360 - Osmometry Flashcards
define molarity
the concentration of a solution expressed as the number of moles of solute per litre of solvent
define molality
the concentration of a solution expressed as the number of moles of solute per kilogram of water or solvent
What is an osmole?
the amount of a substance that dissociates to produce 1 mole of particles in a solution
the concentration of a solution expressed as the number of osmoles of particles per litre of water
Osmolarity
the concentration of a solution expressed as the number of osmoles of particles per kilogram
of water
Osmolality
a phenomenon in which the addition of solutes molecules to a solution lowers
the temperature at which a solution will freeze
Freezing Point Depression
Osmolality is measured in the clinical laboratory to assess several conditions such as….
renal function
ingestion of toxins
the amount of solute that dissociates to produce 1 mole of particles in a solution with each particle in a
solution contributing equally to the total osmolality of a solution
an osmole
a property which is influenced by the number of particles in the
solution
colligative property
define the osmotic pressure of a solution
it is the hydrostatic pressure caused by a difference in the
amounts of solutes between solutions that are separated by a semi-permeable membrane
The four colligative properties
increase in: osmotic pressure and boiling pt [directly proportional]
decrease in: vapour pressure, freezing pt
the measurement
of a solution’s tendency to move from a liquid to a gas state as temperature rises
vapour pressure
the temperature at
which the solution’s vapour pressure is equal to the pressure of the gas above it
boiling pt
the temperature at which the solution changes from a liquid into a solid
freezing pt
The freezing point of a solution
decreases _.__°C for each osmole of particles/kg of solution.
1.86
how is vapour pressure measured?
it’s measured indirectly by measuring the dew point of a solution; the temperature at which condensation of water from a vapour state occurs
T or F. Vapour pressure measurements do not measure volatile substances
T! and the method is sensitive to changes in air temperature
Why is measuring boiling pt not suitable for biological samples
bc endogenous proteins are denatured and alter the nature of the sample
the four main components of freezing pt osmometers
cooling mechanism, a thermistor to measure the temperature, a mechanism to initiate freezing and a readout display
describe the processes of freezing pt depression osmometry
- 20 uL into cooling chamber
- temp decreased below expected freezing pt; approx. -7C (SUPERCOOLING)
- physical shock; vigorous stirring = ice crystal formation (SEEDING)
- sample solidifies and releases heat
- thermistor detects a decrease in resistance as the temperature
increases - temp increases until plateau = true freezing pt of sample
- = osmolality (mOsm/kg)
how is a freezing pt osmometer calibrated?
- sodium chloride solutions of known concentrations to ensure accurate results
- calibration performed at same temp as used for patient samples
- additional calibration of injection syringes and pump systems may also be required
these sources of error can increase the variation in results
- pipetting different sample volumes
- air bubbles in samples
- preparing samples too early = evaporation
presence of particulate matter in a sample may initiate …
seeding and adversely affect osmolality results
T or F. Osmolality measurements are not affected by lipemia or hemolysis
T!
major osmotically active components of serum
sodium
chloride
bicarbonate
glucose
urea
what is osmolality used to evaluate?
- the ability of the kidney to concentrate urine
- monitor kidney disease
- monitor fluid and electrolytes therapy
- aid in the diagnosis of hypernatremia and hyponatremia
- to evaluate the secretion of and renal response to anti-diuretic hormone
define osmolal gap:
the difference between the measured plasma or serum osmolality and the calculated osmolality
- normally close to 0
- increases in presence of unmeasured molecules or ions
- can be used to screen for exogenous substances: ethanol, methanol, ethylene glycol, isopropanol
-may also be increased in the presence of ketones or mannitol
decreased osmolal gaps are usually the result of this
lab error
Unaccounted osmolal gap values above ___ mmol/kg are flagged as High
10 mmol/kg
Causes of an elevated unaccounted osmolal gap
diabetic, alcoholic and starvation
ketoacidosis, sepsis, toxic alcohol poisoning and a variety of drugs (e.g. isoniazid, mannitol,
propylene glycol, contrast dyes, glycerol)
