(USE CSMLS) ELECTROCHEMISTRY & 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
Describe the 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
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
In general terms, describe the Nernst Equation
The relationship between electric potential (E cell) and activity of an electrolyte
What equation describes the selectivity of an ISE ?
Nicolsky-Eisenman
Identify the 3 types of ISE membranes and give examples
- Glass membranes:
- ie. SiO2, Na2O, CaO
- measures H+ and Na+
- Liquid/ Polymer membranes:
a). ionophores dissolved in viscous, water-insoluble solvent OR polyvinyl chloride (PVC)
b). Lipophilic quaternary ammonium salts - Crystalline (solid state) membranes:
- has a thin outer membrane permeable to gas of interest and an internal pH electrode
- CO2 permeable OR urea permeable
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
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
What is an anion gap ?
The gap between measured cations and anions due to unmeasured anions (proteins, sulphates, phosphates…) and unmeasured cations (calcium, magnesium)
List 2 common reference electrodes
- Calomel electrode
- Ag/ Ag electrode
Normal plasma sample is __% water and _% lipid
Normal plasma sample is 92% water and 8% lipid
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 Sodium ISE, including specimen type, and sources of error
- glass or PVC (polymer) membrane
Specimen type: serum or heparin plasma
Sources of Error:
- hyperlipidemia causes false decrease with indirect ISE
- hyper proteinuria
Describe Potassium ISE, including specimen type, and sources of error
- PVC (polymer) membrane; valinomycin
Specimen Type:
- serum or heparin plasma
- serum will have higher [potassium] vs plasma because platelets release potassium during clotting
Sources of Error:
- hemolysized specimen (false increase)
- fist pumping/ prolonged tourniquet in venipuncture (false increase)
- leukocytosis and thrombocytosis (false increase)
Describe Chloride ISE, including specimen type, and sources of error
- quaternary ammonium salt membrane (liquid/polymer)
Specimen Type:
- serum or lithium heparin plasma
Sources of Error:
- halides and organic ions (thiocyanate, lactate) = selectivity decreases
Describe CO2 ISE, including specimen type, and sources of error
- 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
Specimen Type:
- arterial heparin blood, serum or plasma
Sources of Error:
- Air exposure decreases [CO2]; false increase in pH
- Biofouling
- Temperature, barometric pressure, incorrect calibration
Specimen type and sources of error for Total CO2 (bicarbonate) ISEs
Specimen Type:
- serum, lithium heparin whole blood or plasma
Source of Error:
- Air exposure decreases CO2 = false increase in pH
Describe Calcium ISE, including specimen type, and sources of error
- Calcium ionophore (liquid/ polymer) cast on a solid support
Specimen Type:
- collected anaerobically
- dry heparin
- 4°C
Sources of Error:
- first pumping during venipuncture = lactate lowers pH = false increase of ionized calcium
- ethanol, proteins, phosphate, and lactate
Describe Total Calcium ISE, including specimen type, and sources of error
Specimen Type:
- serum or lithium heparin plasma
Sources of Error:
- HIL
- magnesium ions, gadolinium compounds, paraproteins
Describe Phosphorus ISE, including specimen type, and sources of error
Specimen Type:
- serum or lithium heparin
Sources of Error:
- NEVER USE HEMOLYZED specimens = false increase
- Icterus and lipemia
- EDTA, sodium citrate, and potassium oxalate interferes with complex formation = false decrease
- monoclonal free light chains
Describe Total Magnesium ISE, including specimen type, and sources of error
- magnesium binds to calmagite to form a chromogen
Specimen Type:
- serum or lithium heparin plasma
Sources of Error:
- DO NOT USE HEMOLYZED samples = false increase
- bilirubin and lipemia
- EDTA, sodium citrate, potassium oxalate = false decrease
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
What is hypernatremia associated with ?
- Primary aldosteronism (Conn’s syndrome)
- Hyperadrenalism (Cushing’s syndrome)
- Secondary aldosteronism; when water goes to tissues, blood pressure decreases, and RAAS is stimulated = kidney absorbs more sodium to increase bp
- Damage to hypothalamus decreases thirst = dehydration
- Diabetes insipidus
What is hyponatremia associated with ?
- with normal osmolality = electrolyte exclusion effect
- with increased osmolality = hyperglycaemia, uremia, mannitol; due to shift in water or sodium from intracellular to extracellular space = dilution effect decreases serum sodium
- decreased osmolality = compromised RAAS; liver, kidney, Addison’s disease (adrenal insufficiency), SIADH, diuretics and extrarenal fluid loss
How is chloride regulated ?
