Module 10: Osmometry

Question 1

Colligative Properties - 4 things that happen when a solute is dissolved in a solvent

Answer 1

osmotic pressure increased
vapor pressure decreased
boiling point increased
freezing point decreased

Question 2

colligative properties directly relate to

Answer 2

the total number of solute particles per mass of solvent

Question 3

what determines the total osmotic pressure

Answer 3

the total number of individual solute particles present in solution per given mass of solvent (regardless of molecular nature)

Question 4

osmometry

Answer 4

a method for measuring total concentration of solute particles in clinical samples

Question 5

crystalloids

Answer 5

electrolytes

Question 6

colloids

Answer 6

proteins

Question 7

organics

Answer 7

moderate sized carbon containing molecules

Question 8

osmolality

Answer 8

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

Question 9

osmolarity

Answer 9

concentrations per volume of solution

ex. 1 osmolar solution contains 1osmol/L solution

Question 10

1 osmolal solution represents

Answer 10

one mole of particles in solution (1 molar) which will lower the freezing point of 1kg of water by 1.86 deg C

Question 11

For substances that do not dissociate in solution

Answer 11

one mole will produce one osmole

Ex. NaCL –> na + cl = 2 osmol
Glucose –> glucose = 1 osmol

Question 12

primary contributor to total osmolality in plasma

Answer 12

electrolytes (na, cl, bicarbonate in the highest concentrations)

Question 13

Osmolality of a serum sample can be indirectly calculated by

Answer 13

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)

Question 14

2 calculations for osmolality

Answer 14

mOsm/kg H2O = 1.86[Na+] + [glucose] + [urea] + 9

mOsm/kg H2O = 2[Na+] + [glucose] + [urea]

concentrations in mmol/L

Question 15

2 ways to measure total osmolality

Answer 15

vapour pressure (uncommon)
freezing point depression

Question 16

5 parts of a freezing point depression osmometer

Answer 16

freezing bath
sample holder
agitator
thermistor
processor and display

Question 17

Process of freezing point depression measurements

Answer 17

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

Question 18

Sources of error in Freezing Point Depression

Answer 18

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

Question 19

Clinical applications

Answer 19

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

Question 20

osmolal gap

Answer 20

difference between calculated and measured osmolality

Question 21

Increased osmolal gap indicates

Answer 21

presence of unaccounted solutes, which may include ingested volatiles
May also be observed in ketoacidosis

Question 22

If osmolal gap is increased, order

Answer 22

volatile screen (gas chromatography)

Question 23

calculation for osmolal gap

Answer 23

osmolal gap = measured osmolality - calculated osmolality

Question 24

Unaccounted osmolal gap calculation

Answer 24

unaccounted osmolal gap = measured Osm - (calculated osm + ethanol)

Question 25

Normal serum osmolality reference range

Answer 25

280-300 mOsm/kg

Question 26

Normal random urine osmolality reference range

Answer 26

300-900 mOsm/kg

Question 27

Osmolal gap reference range

Answer 27

0-10mOsm/kg

if gap is higher than 10, gap is INCREASED

Question 28

evaluation statement for normal osmolal gap

Answer 28

Osmolality and small gap within reference ranges; no unexpected analytes or volatiles present