Strength of Acids and Bases: \({\rm{pH}}\) Scale, Universal indicator, Role of \({\rm{pH}}\) in Everyday life

Strength of Acids and Bases: We know how acid-base indicators can be used to distinguish acid from the base. We also already know the dilution and reduction of the concentration of \({{\rm{H}}^{\rm{ + }}}\) or \({\rm{O}}{{\rm{H}}^{\rm{ – }}}\) ions in the solution. Can we find a way to quantify the amount of these ions present in a solution? Can we judge how strong an acid or base is? Yes, we can do this with the \({\rm{pH}}\) scale. In this article, we’ll take a closer look at the strength of acids and bases.

Strength of Acids and Bases solution

In pure water or a neutral solution, the concentration of \({{\rm{H}}^{\rm{ + }}}\) ions and the concentration of \({\rm{O}}{{\rm{H}}^{\rm{ – }}}\) ions are the same, so pure water is of a neutral nature. Acids produce \({{\rm{H}}^{\rm{ + }}}\) ions in an aqueous solution. The acidic solution contains more \({{\rm{H}}^{\rm{ + }}}\) ions than \({\rm{O}}{{\rm{H}}^{\rm{ – }}}\) ions and is therefore naturally acidic. Note that even acidic solutions contain \({\rm{O}}{{\rm{H}}^{\rm{ – }}}\) ions resulting from the ionization of water, but the \({\rm{O}}{{\rm{H}}^{\rm{ – }}}\) ion concentration is lower than that of \({{\rm{H}}^{\rm{ + }}}\) ions in acidic solutions. Bases produce \({\rm{O}}{{\rm{H}}^{\rm{ – }}}\) ions in an aqueous solution. The basic solution contains more \({\rm{O}}{{\rm{H}}^{\rm{ – }}}\) ions than \({{\rm{H}}^{\rm{ + }}}\) ions and is therefore basic. Note that even basic solutions contain \({{\rm{H}}^{\rm{ + }}}\) ions resulting from the ionization of water, but the concentration of \({{\rm{H}}^{\rm{ + }}}\) ions is lower than that of \({\rm{O}}{{\rm{H}}^{\rm{ – }}}\) ions in basic solutions. From this, we conclude that both acidic and basic solutions contain \({{\rm{H}}^{\rm{ + }}}\) ions.

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Concept of the \({\rm{pH}}\) Scale

A litmus solution or litmus paper can be used to determine whether a given solution is acidic or basic. Assuming you have two acidic solutions that contain different amounts of acids, how can you tell which solution is more acidic? Similar is the case with the basic solutions. This problem is solved with a scale known as the \({\rm{pH}}\) scale, which was introduced by SP Sorensen in \(1909.\) The \({\rm{pH}}\) of a solution indicates which solution is more acidic or more basic than the other. The acidity or basicity of a solution is usually expressed as a function of the \({{\rm{H}}^{\rm{ + }}}\) ions concentration. This function is known as the \({\rm{pH}}\) of a solution. The \({\rm{pH}}\) of an aqueous solution is the negative logarithm of its \({{\rm{H}}^{\rm{ + }}}\) ions concentration.

That is,

\({\rm{pH  =   – log}}\left[ {{{\rm{H}}^{\rm{ + }}}} \right]\)

Similarly,

\({\rm{pOH  =   – log}}\left[ {{\rm{O}}{{\rm{H}}^{\rm{ – }}}} \right]\)

Define \({\rm{pH}}\) Scale

The strength of an acid or base is measured on a numerical scale called the \({\rm{pH}}\) scale. The \({\rm{pH}}\) scale has values ​​from \(0\) to \(14.\) Note that \({\rm{pH}}\) is a pure number and has no units.

According to the Rules of the \({\rm{pH}}\) Scale

1. Neutral substances have a \({\rm{pH}}\) value of exactly \(7.\) Pure water is a neutral substance (it is neither acid nor basic). Therefore, the \({\rm{pH}}\) of pure water is \(7.\) A sugar solution and a sodium chloride solution are also neutral, each with a \({\rm{pH}}\) of \(7.\) As long as the \({\rm{pH}}\) of a solution is \(7,\) it is a neutral substance. A substance with a \({\rm{pH}}\) of \(7\) has no effect on litmus or other common indicators such as methyl orange or phenolphthalein, etc.

