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The Difference Between Actual and Calculated Osmolality of IU Solutions Should Not Be Overlooked

Williams, E. Lynne MB, BS, FRCA

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doi: 10.1213/00000539-199912000-00072
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Thank you for the opportunity of replying to Dr. Swank. He is correct in pointing out that the measured osmolality of a 0.9% sodium chloride solution (or normal saline) is less than the calculated 308 mOsm/kg. We verified this by measuring the osmolality with the microsomometer (model 3MO) manufactured by Advanced Instruments, Inc., (Norwood, MA) which uses the depression of freezing point to determine osmolality. This demonstrated the osmolality of 0.9% sodium chloride to be 286 mOsm/kg, which implies that sodium chloride is not completely ionized at this concentration.

E. Lynne Williams MB, BS, FRCA

© 1999 International Anesthesia Research Society