The pH Scale

What is the pH scale?

The pOH scale measures hydroxide ion (OH-) concentration in a solution, inversely to the pH scale. Lower pOH values indicate higher alkalinity, while higher values suggest greater acidity. pH + pOH always equals 14 for neutral solutions at a specific temperature, aiding in acidity and basicity assessment.

However, it is customary to use the pH to measure the acidity of a solution. It was proposed by Sörensen who defined pH as the “logarithm of the inverse of the concentration of hydronium ions contained in a solution”. The pH scale is used to measure the acidity and basicity of solutions of pure water is neutral, acidic and basic. Its Chemical principle is based on the relative concentration of hydrogen ions (H^⊕) and hydroxyl (HO^⊖), and is used primarily in Chemistry, Biology, and soil science. Terms pH and pOH they are widely used and are key for measuring chemical, and biological health of the soil to translate the values of the concentration of hydrogen ions or hydroxyl or use pH paper. S. P. L. Sørensen invented or first used the term known as scale or chart of pH to measure the concentration of hydrogen ions [H^⊕] and hydroxyl [HO^⊖] in a solution of pure water, acid and base. Sørensen defines the pH of an absolute manner as the potential of hydrogen and is equivalent to the logarithm of the inverse of the activity or concentration of the hydrogen ion content in a solution. Therefore, pH = -log AH^⊕ , which can also be expressed as AH^⊕ = 10 – pH. When the solution is very dilute, toH^⊕ = [H^⊕], that is to say the activity is almost equal to the concentration of the iónes hydrogen. Therefore: pH = – log [H^⊕] The definition of Sørensen it is used as a mathematical calculation of the simple value scale pH and pOH but , in practice, we use an instrument such as indicator paper or pH meter for the determination of the values.

What is the Scale of pOH?

In the teaching of Chemistry, the concentration value of the ion hydroxyl [HO^⊖] represents the term known as scale pOH in place of pH. Therefore: pOH = -log[HO^⊖] As the above equation using logarithms decimal places, if a dissolution presents a pH = 1 and the acidity of the solution decreases 100 times, the range of the pH scale increases by two units. In the same way, when the ion hydroxyl decreased two units, the values of pOH decreased from 13 to 11. PH scale: heartburn once seen the basic concepts of the pH scale, you can differentiate between solutions of pure water is neutral, acidic and basic in function of the relative concentrations of these ions [H^⊕] and hydroxyl [HO^⊖] or values pH scale. The level of acidity means reading the value of the table of the scale of pH and pOH of the solution of pure water is neutral, acidic or alkaline in chemistry (that is shown in the Figure). The table scale of pH and the pOH define the level of acidity or basicity.

Dissociation of water

A neutral solution or solution of pure water, that is to say, the solution where the concentrations of hydrogen ions and hydroxyl are the same. Therefore, [H^⊕] = [HO^⊖] = 10^-7 M Accordingly, the above expression can be expressed as follows from the definitions of pH and pOH: pH = pOH = 7 significca that this range or level of scale explains that the solution to neutral water has the same value of pH and pOH, and this implies that its value is 7. In an acidic solution the concentrations of hydrogen ion is greater than the ion hydroxyl [H^⊕] > [HO^⊖]. In this way, it is equivalent to saying that pH 7. From this equation, the scale range of pH of a solution of pure water acid is between one and less than seven, and the range of pOH is between seven and fourteen. But in alkaline solution or basic, the concentration of hydrogen is lower than that of the ion, hydroxyl or [HO^⊖]>[H^⊕] . Therefore, it can also be expressed as pOH 7.

Value of the ion product of pure water

The water molecule is ionized weakly to form a conjugate acid (proton) and a base (hydroxyl ion) in solution. In this way, there will always be a balance between the proton and hydroxyl ions in the water molecule.

H₂O  ⇄ H^⊕  + HO^⊖

This equilibrium reaction for the dissociation of water has a value of equilibrium constant:

k × [H₂O] = [H^⊕] × [HO^⊖]

where [H₂O], [H^⊕] and [HO^⊖] are the concentrations of water, protons, and ions, hydroxyl, respectively. But in any dilute aqueous solution, the concentration of water is equal to 55.5 moles/liter, and can be taken as a constant. Therefore:

k × [H₂O] = K_w = [H^⊕] × [HO^⊖]

where K_w it is equal to the ion product of water. The chemical reactions in which the system absorbs a specific heat capacity of the environment are known as endothermic reactions in thermodynamics. A heat-or energy-specific (13.7 kcal) to be absorbed from the environment to the ionization of water. Therefore, in accordance with the principle of Le-Chatelier the increase of the temperature will facilitate the dissociation and give higher values of K_w. The product of hydrogen ion, hydroxyl has a constant that has a value of 10-14. Thus, from the definition of Sørensen, it follows that pH + pOH = 14, and hence pOH = 14 – pH. So this pH scale varies from 0 to 14. pH of dissolution of pure water

Examples of measure values on the pH scale

• For sulfuric acid (H₂SO₄) 0.01 M:

Sulfuric acid causes acid rain, is an acid diprótico. The molarity of the sulfuric acid = 2 × molarity of the acid solution and the sulfuric acid. Therefore, the sulfuric acid scale of acidity 0.1 M and 0.2 N are equal. Therefore, the value of the scale pH in Chemistry represents sulfuric acid 0.1 M or 0.2 N = – log [H^⊕] = -log (0.2) = 0.699.

• For hydrochloric acid (HCl) 0.02 M:

Hydrochloric acid is an electrolyte strong and it is completely dissociated in the solution. Therefore, the value of pH measured from the solution of hydrochloric acid (HCl) 0.02 M = – log [H^⊕] = -log (2 × 10^-3) = (3 – log 2) = 2.7.

• For the acetic acid (CH₃COOH) 0.02 M:

Acetic acid is an organic acid, weak and the concentration of hydrogen ions in the acetic acid is 0.02 M. therefore, the value of pH measured is defined to 0.002 acetic acid (CH₃COOH) = – log [H^⊕] = -log (2 × 10^-4) = 3.7.

• For pure water (H₂O):

The concentration of proton and hydroxyl ion, or scale of pH in pure water, has been defined by the ion product of water. The ion product of pure water is equal to 14 and the concentration of hydrogen and hydroxyl are the same. Therefore,

K_w = [H^⊕] × [HO^⊖] = 10^-7 × 10^-7 = 10^-14

In this way describes the scale level of pH of the solution of pure water. This equation shows that the concentration of protons and ions, hydroxyl are inversely proportional to each other. To maintain K_w constant, if the ion concentration of protons increases by 100 times, hydroxyl ions decreases 100 times. (H₂O) = – log [H^⊕] = -log (10^-7) = 7.

Summary

The ranges of pH, pOH, and concentrations of hydrogen ions and hydroxyl, are assigned a color that can be summarized in the following figure.

Video on the pH Scale

References

• Isac-García, J.; Dobado, J. A.; Calvo-Flores, F. G.; and Martínez-García, H. (2015). Experimental Organic Chemistry Laboratory Manual. Elsevier Science & Technology. ISBN: 978-0-12-803893-2