Osmolarity and pH of 0.9% Normal Saline
0.9% normal saline has an osmolarity of 308 mOsm/L and a pH of approximately 5.5.
Osmolarity
Normal saline (0.9% NaCl) has an osmolarity of 308 mOsm/L, making it isotonic compared to plasma (275-295 mOsm/L). 1
This isotonic property is why 0.9% saline is considered the crystalloid of choice in patients with traumatic brain injury or acute brain injury, where hypotonic solutions must be avoided to prevent cerebral edema. 2, 1
Solutions are considered isotonic when their osmolarity ranges from 280 to 310 mOsm/L, and 0.9% NaCl falls within this range. 2
pH Characteristics
Commercial 0.9% saline solution has a pH around 5.5, making it significantly acidic. 3
The acidity of the saline solution itself is essentially unrelated to the metabolic acidemia that can complicate large-volume saline infusion. 3
The metabolic acidosis associated with normal saline administration is primarily due to its supraphysiologic chloride content (154 mEq/L) rather than its acidic pH, which can impair renal function and coagulation when given in large volumes. 1
Clinical Implications
For brain-injured patients, 0.9% saline is the recommended isotonic crystalloid specifically because of its osmolarity of 308 mOsm/L. 1
In contrast, Lactated Ringer's solution has an osmolarity of 273-277 mOsm/L, making it hypotonic and contraindicated in severe head trauma due to risk of worsening cerebral edema. 1
When managing hyperosmolar states, the corrected sodium level (not the pH) determines fluid selection—if corrected sodium is low, 0.9% NaCl should be used; if normal or elevated, 0.45% NaCl is preferred. 4
Common Pitfalls
Do not assume that the acidic pH of 0.9% saline (5.5) directly causes the hyperchloremic metabolic acidosis seen with large-volume resuscitation—these are separate phenomena. 3
The term "normal" saline is a historical misnomer with little scientific basis, as its composition (154 mEq/L each of sodium and chloride) does not match physiologic plasma concentrations. 5
Despite its limitations, 0.9% saline remains the preferred crystalloid in specific scenarios requiring isotonic fluid, particularly acute brain injury where maintaining osmolarity is critical. 2, 1