Considerations for osmolality measurement under elevated pCO2: Comparison of vapor pressure and freezing point osmometry

Albert E. Schmelzer, Vivian M. DeZengotita, William M. Miller*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

22 Scopus citations


Osmolality increases with pCO2 in bioreactors with pH control, and it has been shown that osmolality compensation by decreasing the basal NaCl concentration partially mitigates the adverse effects of elevated pCO2 on animal cell growth, protein production, and glycosylation. Thus, measurement of osmolality is important for a complete characterization of the culture environment under elevated pCO2. However, osmolality measurement may be compromised by CO2 evolution. Freezing point depression and vapor pressure depression osmometry were directly compared for the measurement of osmolality in samples at elevated pCO2 (up to 250 mmHg) and at a variety of pH values (6.7-7.5). More extensive degassing may be expected with the vapor pressure osmometer due to the smaller sample volume and larger surface area employed. However, both types of osmometer yielded similar results for all pCO2 and pH values studied. Moreover, the measured values agreed with osmolality values calculated using a semi-empirical model. Further analysis showed that, while sample degassing may result in a large decrease in pCO2, there is little associated decrease in osmolality. The great majority of total CO2 in solution is present as bicarbonate (HCO3-). Although a small amount of HCO3- is converted to CO2 to compensate for CO2 evolution, further depletion of HCO3- is inhibited by the associated increase in medium pH and by the need for HCO3- to maintain charge neutrality in solution. This explanation is consistent with the observed similarity in osmolality values for the two types of osmometer. It was also observed that osmolality did not change in samples that were frozen at -20°C for up to 1 year.

Original languageEnglish (US)
Pages (from-to)189-196
Number of pages8
JournalBiotechnology and Bioengineering
Issue number2
StatePublished - Jan 20 2000
Externally publishedYes


  • Bicarbonate
  • CO
  • Carbon dioxide
  • Freezing point osmometry
  • Vapor pressure osmometry

ASJC Scopus subject areas

  • Applied Microbiology and Biotechnology
  • Bioengineering
  • Biotechnology


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