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PH and pOH
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Because the constant of water, Kw is always 1.0·10-14, the pKw is 14, the constant of water determines the range of the pH scale. To understand what the pKw is, it is important to understand first what the " p" means in pOH, and pH. The danish biochemist Soren Sorenson proposed the term pH to refer to the " potential of hydrogen ion. " He defined the " p" as the negative of the logarithm, -log, of [H+]. Therefore the pH is the negative logarithm of the molarity of H. The pOH is the negative logarithm of the molarity of OH- and the pKw is the negative logarithm of the constant of water. These definitions give the following equations:
pH= -lg [H+]
pOH= -lg [OH-]
pKw= -lg [Kw]
The constant of water is always 1.0·10-14. So pKw=-log [1.0 x 10-14]. Using what we know about Logarithms, we can write this as 10-pKw=10-14. By substituting we see that pKw is 14. The equation also shows that each increasing unit on the scale decreases by the factor of ten on the molarity. For example, a pH of 1 has a molarity ten times more concentrated than a solution of pH 2. Also, the pKw of water is 14 and the addition of pH and pOH is always 14 at 25° Celsius.
pKw = pH + pOH = 14 (at 25°C)
- From the range 7-14, a solution is basic. The pOH should be looked in the perspective of OH- instead of H+. Whenever the value of pOH is greater than 7, then it is considered basic. And therefore there are more OH- than H+ in the solution
- At pH 7, the substance or solution is at neutral and means that the concentration of H+ and OH- ion is the same.
- From the range 1-7, a solution is acidic. So, whenever the value of a pH is less than 7, it is considered acidic. There are more H+ than OH- in an acidic solution.
45. pH scale.
Chemically pure water conducts an electric current very poorly and has an electrical conductivity of 0.055 µS∙ cm− 1. But nevertheless it has a measurable electrical conductivity that is explained by the slight dissociation of water into hydrogen and hydroxide ions (According to the theories of Svante Arrhenius):
H2O → H+ + OH–
The electrical conductivity of pure water can be used to calculate the concentration of hydrogen and hydroxide ions in water. Let us write an expression for the dissociation constant of water:
We can rewrite this equation as follows:
[H+]∙ [OH–] = [H2O]∙ K
Replacing the product [H2O]∙ K in the last equation with the new constant Kw, we have:
[H+]∙ [OH–] = Kw
The latter Kw is called the ion product of water (or ionization constant, dissociation constant, self-ionization constant). The Kw value is depended of temperature. For pure water at 25°C Kw=10-14, we have
[H+] = [OH-] = 1·10-7 mol/L.
Hence, for this temperature:
Kw = 10-7 ∙ 10-7 = 10-14
Solutions in which the concentrations of the hydrogen ions and hydroxide ions are the same are called neutral solutions.
This equation also applies to all aqueous solutions. However, Kw does change at different temperatures, which affects the pH range discussed below. Note: H+ and H3O+ are often used interchangeably. The equation for water equilibrium is:
H 2 O ⇌ H ++ OH −
- If an acid (H +) is added to the water, the equilibrium shifts to the left and the OH − ion concentration decreases
If base (OH − ) is added to water, the equilibrium shifts to left and the H + concentration