(CSMLS) ELECTROCHEMISTY & ELECTROLYTES Flashcards
Define “activity” in electrochemsitry
Concentration of an electrolyte measured in an electrochemical cell (used in Nernst equation)
Define “activity coefficient”
Activity of an electrolyte divided by molar concentration
- measurement of the interaction of selected electrolytes with other species in the solution
Define “potentiometry”
Measures electric potential (E) between two electrodes under equilibrium conditions
Components of a potentiometric ion-selective electrode (ISE)
(Ref)erence electrode: stable/ constant potential relative to sample solution
- has a junction (frit) to allow electrical, ionic conductivity (E jxn) between sample and internal chloride sol’n while preventing large convective mixing of sol’ns
(Ind)icator electrode: has an ion selective membrane
- potential occurs when there is a difference in activity of ions on either side of membrane
Physical vs Chemical Properties of an Electrochemical cell
What equation describes the selectivity of an ISE ?
Nicolsky-Eisenman
List 6 general sources of error/ limitations for ISE
- Temperature dependent
- Ionic strength affects activity coefficient
- pH - inadequate conversion of analyte to one form ie. calcium
- Biofouling - protein buildup on membrane
- Cross-reacting ions
- Electrolyte exclusion effect
What is an anion gap ?
The gap between measured cations and anions due to unmeasured anions (proteins, sulphates, phosphates…) and unmeasured cations (calcium, magnesium)
__ is a common reference electrode
Ag/Ag electrode
Normal plasma sample is __% water and _% lipid
Normal plasma sample is 92% water and 8% lipid
Describe pH ISE, including specimen type, and sources of error
glass membrane
ion-exchange along membrane alters electrical potential
change in potential is correlated to H+ activity
Sources of Error:
- temperature dependent; as T increases, pH decreases
- exposure to air decreases CO2 and increases pH
Lipemia plasma sample is __% water and __% lipid
Lipemia plasma sample is 75% water and 25% lipid
What is an ionophore ?
neutral ion carrier
reversibly bind ions at membrane surface while counter-ions remain in solution = electric potential
Eg. valinomycin binds potassium
Describe principle of CO2 ISE
- gas permeable membrane; Teflon or silicon rubber
- CO2 passes membrane and dissolves within inner electrolyte sol’n
- bicarbonate and H+ form
- H+ is detected by interior pH ISE; change in membrane potential is proportional to pCO2
Sources of error for CO2 ISE
- Air exposure decreases [CO2]; false increase in pH
- Biofouling
- Temperature, barometric pressure, incorrect calibration
Sources of error for ISEs
- Fist pumping = increased Ca2+, K+
- Hemolysis = increased K+, phosphorus, Mg+
- Icterus = falsely increased phosphorus
- Lipemia (Na+ falsely decreased in indirect ISE)
Describe the electrolyte exclusion effect
- Exclusion of electrolyte from the fraction of total plasma volume that is occupied by solids
- Electrolytes are accounted for in the water content of sample, not solid
- Sodium concentration is falsely decreased in samples with increased proportions of solids
Causes of increased anion gap
- diabetic ketoacidosis
- lactic acidosis
- renal failure
- renal tubular acidosis
- diarrhea, starvation
- decreased renal tubular reabsorption of bicarbonate
- intoxication (ethanol, methanol, ethylene glycol)
- metabolic alkalosis; proteins gain negative charges = increased unmeasured anions
Causes of decreased anion gap
- hypoalbuminemia (primary unmeasured anion)
- increased cations; hypercalcemia and hypermagnesemia
- hypergammaglobulinemia; paraproteins (ie. IgG) have a positive charge
- laboratory error
How is sodium regulated ?
By the kidneys and renin angiotensin aldosterone system:
- DROP IN BLOOD PRESSURE AND FLUIDS causes kidneys to release renin
- liver releases angiotensin = activated by renin
- angiotensin II stimulates adrenal grand to release aldosterone
- aldosterone = KIDNEYS REABSORB SALT and WATER
How is chloride regulated ?
- by the kidney and the renin angiotensin aldosterone system (same as sodium)
- excess chloride is excreted in sweat and urine
How is potassium regulated ?
By the kidney, aldosterone and insulin
How is bicarbonate regulated ?
The kidneys and lungs regulate bicarbonate and CO2 levels
What is metabolic alkalosis associated with ?
Increase in bicarbonate
What is metabolic acidosis associated with ?
decrease in bicarbonate
How is calcium regulated ?
