Osmometer

Excerpt

The osmometer is a device used in clinical laboratories for measuring the concentration of particles in a solution, known as the osmolar concentration. This quantity can be expressed as osmolality (in units of mmol/kg) or osmolarity (in units of mmol/L). Clinical laboratories usually measure osmolality. Osmolality is considered more precise because weight is temperature-independent. In laboratory analysis, a dissolved substance is referred to as a solute, and the substance in which the solute is dissolved is referred to as a solvent. A solute dissolved into a solvent creates a solution. The unit for osmolar concentration or osmolality is milliosmole (abbreviated mOsm or mOsmol). For nonelectrolytes, 1 mmol equals 1 mOsm, while for electrolytes, the number of particles in a solution depends on the electrolyte's dissociation.

Four colligative properties of a solvent change if a solute is dissolved into a solvent. These properties are:

1. Osmotic pressure

2. Vapor pressure

3. Boiling point

4. Freezing point

These properties are tied to osmolality and depend on the solution's number of solute particles. Dissolving a solute into a solvent generally increases the osmotic pressure and boiling point and decreases the vapor pressure and freezing point of a solution. While any of the four colligative properties could be used to determine the osmolality of a solution, technical limitations restrict the commercial measurement of osmolality to freezing point, vapor pressure, and membrane osmometers. The freezing point depression method is the most common in clinical laboratories because it is the most accurate and simple method.

The general principle of freezing point depression osmometers involves the relationship between the number of moles of dissolved solute in a solution and the change in freezing point. For example, one mole of a dissolved solute reduces the freezing point of water by approximately 1.86 degrees Celsius (~35.35 degrees Fahrenheit). Therefore, freezing and thawing a solution and comparing the relative change in freezing point in reference to a pure solvent allows for the determination of the approximate number of moles of dissolved solute in a sample. In clinical laboratory analysis, most samples are in water-based aqueous solutions, and the reference for solutions is generally pure water.

The setup of a freezing point depression osmometer includes a temperature-controlled bath to allow for sub-freezing temperatures, a thermistor probe connected to a Wheatstone bridge circuit to measure the temperature of a clinical sample, and a thermistor readout circuit, which represents a combination of a galvanometer and a potentiometer. While many osmometers are individual tabletop devices, computerized and automatic osmometers, and handheld osmometers do exist. Vapor pressure osmometers measure the osmolality of a solution by measuring the voltage difference between two thermistors, one exposed to a sample solution and the other exposed to only the pure solvent corresponding to the sample solution; this allows for a relationship between voltage and solute concentration to be determined, and thus the osmolality of a sample.

On the other hand, membrane osmometers measure the flow of solvent (often water) from a pure solvent container across a semipermeable membrane into a solution containing a solute and the same solvent. The flow across the semipermeable membrane can be measured as the osmotic pressure of a sample and is related to the concentration of solute in the sample solution. The semipermeable membrane in this device only allows the flow of solvent and blocks the flow of dissolved particles. These osmometers are limited in their application by the range of osmolality they can measure and the membrane material with which the semipermeable membrane can be made.

Alternative methods to osmometers can be used to measure osmolality, including electrical conductivity and specific gravity. Specific gravity was utilized before osmometers were practical to measure the osmolality of urine, utilizing various instruments such as a refractometer (the measurement of the refraction of light through a fluid) or a hydrometer. The applicability and accuracy of methods besides osmometry to determine a sample's osmolality depend on the specific method and the sample.