Which anion counterbalances sodium in the extracellular space?

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In the extracellular fluid, sodium is the predominant positively charged ion, and electroneutrality is kept by the accompanying negative ions. The main anion that counterbalances sodium in this space is chloride, which together with sodium helps maintain charge balance and osmotic stability in the plasma and interstitial fluid. Bicarbonate also contributes as an extracellular anion for buffering, but chloride is the primary counterion to sodium. Potassium and magnesium are cations, mostly inside cells, so they don’t serve as the counterbalance to extracellular sodium.