Freezing-point osmometry

Whole blood and urine pH and Pco2 are measured quantitatively by an electrode blood-gas system and HCO3 concentrations are calculated. Plasma and urine Na+ and K+ concentrations are measured by flame photometry and CP concentrations by electrotitration. Plasma total protein concentrations and urine specific gravity are measured by refractometry, and hematocrits are determined by a microcapillary reader. Urine osmolalities are determined by freezing-point osmometry. Plasma creatinine concentrations are determined by the Jaffe method without deproteiniza-tion. 

Abel JE. The physical background to freezing-point osmometry and its medical-biological applications. Amer J Med Electronics 1963 2 32-41. 

Mercier DE, Feld RD, Witte DL. Comparison of dewpoint and freezing point osmometry. Amer J Med Technol 1978 44 1066-9. 

This expression can be modified to apply directly to any of various techniques used to measure the interaction parameter, including membrane and vapor osmometry, freezing point depression, light scattering, viscometry, and inverse gas chromatography [89], A polynomial curve fit is typically used for the concentration dependence of %, while the temperature dependence can usually be fit over a limited temperature range to the form [47]... 

Methods for the determination of Molecular weight based on colligative property are vapour-pressure lowering, boiling point elevation (ebulliometry), freezing-point depression (cryoscopy), and the Osmotic pressure (osmometry). 

Colligative properties are dependent on the number of particles present and are thus related to M . M values are independent of molecular size and are highly sensitive to small molecules present in the mixture. Values of are determined by Raoult s techniques, which are dependent on colligative properties such as ebulliometry (boiling point elevation), cryometry (freezing point depression), osmometry, and end-group analysis. 

Explain how osmometry is used to measure molar mass. What are the advantages of osmometry over measurements of boiling points and freezing points of solutions ... 

The up to now most frequently used techniques as, for example, vapour pressure osmometry (VPO) or freezing point depression (with its limitation regarding the solvent dependent measuring temperature) are based upon the colligative properties of the system the classical absolute light-scattering and ultracentrifugation techniques are only occasionally and approximately applicable with respect to the determination of CMC values. Evaluation of critical micelle concentrations which are based on these latter methods suffer considerably from the insensitivity of these techniques if measurements below the CMC, i.e., below about 10-3 mol dm-3, are carried out. More sensitive methods will be discussed below. 

Both vapour pressure osmometry and depression of the freezing point are the standard techniques probably most frequently used to determine the apparent number average molecular weight of the aggregates. The former method is preferred since the temperature of the sample is easily varied, thus allowing the investigation of the temperature dependence of the aggregate size. The accuracy of the commercially available equipment is rather different, and this has to be carefully considered below... 

Colligative properties reflect the chemical potential of the solvent in solution. Alternatively, a colligative property is a measure of the depression of the activity of the solvent in solution, compared to the pure state. Colligative properties include vapor pressure lowering, boiling point elevation, freezing point depression, and membrane osmometry. The latter property is considered here, since it is the most important of the group as far as synthetic polymers are concerned. 

An osmometer is an instrument which measures the osmolality of a solution, usually by determining the freezing point depression of the solution in relation to pure water, a technique known as cryoscopic osmometry. A small amount of sample is cooled rapidly and then brought to the freezing point (Fig. 6.1), which is measured by a temperature-sensitive thermistor probe calibrated in mosmol kg . An alternative method is used in vapour pressure osmometry, which measures the relative decrease in the vapour pressure produced in the gas phase when a small sample of the solution is equilibrated within a chamber. 

Another method of determining the molecular mass of a substance is to measure the freezing-point depression of a solvent that is brought about when a known quantity of test substance is added. This is known as a cryoscopic method. Another method, which is used occasionally, is vapor pressure osmometry, in which the molecular weight of a substance is determined through an examination of the change in vapor pressure of a solvent when a test substance is dissolved in it. 

As a result of polydispersity of the compounds, the measured relative molecular masses are weighted averages of the relative molecular masses of all the individual components. Depending on the method used, the weighting can be based on the number of molecules containing the sample (osmotic pressure, vapor pressure osmometry, depression of freezing point), on their weight proportion with respect to the... 

The parameter x can be obtained by several techniques including osmometry, vapor sorption, gas-liquid chromatography, freezing point depression of solvent, swelling equilibrium, intrinsic viscosity, and critical solution temperatures... 

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