{ "id": "ep39nkxgtk", "slug": "vapor-pressure-osmometry", "title": "Vapor pressure osmometry", "updated": "2026-10-15", "topic_path": [ { "id": "physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/topics/physical" }, { "id": "physical.chemistry", "label": "Chemistry", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.chemistry" }, { "id": "physical.chemistry.methods", "label": "Chemical principles and methods", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.chemistry.methods" }, { "id": "physical.chemistry.methods.analytical-chemistry", "label": "Analytical chemistry", "api_url": "https://www.edgepedia.org/api/v1/topics/physical.chemistry.methods.analytical-chemistry" } ], "geo": [ { "id": "geo.nongeo.t1800.physical.chemistry.methods.analytical-chemistry", "label": "Non-geographic · 1800 to 1945: Analytical chemistry", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical.chemistry.methods.analytical-chemistry", "path": [ { "id": "geo.nongeo", "label": "Non-geographic", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo" }, { "id": "geo.nongeo.t1800", "label": "Non-geographic · 1800 to 1945", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800" }, { "id": "geo.nongeo.t1800.physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical" }, { "id": "geo.nongeo.t1800.physical.chemistry", "label": "Chemistry", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical.chemistry" }, { "id": "geo.nongeo.t1800.physical.chemistry.methods", "label": "Chemical principles and methods", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical.chemistry.methods" }, { "id": "geo.nongeo.t1800.physical.chemistry.methods.analytical-chemistry", "label": "Analytical chemistry", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1800.physical.chemistry.methods.analytical-chemistry" } ] }, { "id": "geo.nongeo.t1946.physical.chemistry.methods.analytical-chemistry", "label": "Non-geographic · 1946 to 2000: Analytical chemistry", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.chemistry.methods.analytical-chemistry", "path": [ { "id": "geo.nongeo", "label": "Non-geographic", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo" }, { "id": "geo.nongeo.t1946", "label": "Non-geographic · 1946 to 2000", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946" }, { "id": "geo.nongeo.t1946.physical", "label": "Physical world and mathematics", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical" }, { "id": "geo.nongeo.t1946.physical.chemistry", "label": "Chemistry", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.chemistry" }, { "id": "geo.nongeo.t1946.physical.chemistry.methods", "label": "Chemical principles and methods", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.chemistry.methods" }, { "id": "geo.nongeo.t1946.physical.chemistry.methods.analytical-chemistry", "label": "Analytical chemistry", "api_url": "https://www.edgepedia.org/api/v1/geo/geo.nongeo.t1946.physical.chemistry.methods.analytical-chemistry" } ] } ], "excerpt": "Vapor pressure osmometry (VPO) is an analytical technique that determines the number-average molecular weight of dissolved solutes, or osmolality, from the depression of the solvent's vapor pressure.", "snippet": "Vapor pressure osmometry (VPO) is an analytical technique that determines the number-average molecular weight of dissolved solutes, or osmolality, from the depression of the solvent's vapor pressure.", "node": "physical.chemistry.methods.analytical-chemistry", "markdown": "# Vapor pressure osmometry\n\nVapor pressure osmometry (VPO) is a colligative-property technique that determines the number-average molecular weight \\( M_{\\mathrm{n}} \\) of dissolved solutes, or the osmolality of a solution, from the depression of the solvent's vapor pressure above the solution compared with the pure solvent.[1](https://www.emeraldcloudlab.com/helpfiles/experimentmeasureosmolality)\n\nThe method belongs to the osmometric family of molecular-weight techniques, alongside membrane [osmometry](https://www.edgepedia.org/osmometry), end-group analysis, light scattering, and [viscometry](https://www.edgepedia.org/viscometry), but it uses no membrane, which distinguishes it from membrane osmometry.