Question

For testing alkalinity for a water sample, first phenolphthalein indicator is added. The water remains colorless. However, when a few drops of methyl orange are added to the sample, the color turns yellow. As per these observations, the correct choice is:

• A. Absence of CO$_3^{2-}$ and/or HCO$_3^-$ but the presence of OH$^-$ ions in the sample
• B. Presence of CO$_3^{2-}$ and/or HCO$_3^-$ but the absence of OH$^-$ ions in the sample
• C. Absence of CO$_3^{2-}$, HCO$_3^-$ and OH$^-$ ions in the sample
• D. Presence of CO$_3^{2-}$, HCO$_3^-$ and OH$^-$ ions in the sample

Hint:

Phenolphthalein detects OH$^-$ and CO$_3^{2-}$ strongly; methyl orange detects all alkalinity (OH$^-$, CO$_3^{2-}$, HCO$_3^-$). If phenolphthalein is colorless but methyl orange shows yellow, the alkalinity is due only to CO$_3^{2-}$ or HCO$_3^-$.

Answer 1

The Correct Option is B

Step 1: Recall behavior of phenolphthalein.
Phenolphthalein turns pink in basic solutions (pH>8.3). If the solution remains colorless, it means OH$^-$ ions are absent.
Step 2: Recall behavior of methyl orange.
Methyl orange turns yellow in basic solutions (pH>4.4). Since the solution turns yellow, the sample must be basic in nature, but not strongly alkaline.
Step 3: Link with carbonate/bicarbonate ions.
If phenolphthalein gives no color (absence of OH$^-$), but methyl orange turns yellow (basicity present), then the alkalinity is due to CO$_3^{2-}$ or HCO$_3^-$. Thus, OH$^-$ ions are absent, but carbonate/bicarbonate ions are present. Final Answer: \[ \boxed{\text{Presence of CO$_3^{2-}$ and/or HCO$_3^-$ but the absence of OH$^-$ ions in the sample.}} \]