Acidity and alkalinity of water

Acidity

Pure water is partially ionized and produces equal amounts of OH– and H+ ions. The concentration of the ions produced is a function of temperature and at 20 degrees Celsius is as follows:

H⁺ + OH^– → H₂O

[ H⁺ =[OH^–] = 1×10^-7 ion/L ]

The product of the ion dissociation of water is a function of temperature and is a constant value at any temperature, at twenty degrees Celsius it is equal to:

[H⁺] × [OH^–] = kw = 1 × 10^-14

According to the above relationship, which is true for all aqueous solutions with different degrees of acidity or alkalinity, the concentration of one of the two ions can be obtained by knowing the concentration of the other ion.

The relationship between acidity and pH

pH is defined as the following relationship and is used as a criterion for measuring the acidity or alkalinity of a solution.

pH= -log[H]⁺

For pure water at 20°C where [H⁺ =[OH^–] = 1×10^-7 ion/L], pH=7 indicates neutrality. The lower the pH, the higher the H+ ion concentration, and the more acidic the solution. Conversely, the higher the pH, the higher the OH^– ion concentration, and the more alkaline the solution. According to the above relationship, pH can vary from zero to fourteen, but it should be noted that pH depends on temperature and the amount of ionization of water changes with temperature.

The relationship between sedimentation and pH

Knowing the pH value is necessary to determine the sedimentation of water salts or its corrosiveness. High pH causes calcium carbonate to precipitate, resulting in sedimentation in heat exchange devices. Low pH gives water corrosive properties.

Alkalinity

Chemically, the alkalinity of water indicates the amount of its neutralization capacity by acid. Neutralization of water means that the pH of water reaches about 5.4.

The alkalinity of natural waters is due to the presence of hydroxides, carbonates and bicarbonates. Of course, other ions such as phosphates, silicates and borates can also cause alkalinity, but due to their very low concentration compared to the first series ions, the alkalinity caused by them can be ignored.

Methods for measuring water alkalinity:

a) Simple alkalinity or phenolphthalein alkalinity (P):

This alkalinity includes anions that are titrated at pH higher than 3.8 by a standard acid and in the presence of phenolphthalein reagent.

b) Total alkalinity or alkalinity relative to methyl orange (M):

This alkalinity includes all anions that are titrated at pH higher than 3.4 by a standard acid and in the presence of methyl orange reagent.

How does alkalinity affect the corrosion and sedimentation of water?

The factor affecting the corrosion and sedimentation of water is alkalinity. For example, it can be said that the alkalinity of water used in steam boilers must be high enough to prevent corrosion of the boiler walls. Also, alkalinity should not be so high as to cause solids to be transported by steam or to cause alkali brittleness and cracks in the boiler walls. Alkalinity is closely related to pH, such that the higher the simple alkalinity, the higher the concentration of hydroxides and carbonates, so the higher the pH and the more alkaline the solution becomes.