Here's why:
* Reaction: The reaction between sulfuric acid (H₂SO₄) and sodium carbonate (Na₂CO₃) produces sodium sulfate (Na₂SO₄), water (H₂O), and carbon dioxide (CO₂):
H₂SO₄ (aq) + Na₂CO₃ (aq) → Na₂SO₄ (aq) + H₂O (l) + CO₂ (g)
* Equivalence Point: The equivalence point of this titration is reached when the moles of H₂SO₄ are equal to the moles of Na₂CO₃. This point is slightly acidic due to the presence of the weak acid, carbonic acid (H₂CO₃), formed by the reaction of CO₂ with water.
* Indicator Choice: Methyl orange is a suitable indicator because its color change occurs at a pH close to the equivalence point of the titration. It changes color from yellow in basic solutions to red in acidic solutions. The endpoint (observed color change) will occur when the solution becomes slightly acidic, which closely approximates the equivalence point.
Other Indicators:
While methyl orange is the most common, other indicators can be used, but they may not be as precise:
* Phenolphthalein: This indicator changes color from colorless in acidic solutions to pink in basic solutions. However, it changes color at a higher pH than methyl orange, and therefore may not be as accurate.
* Bromocresol Green: This indicator changes from blue in basic solutions to yellow in acidic solutions. It changes color at a lower pH than methyl orange, potentially causing early endpoint detection.
In summary: Methyl orange is the most suitable indicator for standardizing sulfuric acid against sodium carbonate because its color change is close to the equivalence point of the titration.
