Question

The amount of sodium hydrogen carbonate, $NaHC{O_3}$ in an antacid tablet is to be determined by dissolving the tablet in water and titrating the resulting solution with hydrochloric acid. Which indicator is the most appropriate for this titration?
$\text{Acid} \;\;\;\;\;\;\;\;\;\;\;\;\;\;\; K_a$​
${H_2}C{O_3} \;\;\;\;\;\;\;\;\; 2.5 \times {10^{ - 4}}$
$HC{O_3}^{-} \;\;\;\;\;\;\;\; 2.4 \times {10^{ - 8}}$

a.) Methyl Orange, $p{K_{In}} = 3.7$
b.) Bromothymol blue, $p{K_{In}} = 7.0$
c.) Phenolphthalein, $p{K_{In}} = 9.3$
d.) Alizarin yellow, $p{K_{In}} = 12.5$

Answer 1

Hint: Since the given question is a clear case of acid-base titration, and while selecting an indicator for acid-base titrations, we need to choose an indicator whose pH range falls within the pH change of the corresponding acid-base reaction. So firstly we need to find the pH of the given reaction, based on that we can select the appropriate indicator with that pH.

Complete step-by-step answer:
Let us first understand what titration is –
Titration is defined as a process of chemical analysis in which the quantity of some constituent of a sample is determined by adding to the measured sample of an exactly known quantity of another substance with which the desired constituent reacts in a definite and known proportion.
The above process is generally carried out by gradually and slowly adding a standard solution (that is a solution of known concentration) of titrating reagent, or titrant, from a burette which is essentially a long and graduated measuring tube with a stopcock and a delivery tube at its lower end. The addition is eventually stopped when the equivalence point is reached and that is when our titration reaches an end.
At the equivalence point of a titration, an exactly equivalent amount of titrant has been added to the sample as is clear by the definition of titration. That experimental point at which the completion of the reaction is marked by some signal is known as the end point. This signal may be a colour change of an indicator or a change in some electrical property that is also measured during the process of titration.
In an acid-base titration (i.e., the titration of an acid with a base, or vice versa could take place), the indicator is that substance which may exist in two forms, an acid form and a basic form though which eventually differ in colour.
The indicator for the above problem should be used in such a way that it shows change in colour in the same pH range as required around the equivalence point or the neutralization point.
Now when solution of the $NaHC{O_3}$ which is actually a weak base is titrated against HCl solution which is a strong acid just after equivalence point there will be presence of very low amount of HCl and pH will be around ∼3.6 that is almost the end of the titration.
$pH = p{K_a} + \log \dfrac{{\left[ {HC{O_3}^ - } \right]}}{{\left[ {{H_2}C{O_3}} \right]}}$
Putting the value of $p{K_a}$ that is 3.6
$\Rightarrow 3.6 + \log \dfrac{{\left[ {HC{O_3}^ - } \right]}}{{\left[ {{H_2}C{O_3}} \right]}}$ Greater than 3.6 obviously.

∴ So Methyl orange having $p{K_{In}}$ 3.7 is the most appropriate for this titration.

Methyl orange is a pH indicator which is most commonly used in the process of titration because of its property of showing clear and distinct colour variance at varied pH values. Methyl orange exhibits a colour red in acidic medium and colour yellow in basic medium. Because it changes colour at the pKa of a mid strength acid, it is usually used in titration for acids with a weak base.
Therefore, the correct option for this question is option (A) ie. Methyl Orange.

Note – The litmus paper is blue in alkaline solution and is red in acid solution. Phenolphthalein is colourless in acid solution and demonstrated a colour of red in alkaline solution. A wide choice of acid-base indicators is available, varying not only in the colours of the two forms but also in their sensitivity toward acid or base but since we needed an indicator with pH of 3.6 at least in this case methyl orange was the ideal case .