What is pH

Pure water self-ionizes slightly to form HgO and OH ions, as shown in this equation. 

The equation for the equilibrium can be simplified by removing one water molecule from each side. 

A special equilibrium expression for the self-ionization of water is defined as follows. 

called the ion product constant for water, is the value of the equilibrium constant expression for the self-ionization of water. In pure water at 298 K, the concentrations of H+ ions and OH ions both equal 1.0 X 10 M, so the value of can be calculated. 

At 298 K, the product of [H ] and [OH ] always equals 1.0 X 10 . Therefore, if the concentration of one of these ions increases, the concentration of the other ion must decrease. The following example problem shows how you can use to find either [H ] or [OH ] if the other concentration is known. 

Water not only serves as the solvent in solutions of acids and bases, it also plays a role in the formation of the ions. In aqueous solutions of acids and bases, water sometimes acts as an acid and sometimes as a base. You can think of the self-ionization of water as an example of water assuming the role of an acid and a base in the same reaction. 

Recall from Section 19.1 that pure water contains equal concentrations of H+ and OH ions produced by self-ionization. One molecule of water acts as a Br0nsted-Lowry acid and donates a hydrogen ion to a second water molecule. The second molecule of water accepts the hydrogen ion and becomes a hydro-nium ion. The 1 1 ratio between the products means that equal numbers of hydronium ions and hydroxide ions are formed. 

The double arrow indicates that this is an equilibrium. Recall that the equilibrium constant expression is written by placing the concentrations of the products in the numerator and the concentrations of the reactants in the denominator. In this example, all terms are to the first power because all the coefficients are 1. 

The concentration of pure water is constant so it can be combined with by multiplying both sides of the equation by [H2O]. 

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What is pH Chemists express the concentration of hydrogen ions using a pH scale based on common logarithms. The pH of a solution is the negative logarithm of the hydrogen ion concentration. 

What is pH The mathematical definition of pH is the negative of the logarithm of the hydrogen ion concentration. It is a measure of the acidity of the solution. The lower the pH, the more acidic the solution. Because of the log term, a pH change of one unit means a tenfold change in hydrogen ion concentration. 

What is pH, and how is it used to describe acid and base solutions ... 

Hollander F (1945) What is pH An explanation of the various measures of acidity employed in gastroenterology. Gastroenterology 4 497-508... 

Using a glass electrode at 15°C, pH and potential (Eg) of the standard solution are 8 and400mV, respectively. What is pH of a test solution if its potmtial (Ex) is 800mV ... 

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