Freezing point depression osmometer

Figure 27-2 Block diagram of a freezing point depression osmometer. /, Cooling fluid 2, stirring rod 3, thermistor 4, galvanometer 5, potentiometer with direct readout.The test tube is shown above the liquid in the cooling bath fso/id line) and inside the cooling liquid (dashed line).

Day-to-day precision of 2mOsmol/kg H2O should be obtainable by todays osmometers. More than 98% of the laboratories in the 2003 CAP surveys use freezing point depression osmometers, with the automated instrument from Advanced Instruments (Norwood, Mass.) being the most common. ... 

In clinical laboratories, the vapor pressure osmolality technique has been reported to be less precise than the freezing point depression method. For serum samples, the coefficients of variation obtained for the vapor pressure osmometer are about twice those obtained for the freezing point depression osmometer. The lesser degree of precision is related to the lower slope of dew point decrease compared with freezing point decrease (i.e., 0.303 °C versus 1.86 °C per osmol/kg H2O). 

An important clinical difference between the vapor pressure technique and the freezing point depression osmometer is the failure of the former to include in its measurement of total osmolality any volatile solutes present in the serum. Substances such as ethanol, methanol, and iso-propanol are volatile, and thus escape from the solution and increase the vapor pressure instead of lowering the vapor pressure of the solvent (water). This makes use of vapor pressure osmometers impractical for identifying osmolal gaps in acid-base disturbances (see Chapter 46). Thus use of this type of osmometer cannot be recommended for most clinical laboratories. 

B. Serum osmolality and osmolar gap. Serum osmolality may be measured in the laboratory with the freezing-point-depression osmometer or the heat-of-vaporization osmometer. Under normal circumstances the measured semm osmolality is approximately 290 mOsm/L and can be calculated from the results of the sodium, glucose, and BUN tests. The difference between the calculated osmolality and the osmolality measured in the laboratory is the osmo-lal gap, more commonly referred to as the osmolar gap (Table 1-22). 

There are many measurement techniques for activity coefficients. These include measuring the colligative property (osmotic coefficients) relationship, the junction potentials, the freezing point depression, or deviations from ideal solution theory of only one electrolyte. The osmotic coefficient method presented here can be used to determine activity coefficients of a 1 1 electrolyte in water. A vapor pressure osmometer (i.e., dew point osmometer) measures vapor pressure depression. 

The collected urine is centrifuged to remove solid debris and analyzed by standard methods for sodium, potassium and chloride (Durst and Siggard-Andersen, 1999, Scott et al. 1999). Osmolality is also measured with an osmometer (the freezing point depression type of instrument is recommended (Scott et al. 1999). 

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. 

The instrument used is a freezing point depression osmome-ter but it is often referred to simply as an osmometer. The... 

Because of their large molecular weight, proteins contribute only about 1 mOsmol/kg H2O to the total serum osmolality measured by freezing point depression. Occasionally, one may be asked to determine the contribution of macromolecules to the serum osmolality. Colloid osmotic pressure (COP) is a direct measure of the contribution of macromolecules (primarily proteins) to the serum osmolality. It is used primarily in the assessment of pulmonary edema or other abnormahties of water balance and serum protein concentrations. However, its utility has been questioned and the method is seldom used. Previous editions of this textbook describe the principles of a COP osmometer. 

Osmolality determinations are usually carried out using a cryoscopic osmometer, which is calibrated with deionized water and solutions of sodium chloride of known concentration. Using this technique, the sodium chloride equivalents and freezing point depressions for more than 500 substances have been determined and reported in a series of papers by Hammerlund and co-workers (e.g., see Hammerlund 1981). Figure 6.18 shows the osmolality of mannitol-water solutions. 

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