pH - Understanding the Scale

The word acid is often used in movies and books as a synonym for “scary stuff,” while the truth is that many acids are common and completely safe. At the same time the word base is virtually never mentioned, although the most dangerous things in your house are probably bases.

The pH scale is actually a mathematical tool, but for most of the world, its purpose is to help people understand the safety or dangers of a solution. On this page we’ll look at the pH scale and get a sense of what the numbers on the scale tell us. We’ll even look at common household items and place them on the scale to help you make sense of it.

A Scale With No Ends
The pH scale is often drawn as a number line with ends at 0 and 14. This is WRONG. Because pH is actually a mathematical definition (based on the concentration of hydronium and hydroxide ions) the scale does not actually have ends. It is true that few people outside of a chemistry lab will ever encounter something with a negative pH or with a pH above 14. Even Chemists are unlikely to ever encounter anything with a pH below -1.5 or above 15, but the point remains. This is a much more accurate picture of the scale

So How Does the Scale Work?
The pH scale may not have ends, but it does have a middle, 7, the pH of pure water. Water is safe. (Actually, water is the cause of lots of problems from burns to drowning, but that’s biology.) Solutions with a pH LOWER than 7 are considered acidic. Solutions with a pH HIGHER than 7 are considered basic. As you move further away from water in either direction on the scale the solution becomes more dangerous. It might help to think of the pH scale as a valley rather than a line...something like this: 

Why the Mathematical Definition of pH Should Matter to Everyone
The pH scale is a logarithmic scale. That means that every step on the scale is 10x’s the previous step. So, a pH of 2 is 10 times more acidic than a pH of 3, and 3 is 10 times more acidic than 4. The result of this is that the numbers “near” 7 are all pretty safe. People regularly eat foods with pH’s in the 3 range and some people brush their teeth with baking soda, with a pH of 10.

If such a scale seems odd, or doesn’t make sense, there is another logarithmic scale that people encounter - the Richter scale, used to measure earthquakes. The analogy isn’t perfect (the Richter scale starts at 0 and increases, the pH scale “starts” at 7 and goes both up and down), but it may help.

Many people will not even notice an earthquake with a value less than 3. A quake up to 5 may knock things off the wall and feel frightening, but generally will not do serious damage to buildings. Each step above that seems dramatically worse. A “6” quake may shatter windows and do other damage to structures. A “7” may destroy homes and damage larger buildings. An “8” can cause massive damage to buildings, roads and bridges. A “9” has the potential to destroy a city.

In the same way, most people cannot taste the “sourness” in anything with a pH between 7 and 5. Most people will begin to notice the acidic nature of pH 4 and by the time we get to a solution with a pH of “3” everyone can tell you that the solution is sour. A solution with a pH of 2 will pucker your lips and a pH of one is painful - think about how your throat feels when you vomit. 

Let’s attach some household solutions to the pH scale so that you can get a sense of what these numbers mean.