Have you ever been curious about how antacid tablets work? Or what ingredients are in toothpaste? And that cream that helps with wasp stings - ever wonder what it's made of? All of these things involve a process called neutralisation, which happens when acids and bases react with each other.

What are acids and bases?

Acids and bases can be defined in different ways. Swedish physicist and chemist, Svanthe Arrhenius, was the first to define an acid in 1884 as a substance that breaks up into hydrogen ions in solution. In 1923, Johannes Nicolaus Brønsted and Thomas Martin Lowry developed a different theory that defined an acid as a proton donor. Gilbert N. Lewis also defined an acid in 1923 as an electron pair acceptor.

For the purpose of this article, we will focus on Brønsted and Lowry's definition of an acid, which is a proton donor. Monoprotic acids donate one proton per acid molecule in a solution, while diprotic acids donate two.

The word "acid" comes from the Latin term "acidus," which means sour. Acids turn damp blue litmus paper red. In contrast, bases turn red litmus paper blue and have a soapy texture. A base is a proton acceptor. Both acids and bases dissociate in solution, which means they split up into ions. Acids always split up into protons and a negative ion, while bases dissociate into hydroxide ions and a positive ion. It's important to note that not all bases contain the OH group. Other bases, such as ammonia, will be explored in the Brønsted-Lowry Acids and Bases.

Conjugate acids and bases

In addition to acids and bases, there are also conjugate acids and bases.

A conjugate acid is a base that has gained a proton, while a conjugate base is an acid that has lost a proton.

Every acid and base has a conjugate acid or base paired with it. For instance, the conjugate acid of ammonia is ammonium, and the conjugate base of water is the hydroxide ion.

Here is a table showing some examples of bases and their corresponding conjugate acids:

Base Conjugate acid
Ammonia Ammonium ion
Water Hydrogen ion (hydronium ion)
Hydroxide ion Water

Neutralisation reactions

Acids and bases can react together in neutralization reactions, which form salts. A neutralization reaction is a reaction between an acid and a base.

Salts are ionic compounds that consist of positive and negative ions held together in a giant lattice. When naming salts, we state the cation (positive ion) first, followed by the anion (negative ion). For example, common table salt, which is sodium chloride. Another example is calcium chloride, which is used to de-ice roads.

During a neutralization reaction, the acid and base react to form water and a salt. The hydrogen ions from the acid combine with the hydroxide ions from the base to form water, while the remaining ions combine to form a salt. For example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), the products are water (H2O) and sodium chloride (NaCl), which is a salt. Neutralization reactions are important in many chemical processes, including the digestion of food in our stomachs, the treatment of wastewater, and the production of medicines and cleaning products.

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To completely neutralize a solution, you need to add just enough base to react with all of the acid, leaving neither acid nor base left over.

In a neutral solution, the concentrations of hydrogen ions (H+) and hydroxide ions (OH-) are equal, leading to a pH of 7.

As mentioned previously, neutralization reactions are involved in a variety of everyday activities, such as taking antacid tablets, brushing teeth, and soothing wasp stings. Antacid tablets contain bases like magnesium hydroxide, which neutralize excess hydrochloric acid produced by the stomach. Toothpaste is alkaline and reacts with the acids produced by bacteria in the mouth. Wasp stings are also alkaline, so creams and balms often contain acids to neutralize the sting and calm the affected area.

An alkali is a base that is soluble in water. Common examples of alkalis include sodium hydroxide (NaOH), potassium hydroxide (KOH), and calcium hydroxide (Ca(OH)2). Alkalis are often used in cleaning products and in the production of soaps and detergents due to their ability to neutralize acids and break down oils and fats.

Concentrated and dilute acids and bases

You’ve probably heard of the dangers of acids and images of corrosive warning signs fill your head. Whilst it is true that both acids and bases can be extremely dangerous, you generally only have to worry about concentrated acids and bases. Concentration refers to the number of acid or base molecules in solution. A concentrated acid or base contains a lot of molecules dissolved in solution whereas a dilute acid or base contains fewer.

Strong and weak acids and bases

The equations shown demonstrate the difference between strong and weak acids. Both hydrochloric acid (HCl) and ethanoic acid (CH3COOH) are acids that donate protons in solution. However, the reaction with hydrochloric acid goes to completion while the reaction with ethanoic acid is reversible. This is because hydrochloric acid is a strong acid, meaning it fully dissociates in solution, while ethanoic acid is weak, meaning it only partially dissociates in solution.

Strong acids include hydrochloric acid, which is found in gastric juices, and sulfuric acid. Weak acids include ethanoic acid, which is found in malt vinegar, and citric acid, which is found in citrus fruits like lemons. Strong bases include sodium hydroxide, while weak bases include ammonia.

