What are Acid -base indicators? Acid-base indicators are dyes that can change color according to the pH of the medium they are found. The most common indicator is litmus which is red in an acidic medium and blue in an alkaline medium. This indicator changes from a color gradation of red through purple to blue over a pH range of 5.0 to 8.0 confirming the fact that each pH has its own specific range of pH over which it changes color.
The pH of a solution can be measured using the following methods:
(i) Universal indicator
(ii) pH meters.
A universal indicator is made up of a mixture of various indicators that work at different pH ranges. Through a series of successive color changes, the universal indicator can indicate pH values of 3 to 11 and these pH changes can be obtained by comparing the color seen with that of the standards given. However, this method of measuring pH is not very accurate for scientific precision.
On the other hand, the pH of a solution can be measured accurately with the aid of a pH meter. A p meter can accurately measure the pH of even very dilute solutions as well as that of opaque and colored liquids.
A universal indicator can be used to compare the pH values of 1.0M solutions of the following
(i) Strong alkalis such as sodium hydroxide solution
(ii) Strong acids such as hydrochloric acid
(iii) Weak alkalis such as aqueous ammonia
(iv) Weak acids such as ethanoic acid
(v) Pure distilled water
It can be observed that strong acids and strong alkalis have pH values that are at the extremes of the pH scale, whereas weak acids and weak alkalis have pH values close to 7. For pure distilled water, its pH value is 7. Indicators are used in acid-base titrations and the best indicator that can be used for this is the universal indicator.
A Buffer solution is a solution that resists changes in pH on dilution or on addition of small amounts of acids or alkalis. Buffer solutions usually consist of a weak base or a weak acid in the presence of one of its salts. Some examples of buffers are:– Ethanoic acid and sodium ethanoate, trixocarbonate(IV) acid and sodium hydrogen trioxocarbonate(IV) and aqueous ammonia and ammonium chloride.
Buffer solutions are of huge significance in fields such as biochemistry and medicine. This is because pH values are often very critical and germane and therefore have to be maintained at a steady value for biological systems to function optimally. For instance, blood has a pH value of 7.4 and even a slight change of +0.5 may prove to be very fatal. Therefore, injections into the blood stream needs to be buffered so as not to alter this delicate balance. Also, many fermentation processes and enzyme facilitated reactions depend largely on pH, which can only possible vary within narrow limits. Buffer solutions are widely used in processed drinks and foods to prevent acidity.
The completion of many chemical reactions depends largely on the pH of the solutions in which they take place. The pH of aqueous solutions is extremely sensitive to the addition of small amounts of alkalis or acids. For instance, when 0.1cm3 of 1.0M hydrochloric acid is added to 1dm3 of distilled water or sodium chloride solution, the pH changes from 7.0 to 4.0 and this sort of sharp pH change would prove to be very fatal to living organisms. However, animals are protected from this sort of sharp pH changes by the presence of buffers.