculator
The Langelier Saturation Index is based on the study of the carbonate equilibrium in water.
For the Langelier Saturation Index use calculator The elements that are important for the carbonate equilibrium are gaseous carbon dioxide CO_{2}, aqueous carbon dioxide (CO_{2})_{aq}, carbonic acid H_{2}CO_{3}, bicarbonate HCO_{3}^{}, carbonate CO_{3}^{2} and solids containing carbonate like calcium carbonate CaCO_{3} or magnesium carbonate MgCO_{3}. Below you can see all the equations that matters for the carbonate equilibrium. These are all equilibrium reactions symbolized by double arrows. 
(CO_{2})_{g} (CO_{2})_{aq
}
(CO_{2})_{aq} + H_{2}O (H_{2}CO_{3})_{aq }
(H_{2}CO_{3})_{aq }_{ }(H^{+})_{aq} + (HCO_{3}^{})_{aq
}
(HCO_{3}^{})_{aq }_{ }(H^{+})_{aq} + (CO_{3}^{2}^{})_{aq} (A)
(CO_{3}^{2}^{})_{aq} + (Ca^{2+})_{aq} (CaCO_{3})_{5} (B)
Figure 1
Figure 1 can give you the filling that you have to start with gaseous CO_{2} in your water to obtain at the end calcium carbonate CaCO_{3}. This is not the right interpretation. You can have water that naturally contains bicarbonate ions HCO_{3}^{}. So equilibrium equation (A) may occur without the earlier equilibrium reactions with carbon dioxide.
The formula to calculate the Langelier Saturation Index is: where:
The pH of a solution is usually known. Equations (A), (B) and (C) are used to compute the saturation pH (pH_{s}). Equation (C) is a result of the combination of equations (A) and (B).
The equilibrium constant K_{a }of equation (A) can be calculated with the following formula:
The equilibrium constant K_{sp }of equation (B) can be calculated with the following formula:
The equilibrium constant K of equation (C) can be calculated with the following formula: From this point untill the formula of pH_{s} you only have to use mathematics. Simplify If X=Y then log(X) = log(Y) So: log (X * Y) = log(X) + log(Y) and log(X/Y) = log (X)log(Y) so: The formula for pH_{s} is then: If you have a water analysis de calcium and bicarbonate concentration are known so you have to calculate the activity coefficients. These coefficients are approximately equal to one but it is better for the accuracy of pH_{s} to calculate them. The activity coefficient can be calculated by the following formula:
The ionic strength can be calculated with the following formula:
All the parameters are now known and the Langelier Saturation Index can be calculated: The equilibrium constants Ka and K_{sp} change with temperature. The following values for the equilibrium constants are given in [1]:
The relation between the constant solubility product K_{sp} of calcium carbonate and temperature is as follows This equation is found by adding an exponential trendline in the graph of the solubility product constant as a function of the temperature in degree Celsius. The relation between the equilibrium constant Ka of carbonate/bicarbonate (CO_{3}^{2}/HCO_{3}^{}) and temperature is as follows:

A boiler is a device for generating steam, which consists of two principal parts: the furnace, which provides heat, usually by burning a fuel, and the boiler proper, a device in which the heat changes water into steam. The steam or hot fluid is then recirculated out of the boiler for use in various processes in heating applications. The water circuit of a water boiler can be summarized by the following pictures: 

The boiler receives the feed water, which consists of varying proportion of recovered condensed water (return water) and fresh water, which has been purified in varying degrees (make up water). The makeup water is usually natural water either in its raw state, or treated by some process before use. Feedwater composition therefore depends on the quality of the makeup water and the amount of condensate returned to the boiler. The steam, which escapes from the boiler, frequently contains liquid droplets and gases. The water remaining in liquid form at the bottom of the boiler picks up all the foreign matter from the water that was converted to steam. The impurities must be blown down by the discharge of some of the water from the boiler to the drains. The permissible percentage of blown down at a plant is strictly limited by running costs and initial outlay. The tendency is to reduce this percentage to a very small figure. 

Proper treatment of boiler feed water is an important part of operating and maintaining a boiler system. As steam is produced, dissolved solids become concentrated and form deposits inside the boiler. This leads to poor heat transfer and reduces the efficiency of the boiler. Dissolved gasses such as oxygen and carbon dioxide will react with the metals in the boiler system and lead to boiler corrosion. In order to protect the boiler from these contaminants, they should be controlled or removed, trough external or internal treatment. In the following table you can find a list of the common boiler feed water contaminants, their effect and their possible treatment. Find extra information about the characteristics of boiler feed water.

Characteristics of boiler feed water
Water absorbs more heat for a given temperature rise than any other common inorganic substance. It expands 1600 times as it evaporates to form steam at atmospheric pressure. The steam is capable of carrying large quantities of heat. These unique properties of water make it an ideal raw material for heating and power generating processes. Feedwater purity is a matter both of quantity of impurities and nature of impurities: some impurities such as hardness, iron and silica are of more concern, for example, than sodium salts. The purity requirements for any feedwater depend on how much feed water is used as well as what the particular boiler design (pressure, heat transfer rate, etc.) can tolerate. Feedwater purity requirements therefore can vary widely. A lowpressure firetube boiler can usually tolerate high feedwater hardness with proper treatment while virtually all impurities must be removed from water used in some modern, highpressure boilers. The following tables are extracts of recommended levels from APAVE (Association of electrical and steam unit owners), up to pressures of 100 bar for medium steaming rates and for volumes of water in the chambers sufficient to properly control the blow down rates, and from ABMA (American Boiler Manufacturers Association) in its standard guarantee of steam purity. 

