pH in water.
The pH is a logarithmic expression of the concentration of hydronium ions.
Any substance dissolved in water can be classified into two different substances: electrolytes and non-electrolytes. Electrolytes are those molecules that dissociate into electrically charged ions, which add conductivity to water. The non-electrolytes, are molecules dissolved in water that have neutral charge and do not add conductivity to water.
That said, water can also dissociate into electrically charged ions by being in contact with other water molecules. The equilibrium reaction of this dissociation breaks down the water molecule into a hydronium ion and a hydroxyl ion. It is represented as follows:
H2O -> H+ + OH–
Although this equation is not entirely correct, we express it this way for better understanding, the correct expression of the hydronium ion is as follows: H3O+.
Most dissociation reactions have a chemical equilibrium, where there’s a relation in the production of both H3O+ and OH–
Kw = [H+] [OH–] = 10-14
This relationship indicates that if the concentration (mol / l) of the hydronium ion decreases, the concentration of the hydroxyl ion increases, respecting the value that your product has by being equal to 10-14 at 25 °C (although this value depends strongly on the temperature). With this expression of equilibrium, we can conclude that an equal concentration of both ions to reach this value, can be considered that:
[H+] [OH–] = (10-7) (10-7) = 10-14
[H+] = 10-7 mol / l
[OH–] = 10-7 mol / l
We can see an exponential relation based on 10, so it was considered to use a relation to obtain a more punctual value, thus the term pH was born.
The pH is considered as the negative logarithm in base 10 of the hydrogen concentration.
pH = -Log [H+]
Thus, this point value will indirectly indicate the hydrogen concentration contained in the water. We can apply the same principle for hydroxyl ion and the constant Kw
pOH = -Log[OH–]
pKw = -Log[Kw]
pH + pOH = pKw = 14
The high concentration of hydrogens, i.e. a low pH, gives water the property of acidity, and conversely a low concentration of these ions, increases a property of water called alkalinity.
Commonly there are devices capable of measuring pH, called pH meters (Potentiometer), each complete pH value is 10 times larger for each reduced pH value. For example, we have 10 times the concentration of hydrogen ions in a water of pH = 5 than in one of pH = 6.
As mentioned above, Kw depends on temperature, which is very important, since in many processes it is something that is not taken into account. In other words, if a pH measurement is made on a process water at 25 °C and another measurement is made on this same water at a temperature of 50 °C, the pH value will be different. Table 1 shows pKw values as a function of temperature.
Table 1.- Changes in the pKw values as a function of temperature.
pKw = -log(Kw)
This exponential behavior indicates that, at low temperature values, we will have high pH values, so it is established that at pH values below 4.3, the method for determining pH changes to a titration, since the concentrations start to be considerable, as expressed in Table 2.
Table 2.- Concentration of H+ ions as a function of CaCO3 as a function of pH.
H+ mg / l as CaCO3
Many industrial processes, chemical, biological, etc., are directly affected by the pH contained in the water to be used, since we can establish how strong are the properties of acids and alkalinity in the water.
According to NOM-127-SSA1-1994, the pH of a water that can be considered potable is in the range of 6.5 to 8.5, so that it does not directly affect health and / or the way of disposing of this type of water, although normally it can It is considered that water below 6.5 is considered acidic or corrosive, due to the oxidizing power it can have with metals, while if it has a pH above 8.5 it is very likely that it is water that can embed pipes, although the pH is not a definitive value for these parameters, if it provides important information to classify the corrosive or fouling power of the water.