[2](https://www.degruyterbrill.com/document/doi/10.1515/9783112531846-005/html)\n\n| Key fact | Detail |\n|---|---|\n| What it measures | Vapor pressure depression of the solvent, related to osmolality by Raoult's law and, for polymers, to number-average molecular weight \\( M_{\\mathrm{n}} \\)[1](https://www.emeraldcloudlab.com/helpfiles/experimentmeasureosmolality) |\n| Molecular-weight range (thermistor instruments) | 100–25,000 Da in toluene and 100–5,000 Da in water (Model 833)[3](https://www.uicinc.com/wp-content/uploads/2019/11/Model-833-VPO.pdf); 40–50,000 g/mol in organic solvents and up to 5,000 g/mol in water (OSMOMAT 010)[4](https://labteamet.com/wp-content/uploads/labteamet/Gonotec/Technical-data-Gonotec-Osmomat.pdf) |\n| Upper limit (early thermocouple work) | Around 30,000 for polystyrene, with an accuracy of about 10%[5](https://doi.org/10.1002/recl.19640830205) |\n| Signal | Temperature (resistance) difference between two matched thermistors[3](https://www.uicinc.com/wp-content/uploads/2019/11/Model-833-VPO.pdf) • [4](https://labteamet.com/wp-content/uploads/labteamet/Gonotec/Technical-data-Gonotec-Osmomat.pdf), or dew-point temperature depression on a thermocouple[6](http://faculty.smcm.edu/wihatch/courses/436web/436resources/wescor_man5520.pdf) |\n| Measurement time | 2–5 minutes per sample (thermistor instruments)[3](https://www.uicinc.com/wp-content/uploads/2019/11/Model-833-VPO.pdf); 80–90 seconds (dew-point clinical instruments)[6](http://faculty.smcm.edu/wihatch/courses/436web/436resources/wescor_man5520.pdf) • [7](https://sanova.at/wp-content/uploads/2020/08/Sanova-RoutinePOC-Vapro5600-1.pdf) |\n| Osmolality range (VAPRO 5600) | Typically 20 to 3200 mmol/kg, resolution 1 mmol/kg[7](https://sanova.at/wp-content/uploads/2020/08/Sanova-RoutinePOC-Vapro5600-1.pdf) |\n| Principal restriction | Cannot measure solutions containing volatile solutes; lower precision than freezing-point osmometry[8](https://ncbi.nlm.nih.gov/books/NBK589659/) |\n\n## How it works\n\nThe thermodynamic basis is [Raoult's law](https://www.edgepedia.org/raoults-law), formulated in 1886: the vapor pressure of a solution containing a non-volatile solute is proportional to the mole fraction of the solvent, so dissolved solute molecules lower the solvent's vapor pressure in proportion to their number.[9](https://www.osti.gov/servlets/purl/2006587)\n\nIn a thermistor instrument, two matched thermistors are suspended in a thermostated chamber filled with saturated solvent vapor.[4](https://labteamet.com/wp-content/uploads/labteamet/Gonotec/Technical-data-Gonotec-Osmomat.pdf) Both carry a droplet of pure solvent and read the same temperature. When the solvent droplet on one probe is exchanged for a droplet of solution, the lower vapor pressure of the solvent above the solution drives condensation of vapor onto that droplet.[4](https://labteamet.com/wp-content/uploads/labteamet/Gonotec/Technical-data-Gonotec-Osmomat.pdf) The condensation releases latent heat and warms the droplet until its vapor pressure rises to that of the pure solvent, at which point condensation stops.[3](https://www.uicinc.com/wp-content/uploads/2019/11/Model-833-VPO.pdf) The steady temperature difference between the two probes changes the thermistor resistances, and the difference is read on a bridge circuit.[3](https://www.uicinc.com/wp-content/uploads/2019/11/Model-833-VPO.pdf) That difference is related to the molal concentration of the solution through a calibration curve,[3](https://www.uicinc.com/wp-content/uploads/2019/11/Model-833-VPO.pdf) and for polymers of known mass concentration it yields \\( M_{\\mathrm{n}} \\).[1](https://www.emeraldcloudlab.com/helpfiles/experimentmeasureosmolality)\n\n## How it is done\n\nA typical polymer measurement proceeds as follows. The chamber is thermostated (ambient to 130 °C on the Model 833) and saturated with the chosen solvent's vapor. A 0.25 mL sample volume is used; minimum measurable concentrations are 5 × 10⁻⁵ mol/L in toluene and 2.5 × 10⁻⁴ mol/L in water.[3](https://www.uicinc.com/wp-content/uploads/2019/11/Model-833-VPO.pdf) Solutions are prepared at several concentrations, droplets are placed on the probes, and the bridge reading is recorded after equilibration, typically 2–5 minutes per sample.[3](https://www.uicinc.com/wp-content/uploads/2019/11/Model-833-VPO.pdf) Readings are extrapolated to infinite dilution and converted to \\( M_{\\mathrm{n}} \\) with a calibration factor established from standards of known molecular weight.