The pH scale helps us measure the acidity, basicity or neutrality of a solution on a scale of 0 to 14, with 7 being the neutral marker. With this, a pH above 7 is basic, while a pH below 7 is acidic.
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.
The question is, how does the pH scale work, and how do we use it?
In this post:
Key Takeaways
The pH scale measures a solution’s acidity or alkalinity based on hydrogen ion concentration
pH ranges on a scale from 0 (acidic) to 14 (alkaline or base), with 7 being the neutral middle marker
A solution’s pH is logarithmic, meaning each unit is a tenfold difference between them
Strong acids dissociate fully in water, while weak acids don’t
pH indicators visually represent pH levels and can predict chemical reactions
What Does pH in Chemistry Stand For?
In chemistry, pH stands for ‘potential of hydrogen’, but it can also mean the 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 is still disputed. The ambiguity of the “p” term 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’.
Possible meanings include 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, giving precedent for this reading.
In modern science, however, 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).
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.
With this in mind, 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–).
For example, 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. Let’s take hydrochloric acid for instance. It is a strong acid that’s 100% ionised into cations and anions when dissolved in distilled 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, as 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, as an example, 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.
To get a clearer idea of the scale in practice, you can look at this pH chart:
As you can see in the illustration, sea water is slightly alkaline compared to neutral pure water, while a cup of coffee is more acidic. Water is often used as a benchmark because of its neutral pH, sitting right in the middle at 7.
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 a pH Indicator Chart
A pH indicator chart offers a more basic way to gauge pH by relying on colour changes from various indicators. While not as precise as a pH meter, it’s a practical alternative when you don’t have access to one. These charts are often used for calibration or to confirm the accuracy of a pH meter reading.
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, such as how a 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.
Applications of pH Measurements
From environmental monitoring to industrial processes, taking pH measurements is crucial. Whether you’re monitoring the health of ecosystems, ensuring food quality, or controlling industrial reactions, knowing the pH of a solution is often a critical step in ensuring everything happens as it should.
These measurements can help detect contamination, maintain balance, and optimise processes, making them an essential tool in a wide range of industries and disciplines.
Let’s look at some of the most important contexts in which pH measurements are essential.
Measuring pH Levels in the Environment
Environmental pH measurements are vital for monitoring the health of ecosystems around the world. As an example, acid rain is something that can drastically alter the pH of soil and water, harming plant life and aquatic organisms.
By regularly measuring pH in rivers, lakes, and soils, we can track pollution levels and assess the impact of environmental changes. Maintaining the right pH balance in natural habitats ensures that ecosystems remain stable and supports the survival of various species.
On top of that, pH measurements are also used in managing waste disposal and helps with making sure that runoff from industries doesn’t harm local water sources.
Measuring pH in Biological Systems
pH regulation in biological systems is crucial for proper functioning. In human health, for example, blood pH must remain within a narrow range (around 7.35–7.45) for cells and organs to operate effectively.
Deviations away from this range can be a sign of health problems, such as respiratory or metabolic disorders. This is why pH measurements are routinely taken in medical diagnostics and treatments.
In agriculture, pH plays a significant role in soil health, impacting nutrient availability for plants. If the soil pH is too high or too low, plants may not absorb nutrients properly, leading to poor growth or crop failure. Monitoring pH helps farmers adjust soil conditions to improve crop yield and maintain soil fertility.
Measuring pH in Industrial Settings
In industrial settings, pH measurement is essential to control processes and ensure product quality. For example, in the food and beverage industry, maintaining the correct pH is vital to preserve taste, texture, and shelf life.
For many products, like dairy or canned goods, pH also helps prevent spoilage and keeps food products safe to consume for a long time.
Similarly, pH plays a key role in water treatment plants, where it is adjusted to neutralise harmful contaminants before water is released back the waterways
In the manufacturing and chemical industries, pH controls the rate of reactions and the efficiency of various processes. Many chemical manufacturing processes require the use of the pH scale, particularly in preparing food and beverages, which must have the right pH levels in order to prevent spoilage.
Conclusion
Understanding the pH scale is essential for a wide range of applications, from chemistry labs to everyday life. It provides a simple yet powerful way to measure acidity and alkalinity, helping you make informed decisions in areas like food safety, chemical manufacturing, and scientific research.
While the pH scale is a handy tool for comparing solutions, it’s important to remember that it’s just one part of the equation when it comes to understanding acids, bases, and their behaviour in different environments.
