Posted by Kizner on .
Why can either phenolphthalein or methyl orange be used for an HCl-NaOH titration, but only phnolphthalein is suitable for an acetic acid-NaOH titration?
Sol: for a while,I misunderstood the question and suggested that Phenol. has a wider array of pH. But then both can be used for and HCl-NaOH. I can't see any other reason but to compare the solution changes (-1<pKa>+1.
P.S. I found that the pH of methyl orange ranges from 3.1 - 4.4 pH whereas, phenol. is 8.2 - 10. I know that in acidic solution, Phenol is colorless, so I imagine phenolphthalein is more appropriate for HCl-NaOH titration because it will change color. It's perfect for this type of titration??
Your reasoning is incorrect. Did you not see the post in which I answered this question. It is the one involving how to draw a graph to show the points on the titration curve for acetic acid vs NaOH and HCl vs NaOH and where the indicators change in relation to the end point for each titration. I can't find it now but look through your notes. It may have been another student but I think you were the one. If you can find it then tell me what you don't understand about my instructions for drawing the graph and perhaps we can get through another one together. By the way, phenolphthalein changes from approximately 8.3 to approximately 10 or about 9 as a mid-point. The equivalence point for HCl vs NaOH is 7.0 and the equivalence point for acetic acid vs NaOH is close to 9. The range for methyl orange you found is ok.