[10](https://doi.org/10.1002/app.1977.070210210) In clinical use, a full calibration is required at least every 6 months.[8](https://ncbi.nlm.nih.gov/books/NBK589659/)\n\nThe dew-point variant uses a different procedure: a 10 µL sample is pipetted onto a small, solute-free paper disc, sealed into a sample chamber, and the thermocouple is cooled by the Peltier effect below the dew point so that water condenses on it.[6](http://faculty.smcm.edu/wihatch/courses/436web/436resources/wescor_man5520.pdf) [Latent heat](https://www.edgepedia.org/latent-heat) then warms the thermocouple asymptotically toward the dew point,[1](https://www.emeraldcloudlab.com/helpfiles/experimentmeasureosmolality) and the dew-point temperature depression, measured with a resolution of 0.00031 °C, gives the osmolality in an 80-second microprocessor-controlled cycle.[6](http://faculty.smcm.edu/wihatch/courses/436web/436resources/wescor_man5520.pdf)\n\n## Origin\n\nThe thermoelectric vapor phase osmometer was described by J. van Dam in 1964 in *Recueil des Travaux Chimiques des Pays-Bas*; in it, the vapor pressure difference between a solution and the pure solvent is converted into a temperature difference between thermocouple junctions, and the steady-state difference is calculated from heat and mass transfer.[5](https://doi.org/10.1002/recl.19640830205) An earlier thermal method of measuring the vapor pressure of an aqueous solution was published by Archibald Vivian Hill in 1930 in *Proceedings of the Royal Society A*.[11](https://doi.org/10.1098/rspa.1930.0041) Further thermoelectric vapor-pressure osmometry was reported by Arnold Adicoff and Warren J. Murbach in *Analytical Chemistry* in 1967,[12](https://doi.org/10.1021/ac60247a002) and an apparatus for molecular-weight determination on highly diluted (10⁻⁴ M) solutions was described by R. E. Dohner, A. H. Wachter, and W. Simon in *Helvetica Chimica Acta*, also in 1967.[13](https://doi.org/10.1002/hlca.19670500803)\n\nLater work refined the calibration theory: Bruce H. Bersted examined the solute dependence of the calibration constant for high polymers (1973);[14](https://doi.org/10.1002/app.1973.070170509) Kenji Kamide, Toshikazu Terakawa, and Hideo Uchiki reported molecular weight determination of macromolecules by vapor pressure osmometry (1976);[15](https://doi.org/10.1002/macp.1976.021770517) C. E. M. Morris examined drop size, solute volatility, and calibration-factor constancy (1977);[10](https://doi.org/10.1002/app.1977.070210210) Kwang E. Chung, Larry L. Anderson, and Wendell H. Wiser proposed a procedure distinguishing a rectilinear from a curved region of the response (1979);[16](https://www.sciencedirect.com/science/article/abs/pii/0016236179901996) Marianne Marx-Figini and Rubén Victor Figini considered the calibration function (1980);[17](https://doi.org/10.1002/macp.1980.021811118) and L. Mrkvičaková and S. Pokorny examined the reliability of molecular weight determination by vapor pressure osmometry (1985).[18](https://doi.org/10.1002/app.1985.070300325)\n\n## Variants\n\nThermistor VPO, used for polymer \\( M_{\\mathrm{n}} \\) in organic solvents and water, relies on the condensation-heating mechanism described above.[3](https://www.uicinc.com/wp-content/uploads/2019/11/Model-833-VPO.pdf) Dew-point VPO, exemplified by the Wescor Vapro[6](http://faculty.smcm.edu/wihatch/courses/436web/436resources/wescor_man5520.pdf) and Gonotec VAPRO[19](https://www.gonotec.de/en/en_vapro/) clinical osmometers, uses a fine-wire thermocouple hygrometer cooled by the Peltier effect; because the sample is never frozen or boiled, the measurement avoids artifacts that occur when the specimen must be altered physically.[6](http://faculty.smcm.edu/wihatch/courses/436web/436resources/wescor_man5520.pdf) A named research variant, controlled partial pressure-vapor pressure osmometry (CPP-VPO), was applied by M. Ueda and Z. A. Schelly to AOT reverse micellar systems to determine the mean aggregation number and water vapor pressure.[20](https://doi.org/10.1021/la00081a026)\n\n## Applications\n\nThe classic application is the determination of \\( M_{\\mathrm{n}} \\) for polymers and oligomers in solution.