pH and pOH

pH is a measure of hydrogen ion concentration in a solution. pOH is a measure of hydroxide ion concentration.

pH is the more commonly used measure. However, either scale can describe acidity or alkalinity. Even the most strongly basic solutions will contain some hydrogen ions, just as the most acidic solutions will still contain hydroxide ions.

When a solution contains more hydrogen ions than hydroxide ions, it is acidic. An acidic solution has a pH < 7 at 25oC.

When a solution contains more hydroxide ions than hydrogen ions, it is basic. A basic solution has a pH > 7 at 25oC.

A roll of universal indicator paper

Strong acids completely dissociate in H3O+ and an anion. This means that to calculate their pH in a solution we can use their molarity, since all the acid will be dissociated.

The concentration of H3O+ and OH- in solution, which corresponds to x in the ICE table, is used to calculate pH and pOH, respectively. These numbers are related to each other, as hydrogen ions and hydroxide ions co-exist at equilibrium.

pH + pOH = 14

Once you have done this calculation, you can say that the solution is acidic if pH \< 7, whereas pH > 7 indicates that the solution is basic.

pH indicators

First and second transitions of thymol blue solution: acidic, neutraland alkaline from left to right (1); Color of methyl red solution atdifferent acid-base conditions: acidic, neutral and alkaline from leftto right (2). Bromothymol blue indicator in acidic, neutral and alkalinesolutions, from left to right(3)

There are several ways of measuring the approximate pH by mixing the solution with different chemicals that change colors when the solution is acidic or basic. Some are mentioned in table below but can be combined as is done in universal indicator paper of pH. A more accurate way of measuring pH is to use the pH-meter.

Indicator Color at lower pH Transition pH range Color at higher pH
Thymol blue (first transition) Red 1.2 – 2.8 Yellow
Methyl red Red 4.4 – 6.2 Yellow
Bromothymol blue (second transition) Yellow 6.0 – 7.6 Blue
Thymol blue Yellow 8.0 – 9.6 Blue
Phenolphthalein Colorless 8.3 – 10.0 Fuchsia

pH in living systems

Some biological fluids such as blood, act as buffer solutions. In the case of blood, pH is 7.365 (slightly basic) and it is kept between 7.35 and 7.45 to be able to undergo all physiological reactions.

The pH of gastric fluids is highly acidic in order to digest proteins. The acidity is due to its composition: high concentration of hydrochloric acid (HCl) and potassium chloride (KCl) and sodium chloride (NaCl). Cells in the stomach produce mucus to form a protective barrier that prevents damages in the organ. Fluid pH:

  • Gastric acid 1
  • Urine 6.0
  • Cerebrospinal fluid (CSF) 7.5
  • Blood 7.35 – 7.45
  • Pancreas secretions 8.1

Acids and bases