lab quiz #7 electrochemical

what is electrochemistry

an analytical technique that measures ion in solution by electrical properties

what are most commonly measured ions?

sodium (Na+)

potassium (K+)

chloride (Cl-)

bicarbonate (HCo3-)

calcium (Ca+2)

magnesium (Mg+2)

what other non-ionic analytes does electrochemistry also measure?

oxygen or BUN

what does Ohm’s Law describe?

the relationship between electrical potential (E), current (I), and resistance (R)

what is the relationship between E and I in Omh’s law?

directly proportional (increase of E and Increase of I)

if 2 are constant then the 3rd one is measured (R)

which is the most common electrochemical method?

potentiometry

what is potentiometry

measures electrial potential in volts

what is the standard units of potentiometry?

volts

what is conductivity?

combined ability of ion to carry a charge

what is coulometry

measures the flow of electrial charge per unit of time in coulombs

what is the standard unit of coulometry

coulombs

what is amperometry?

meausre current in amperes

what is the standard units of amperometry?

amperes

what is polarography

measures current while controlling an increase in voltage

what is voltammetry?

measures the current at an electrode using a specific voltage generated at another (opposite) electrode

what is osmometry?

measuring dissolved particles in a solution using a series of variable resisters

what is an example of osmometry?

osmolality using the freezing point depression

what is the principle of potentiometry?

is an electrical system in which change in electrical potential is measured between a detecting electrode and standard reference electrode which the current is kept constant

what is detecting electrode?

aka indictor electrode

measures patient’s potential

what is standard reference electrode?

is the known value

the detecting electrode is compared to

what are used to measure the activity of one ion vs the other ion in potentiometry?

ion-selective electrodes

what are electrodes made up of?

made up of materials that are more selective for one ion compared to other similar ions

how is voltage changed measured and what does it reflect in potentiometry?

measured when ion comes in contact with electrode and reflection of the ionic activity

what are ion-selective electrodes (ISE)?

contains a selective membrane or layer that interacts with one particular ion more than others, making it specific to that ion

what is electrical potential set up with?

with a reference elctrode to which a standard voltage is applied and current kept constant

what is the change in potential due to?

due to the presence of ions in the solution

what is a “salt bridge”?

is a circuit between the two electrodes is completed by a liquid junction

what is ISE and reference sample is connected by?

salt bridge (potentiometer)

what is the ISE for SODIUM?

glass membrane

made up of: lithium aluminum silicate or silicon dioxide contained ionophore like monesin (antibiotic)

what is the ISE for POTASSIUM?

liquid membrane

contains: valinomycin (antibiotic)

what is ISE for CO2/HCO3-?

2 membrane separated by buffer

outer: gas permeable

inner: glass membrane

made up of silicon dioxide/lithium and calcium oxide glass

how does the ISE of Co2/HCO3- work?

once diffusion occurs, gass reacts with buffer to produce H+ whichis proportional to total CO2 in plasma

H+ ions are then detected by the glass membrane of silicon dioxide/lithium and calcium oxide glass that is selective for H+

what is the ISE for CLORIDE?

solid-state membrane

contain: silver chloride

what is ISE for CALCIUM?

membrane made up of PVC

contain: neutral liquid carrier (gel)

what 2 ways can ISE measured?

directly and indirectly

what is the direct measurement of ISE?

measures electrolytes in undiluted samples?

what is direct measurement NOT SUBJECTED TO?

measurement of Na

what is indirectly measurement of ISE?

measures electrolytes by diluting the sample with diluent prior to analysis with the electrode

what is the dilutional effect of sodium?

an increase of lipids or protein in liquid space causes less space in sample causing a falsely decrease value

what specimens can be used in ISE?

venous serum or lithium heparin whole blood or plasma.

what is the preferred speciment of ISE

heparinized plasma

b/c less potassium than serum due to platelet release during coagulation

what speciment should be avoided?

hemolysis

due to K+ intracellular causing false increase

fist clenching, leakage from RBCs

what other specimen can be used?

urine, sweat, CSF, gastric fluid but have short stability

what causes a false decrease in ISE?

protein build up on the surface of the electrolyte membrane

what causes a false decrease in NA and CL in indirect ISE systems?

dilutional effected caused by increase lipid and protein levels.

these high amounts displace some of the volume of plasma within the dilution

what are the solutions to the dilutional effect of Na and Cl?

drawing a fasting sample (less triglyerides)

ultracentrifugation (faster, spins component to bottom)

use direct method (instrument based)

what is the principle of conductivity?

is the combined ability of all ions in a solution to conduct or carry an electrical charge

what does the alternating current do in conductivity?

is applied to electrodes that generates the electrical potential in order to prevent polarization of electrodes

how does alternating current relate to the relationship of conductivity and resistance?

allows them to be reciprocal of each other

higher conductivity = lower resistance

what is Ohmic material?

follow Ohm’s law

E= I x R

what is conductivity used to measure?

total CO2 measurement

what are pH meters used for?

adjusting pH of reagents

determine pH of clinically significant body fiulds

what is the principle of amperometry?

a fixed potenial is applied to a electrochemical cell

the applied potential permits some compounds in the sample that come into contact with the elelctrodes to be oxidized or reduced

what does the potential serve as in amperometry?

generating or exictation signal

the reponse is the current measured

what is the most common electrol in amperometry?

Clark PO2

what does Clark PO2 electrode contains?

gas-permeable membrane

allows dissolved oxygen to pass through

what does the membrane of Clark PO2 prevent?

prevents other blood constituents from pass through that may inter fere with the electrode

what happens after oxygen passes through the Clark PO2 membrane?

mixes with a phosphate buffer solution and reacts witha polarized platinum cathode

what does the cathode have?

has an applied voltage which results in the reduction of oxygen

what does the reduction of oxygen produce?

flow of electrons (electrial current) and magnitude of current is proportional to the amount of oxygen present

what is the principle of coulometry?

uses the measurement of coulombs, flow of electrons per unit of time

coulometric tiration

what is titration?

a process of determining an unknown concentration of a substance by reacting it with an equivalent concentration of a known substance

what electrolyte is most commonly measured with columetry?

chloride

what does coulometry use to measure with?

silver anode that release silver ions at a constant rate due to constant electrical potential

what do silver ion react with? and what do they form?

reacts with chloride ions

forms insoluble silver cloride

what happens when all the chloride ions reacted with the generated silver ions?

causes a sharp increase in current conducted

how is current related to the cloride activity?

current relates to electrical charge per unit of time, time of titration proportional to the chloride activity

what analysis is coulometry most commonly used for?

sweat cloride analysis (CF testing in peds pt)

what is another name for voltammetry?

anodic stripping voltammetry

what is the principle of voltammetry?

electrochemical technique that involves electrical portential that varied over time

what is step 1 of voltammetry?

at the reference electode, a direct current is applied initially and large negative portenttial is generated

what are being reduced at the sruface of the reference lectrode? and causes what?

metal ions

causes them to be strongle attracted to surface of electrode

what is step 2 of voltammetry?

the direct current is then applied as a large linear postiive potential causing the metal ions to be gradually reoxidized and stripped from the electrode in teh order of the characcterisitic potential

what records current vs applied ptoential to quantify the substance present in voltammetry?

microammeter

what is polarography?

type of voltammetry

potential is applied constantly and slowly increased

causes reduction to occur which results in the electrical current to be proportional to the amount present.