Unit 4- Osmometry.docx

UNIT 4 Osmometry What is it? A technique for measuring the concentration of solute particles that contribute to the osmotic pressure of a solution Osmotic pressure- governs the movement of a solvent across a membrane that separates two solutions. In physiologic conditions, the solute is water. Clin...

UNIT 4 Osmometry What is it? A technique for measuring the concentration of solute particles that contribute to the osmotic pressure of a solution Osmotic pressure- governs the movement of a solvent across a membrane that separates two solutions. In physiologic conditions, the solute is water. Clinical applications include the analysis of osmolality in urine, plasma, and serum. Osmolality- expresses concentrations relative to the mass of the solvent. A 1 molal solution contains 1 mole of solute per 1 kg of water Measured Osmolality is usually expressed in mOsm/kg of water The concentration of dissolved particles is determined by the measurement of one of four physical properties of the solution called the colligative properties Measures how concentrated a liwuid is, is it really diluted or concentrated. This is a way to be able to determine how concentrated something is for the solutes that contribute to osmotic pressure. This is the pressure that is in your blood pressure, the water in your blood vessels, how much water is in there tells you how concentrated that is based on how much water movement of water between two different areas of soncentration through a semi permeable vessel. Only water moves through your blood vessels not so much other particles. Solutions with low solute concentration have low osmotic pressure. Osmolality is a form of concentration and is relative to mass of molecule which for us is water. We measure one of four properties calle dcalogative properties , these will change depending on the number of solute particles in solution, the size does nojt matter. Glucose to sodium will have equal effect, size doesn’t matter just maount. The more particles the longer to boil eleveate the osmostic pressure, takes longer to freeze the sample. It’ll take longer to vapourize as well. Colligative Properties: Those physical properties that are a function of the number of solute particles suspended in the solution and not the chemical nature of the particles. The colligative properties affected whenever a solute is added to a solvent are: an elevation in the boiling point an elevation in the osmotic pressure of the solution a depression of the freezing point a depression of the vapor pressure we can cool water to negative 40 edegrees as long as there are no particles in it Freezing point Osmometry- Components include: Cooling bath or block Rapid stir mechanism to initiate freezing of the sample A thermistor to measure temperature of the sample- basically a thermometer LED (light emitting diode) display that indicates the final result Plasma or urine for this, very small amount around 5 microlitres How does it work? Put sample in cooling block and it will cool a tiny but, the stirring wire will super cool our sample, it will allow it to freeze at a lower temp then it would on its own, once this happens we take it out of cooling block and stir faster and this will initiate the actuall freezing it. This is in a slush phase. Once it starts to form the slush those molecules ciming together will release heat and it will eventually get to equilibrium between freezinf and melting and this is when the actual reading is taken The sample is first lowered into the bath and, with gentle stirring, is super-cooled to a temperature several degrees below its freezing point When sufficient super-cooling has occurred, the sample is raised to a point above the liquid in the cooling bath The stirrer is changed from a gentle rate to a more vigorous one, which initiates freezing of the super-cooled solution Freezing is only to the slush stage, with about 2% to 3% of the solvent solidifying Released heat of fusion initially warms the solution and then the temperature plateaus and remains stationary The solution has reached a point of equilibrium in which both freezing and thawing are occurring we lowered the temperature of freezing point this is all based on the concentration of solutes in the solution -the higher the freezing point, the lower the solutes less concentrated -lower freezing point our solution has more solutes and is more concentrated This is why salt water freezes at a lower temperature because we are adding more solutes to it. Forced cooling begins Point of super cooling agitation is initiated from two to 3 we are freezing molecules and they are releasing heat Heat of fusion released into sample. Temperature increases Freezing point equilibrium is maintained for a short time.- this is when reading is taken Forced cooling begins again. Vapor Pressure Osmometry Measurement in these instruments is not directly related to a change in vapor pressure, but to the decrease in the dew point temperature of the solvent (water) caused by the decrease in vapor pressure of the solvent by the solutes Substances such as ethanol, methanol, and isopropanol are volatile and thus escape from the solution and increase the vapor pressure instead of lowering the vapor pressure of the solvent This type of osmometry is impractical for most clinical uses We don’t really use in medical lab, this idea is to test something to look at dew point We have to vapourize these samples, if we use thison plasma there are volatiles that we could vapourize off and we will get an inaccurate result. We only use freezing point in our lab Clinical Use: Determination of serum, plasma, and urine osmolality are used to assess fluid and electrolyte balance related to hydration, acid-base balance, and toxic screening, to assist in diagnosing disease such as diabetes, if someone comes in intoxicated they will draw an osmo, this will increase their osmo. There is no critical limits for this. Glucose is also a major contributor for osmolality. Sample type: SST/PST Random urine Reference ranges: Serum: 275-295 mOsm/kg Urine: 250-900 mOsm/kg Calculation The major osmotic substances: Na+, Cl-, glucose and urea (Na+ is the biggest contributor) Serum osmolality can be calculated by using the concentrations of these analytes. In SI units: Calculated serum osmolality (mOsm/Kg)= 2 (Na mmol/L) + Glucose (mmol/L) + Urea (mmol/L) need to know this calculation for the exam 2 x sodium concentration + glucose concentration + urea concentration