In general chemistry, you used phenolphthalein to indicate the end point of a ti
ID: 822744 • Letter: I
Question
In general chemistry, you used phenolphthalein to indicate the end point of a titration of an acid with a base. This indicator is colorless in its acidic form and pink in its basic form. Imagine, instead, that you needed to choose an indicator for the titration of a strong base (like NaOH) with a strong acid (like HNO3). You have the indicators in Table 1 available. Which indicator would you choose? Why?Table 1:
HIn In- pKa
Red red blue 4.3
Yellow red yellow 7.4
Purple blue red 10.5
In general chemistry, you used phenolphthalein to indicate the end point of a titration of an acid with a base. This indicator is colorless in its acidic form and pink in its basic form. Imagine, instead, that you needed to choose an indicator for the titration of a strong base (like NaOH) with a strong acid (like HNO3). You have the indicators in Table 1 available. Which indicator would you choose? Why?
Table 1:
HIn In- pKa
Red red blue 4.3
Yellow red yellow 7.4
Purple blue red 10.5
In general chemistry, you used phenolphthalein to indicate the end point of a titration of an acid with a base. This indicator is colorless in its acidic form and pink in its basic form. Imagine, instead, that you needed to choose an indicator for the titration of a strong base (like NaOH) with a strong acid (like HNO3). You have the indicators in Table 1 available. Which indicator would you choose? Why?
Table 1:
HIn In- pKa
Red red blue 4.3
Yellow red yellow 7.4
Purple blue red 10.5
Explanation / Answer
For a strong acid-strong base titration, a neutral salt is formed:
e.g. NaOH(aq) + HNO3(aq) => NaNO3(aq) + H2O(l)
The ions of the salt (Na+ and NO3-) do not ionize further in water.
Thus the pH at the equivalence point = pH of pure water = 7.0
The ideal indicator for the titration would be the one whose pKa value is closest to the equivalence point pH of 7.0
From the Henderson-Hasselbalch equation:
pH = pKa + log([In-]/[HIn]
If pH = pKa => log([In-]/[HIn] => [In-] = [HIn]
=> indicator will change color at this point as it converts from one form to another in majority
=> end point of titration will correspond to equivalence point in reaction
Thus the best indcator is Yellow indicator with pKa = 7.4
=> color changes from yellow (basic In-) to red (acidic HIn) at end point