What is a "common-ion effect", and how does it affect the molar solubility of a salt?
Why can either phenolphtalein or methyl orange be used for an HCl-NaOH titration, but only phenophtalein is suitable for an acetic acid-NaOH titration?
The answer to your first question is answered in your first post "To DrBob." The second one is hard to explain without a graph but I'll try. Here is what you need to do. On a sheet of paper, make a rudimentary set of y and x axes like so.
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Now, on the y axis, mark the origin as zero, and put dashes at 1, 2, 3, 4, 5, etc until you get to 14. Label this axis as pH. On the x axis, mark off units of 10 such as 10, 20, 30, 40, 50, etc until you get to 100. The x axis is marked as volume. The completed "incomplete" graph would look somthing like this.
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10 pH
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0-10-20-30-40-50-60-70-80-90-100
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Now. Starting at 0 mL and pH 1 draw a rather straight line more or less parallel to the x axis until you get to 50 mL. Make a smooth curve there, upward and continue parallel to the y axis until you get to a pH of 13, make another smooth curve downward (to the right) and continue the line parallel to the x axis. Label that curve the HCl vs NaOH curve. For the acetic acid/NaOH curve, Start at volume of zero mL and a pH of about 3.5, mark a point at 4 mL about pH = 4, mark a point at 45 mL and a pH = 5, mark a point at 49.5 mL and pH = 6, at 50 mL a pH = 9, at 50.5 mL a pH =11 55 mL a pH = 11.5 and at 100 mL pH = 12.5 . I haven't calculated these; rather I have guessed from memory. Connect these dots and label it the curve for acetic acid and NaOH. Now lets look where the indicators change color. Draw a band on the curve covering pH = 3.1 to 4.4 and label that methyl orange. Draw a band on the curve covering pH = 8.3 to 10.0. Label that phenolphthalein. With due respect for the crude way in which this was done, with estimating the volume/pH readings, and your drawing skills for a guesstimated graph, do you see that the vertical part of the curve for the HCl/NaOH starts about 4 or so and continues vertically until we get to about pH = 11 or so. That vertical portion covers both the area in which methyl orange changes as well as the area in which phenolphathalein changes. BUT that is not so for the acetic acid/NaOH curve. The methyl orange curve starts changing long before the vertical part of the curve so it is a slow slow slow change over and one would not be able to see a sharp change with just one or two drops of a titrant. Phenolphthalein, however, changes where the acetic acid/NaOH titration curve is vertical and there the change will be very sharp. We want titrations to be sharp, the end point must change within a drop or two (a half-drop is even better). Most indicators have a range of approximately 2 pH units from one color to the other color and we want that 2 pH unit change to be in the region where the titration curve is changing rapidly; i.e., where the pH is changing at least two pH units per 1 drop of titrant. I hope this helps. Let me know if I need to address any part of this. It would be better if I could have drawn a graph but perhaps this will do. I hope so.
A common-ion effect is the effect of the addition of a common ion, which is an ion that is already present in the solution, on the solubility of a salt. When a salt is dissolved in water, it dissociates into its constituent ions. The solubility of a salt is determined by the equilibrium between the dissolved ions and the undissolved salt.
Adding a common ion to the solution reduces the solubility of the salt. This is because the common ion increases the concentration of one of the ions involved in the dissociation of the salt, which shifts the equilibrium towards the undissolved salt. This effectively decreases the molar solubility of the salt.
For example, let's consider the solubility of silver chloride (AgCl) in water. When AgCl dissolves in water, it dissociates into Ag+ and Cl- ions. If we add a solution of sodium chloride (NaCl), which dissociates into Na+ and Cl- ions, to the AgCl solution, the concentration of Cl- ions in the solution increases due to the common ion (Cl-). This increase in Cl- concentration shifts the equilibrium towards the undissolved AgCl, reducing its solubility.
In the phenolphthalein-HCl-NaOH titration and acetic acid-NaOH titration, different indicators are used because of the different pH ranges of their color changes. Phenolphthalein changes color in the pH range of around 8.2 to 10, while methyl orange changes color in the pH range of around 3.1 to 4.4.
In the HCl-NaOH titration, the pH range where phenolphthalein changes color overlaps with the vertical part of the titration curve, where the pH changes rapidly. This allows for a sharp color change at the equivalence point, making phenolphthalein suitable for this titration.
On the other hand, in the acetic acid-NaOH titration, the pH range where methyl orange changes color does not overlap with the vertical part of the titration curve. This means that the color change with methyl orange would be too slow and gradual, making it difficult to accurately determine the equivalence point. Phenolphthalein, with its color change in the vertical region of the acetic acid-NaOH titration curve, provides a sharp and easily detectable color change, making it the suitable indicator for this titration.
I hope this clarifies the concepts for you. Let me know if you have any further questions!