What is the pH Scale?

Lucy Bell-Young

by Lucy Bell-Young

14th April 2021

Blog

The pH scale is a precise way of classifying the acidity, basicity or neutrality of a solution. As a logarithmic scale, 1 pH unit is ten times stronger, or ten times weaker, than the one below or above it, depending on its position: a pH of 4 is ten times more acidic than a pH of 5, but ten times weaker than a pH of 3. Continue reading for an in-depth look at what the pH scale measures, how it works, and how to use it. 

What Does pH in Chemistry Stand For?

In chemistry, pH stands for ‘potential of hydrogen’. But it can also denote power of hydrogen or per hydronium ion measurement. Søren Peder Lauritz Sørensen, a Danish chemist, introduced the concept in 1909, revising it fifteen years later to the modern understanding we have of it today.

Sørensen didn’t explain why he used the p in the pH notation. Its exact meaning, as originally intended, is still disputed. The ambiguity of the p could come from the fact that Sørensen was fluent in several languages, including Danish, French, and German. The closest possible meaning of p is ‘power’. It could stand for the French puissance, the German potenz, or the Danish potens, which all mean power. On the other hand, the p could just be an arbitrary label; he also used q to refer to test solutions, for no apparent reason.

Nevertheless, the common convention is either to use ‘potential for hydrogen’ or ‘power of hydrogen’ when referencing pH. Both refer to the involvement of hydrogen ions based on the reactivity potential of a solution, i.e. whether it’s a proton acceptor (base) or a proton donor (acid).

The pH scale

What Does the pH Scale Measure?

The pH scale measures the relative acidity and alkalinity of solutions. It’s a negative logarithmic scale of base ten that measures the potential of a solution to accept protons in the form of hydrogen ions. For example, a solution with pH level 8 is ten times more alkaline than pure water, which has a pH level of 7. This implies that the particular solute has ten times more potential to accept protons compared to water. 

Therefore, basic or alkaline solutions have higher pH readings because of their potential to accept hydrogen ions. Acidic solutions already have high concentrations of hydrogen ions, which is why they have lower pH values. The rule of ten still applies here, though, with each integer value having a difference of ten times.

Although pH isn’t the absolute criterion of acid or base strength, it’s a good measurement for the concentration of a particular acid or base solution. The strength of an acid or base is measured by the tendency of its ions to completely dissociate in aqueous solutions. This is measured by the dissociation constant, which is the ratio between the ions and the molecular form of the dissolved substance. An acid always yields hydrogen positive ions (H+) while a base always yields hydroxide negative ions (OH).

Hydrogen ion concentration is calculated in an aqueous solution based on the molarity of the hydronium ions, which is measured in moles per litre. You need to know the molarity to calculate the pH. The formula is simple:

pH = – log [H3O+]

It’s easier to calculate in strong acids because the ions completely dissociate in water. For instance, hydrochloric acid is a strong acid that’s 100% ionised into cations and anions when dissolved in water. If you want to calculate the pH of a hydrochloric acid solution with 0.0025 M concentration, you simply need to add the number to the formula, as in the following:

pH =  – log (0.0025) = – (- 2.60) = 2.60

As you can see, even at very low concentration, the solution is still highly acidic in terms of its pH level. You can experimentally confirm this by preparing the solution and measuring the pH using a pH meter. You can also deduce the concentration if you measure the pH. However, the calculation can be tricky if the acid is weak. You can’t simply rely on the molarity of the acid solution; you need to know the dissociation constant.

How Does the pH Scale Work?

As previously mentioned, a pH scale is very useful as a comparative tool. It’s a continuous, rather than discrete scale, and is so precise that decimal numbers can be used in addition to whole numbers. Orange juice, for instance, has a pH range of 3.3 to 4.2.

You can compare various types of solutions using the pH scale. It’s a visual and numerical sale that can be represented by numbers, graphics, pictures, and colours. In fact, many pH indicators change colours to indicate pH range. The sample chart below compares the acidity and basicity of various common solutions in the home:


Source: Wikipedia

As you can see in the illustration, sea water is basic compared to pure water. Meanwhile, a cup of coffee is more acidic than pure water. Water is often conveniently used as a point of comparison because it has neutral pH.

How to Use the pH Scale

The pH scale is an analytical tool that you can use both theoretically and experimentally. For instance, if the hydronium concentration of an acid is known, you can theoretically calculate the pH of the solution. Conversely, if the pH is precisely measured, you can calculate the concentration of the solution.

As an analytical tool, the pH scale is very useful in comparing various substances. It can also be used to predict chemical reactions, as well as to help determine the chemical composition of analytes in a titration experiment. Many chemical manufacturing processes require the use of the pH scale. It’s also necessary in preparing food and beverages, which must have the right pH levels in order to prevent spoilage.

How to Read pH Indicator Chart

Using a pH indicator chart is a less precise way of measuring the pH level of a solution as it’s only based on the colour changes of various pH indicators. Indicator charts are useful, however, for calibration purposes and if a pH meter isn’t available. 

Even if a pH meter is available, you can use an indicator and a chart to confirm the validity of the pH meter reading. Various chemical indicators gradually change in colours as the pH changes. For instance, crystal violet indicator changes from light green to dark violet between the pH range of 1 and 2.

The table below shows other types of indicators. As you can see, there are various colour transitions and pH ranges depending on the type of pH indicator. The chart can be used as a qualitative reference if you are conducting a titration experiment.

You can divide the range into decimal points for a more precise reading. In reality, however, it would be very difficult to judge the colour transitions if they’re too subtle. It’s only very reliable in the extremes, especially when determining the end point of a titration chemical reaction:

Disclaimer

All content published on the ReAgent.co.uk blog is for information only. The blog, its authors, and affiliates cannot be held responsible for any accident, injury or damage caused in part or directly from using the information provided. Additionally, we do not recommend using any chemical without reading the Material Safety Data Sheet (MSDS), which can be obtained from the manufacturer. You should also follow any safety advice and precautions listed on the product label. If you have health and safety related questions, visit HSE.gov.uk.

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