pH is a major parameter that is measured in determining the water quality. It indicates whether a water source is of good quality to be used for drinking purposes and other house hold needs. In addition, maintaining the appropriate pH is critical for some chemical reactions as well. The pH of a system is dependent on the concentration of hydrogen ions present in that system. The concentration of hydrogen ions in a solution is the amount of hydrogen ions (in the units of mol) present in a liter of solution. pH is the logarithmic value of the inverse of the hydrogen ion concentration. This is the relationship between hydrogen ions and pH.
Key Areas Covered
1. What are Hydrogen Ions
– Definition, Properties
2. What is pH
– Definition, Properties, Scale and Measurements
3. What is the Relationship Between Hydrogen Ions and pH
– Explanation with Examples
Key Terms: Acidity, Alkalinity, Basicity, Hydrogen Ions, Hydronium Ion, pH, Proton
What are Hydrogen Ions
Hydrogen ions are the ionic forms of the element Hydrogen; this is given by the symbol H+. An ion is formed when electrons are removed or added to an atom. When electrons are removed from an atom, it becomes a cation. When electrons are added to an atom, it becomes an anion. Hydrogen has only one electron in its neutral atoms in the gaseous phase. The most stable isotope of hydrogen contains a proton in its nucleus. Therefore, the positive charge of the nucleus (due to the presence of proton) is neutralized by the only electron it has. But when this electron is removed from the hydrogen atom, only the positively charged proton is present. Therefore, hydrogen becomes a hydrogen ion.Thus, hydrogen ion is only a proton.
In aqueous solutions, this hydrogen ion or proton is in combination with H2O (water) molecules. Then it is called a hydronium ion. The symbol for hydronium ion is H3O+. This symbol is created from H+-H2O combination.
Figure 1: The chemical structure of Hydronium ion
Acidity of a compound is determined by its ability to release Hydrogen ions (or protons). Therefore, strong acids are compounds that can completely ionize and release all hydrogen ions they have. Weak acids are compounds that can partially dissociate into its ions and release some of the hydrogen ions. The amount of hydrogen ions present in a system can be determined by looking at the pH of that system. Acid-base titrations are mainly based on hydrogen ions present in a system. A base can be used to determine the number of hydrogen ions present in an aqueous solution of an acid.
Furthermore, hydrogen ions can also exist anions. Since hydrogen atom has an unpaired electron in the 1s orbital, it can get another electron to the s orbital to complete the orbital and obtain the electron configuration of Helium (He). Then it is indicated as H–. This happens because the hydrogen atom is composed of a proton and there is no other positive charge in the atom to neutralize the incoming electron.
Generally, hydrogen tends to form the positive ion since it has a less electronegativity compared to most other elements. But when it is bonded to elements such as metals with a very low electronegativity than hydrogen, it tends to form anions instead of cations.
What is pH
pH is the logarithmic value of the inverse of the hydrogen ion concentration (more accurately the activity of hydrogen ions). pH is actually a scale used to determine the acidity or basicity (Alkalinity) of a system. Logarithmic value is because the hydrogen ion concentration in a solution has a very low amount. For example, the hydrogen ion concentration of pure water at 25oC is about 10-7 mol/L. Therefore, the concentration of hydrogen ions is considered instead of the activity and the logarithmic value is used in order to make it easy to identify and compare these concentrations. When calculating the pH, the units of hydrogen ion concentration should be taken in mol/L.
The pH scale varies from 1 to 14. pH 7 is considered as the neutral pH of a system. Acids have pH values less than 7 and bases have their pH value above pH 7. Strong acids show pH values from 1 to 3 whereas weak acids show pH values from 4 to 6. If a system has a pH value near 14, it is considered as strongly basic.
Figure 2: pH scale
The pH can be measured using various methods. Litmus papers can be used to determine whether a solution is acidic or basic. pH papers can be used to determine the pH value as a whole number. Titration methods can also be used.
Relationship Between Hydrogen Ions and pH
There is a direct relationship between hydrogen ions and pH. The pH of a solution depends on the hydrogen ion concentration in that solution. pH value is the logarithmic value of the inverse of the hydrogen ion activity. Since the concentration of the hydrogen ions is often very low, ion activity is considered as equal to the concentration of hydrogen ions. Then, the pH is the logarithm of the inverse of the hydrogen ion concentration.
pH = -log10[aH+]
Where a is the activity.
pH = -log10[H+(aq)]
Therefore, there is a direct relationship between pH and the concentration of hydrogen ions in a solution.
Conclusion
The pH of a solution is mainly dependent on the hydrogen ion concentration in that solution. The hydrogen ion concentration will vary according to the chemical species present in that solution and the changes in the temperature of that solution.
References:
1. “Hydrogen ion.” Encyclopædia Britannica. Encyclopædia Britannica, inc., n.d. Web. Available here. 26 July 2017.
2. “What is pH.” .N.p., n.d. Web. Available here. 26 July 2017.
Image Courtesy:
1.” Hydronium-3D-balls” By Benjah-bmm27 – Own work, Public Domain) via Commons Wikimedia
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