Alkalinity: Definition & Calculation

Instructor: Emily Lockhart

Emily has taught science and has a master's degree in education.

This lesson looks at the properties of an alkaline solution. Alkalinity and pH are defined, and the calculations for finding the pH of a solution are laid out.

Definition of Alkalinity

While cookies may be sweet, some of their ingredients, like baking soda, are very bitter. The taste of baking soda is due to the fact that its pH is very basic, or alkaline. An alkaline solution has an increased concentration of hydroxide ions. The opposite of alkaline is acidic: sour foods, like lemons and vinegar, are acidic. Acidic solutions have an increased concentration of hydronium ions. You may know that mixing baking soda (which is high in hydroxide ions) with vinegar (which has more hydronium ions) can create a terrific explosion! This is because when you mix an acid with a base, the exchange of atoms releases energy. Cooking is just one situation in which knowing pH is important. Knowing the alkalinity of soils or water can tell farmers or biologists the health of an ecosystem. Testing for alkalinity is important in many different disciplines, which makes this a practical lesson.

The word alkali or alkaline implies that a substance has a pH higher than 7. Both acids and alkali, or bases, have a measurable pH, which is a scale from 0-14. The scale sets 7 as neutral pH: distilled water would be the closest to 7. A pH between 7 and 14 makes a substance alkaline. A pH between 7 and 0 makes a substance acidic. Your blood has a pH of 7.4, which is close to neutral. If your blood pH gets below 7.35 or above 7.45, it is life threatening. A strong base like ammonia has a pH of 10!

shows pH scale of common household items

Why Use Water as Neutral?

Water is the neutral or middle-of-the-road pH. To understand why scientists picked water, let's look at water's structure. Water can be chemically written as H20. That means that two hydrogen atoms are bonded to one oxygen atom to form water (atoms are the smallest unit and make up every object in the universe). Hydrogen and oxygen are found in almost everything on our earth.

A water molecule looks a bit like Mickey Mouse. The ears of Mickey are the two hydrogen atoms, and the face is the oxygen. Sometimes, a hydrogen atom (H+) breaks off of one water molecule and joins another water molecule (written as H3O+). Left behind is a single oxygen bonded to just one hydrogen (written as OH-). Within a neutral substance there are roughly 10-7 concentrations of H3O+ and 10-7 concentrations of OH- . Another way to think of 10-7 is to write it as 0.000001. This is the concentration of H3O+ or OH- in neutral water.

As for why H3O+ and OH- have positive and negative charges, you must understand what makes up hydrogen and oxygen. These, and all other atoms, are made up of three smaller components: electrons, which carry a negative charge, protons, which carry a positive charge, and neutrons, which are neutral. Hydrogen is the smallest of atoms and is made of just one electron and one proton. When a hydrogen atom bonds with oxygen to form water, the hydrogen atom shares its electron with the oxygen atom. When hydrogen breaks from the bond with the water molecule, that little electron leaves the hydrogen and stays with the OH in the water molecule. The hydrogen is now just a single proton and written as H+. In water, though, H+ can't exist on its own, but instead is drawn to another H2O molecule. It is written as H3O+, but that new H is in fact a single proton. The electron from the hydrogen stays with the oxygen bonded to the hydrogen and gives the OH a single negative charge. That is why OH is written as OH-. We refer to these as ions. An ion is any atom or molecule that has lost or gained electrons. H3O+ is a hydronium ion; it sounds like water is 'drowning' in hydrogen. OH- is called a hydroxide ion.

Image shows a water molecule and its ions that contribute to pH

Calculating pH

If another molecule is dissolved in water, it may leave hydronium ions behind. For example, if HCl, hydrogen chloride, dissolves in water, the molecule dissociates into hydrogen ions and chloride ions. More concentrations of hydrogen ions lead to an acidic solution. If OH- ions are added due to dissociating from other molecules like NaOH, sodium hydroxide, the solution will become more alkaline. The reason is that when one of these ions is added, for example OH-, it not only increases the concentration of that ion, but also decreases the concentration of the other ion (H+) by bonding with it to make water, H2O.

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