2. Acids (or acidic solutions) have a \({\rm{pH}}\) of less than \(7.\) As long as a solution has a \({\rm{pH}}\) of less than \(7,\) it is an acidic solution. For example, a solution having a \({\rm{pH}}\) of \(4\) will be acidic in nature. The more acidic a solution is, the lower its \({\rm{pH}}\) will be. Solutions with a \({\rm{pH}}\) of \(0, 1, 2,\) and \(3\) are generally considered strong acids, and solutions with a \({\rm{pH}}\) of \(4, 5,\) and \(6\) are considered weakly acidic solutions. It is clear that the acidity of a substance is related to its \({\rm{pH}}.\) Note that all solutions with a \({\rm{pH}}\) below \(7\) are acidic and therefore change from blue litmus to red. They also turn methyl orange into the red.

3. Bases (or basic solutions) have a \({\rm{pH}}\) greater than \(7.\) If a solution has a \({\rm{pH}}\) greater than \(7,\) it is a basic solution or an alkaline solution. For example, a solution with a \({\rm{pH}}\) of \(11\) is basic. In nature, the more basic a solution is, the higher its \({\rm{pH}}\) value. For example, a solution with a \({\rm{pH}}\) of \(14\) is much more basic than another solution with a \({\rm{pH}}\) of \(10.\) All substances with a \({\rm{pH}}\) of more than \(7\) are basic, and therefore they turn the red litmus into blue. They also make the phenolphthalein indicator turn pink.

Universal Indicator

Common indicators can tell us whether the given substance is an acid or a base; they cannot tell us whether the given substance is a strong acid, a weak acid, a strong base, or a weak base; However, we can measure the strength of an acidic solution or a basic solution by using a special type of indicator called a “universal indicator”.

Therefore, to get an idea of ​​how acidic or basic a substance is, a universal indicator is used. The solution in the school laboratory is to use a universal indicator. The universal indicator is a mixture of many different indicators that give different colours at different \({\rm{pH}}\) values ​​across the \({\rm{pH}}\) scale. Because the \({\rm{pH}}\) of a solution depends on the concentration of hydrogen ions, we can also say that the universal indicator shows different colours in different concentrations of hydrogen ions in the solution.

Measurement of \({\rm{pH}}\) Scale

The \({\rm{pH}}\) of a solution is usually determined using a universal \({\rm{pH}}\) or indicator paper. The \({\rm{pH}}\) paper gives a special colour with a certain \({\rm{pH}}\) solution. The colour is compared to a straw, which has different colours at different \({\rm{pH}}\) levels.

Role of pH in Everyday Life

The role of \({\rm{pH}}\) in our everyday life are explained below:

1. \({\rm{pH}}\) in Our Digestive System

In our digestive system, the hydrochloric acid produced in our stomach helps digest food without harming the stomach, but when the amount of acid exceeds a certain limit due to indigestion, pain, and imitation, develop in the stomach. To neutralize the effects of excess acid, a mild base called an antacid is usually taken. Magnesium hydroxide (milk of magnesia) is a mild base commonly used as an antacid.

2. \({\rm{pH}}\) Change as the Cause of Tooth Decay

Acids cause tooth decay when we eat sugary foods. They are broken down by bacteria in the mouth, and acid is formed. When the \({\rm{pH}}\) becomes less than \(5.5,\) tooth enamel corrodes. Saliva that is slightly alkaline will neutralize some acid that is produced in the mouth, but it will not affect excess acid. So, the cleaning of the tooth by Neem stick also helps to reduce tooth decay.

3. Plants and Animals are Sensitive to \({\rm{pH}}\) Changes

Plants and animals are sensitive to changes in \({\rm{pH}}\) in their environment. In fact, plant growth and animal survival depend heavily on the availability of the right \({\rm{pH}}\) conditions to suit them.