The difference between strong and weak acids becomes important when calculating pH, which is a measure of the acidity or basicity of a solution. pH is defined as the negative logarithm of the concentration of hydrogen ions in a solution. A lower pH indicates a higher concentration of hydrogen ions, making the solution more acidic. A pH of 7 is considered neutral, while a pH above 7 is basic and a pH below 7 is acidic.

Understanding the difference between strong and weak acids is important in calculating pH because the dissociation of weak acids is an equilibrium reaction. This means that the concentration of hydrogen ions is not solely determined by the concentration of the acid, but also by the equilibrium constant of the reaction. This makes calculating the pH of weak acid solutions more complex than calculating the pH of strong acid solutions.

What is the pH scale?

It is a measure of hydrogen ion concentration in solution.

The pH scale was invented by a Danish brewer and chemist named Søren Peder Lauritz Sørensen, who was looking to control the acidity of his beer. Solutions with a high hydrogen ion concentration have a low pH and vice versa. You now know that acids release protons (hydrogen ions) in solution. This means that acids have a low pH. On the other hand, bases have a high pH.

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Calculating pH

Calculating pH can get a little tricky. There are lots of equations you need to learn, and it is easy to get confused between moles and concentrations. The following table gives you some of the values you need to understand in order to calculate pH, as well as equations linking them.

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In later articles, we’ll explore all these values in greater detail and walk you through the different methods of calculating pH. However, the process can be summarised with the following flow chart:

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Measuring pH

When carrying out an acid-base reaction, such as a neutralization reaction, it may be necessary to determine the pH of the solution at regular intervals. Calculating the pH each time can be time-consuming and laborious, but there are alternative methods for finding the pH instantaneously.

Indicators are substances that show a distinct observable change when the conditions they are in change. In the case of acid-base reactions, indicators typically display a color change when the pH of the solution changes. One example is the universal indicator, which is a mixture of different indicators that spans the whole color spectrum according to whether a substance is acidic or alkaline. The universal indicator can be used to determine the approximate pH of a solution based on its color.

Another option is to use a pH meter, which is a digital instrument that accurately measures the pH of a solution. pH meters work by measuring differences in electrical activity between a reference electrode and a pH-sensitive electrode. The pH-sensitive electrode detects the concentration of hydrogen ions in the solution and converts this information into a pH reading. pH meters are highly accurate and can provide precise pH measurements. They are commonly used in scientific research and industrial applications where precise control of pH is important. In summary, indicators and pH meters are two methods for determining the pH of a solution. Indicators show a color change when the pH changes, while pH meters use electrical activity to measure the pH of a solution.

What are buffer solutions?

A buffer solution is a solution that can resist changes in pH when small amounts of acid or alkali are added to it. These solutions are extremely useful in many systems that require a constant pH, such as the circulatory system in the human body.

The pH of blood must be maintained within a narrow range of around 7.4 for optimum enzyme activity. This is achieved by several buffer systems, with the bicarbonate buffer system being the most notable. When cells respire, they release carbon dioxide (CO2) into the bloodstream. This CO2 reacts with water to form the bicarbonate ion (HCO3-), which exists in equilibrium with carbonic acid (H2CO3). Any acids produced by cellular activity, such as lactic acid, are neutralized by bicarbonate ions, while any bases are neutralized by carbonic acid. Overall, this maintains a steady pH in the blood.

Buffer solutions are also commonly used in chemical and biological processes to maintain a constant pH. For example, in biotechnology, buffer solutions are used to maintain the pH of cell cultures and enzyme reactions. In the manufacturing of pharmaceuticals and other chemicals, buffer solutions are used to control the pH of reactions and ensure the desired product is obtained. In summary, buffer solutions are essential for maintaining a constant pH in many systems, including the human body. The bicarbonate buffer system plays a crucial role in maintaining the pH of blood, while buffer solutions are also widely used in chemical and biological processes.

What are titrations?

If you have a solution of hydrochloric acid but don't know its concentration, you can determine its concentration using a titration reaction. In this process, a fixed volume of an acid or base of known concentration is neutralized with the acid or base of unknown concentration, and the volume of the unknown substance required is measured.

To accurately carry out a titration, it is important to know the equivalence point. The equivalence point is the point where just enough base has been added to neutralize an acid in solution or vice versa. To determine when the equivalence point has been reached, an indicator is used. Indicators change color at a specific pH, which is known as the end point.

