360 - Osmometry

Question 1

define molarity

Answer 1

the concentration of a solution expressed as the number of moles of solute per litre of solvent

Question 2

define molality

Answer 2

the concentration of a solution expressed as the number of moles of solute per kilogram of water or solvent

Question 3

What is an osmole?

Answer 3

the amount of a substance that dissociates to produce 1 mole of particles in a solution

Question 4

the concentration of a solution expressed as the number of osmoles of particles per litre of water

Answer 4

Osmolarity

Question 5

the concentration of a solution expressed as the number of osmoles of particles per kilogram
of water

Answer 5

Osmolality

Question 6

a phenomenon in which the addition of solutes molecules to a solution lowers
the temperature at which a solution will freeze

Answer 6

Freezing Point Depression

Question 7

Osmolality is measured in the clinical laboratory to assess several conditions such as….

Answer 7

renal function
ingestion of toxins

Question 8

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

Answer 8

an osmole

Question 9

a property which is influenced by the number of particles in the
solution

Answer 9

colligative property

Question 10

define the osmotic pressure of a solution

Answer 10

it is the hydrostatic pressure caused by a difference in the
amounts of solutes between solutions that are separated by a semi-permeable membrane

Question 11

The four colligative properties

Answer 11

increase in: osmotic pressure and boiling pt [directly proportional]

decrease in: vapour pressure, freezing pt

Question 12

the measurement
of a solution’s tendency to move from a liquid to a gas state as temperature rises

Answer 12

vapour pressure

Question 13

the temperature at
which the solution’s vapour pressure is equal to the pressure of the gas above it

Answer 13

boiling pt

Question 14

the temperature at which the solution changes from a liquid into a solid

Answer 14

freezing pt

Question 15

The freezing point of a solution
decreases _.__°C for each osmole of particles/kg of solution.

Answer 15

1.86

Question 16

how is vapour pressure measured?

Answer 16

it’s measured indirectly by measuring the dew point of a solution; the temperature at which condensation of water from a vapour state occurs

Question 17

T or F. Vapour pressure measurements do not measure volatile substances

Answer 17

T! and the method is sensitive to changes in air temperature

Question 18

Why is measuring boiling pt not suitable for biological samples

Answer 18

bc endogenous proteins are denatured and alter the nature of the sample

Question 19

the four main components of freezing pt osmometers

Answer 19

cooling mechanism, a thermistor to measure the temperature, a mechanism to initiate freezing and a readout display

Question 20

describe the processes of freezing pt depression osmometry

Answer 20

• 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)

Question 21

how is a freezing pt osmometer calibrated?

Answer 21

• 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

Question 22

these sources of error can increase the variation in results

Answer 22

• pipetting different sample volumes
• air bubbles in samples
• preparing samples too early = evaporation

Question 23

presence of particulate matter in a sample may initiate …

Answer 23

seeding and adversely affect osmolality results

Question 24

T or F. Osmolality measurements are not affected by lipemia or hemolysis

Answer 24

T!

Question 25

major osmotically active components of serum

Answer 25

sodium
chloride
bicarbonate
glucose
urea

Question 26

what is osmolality used to evaluate?

Answer 26

• 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

Question 27

define osmolal gap:

Answer 27

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

Question 28

decreased osmolal gaps are usually the result of this

Answer 28

lab error

Question 29

Unaccounted osmolal gap values above ___ mmol/kg are flagged as High

Answer 29

10 mmol/kg

Question 30

Causes of an elevated unaccounted osmolal gap

Answer 30

diabetic, alcoholic and starvation
ketoacidosis, sepsis, toxic alcohol poisoning and a variety of drugs (e.g. isoniazid, mannitol,
propylene glycol, contrast dyes, glycerol)