CHEM Osmometry Flashcards
Describe the four colligative properties
Increased osmolality:
1. Increases boiling point
2. Decreases vapour pressure
3. Increases osmotic pressure
4. Decreases freezing point depression
Differentiate Molarity vs Molality; Osmolality vs Osmolarity
Molarity: mol/ L
Molality: mol/ kg
Osmolarity: number of particles/ L
Osmolality: number of particles/ kg
Describe the mathematical relationship between freezing point and osmolarity
- “freezing point depression”
- freezing point of a solution decreases by -1.86°C for each Osm of particles/kg
[(1000 mOsm/kg)/ -1.86°C] = [(x mOsm/kg)/ measured freezing point of sample]
Describe the steps of freezing point depression measurements
- 20 μl of sample is placed in cooling chamber via syringe injection
- “supercooling” occurs; temp drops below expected freezing point (-7°C)
- “seeding”; a physical shock initiates formation of ice crystals
- sample begins to solidify and releases heat
- thermistor detects decrease in resistance as temp increases
- true freezing point (mOsm/kg) = where temp plateaus
Identify the 4 parts of a freezing point depression osmometer
- Cooling mechanism
- Thermistor; measures temp
- Mechanism for “seeding”
- Display
Discuss calibration of a freezing point osmometer
- calibrated using NaCl of known concentrations
- must be performed at same temp as patient samples
Ref: Serum Osm
280 - 300 mOsm/kg
Ref: Osmol Gap
<10 mmol/kg
What is an osmole (Osm) ?
The amount of solute that dissociates to produce 1 mole of particles in a solution
NOTE: osmolality is per kg of water
Osm of NaCl
2 Osm; NaCl dissociates into individual ions
Osm of CH4N2O
1 Osm; urea does not dissociate
Define osmotic pressure
The hydrostatic pressure caused by a difference in the amounts of solutes between solutions that are separated by a semipermeable membrane
Vapour pressure is measured __ by measuring the __ of a solution; the temperature at which condensation of water from a vapour state occurs.
Vapour pressure is measured INDIRECTLY by measuring the DEW POINT of a solution; the temperature at which condensation of water from a vapour state occurs.
Why is freezing point depression the most common method to measure osmolality ?
- detects volatile substances in biological substances; does not denature proteins
- not sensitive to variation in atmospheric temperature
Sources of Error in determining Freezing point
- variation in pipetting sample volumes
- air bubbles in sample
- preparing sample too early and evaporation occurs
- contamination can initiate seeding = inaccurate results
Formula for Osmolal Gap
Osmolal gap (mOsm/kg) = Measured osmolality (mOsm/kg) - Calculated osmolality (mmol/L)
The osmolal gap is used to screen for exogenous substances such as __, __, __ and __.
The osmolal gap is used to screen for exogenous substances such as ETHANOL, METHANOL, ISOPROPANOL and ETHYLENE GLYCOL.
Formula for Calculated Osmolality
[2Na+] + [glucose] + [urea] = mmol/L
List some clinical utility of osmometry
- evaluate ability of kidney to concentrate urine
- monitor kidney disease
- monitor fluid and electrolytes therapy
- aid in diagnosis of hypernatremia and hyponatremia
- evaluate secretion and renal response of ADH
Which of the following can be assessed by measuring urine osmolality?
a.
filtering capacity of glomeruli
b.
renal blood flow
c.
renal concentrating ability
d.
renal tubular secretion
c.
renal concentrating ability
Which of the following is an advantage of osmometry when compared to specific gravity?
a.
each solute contribution is proportional to its molecular weight
b.
all solutes contribute equally regardless of molecular weight
c.
only the contribution of ionic solutes is measured
d.
only the contribution of non-ionic substances is measured
b.
all solutes contribute equally regardless of molecular weight
What component in an osmometer that measures the heat of fusion?
a.
barometer
b.
thermometer
c.
thermistor
d.
manometer
c.
thermistor
What is the mOsm equivalent of 2 mol of sodium chloride in 1kg of water?
a. 1
b. 2
c. 2000
d. 4000
d. 4000
Which of the following warms the sample in a freeze point osmometer?
a.
heat of vaporization
b.
heat of fusion
c.
heat of condensation
d.
heat of sublimation
b.
heat of fusion
NOTE: the amount of energy required to convert one mole of solid into liquid
Calculate the change in freezing point (°C) when 1g of sodium chloride is added to 1kg of water?
a.
-1.86°C
b.
+1.86°C
c.
-3.72°C
d.
-0.93°C
c.
-3.72°C
NOTE:
[(1000 mOsm/kg)/ -1.86°C] = [(x mOsm/kg)/ measured freezing point of sample]
Which of the following can falsely decrease osmolality reading when using a freezing point osmometer?
a.
ethylene glycol
b.
dust
c.
serum stored at room temperature
d.
hemolysis
b.
dust
NOTE: particulate matter can induce ice seeding prematurely = falsely decreased
Which of the following can be assessed by measuring urine osmolality?
a.
glomerular filtration
b.
renal tubular secretion
c.
renal blood flow
d.
renal tubular function
d.
renal tubular function; ability to concentrate urine
NOTE: glomerular filtration is measured by clearance