I. Soil \({\rm{pH}}\) and plant growth: Most plants grow best when the \({\rm{pH}}\) of the soil is close to \(7.\) If the soil is too acidic or too basic (too alkaline), the plants will grow poorly or not at all. If the soil is too acidic (low \({\rm{pH}}\)), treat it with materials such as quicklime (calcium oxide), hydrated lime (calcium hydroxide), or chalk (calcium carbonate). If the soil is too alkaline, the alkalinity can be reduced by adding decaying organic matter (manure or compost), which contains acidic substances.

II. \({\rm{pH}}\) Variation and Animal Survival: \({\rm{pH}}\) plays an important role in the survival of animals, including humans. Our body works well within a narrow \({\rm{pH}}\) range of \(6.8\) to \(7.4.\) If, for some reason, this \({\rm{pH}}\) range in the human body changes, many diseases can occur. Aquatic animals (such as fish) can survive in lake or river water over a narrow range of \({\rm{pH}}\) changes. For example, when the \({\rm{pH}}\) of rainwater is around \(5.6,\) it is called acid rain. Too much acid rain can lower the \({\rm{pH}}\) of a lake or river to such an extent (and make it acidic) that it becomes difficult for aquatic animals to survive. The high acidity of the lake or river water can even lead to the death of aquatic animals (for example, fish). Calcium carbonate is often added to acidic lake water to neutralize acids from acid rain. This prevents the fish in the lake from being killed.

4. Self Defence by Animals and Plants Through Chemical Warfare

Some animals and plants contain acids. The bee injects an acid through its stings, which causes pain and irritation. Similarly, nettle leaves, which have stinging hairs, when touched inject formic acid into our body. This causes burning pain.

Summary

In pure water or a neutral solution, the concentration of \({{\rm{H}}^{\rm{ + }}}\) ions and the concentration of \({\rm{O}}{{\rm{H}}^{\rm{ – }}}\) ions are the same, so pure water is of a neutral nature. The \({\rm{pH}}\) of a solution indicates which solution is more acidic or more basic than the other. The \({\rm{pH}}\) of an aqueous solution is the negative logarithm of its \({{\rm{H}}^{\rm{ + }}}\) ion concentration. Neutral substances have a \({\rm{pH}}\) value of exactly \(7.\) Acids (or acidic solutions) have a \({\rm{pH}}\) of less than \(7.\) Bases (or basic solutions) have a \({\rm{pH}}\) greater than \(7.\) \({\rm{pH}}\) is very important to our daily life, such as in our digestive system, tooth decay, etc.

FAQs on the Strength of Acids and Bases

Q.1. What is the strength of Acids and Bases?
Ans: The strength of an acid and a base is a measure of how easily the molecule ionizes in water and produces \({{\rm{H}}^{\rm{ + }}}\) or \({\rm{O}}{{\rm{H}}^{\rm{ – }}}\) ions.

Q.2. How is the strength of an acid and base determined?
Ans: The strength of an acid or base is measured on a numerical scale called the \({\rm{pH}}{\rm{.}}\) scale \(\left( {0 – 14} \right).\) The more acidic a solution, the lower its \({\rm{pH}}{\rm{.}}\) Solutions with \({\rm{pH  =  0,1,2,3}}\) are considered as strong acids. Solution with \({\rm{pH  =  4,5,6}}\) are considered as weak acids. The more basic a solution, the higher is its \({\rm{pH}}{\rm{.}}\) Solutions with \({\rm{pH  =  12, 13, 14}}\) are considered as strong bases. Solutions with \({\rm{pH  =  7, 8, 9}}\) are considered weak bases.

Q.3. What is the strength of an acid?
Ans: The strength of an acid is the measurement of hydronium ions concentration given by acid after dissociation into an aqueous solution.

Q.4. Which acids and bases are strong?
Ans: The acids which dissociate completely in water to produce hydrogen ions are called strong acids. The bases which dissociate completely in water to produce hydroxide ions are called the strong bases.

Q.5. On which factor the strength of acids and bases depends?
Ans: Acid and base strength is determined by how much an acid or base ionises in solution.

Q.6. Which is the strongest acid?
Ans: The acids which dissociate completely in water to produce hydrogen ions are called strong acids.

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