- by the kidney and the renin angiotensin aldosterone system (same as sodium)
- excess chloride is excreted in sweat and urine
What is hyperchloremia associated with ?
- same as hypernatremia
- increased in respiratory alkalosis where HCO3- is excreted in the kidney with Na+ instead of Cl-
What is hypochloremia associated with ?
- same as hyponatremia
- furosemide (antidiuretic) inhibits Cl- reabsorption in the kidney
How is potassium regulated ?
By the kidney, aldosterone and insulin
What is hyperkalemia associated with ?
- pseudohyperkalemia; hemolysis, thrombocytosis, leukocytosis
- redistribution in acidosis, IVH, rhabdomyolysis, burns, and tissue hypoxia
- increased retention in Addison’s disease (hypoaldosteronism), and treatment with ACE inhibitors
What is hypokalemia associated with ?
- insulin therapy for diabetic hyperglycaemia causes redistribution of potassium into cells
- renal tubular acidosis, tubular necrosis, and corticoid hormone excess with metabolic acidosis; inability to excrete H+ causes decreased reabsorption of potassium
- decreased potassium intake and excessive loss (diarrhea, sweating)
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
What is hypercalcemia associated with ?
- primary hyperparathyroidism (parathyroid adenoma)
- malignancy ( breast cancer)
- tumor invades bone OR produce a PTH-related protein = bone resorption
- renal failure
- endocrine disorders ie. hyper/hypo parathyroidism and acromegaly
What is hypocalceamia associated with ?
- hypoalbuminemia (pseudohypocalcemia)
- proteinuria = hypoalbuminemia; low production of 1 α-hydrolase activity
- lower total calcium levels with normal free calcium levels
- liver, renal, and heart disease
- hyperphosphatemia
- hypoparathyroidism
- parathyroid gland destroyed by neck surgery
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
What is hyperphosphatemia associated with ?
- hypoparathoidism
- pseudohypoparathyroidism
- acromegaly
- renal failure; reduced excretion
What is hypophosphatemia associated with ?
- shift from ECF to ICF = respiratory alkalosis; glucose administration; insulin
- renal wasting ?
- hyperparathyroidism
- Fanconi’s syndrome, inherited rickets, osteomalacia
How is magnesium regulated ?
NONE
- but magnesium does influence PTH secretion
What is hypermagnesemia associated with ?
- hospital patients with renal failure and excessive administration of antacids, enemas, and fluids containing magnesium
What is hypomagnesemia associated with ?
- hospital patients with a shift from ECF to ICF
- increased loss by intestinal origin (diarrhea, bowel surgery, vomiting)
- increased loss by renal origin (diabetes mellitus, diuretics, antibiotics, alcoholism)
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 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 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
ISE membranes for CO2
Teflon and Silicon rubber
ISE membrane for Hydrogen
Lipophilic amine type neutral ionophore
ISE membrane for Calcium
ETH 1001
ISE membrane for Magnesium
ETH 1117
ISE membrane for Chloride
Lipophilic quaternary ammonium salt
ISE membrane for Potassium
Valinomycin
ISE membrane for Trifluoroacetophenone
Carbonate
ISE membrane for Nonactin
Ammonia
ISE membrane for Sodium
ETH157
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
Which of the following methodologies is used to measure oxygen?
a.
conductometry
b.
potentiometry
c.
coulometry
d.
amperometry
d.
amperometry
Which of the following materials is common component of chloride ISE membranes?
a.
silicon dioxide
b.
polyvinyl chloride
c.
quartz
d.
polystyrene
b.
polyvinyl chloride
T or F: A Ag/AgCl external reference electrode is commonly coupled with a saturated KCl solution to ensure equilibrium
TRUE; A Ag/AgCl external reference electrode is commonly coupled with a saturated KCl solution to ensure equilibrium
Where is the majority of the body’s magnesium and phosphate located ?
bones
Which of the following analyte measurements may be adversely effected by gadolinium?
a.
bicarbonate
b.
calcium
c.
phosphate
d.
sodium
b.
calcium
- negative interferent with o-cresolphthalein method
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
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
T or F: The first step in enzymatic method for total carbon dioxide is acidification to form carbonic acid.
FALSE; The first step in enzymatic method for total carbon dioxide is acidification to form BICARBONATE
