Week 4

Question 1

What do electrochemical systems measure

Answer 1

• largest group of chemistry testing procedures after light measuring systems

-measures lytes, gases, glur, urea, lact, and you can use whole blood specimen

Question 2

what is electric potential energy

Answer 2

Force that causes electrons to move in a conductor – measured in volts. Energy needed to move a charge against an electric field

Question 3

what is a potentiometer as an intrument not theory

Answer 3

Instrument that measures and displays value for electrical potential in an electrical circuit

Question 4

What is an Ion-selective electrode (ISE)

Answer 4

An electrode used for the measurement of the ion concentration
-electrodes are able to respond to only specific ions in a specimen
-ELECTRODE generated potentials and MEMBRANE generated potentials
-some analyzers will have different electrodes for different analytes - pH, Na, K etc

Question 5

what is electrochemistry

Answer 5

-relationship between chemical reaction and chemistry since chemical reactions can produce an electric current
-in order for an electrical current to be generated in a reaction there has to be movement of electrons
-this movement or flow is dependent on concentration which helps to quantitate how many ions are in a patient sample

-ions carry current in solution while electrons carry current in wires and electrodes in the middle of the two an electron is formed that changes the charge of the ion

Question 6

What are two types of electrochemical cells

Answer 6

• galvanic and electrolytic

Question 7

what is a galvanic cell or voltaic cell

Answer 7

-connected electrodes spontaneously move electrons from the electrode with lower affinity molecules
-converts chemical energy to electrical energy since moving electrons make electricity
-the reaction continues until the cell in dead it cannot produce electrical energy

Question 8

What is an electrolytic cell

Answer 8

–non spontaneous and needs external electromotive force to move current through a dead cell
-converts electrical energy into chemical energy
-electrons pass through the external meter to the cathode (reduction) where OH ions are released

Question 9

What is a half cell

Answer 9

-one half of either the electrolytic or galvanic cell where oxidation or reduction occur
-anode is oxidation - Zinc electrode in zinc sulfate solution
-cathode is reduction - copper electrode in copper sulfate solution
-must be separate with nothing connecting them to prevent direct contact and create a potential difference

Question 10

How can two half cells be connected and is the purpose of the connection

Answer 10

• using a salt bridge so ions can be transported freely between the cells

purpose
-since its a porous barrier it prevents mixing of either compartments - go from anode to cathode (OX to Red)
-join both half cells
-made from a strong electrolyte solution like KCl to maintain cell neutrality
- the bridge can also be filter paper saturated with electrolytes or a large beaker with salt solution
OIL RIG

Question 11

What is a reducing agent vs a oxidizing agent?

Answer 11

a reducing agent losses electrons and is oxidized

an oxidizing agent gains electrons and is reduced

Question 12

what will happen if you put Zn metal in Cu solution

Answer 12

If the Zn electrode is placed in Cu electrolyte solution will form an oxidation reduction or redox reaction
-the Zn metal will be covered with black coating of Cu
-unless an external energy source is applied the reaction will move to completion and the energy will dissipate as heat

at the anode (Zn) with positive electrodes anions will be attracted and vice versa
-A volt meter can measure the difference in potential

Question 13

what is standard reduction potential

Answer 13

-measures in volts the tendency of a chemical to be reduced , this is the electrode potential for redox reaction in a half cell
-half cell potential is measured by comparison with standardized hydrogen electrode with a 0.00V potential

Question 14

what is the potential of a complete cell

Answer 14

• add the standard reduction potentials for the two half cells
  -cell potential in V indicates how easily a reaction will occur. If cell voltage is + = the reaction will be spontaneous like car batteries, flashlights, or calculators. If cell voltage is - ve then reaction will not be spontaneous

Question 15

what is the difference between activity and concentration

Answer 15

-when youre doing any sort of substance measurement you need to differentiate between total concentration and degree of activity in the solution

Activity
-free form of analyte which is active and able to participate in a reaction. It is related to concentration

Concetration:
-total substance amount both bound and free

when we talk about electrode measurement and ion analysis we refer to activity not concentration

a = yC
ɣ - activity coefficient
a is activity
C - concentration

if every ion in the solution can participate in a reaction then ɣ =1 and a = c

Question 16

What is the activity coefficient ɣ

Answer 16

-activity of the substance depends on its environment for example Calcium, in blood it is bound to protein so its activity is half its concentration. Being bound to another substance affects measurement and in its ionized form ɣ will be <1
-inversely related to ionic strength if its bound its strength is decreased

Question 17

what is potentiometry

Answer 17

-measures the electrical potential difference (voltage) between indicator electrode and reference electrode in an electrochemical cell without current flow
-an electrochemical cell contains electrodes and solution with each electrode representing a half cell with half cell potential
-youre comparing the potential of a known calibrator to an unknown. the potential of unknown measured is proportional to the activity of the ion which is related to its concentration

-measured potential is dependent on chemical nature of metal and ion concentration in solution

Question 18

what is potential difference (PD)

Answer 18

measured using ion selective membrane on the outside surface on an ISE indicator/measuring electrode

-if the specimen is the same as internal electrolyte and PD is 0, but the specimen has more of the substance of interest =PD