[1](https://www.emeraldcloudlab.com/helpfiles/experimentmeasureosmolality) Clinical and bioprocess laboratories use dew-point osmometers for osmolality, with the VAPRO 5600 covering 20 to 3200 mmol/kg (up to 3600 mmol/kg extended) in 90 seconds.[7](https://sanova.at/wp-content/uploads/2020/08/Sanova-RoutinePOC-Vapro5600-1.pdf) For high-concentration protein formulations and formulations containing sugars or highly water-soluble polymers, vapor-pressure-based osmometers appear to provide a more reliable alternative to freezing-point-based osmometers, although both types give satisfactory results at low concentrations.[21](https://www.bioprocessintl.com/formulation/osmolality-measurements-for-high-concentration-protein-polymer-solutions-variation-based-on-working-principles-of-osmometers)\n\n## Limitations and alternatives\n\nVPO fails for volatile solutes: solutes with vapor pressures as low as 0.3 mm Hg under the operating conditions give unsatisfactory results,[10](https://doi.org/10.1002/app.1977.070210210) and clinical guidance states that vapor pressure osmometers cannot measure solutions containing volatile solutes.[8](https://ncbi.nlm.nih.gov/books/NBK589659/) [Calibration](https://www.edgepedia.org/calibration) is empirical and solute-dependent, and there is also a drop size effect dependent on solute concentration.[10](https://doi.org/10.1002/app.1977.070210210)\n\nPrecision is lower than that of freezing-point depression osmometry.[8](https://ncbi.nlm.nih.gov/books/NBK589659/) Compared with direct membrane osmometry, VPO and freezing-point osmometry consistently give higher osmotic pressure values for concentrated solutions, whether the solutes are small molecules, carbonated solutions, or polymers.[9](https://www.osti.gov/servlets/purl/2006587) Most VPO instruments are not isopiestic, giving low accuracy at low osmotic pressure because osmotic pressure depends logarithmically on activity.[9](https://www.osti.gov/servlets/purl/2006587)\n\nPublished upper molecular-weight limits differ: van Dam reported an upper limit around 30,000 with about 10% accuracy for polystyrene,[5](https://doi.org/10.1002/recl.19640830205) while the OSMOMAT 010 documentation gives 40 to 50,000 g/mol in organic solvents;[4](https://labteamet.com/wp-content/uploads/labteamet/Gonotec/Technical-data-Gonotec-Osmomat.pdf) no single figure is settled by the published literature. For polyelectrolytes, size-exclusion chromatography carries significant uncertainty from chain–column interactions and reliance on neutral polymer standards, and a proof-of-concept in *Analytical Chemistry* introduced a calibration-free LSPR kinetics method for polyelectrolyte \\( M_{\\mathrm{n}} \\), calculating the \\( M_{\\mathrm{n}} \\) of PAZO as 257,400 g mol⁻¹ (degree of polymerization 642) from the known \\( M_{\\mathrm{n}} \\) of poly(ethylenimine).[22](https://pubs.acs.org/doi/full/10.1021/acs.analchem.5c05449)\n\n## References\n\n1. [MeasureOsmolality protocol documentation (Emerald Cloud Lab)](https://www.emeraldcloudlab.com/helpfiles/experimentmeasureosmolality)\n2. [Determination of Number-average Molecular Weight of Polymers by Osmotic Measurement, Vapour-pressure Osmosis (Schröder, Müller, Arndt, Polymer Characterization)](https://www.degruyterbrill.com/document/doi/10.1515/9783112531846-005/html)\n3. [Model 833 Vapor Pressure Osmometer (UIC, manufacturer technical note)](https://www.uicinc.com/wp-content/uploads/2019/11/Model-833-VPO.pdf)\n4. [Gonotec OSMOMAT 010 technical data](https://labteamet.com/wp-content/uploads/labteamet/Gonotec/Technical-data-Gonotec-Osmomat.pdf)\n5. [J. van Dam (1964). Determination of molecular weights by means of thermoelectric vapour phase osmometry. Recueil des Travaux Chimiques des Pays-Bas.](https://doi.org/10.1002/recl.19640830205)\n6. [Wescor Vapro 5520 vapor pressure osmometer manual](http://faculty.smcm.edu/wihatch/courses/436web/436resources/wescor_man5520.pdf)\n7. [VAPRO 5600 specifications (ELITech/Gonotec distributor datasheet)](https://sanova.at/wp-content/uploads/2020/08/Sanova-RoutinePOC-Vapro5600-1.pdf)\n8. [Osmometer - StatPearls (NCBI Bookshelf)](https://ncbi.nlm.nih.gov/books/NBK589659/)\n9. [Comparison of vapour pressure, freezing point, and direct membrane osmometry for aqueous solutions (OSTI report)](https://www.osti.gov/servlets/purl/2006587)\n10. [C. E. M. Morris (1977). Aspects of vapor pressure osmometry. Journal