Which is the chief cation of intracellular fluid for which maintenance of normal levels is essential to life of the cells?

Potassium

Explanation:

Potassium is the major cation of intracellular fluid and is involved in the normal functioning of neurons and muscles. Adequate intracellular levels of potassium are also required for many vital metabolic activities.

Nerve and muscle cells need to balance the intracellular potassium ion concentration as relative concentrations of potassium ions in intracellular and extracellular fluid regulate their resting membrane potential. Increased extracellular concentration of potassium depolarizes these cells while its reduced levels cause hyperpolarization making the nerve cells and muscle cells non-responsive.

Similarly, heart cells need to maintain the potassium levels as its imbalance disrupts the electric conduction in the heart and may even cause sudden death. Being a part of buffer system of cells, potassium ions maintain the pH of body fluids.