Module 10: Osmometry Flashcards
Colligative Properties - 4 things that happen when a solute is dissolved in a solvent
osmotic pressure increased
vapor pressure decreased
boiling point increased
freezing point decreased
colligative properties directly relate to
the total number of solute particles per mass of solvent
what determines the total osmotic pressure
the total number of individual solute particles present in solution per given mass of solvent (regardless of molecular nature)
osmometry
a method for measuring total concentration of solute particles in clinical samples
crystalloids
electrolytes
colloids
proteins
organics
moderate sized carbon containing molecules
osmolality
concentrations in terms of mass of solvent
ex. 1 osmolal solution contains 1 osmol/kg H2O
this is more accurate because it does not vary with temp differences
osmolarity
concentrations per volume of solution
ex. 1 osmolar solution contains 1osmol/L solution
1 osmolal solution represents
one mole of particles in solution (1 molar) which will lower the freezing point of 1kg of water by 1.86 deg C
For substances that do not dissociate in solution
one mole will produce one osmole
Ex. NaCL –> na + cl = 2 osmol
Glucose –> glucose = 1 osmol
primary contributor to total osmolality in plasma
electrolytes (na, cl, bicarbonate in the highest concentrations)
Osmolality of a serum sample can be indirectly calculated by
using the concentration of the osmolutes that contribute most to the total osmolality: na, cl, bicarbonate, glucose, urea
Cl and bicarbonate not used directly in calculation (instead represented by an osmotic coefficient)
2 calculations for osmolality
mOsm/kg H2O = 1.86[Na+] + [glucose] + [urea] + 9
mOsm/kg H2O = 2[Na+] + [glucose] + [urea]
concentrations in mmol/L
2 ways to measure total osmolality
vapour pressure (uncommon) freezing point depression
5 parts of a freezing point depression osmometer
freezing bath sample holder agitator thermistor processor and display
Process of freezing point depression measurements
Sample cooled rapidly to close to freezing point
Cooled slowly with gentle aggitation to a temp BELOW freezing point (supercooled)
Agitator is activated to induce crystallization and freezing (“seeding”)
Temp of sample RISES due to heat of fusion released during crystal formation- sample reaches point of equilibrium; both freezing and thawing occurs
This represents actual freezing point of solution which is dependent on the total concentration
Sources of error in Freezing Point Depression
prefreeze (sample freezes during cooling stage or prior to seeding)
incorrect sample volumes
performed maintenance using an alcohol solution and inadequately rinsed before patients (falsely increased results)
Turbid sample
Clinical applications
hyper and hyponatremia diuretic and IV fluid therapy Dehydration Diabetic ketoacidosis Diabetes insipidus Renal disease Traumatic shock Screen for volatiles and other toxic substances osmotherapy for treatment of cerebral edema
osmolal gap
difference between calculated and measured osmolality
Increased osmolal gap indicates
presence of unaccounted solutes, which may include ingested volatiles
May also be observed in ketoacidosis
If osmolal gap is increased, order
volatile screen (gas chromatography)
calculation for osmolal gap
osmolal gap = measured osmolality - calculated osmolality
Unaccounted osmolal gap calculation
unaccounted osmolal gap = measured Osm - (calculated osm + ethanol)
