Why is methyl orange indicator used instead of phenolpthalein when determing calcium content in a chalk by back titration
Back titration with what? Probably because MO indicator changes color approximately 4 while phenolphthalein changes color approximately 8.3. If you are titrating CO3^2- with acid, using phenolphthalein the end point is not as sharp AND it titrates only 1 H^+ while the MO is a sharper end point and it titrates two Hs.
Methyl orange indicator is often used instead of phenolphthalein when determining calcium content in a chalk by back titration due to several reasons.
Firstly, let's understand the principle behind back titration. Back titration involves the reaction of an excess known solution (reagent) with the analyte (the substance being analyzed) followed by the titration of the excess reagent with a standard solution (titrant) to determine the amount of excess reagent that reacted with the analyte.
In the case of determining calcium content in a chalk, the excess reagent is usually hydrochloric acid (HCl), which reacts with the calcium carbonate (CaCO3) present in the chalk to form calcium chloride (CaCl2), water (H2O), and carbon dioxide (CO2).
Now, let's discuss the differences between methyl orange and phenolphthalein indicators and why methyl orange is preferred in this specific scenario:
1. pH Range: Methyl orange has a pH range between 3.1 and 4.4, whereas phenolphthalein has a pH range between 8.2 and 10.0. In the back titration of calcium carbonate with hydrochloric acid, the reaction produces carbon dioxide gas. This causes the solution to be acidic, and the pH decreases. Therefore, methyl orange, which has a lower pH range, is more suitable for detecting the endpoint of the reaction.
2. Color Change: Methyl orange changes color from red to yellow (acidic solution) around a pH of 3.7, while phenolphthalein changes color from colorless to pink (basic solution) around a pH of 8.2. Since the reaction in back titration results in an acidic condition, the color change of methyl orange is more distinct and easily noticeable than that of phenolphthalein.
By using methyl orange as an indicator, the endpoint of the reaction (when all the calcium carbonate has reacted with hydrochloric acid) can be accurately determined, allowing for precise calculations of the calcium content in the chalk sample.
So, in summary, methyl orange is preferred over phenolphthalein in determining calcium content in a chalk sample by back titration due to its suitable pH range and distinct color change under acidic conditions.