What Is the Meaning of a Ph Scale?

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You may know that the pH level of pure water is 7, the pH of vinegar is about 3 and the pH of sodium hydroxide is around 13, but what do these numbers actually mean? They tell you how acidic or alkaline an aqueous (water-based) solution is, on a scale of 0 to 14. This scale is known as the pH scale, where pH is the abbreviation for "power of hydrogen."

The pH Scale Definition

When you immerse acids and alkalis in solution, they release free ions. In a water-based solution, an acid releases positive hydrogen (H+) ions while an alkali releases negative hydroxide (OH-) ones. This means when an acid is dissolved in water, the balance between hydrogen ions and hydroxide ions changes, resulting in more hydrogen ions than hydroxide ions in the solution (an acidic solution). The balance also changes when an alkali is dissolved in water, but in the the opposite way. In this case, the solution ends up with more hydroxide ions than hydrogen ions (an alkaline solution).

The pH scale measures how strong an acid or alkali is. If it is midpoint on the scale, it is considered neutral – the concentration of hydrogen ions is equal to the concentration of hydroxide ions.

The definition of pH is the negative log of hydrogen ion concentration. The Danish biochemist Søren Peter Lauritz Sørensen was responsible for this term, which he created in 1909 as an abbreviation for "power of hydrogen." "P" stands for the German word for power (potenz), and H is the element symbol for hydrogen.

Sørensen came up with the following equation to calculate pH:

pH = -log[H+]

Log is the base-10 logarithm, and [H+] stands for the hydrogen ion concentration in units of moles per liter solution.

The Purpose of the pH Scale

The pH scale runs from 0 to 14, with 7 being a neutral pH, anything under 7 being acidic and anything over 7 being alkaline (sometimes referred to as basic). The pH scale is logarithmic, meaning every whole value below 7 is 10 times more acidic than the higher value, and every whole value above 7 is 10 times less acidic that the lower value. For example, a pH of 2 is 10 times more acidic than a pH of 3 and 100 times more acidic than a pH of 4. In other words, the stronger the acid, the lower the pH number, and the stronger the alkali, the higher the pH number.

Small changes in pH can have large effects. For example, acid rain, which generally has a pH of 4.2 to 4.4, is more than 10 times more acidic than clean rain, which usually has a pH of 5.6.

A substance with a pH of 1 to 2 is considered to be a strong acid, while one with a pH of 13 to 14 is a strong alkali. If an acid is very strong, it may have a negative pH, while very strong bases may have a pH higher than 14. A substance that is neither acidic or alkaline, such as pure water, is neutral. Human blood has a slightly higher-than-neutral pH of about 7.4.

Only aqueous solutions have pH levels, meaning chemicals, including some liquids, do not have pH values. For example, pure alcohol, vegetable oil and gasoline have no pH level.

Examples of Acidic Substances

Acidic solutions have more hydrogen ions than alkaline or neutral solutions. Acids also have a sour taste and react very strongly to metals. When concentrated, they may be very corrosive. Some common acids include orange juice, vinegar, lemons and sulfuric acid.

Examples of Alkaline Substances

Alkaline solutions have fewer hydrogen ions than neutral or acidic solutions or acids. Bases tend to feel slippery, and they usually have a bitter taste. Like acids, strong alkalis can burn your skin. Some common bases include ammonia, lye, baking soda, soapy water, bleach and milk of magnesia.

Mixing Acid and Alkali

If you mix equal amounts of a strong acid and a strong alkali, the two chemicals essentially cancel each other out, and the result is a salt and water. Mixing the same amounts of a strong acid and a strong alkali also produces a neutral pH solution. This is known as a neutralization reaction and looks like this:

HA + BOH → BA + H2O + heat

For example, the reaction between the strong acid HCl (hydrochloric acid) and the strong alkali NaOH (sodium hydroxide) is:

HCl + NaOH → NaCl + H2O + heat

This reaction produces sodium chloride (table salt). If you had more acid than alkali in the reaction, not all of the acid would react, so the result would be salt, water and leftover acid, and the solution would still be acidic (with a pH lower than 7). However, if you had more alkali than acid, there would be leftover alkali, and the final solution would still be alkaline (with a pH greater than 7).

Because the mixture warms up during the reaction, neutralization is known as an exothermic reaction. Neutralization is used for many things. Farmers may use lime (calcium oxide) to neutralize acid soils. You may use baking powder, which contains sodium hydrogen carbonate, to neutralize an acidic bee sting.

Something similar happens when one or both of the reactants are weak. A weak acid or alkali doesn't completely dissociate in water, so there may be leftover reactants at the end of the reaction, which affect the pH. Also, water may not be created because most weak alkalis are not hydroxides, so there is none of the OH- necessary to make water.

How to Measure pH

You can measure the pH level of a solution in different ways. The simplest method involves pH testing strips, which are made of a special paper called litmus paper. This is filter paper that has been treated with dyes made from lichens. This paper changes color when it comes into contact with an acid or alkaline. When placed in an acidic solution, blue litmus paper turns red, and when placed in an alkaline solution, red litmus paper turns blue. (As you would expect, when blue litmus paper is placed in a neutral solution, it stays blue, and when red litmus paper is placed in a neutral solution, it stays red.)

Some pH testing strips contain indicator bars that each change color depending on the solution the strip is exposed to. When you cover the testing strip with your solution (in a clean container) for a few seconds and then remove it, you can compare the end of the testing strip with the color chart you received with the paper to determine the solution's pH level.

Another way to measure pH requires a probe and meter. Before you use these tools, you must calibrate the meter by testing it in a substance with a known pH level (such as distilled water with a pH of 7). After you've made any necessary adjustments to the meter, and rinsed and dried the probe and meter, you can perform your pH test on your liquid sample in a clean container deep enough to completely cover the tip of the probe. Check the temperature of the sample with a thermometer and ensure the meter matches this temperature. Place the probe into your sample and wait for the measurement to become constant (this means the meter has reached equilibrium) before recording the pH level.


About the Author

Claire is a writer and editor with 18 years' experience. She writes about science and health for a range of digital publications, including Reader's Digest, HealthCentral, Vice and Zocdoc.