Question

The titration shown in the plot has an equivalence point at pH = 3.2. 

Identify which of the indicators would be appropriate for this titration.

Answer 1

The \(\mathrm{pH}\) at the equivalence point of the titration is \(3.2\). An indicator with the color change near the \(\mathrm{pH}\) is useful for an endpoint.

The indicators with the \(\mathrm{pH}\) ranges are as follows:

Bromothymol bleu: \(6.0-7.6\) Phenolphthalein: \(8.0-9.6\) Ethyl red: \(4.0-5.8\) Metacresol purple: \(1.2-2.8 ; 7.4-9.0\) Quinalidine red: \(1.4-3.2\) Thymol blue: \(1.2-2.8 ; 8.0-9.6\)

In the given figure, the \(\mathrm{pH}\) drops steeply (from \(4.2\) to 2.2) over a small volume interval. So, any indicator with a color change in this pH interval will provide a fair approximation to the equivalence point. The closer the point of the color changes is to \(\mathrm{pH} 3.2\), the more accurate will be the end point.

Thus, the indicators that are close to the given \(\mathrm{pH}\) range are metacresol purple, quinaldine red and thymol blue.

Answer 2

ANSWERS-

• Thymol blue

Equivalence point pH must be within the pH range of indicators. Thymol blue has a pH range of 1.2 to 3.2 hence it is a suitable indicator.

Answer 3

This can be explained an the basis of purange of indicator. the plot has equivalence That mean indicator show point at þu=3.2 colour change at pu= 3.2 or greater than 3.2. Change to red and pu73.2 1. colourless At by <3.2 quinaldine red is pn 1.4 to 3.2 so Colourless tored so it can be used as an indicator. bh - 8.3 to 104 it remain ph=3.2 colourless at so it cannot be : Phenolphthalein used. bu=4.0-5.8 colourles to red it can also be used 1. 3. ethyl red it cannot be used. pk = 6:0 to 7.6 - 4. bromothymol blue No it cannot be used. 0,5 to 2.5 meta (resol purple No it can not be used 1.2 to 2.8 thymol blue can be used s ethyl red so quin aldine red

Answer 4

According to the indicator sheet provided in the appendix of the text, the correct answers would be...Thymol Blue, Metacresol Purple, and Quinaldine Red.

Answer 5

In titration, the volume of titrant required to reach the equivalence point is needed to determine the concentration of the unknown being titrated. One way to observe the equivalence point is to use an indicator that changes color near the equivalence point. Notice on the graph that the change in titrant volume (?$x$‑axis) between pH=4.2$\text{pH}=4.2$ and pH=2.2$\text{pH}=2.2$ is very small. Thus, the indicator does not need to change color at the exact equivalence point, just somewhere in the endpoint pH range.

In this case, the pH decreases as titrant is added. Thus, the upper end of the indicator pH range needs to be within the endpoint pH range. Phenolphthalein, bromothymol blue, and ethyl red would begin changing color too soon (above pH=4.2$\text{pH}=4.2$) and therefore are not appropriate for this titration. Of the given choices, quinaldine red, thymol blue, and metacresol purple would work.

For titrations involving a weak acid or base, the endpoint pH range is generally ±1 pH unit of the equivalence point. For strong acid and strong base titrations, the endpoint pH range can be as wide as ±3 pH units of the equivalence point.