What is the counterion of sodium in extracellular fluid?

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Sodium is the main cation in extracellular fluid, and electroneutrality means there must be balancing negative charges. The primary anion that balances Na+ in this compartment is chloride, forming the abundant NaCl component of plasma. While bicarbonate is also a significant extracellular anion and participates in acid–base balance, it is not the main counterion for sodium under normal conditions. Potassium and magnesium are cations (not anions) and are more prominent in other contexts (potassium mainly intracellular). So the best answer is chloride.