- parathyroid hormone (PTH):
- directly increases Ca2+ levels by increasing calcium reabsorption in the distal convoluted tube
- indirectly increases calcium by inducing 1 α-hydrolase activity = production of calcitrol
- directly increases bone resorption - Calcitrol:
- synthesized from vitamin D by actions of liver and kidney (1 α-hydrolase)
- increases Ca2+ levels by increasing vitamin D absorption and increasing bone resorption
How is phosphate regulated ?
Regulated by PTH and calcitrol
- PTH decreases blood phosphate by decreasing phosphate reabsorption in the PCT
- calcitrol increases blood phosphate by increasing phosphate absorption in the intestines; at high levels = bone resorption
How is magnesium regulated ?
NONE
- but magnesium does influence PTH secretion
What is Conn’s syndrome ?
- a type of hyperadrenalism
- primary hyperaldosteronism; hypersecretion of aldosterone by adrenal adenoma
- electrolyte imbalance = increased plasma Na+, decreased plasma K+
What is Cushing’s syndrome ?
- a type of hyperadrenalism
- increased plasma cortisol; can suppress ADH
- linked to pituitary tumors
- electrolyte imbalance = increased plasma Na+, decreased plasma K+
What is Secondary Hyperaldosteronism ?
- a type of hyperadrenalism
- caused by decreased blood flow to kidney (ie. CHF, nephorotic syndrome, stenosis)
- electrolyte imbalance = increased plasma Na+, decreased plasma K+
What is Addison’s disease ?
- a type of hypoadrenalism
- primary adrenal disease affecting entire adrenal cortex
- decreased cortisol and aldosterone
- increased ACTH; by pituitary to compensate for decreased adrenal hormones
- electrolyte imbalance = decreased plasma Na+, increased plasma K+
- also associated to hypoglycemia
What is SIADH ?
- syndrome of inappropriate antidiuretic hormone
- ectopic overproduction of ADH; associated with lung cancer
- electrolyte imbalance = decreased plasma Na+, increased plasma K+
- increased urine osmolality
What is Rhabdomyolysis ?
- breakdown of skeletal muscle (after trauma)
- myoglobinuria
- electrolyte imbalance = hyperkalemia
What is Fanconi’s Syndrome ?
- renal tubular defects
- electrolyte imbalance = hypophosphatemia
What are Rickets and Osteomalacia ?
- metabolic bone disorders; imbalanced bone formation and resorption
- defective mineralization of bone due to vitamin D deficiency OR phosphate depletion
- electrolyte imbalance = hypophosphatemia
Which of the following can increase the electrolyte detection limits of a polymer ISE?
a.
decreasing the concentration of electrolyte in the electrode internal solution
b.
increasing the size of the frit
c.
recharging the electrode in an acidic buffer between samples
d.
substituting polystyrene for polyvinylchloride
a.
decreasing the concentration of electrolyte in the electrode internal solution
Membrane potential is proportional to the __ of the ion concentration
Membrane potential is proportional to the LOGARITHM of the ion concentration
Which of the following chemicals is a main component of glass ISE membranes?
a.
tellurium dioxide
b.
lead (II) oxide
c.
silicon dioxide
d.
sodium carbonate
c.
silicon dioxide
Where is the majority of the body’s magnesium and phosphate located ?
bones
T or F: If the arm tourniquet is not released before the beginning of the draw, the plasma total calcium concentrations will be falsely decreased.
FALSE; If the arm tourniquet is not released before the beginning of the draw, the plasma IONIZED calcium concentrations will be falsely INCREASED.
Which condition could be associated with a sodium of 151 mmol/L?
a.
normal health
b.
mannitol therapy
c.
Conn syndrome
d.
electrolyte exclusion effect
c.
Conn syndrome
Reference Interval: 133-146 mmol/L
Critical Value: <125 or > 155
T or F: Hemolysis will cause falsely elevated sodium concentrations.
FALSE; hemolysis and icterus DOES NOT cause interference in sodium levels
T or F: Icterus does NOT interfere with the ammonium molybdate method (340 nm) for measuring phosphate.
FALSE; Icterus DOES interfere with the ammonium molybdate method (340 nm) for measuring phosphate.
Which of the following indices is routinely measured using conductometry?
a.
hematocrit
b.
anion gap
c.
TIBC
d.
osmolal gap
a.
hematocrit
