What indicator would be used if we were titrating with HCl in the buret and NaOH in the flask since phenolphthalein is not appropriate?
In most cases of strong acid-strong base titrations phenolphthalein is appropriate so to answer this question best I would need to know the concentrations being titratrated. Bromothymol blue might be good. Phenol red might be good. You want an indicator that will change pH at 7.
If phenolphthalein is not appropriate, an alternative indicator that can be used when titrating with HCl in the buret and NaOH in the flask is bromothymol blue (BTB).
To understand why bromothymol blue is a suitable indicator for this specific titration, let's examine the color change properties of BTB.
Bromothymol blue is a pH indicator that transitions between different colors at different pH levels. In an acidic solution, it appears yellow or yellow-green, while in a basic solution, it changes to blue. This makes it ideal for acid-base titrations, as it can accurately signal when the solution has reached the endpoint.
When titrating HCl with NaOH, the reaction involves the neutralization of an acid (HCl) with a base (NaOH) to produce water and a salt. The equivalence point occurs when stoichiometrically equivalent amounts of acid and base have reacted, resulting in a neutral solution with a pH close to 7.
At the beginning of the titration, before adding any NaOH, the HCl is in excess, resulting in an acidic solution. As you gradually add NaOH, the pH of the solution will increase, indicating the formation of water and a salt. Initially, the color of the bromothymol blue indicator will be yellow due to the acidic environment.
As you continue to add NaOH, the base will neutralize the acid, gradually bringing the pH closer to 7. At some point, when the stoichiometric ratio of HCl to NaOH is achieved, the solution will become neutral, and the indicator will undergo a color change from yellow to blue.
Therefore, bromothymol blue can be used as an indicator in this HCl-NaOH titration. It will exhibit a color change from yellow to blue as the solution approaches neutrality, providing a visual indication of reaching the endpoint.
It is important to note that in some cases, the choice of indicator may depend on the specific conditions and requirements of the experiment or analysis. Thus, it is always advisable to consult appropriate references, guidelines, or experimental procedures to ensure the selection of the most suitable indicator for a particular titration.