The end point of a titration is the point where the indicator just changes color. If the end point of a titration is the same as its equivalence point, the color change of the indicator can be used to tell when just enough base has been added to neutralize the acid or vice versa. By using the chemical equation for the reaction, it is then possible to determine the concentration of the unknown acid or base. Titration is a fundamental technique used in analytical chemistry to determine the concentration of unknown substances. It is widely used in many industries, including pharmaceuticals, food, and environmental testing. In summary, titration is a useful method for determining the concentration of an unknown acid or base. By using an indicator to determine the equivalence point and end point, the concentration of the unknown substance can be calculated using the chemical equation for the reaction.

pH curves and suitable indicators

When plotting the pH change in a neutralization reaction against the volume of acid or base added, a curved graph known as a pH curve is produced. This graph has three distinct sections.

The first section is an initial shallow-sloping section, where the pH barely changes. This is due to the buffering capacity of the solution. The second section is a sharply-sloping section, where the pH changes rapidly. This section is known as the equivalence point and is the point where the acid and base have been neutralized. The third section is another shallow-sloping section, where the pH barely changes. This is due to the excess of the added acid or base.

The equivalence point of a titration lies in the middle of the sharply-sloping section of the pH curve. If an indicator's end point also lies in this section, it can be used in the titration to determine the equivalence point. The choice of indicator is critical in determining the accuracy and precision of the titration.

For example, in the addition of a strong base to a weak acid, the pH curve would have an equivalence point in the middle of the sharply-sloping section. The end points of two indicators are shown on the graph. In this case, phenolphthalein would be a suitable indicator as its end point lies in the sharply-sloping section of the graph, which also contains the equivalence point.

Understanding pH curves is essential in accurately performing and interpreting titration experiments. By using the correct indicator and analyzing the pH curve, the concentration of an unknown substance can be determined with high precision and accuracy.

Carrying out titrations

A fun practical activity could be carrying out a simple titration or general neutralisation reaction. In fact, you’ll probably do many titrations over the course of your studies. If you are carrying out a titration, make sure you use a suitable indicator, but you could also use a pH meter. Let’s walk through the process using hydrochloric acid and sodium hydroxide.

Measure out 30  of  using a volumetric flask. Pour the solution into a conical flask. Add 2-3 drops of your indicator and swirl the flask. Rinse a burette, first with distilled water and then with . Set up the burette using a stand and clamp so that it is suspended above the conical flask. Fill the burette with your titrant. Note the value of this solution shown on the burette. This is your start value. Add the titrant to the conical flask in  intervals, swirling after each addition, until the solution in the conical flask changes colour. Note the value shown on the burette. This is your end value. To work out the titre, subtract your end value from your start value. This gives you the volume of titrant added to the flask. Repeat the experiment again until you have three titre values within  of each other. As you reach the end of the titration, near the point of colour change, add the titrant dropwise. The colour change takes place over a very small volume range and so adding the titrant in reduced amounts allows you to be more precise.

If you are using a pH meter, use it to measure the pH of the solution in the conical flask each time you add more of the titrant. As you near the point of colour change, add the titrant in smaller quantities as explained above.

This is the set up for a typical titration.

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Titrations have many useful applications in everyday life. For example, they’re used to determine the degree of contamination of wastewater and to find out the nutritional content of certain foods, such as the proportion of saturated and unsaturated fatty acids present. The cosmetic industry also used titrations to make sure the pH of their products stays within a safe range for human skin.

To summarize, here are the key takeaways about acids and bases:

• There are different definitions of acids and bases, with the Brønsted-Lowry definition being one of the most commonly used.
• Acids donate protons and bases accept protons.
• Acids and bases dissociate in solution, with strong acids and bases fully dissociating and weak acids and bases only partially dissociating.
• Concentration is a measure of the amount of acid or base molecules in solution.
• Neutralization reactions involve the reaction of an acid and a base to form a salt and water.
• A neutral solution has an equal concentration of hydrogen and hydroxide ions.
• pH is a measure of hydrogen ion concentration in solution and can be calculated using different values and formulas.
• Titration reactions can be used to determine the concentration of an unknown acid or base and use indicators to show when the equivalence point has been reached.
• Buffer solutions can maintain a constant pH even when small amounts of acids or bases are added. 

Acids and Bases

What are acids and bases?

There are multiple different definitions of acids and bases. However, the Brønsted-Lowry definition defines acids as proton donors and bases as proton acceptors.

How do you distinguish between acids and bases?

You can use differences in pH to distinguish between acids and bases. Acids have a low pH of below 7 whilst bases have a high pH of above 7. To measure pH, we use a universal indicator or a pH meter.

What are conjugate acids and bases?

Conjugate acids are bases that have gained a proton whilst conjugate bases are acids that have lost a proton. Every acid and base has a paired conjugate acid or base.

What are alkalis?

Alkalis are bases that are soluble in water. This means that all alkalis are bases but not all bases are alkalis!

What are Arrhenius acids and bases?

One way of defining acids and bases is by using the Arrhenius definitions. An Arrhenius acid is a substance that donates a proton in solution while an Arrhenius base gives hydroxide ions in solution.

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