a)For the titration of an acid analyte with a NaOH titrant, a phenolphthalein endpoint will change from colorless to pink. If the "pink" resulted from perhaps only a half-drop(or less) of NaOH titrant and the Erlenmeyer flask is set aside on the laboratory bench, the pink color may disappear after an hour or so. Explain.
b) Explain why it is quantitatively not acceptable to titrate each of the vinegar samples with the NaOH titrant to the same dark pink endpoint.
a) My experience is that it doesn't take an hour or so but the pink color disappears after a VERY short time. The reason is absorption of CO2 from the air which produces an acid.
b) You probably should have explained a little better since we don't know what you're doing experimentally; however, I assume the question is asking why not titrate to a DARK pink end point. The reason is that the eye is more sensitive to the faintest pink visible (from colorless) versus the intensity or hue of a darker pink color.
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a) The pink color produced during titration with phenolphthalein is due to the presence of excess hydroxide ions (OH-) from the NaOH titrant. Phenolphthalein is a pH indicator that changes color in the pH range of around 8.2 to 10, which corresponds to the basic region.
When a small amount of NaOH titrant is added to the acid analyte, it neutralizes a part of the acid, resulting in the formation of water and a salt. However, since only a half-drop (or less) of NaOH titrant is added, there may not be sufficient hydroxide ions to completely convert all the acid to water and salt. As a result, the pH of the solution may only partially reach the basic region, causing the pink color to appear.
Over time, the pink color may disappear because the excess hydroxide ions that were responsible for the color will react with carbon dioxide from the air, forming bicarbonate ions. This reaction is known as the absorption of carbon dioxide, which gradually reduces the concentration of hydroxide ions in the solution. As the concentration decreases, the pH returns to a level below the range where phenolphthalein shows a pink color, and therefore, the pink color fades away.
b) Titrating each vinegar sample to the same dark pink endpoint would not be quantitatively acceptable because it indicates that each sample has the same amount of acetic acid, which may not be true. Vinegar samples can vary in their acetic acid concentration, as it depends on factors like the brand, manufacturing process, or storage conditions.
When titrating with NaOH, the goal is to determine the concentration of acetic acid in the vinegar. The dark pink endpoint signifies that all the acetic acid has reacted with the NaOH titrant. If different vinegar samples with varying concentrations of acetic acid were titrated to the same dark pink endpoint, the calculated values would not accurately reflect the actual acetic acid content.
To obtain accurate and quantitative results, it is essential to perform a proper calibration. This can be achieved by preparing a standardized solution of acetic acid, determining its concentration accurately through other means (e.g., mass measurement or a certified reference material), and titrating it with the NaOH titrant. This calibration curve can then be used to determine the concentration of acetic acid in the vinegar samples.