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18 Cards in this Set

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Potentiomtry-
- Use of electrodes to measure voltages that provide chemical information.
Indicator electrode-
Use an electrode that responds directly to the analyte.
Reference electrode-
Second half-cell has a fixed composition so that it will have a constant potential.
Silver-Silver chloride Electrode-
A common reference electrode containing a silver wire coated with AgCl paste and dipped in a solution saturated with AgCl and (usually) KCl.
Metal electrodes-
Develope an electric potential in response to a redox reaction at the metal surface.
Ion selective electrodes-
Not based on redox processes; instead selective migration of one type of ion across the electrode membrate generates an electric potential.
WHy are various types of carbon used as indicator electrodes?
B/c the rates of many redox rxns on carbon surfaces are fast.
Junction Potential:
-Any time two dissimilar electrolyte solutions are in contact, a voltage difference (called the ___) develops at their interface.
- this small voltage (mV range) found at each end of a salt bridge connecting two half cells.
- limits accuracy of direct potentiometric measurements, b/c contribution of the junction potential to the measured voltage is unknown
[Errors in pH measurement]
(1) Standards-
-pH measurement cannot be more accurate than our standards, which are typically +/-0.01 pH unit.
[Errors in pH measurement]
(2)Junction potential-
Exists at porous plug near bottom of electrode; if ionic composition of analyte soln diff. from standard buffer, junction potential will change even if pH of two solns. are same.
[Errors in pH measurement]
(3) Junction potential drift-
Precipitates may form in plug of electrode which change junction potential, causing a slow drift of pH reading of a long period of time
- solve problem by recalibrating periodically.
[Errors in pH measurement]
(4) Sodium Error-
When concentration of H+ is very low and conc. Na+ high, electrode responds to Na+ well; apparent pH is lower than the true pH; also known as alkaline error.
[Errors in pH measurement]
(5) Acid Error-
In strong acid, measure pH is higher than the actual pH, perhaps b/c glass surface is saturated with H+ and cannot be protonated at any more sites.
[Errors in pH measurement]
(6) Equilibrium time-
It takes time to equilibrate with a buffer; a well buffered soln. takes about 30 secs.
[Errors in pH measurement]
(7) Hydration of glass-
A dry electrode requires several hours of soaking before it responds to H+ correctly.
[Errors in pH measurement]
(8) Temperature-
A pH meter should be calibrated at the same temp. at which the measurement will be made.
Errors occur with what in pH measurement?
(1) Standards
(2) Junction potential
(3) Junction potential drift
(4) Sodium Error
(5) Acid Error
(6) Equilibrium time
(7) Hydration of glass
(8) Temperature
Chem 7 test-
Constitutes up to 70% of tests performed in hospital lab;
- Measures Na+, K+, Cl-, tot. CO2, glucose, urea, and creeatinine, four of which analyzed with ion-selective electrodes.