of Applied Polymer Science.](https://doi.org/10.1002/app.1977.070210210)\n11. [Archibald Vivian Hill (1930). A thermal method of measuring the vapour pressure of an aqueous solution. Proceedings of the Royal Society of London Series A Containing Papers of a Mathematical and Physical Character.](https://doi.org/10.1098/rspa.1930.0041)\n12. [Arnold. Adicoff, Warren J. Murbach (1967). Thermoelectric vapor-pressure osmometry. Analytical Chemistry.](https://doi.org/10.1021/ac60247a002)\n13. [R. E. Dohner, A. H. Wachter, W. Simon (1967). Apparatur zur Molekulargewichtsbestimmung an hochverdünnten (10−4M). Lösungen mittels Dampfdruckosmometrie. Helvetica Chimica Acta.](https://doi.org/10.1002/hlca.19670500803)\n14. [Bruce H. Bersted (1973). Molecular weight determination of high polymers by means of vapor pressure osmometry and the solute dependence of the constant of calibration. Journal of Applied Polymer Science.](https://doi.org/10.1002/app.1973.070170509)\n15. [Kenji Kamide, Toshikazu Terakawa, Hideo Uchiki (1976). Molecular weight determination of macromolecules by vapor pressure osmometry. Die Makromolekulare Chemie.](https://doi.org/10.1002/macp.1976.021770517)\n16. [New procedure for molecular-weight determination by vapour-phase osmometry (Anderson, 1979, Fuel)](https://www.sciencedirect.com/science/article/abs/pii/0016236179901996)\n17. [Marianne Marx‐Figini, Rubén Victor Figini (1980). On the molecular weight determination by vapour pressure osmometry, 1. Consideration of the calibration function. Die Makromolekulare Chemie.](https://doi.org/10.1002/macp.1980.021811118)\n18. [L. Mrkvičaková, S. Pokorny (1985). On the reliability of molecular weight determination by vapor pressure osmometry. Journal of Applied Polymer Science.](https://doi.org/10.1002/app.1985.070300325)\n19. [Gonotec VAPRO osmometer product page](https://www.gonotec.de/en/en_vapro/)\n20. [M. Ueda, Z. A. Schelly (1988). Mean aggregation number and water vapor pressure of AOT reverse micellar systems determined by controlled partial pressure-vapor pressure osmometry (CPP-VPO). Langmuir.](https://doi.org/10.1021/la00081a026)\n21. [Osmolality Measurements for High-Concentration Protein–Polymer Solutions: Variation Based on Working Principles of Osmometers (BioProcess International)](https://www.bioprocessintl.com/formulation/osmolality-measurements-for-high-concentration-protein-polymer-solutions-variation-based-on-working-principles-of-osmometers)\n22. [An LSPR-Based Kinetic Framework for Polyelectrolyte Molecular Weight Determination: A Proof-of-Concept Study (Analytical Chemistry, 2025)](https://pubs.acs.org/doi/full/10.1021/acs.analchem.5c05449)\n\n---\n*Topic: Encyclopedia › Physical world and mathematics › Chemistry › Chemical principles and methods › Analytical chemistry*\n\n*Initially written Sep 29, 2026 · Edited: Oct 15, 2026*\n\n*Copyright 2026 EdgeChat AI, a subsidiary of Biostate AI.*\n\nLicense: Edgepedia Community License 1.0, https://www.edgepedia.org/edgepedia/license\n", "same_as": [], "url": "https://www.edgepedia.org/vapor-pressure-osmometry", "markdown_url": "https://www.edgepedia.org/vapor-pressure-osmometry.md", "license": { "name": "Edgepedia Community License 1.0", "url": "https://www.edgepedia.org/edgepedia/license", "summary": "Free with credit, commercial use included. AI training is open to everyone. For other uses, organizations over USD 100M in revenue or 100M monthly users license separately.", "spdx": "LicenseRef-Edgepedia-Community-1.0" }, "credit": "\"Vapor pressure osmometry\", Edgepedia (EdgeChat), https://www.edgepedia.org/vapor-pressure-osmometry. Edgepedia Community License 1.0.", "credit_md": "\"[Vapor pressure osmometry](https://www.edgepedia.org/vapor-pressure-osmometry)\", Edgepedia (EdgeChat), [https://www.edgepedia.org/vapor-pressure-osmometry](https://www.edgepedia.org/vapor-pressure-osmometry). [Edgepedia Community License 1.0](https://www.edgepedia.org/edgepedia/license).", "credit_html": "\"Vapor pressure osmometry\", Edgepedia (EdgeChat), https://www.edgepedia.org/vapor-pressure-osmometry. Edgepedia Community License 1.0.", "speakable": "Vapor pressure osmometry is an analytical technique that determines the number-average molecular weight of dissolved solutes, or osmolality, from the depression of the solvent's vapor pressure." }