Amount of PD on membrane is proportional to activity of the ion of interest

Question 19

what is a selective membrane for on the outside of a ISE

Answer 19

-limits ion permeability and movement based on size, concentration and charge

Question 20

What is the liquid junction

Answer 20

-electrical connection between indicator and reference electrode that is maintained by a flow of electrolyte from tip of reference electrode
-sets boundary between two solutions
-can increase or decrease reference electrode potential

Question 21

What is the standard H2 electrode

Answer 21

-type of inert metal electrode
-reference half cell that standardizes other reference electrodes
-has a potential of 0.00 V at all temperatures

Question 22

what are electrode generated potentials

Answer 22

-ability to conduct electricity - mV
- a metal in a solution with its own ions produces electrical potential related to activity of ions in solution
-an electrode allows you to compare the potential of a standard solution vs an unknown

Question 23

What are membrane generated potentials

Answer 23

• potential that occurs across a membrane that separates 2 solutions with 2 different ionic activities and allows them to diffuse back and forth
  -areas beside the membrane have slightly different composition than the rest of the solution
  -the membrane allows for electrical communication between the two solutions without mixing

Question 24

what are the three parts of a simple electrode system

Answer 24

Measuring (indicator) electrode
The part of the cell that contains the solution being measured

Reference electrode
Part of the cell that’s constant

Voltmeter/potentiometer

when a reactive surface is put in contact with material to be analyzed, a junction potential is produced

Question 25

What is a measuring (indicator) electrode

Answer 25

-has a membrane that is reactive or specific to the ion being measured
-put in contact with material being measured
-ions from the membrane will react with the internal electrolyte solution producing a junction potential proportional to activity of the ion
-Concentration calculated with Nernst equation

Question 26

what does a metal or metal salt electrode contain in a measuring electrode

Answer 26

-a metal in a solution of the metal salt called the internal reference element Ag (metal) in AgCl and HCl electrolyte solution
-gives electrical contact to the meter
-has a baseline indicator potential that changes based on changes in electrolyte concentration

Question 27

what is the filling solution in a measuring electrode

Answer 27

-electrolyte solution with low concentration
-the solution reacts with the inner metal element and the inner surface of the ion selective membrane causing changes in ion concentration
-when ions are attracted to the membrane more ions will leave the metal element and any ions forced away from the membrane will make ions go back into the metal element

Question 28

what is the ion sensitive or ion selective membrane in a measuring electrode

Answer 28

-very sensitive towards the ion being measure and zero sensitivity to anything else
-outer surface will attract or repulse ions from internal electrolyte
-high cation con’n on outer surface will attract Cl- toward the membrane

-must generate a potential that is proportional to the activity of ion being measure

Question 29

What are the measuring electrode system affected by

Answer 29

-contamination
-gradual loss of membrane components
-lipophilic (lipid-loving) specimens that extract membrane components more readily than aqueous solutions

-typical response time is 30s

-must correct “signal” drift during calibration

Question 30

What is a reference electrode in a simple electrode system

Answer 30

-maintains a constant potential independent of what is being measured
-have no membrane or reactive surface so it is unresponsive to test solution
-must be able to produce stable voltage (stable half cell potential)
-potential is generated due to contact between inner element and surrounding solution

Types:
Calomel electrode (inner element has mercury/ mercurous chloride or Hg/Hg2Cl2) - contains 2 fibers one is an asbestos fiber (bridge) in KCl solution
-chloride maintains constant ionic strength

Silver/silver chloride or Ag/AgCl electrode (KCl solution surrounds the electrode)

The fibers create a bridge between inner solution and test so there isnt uneven diffusion of ions

Question 31

What does a reference electrode contain?

Answer 31

1-Metal /Metal salt internal reference
-stable electrical contact to meter

2-Filling solution
-internal high concentration electrolyte to create stability
-has ions that carry charges to the test solution without disturbing the reference potential - KCl solution

3-Salt bridge junction
-contact between half cells without disturbing measured potential
-electrical continuity with external solution
-creates path for ions from reference electrode to move to external solution

Question 32

What is a Voltmeter OR Potentiometer in a simple electrode system

Answer 32

-must be calibrated with a standard of known concentration
-connected to reference and measuring (indicator) electrodes mV
-if potential between Ref and Meas is constant then the PD between the 2 is based on the PD across the membrane on ISE (indicator) electrode
-a change in ion activity will change the potential being measured

Question 33

what is the equation to measure voltage changes related to change in ion concentration

Answer 33

Ecell = Eind – Eref + Eljp

Eind = half-cell potential of the indicator (ISE) electrode
Eref = half cell potential of the reference electrode
Eljp = liquid junction potential (develops at the interface between 2 liquids that have differences in charge)

-potential in mV is used to determine ion activity in the solution

Question 34

What is the nernst equation

Answer 34

-predicts change in potential (electrode response) to change in ion activity
-factors in temperture, pressure, valence thermodynamic activity variables
-the formula is able to convert potential in V to concentration of specific ion

Electrical potential that develops at a membrane is directly proportional to the log of the ion activity

                  Em = Eo' + S [log (ɣ c) ]

Em = measured potential
Eo ‘ = constant reference potential
S = slope* of electrode in mV (electronic variable)
*comes from the calibration data (stored in software)
ɣ = activity coefficient- % of ions in specimen that
contribute to the potential
c = concentration of ion being measured

if electrode follows Nernst the response will be linear