Quantitative Analysis Test 2 Review

- Use of electrodes to measure voltages that provide chemical information.

Use an electrode that responds directly to the analyte.

Second half-cell has a fixed composition so that it will have a constant potential.

A common reference electrode containing a silver wire coated with AgCl paste and dipped in a solution saturated with AgCl and (usually) KCl.

Develope an electric potential in response to a redox reaction at the metal surface.

Not based on redox processes; instead selective migration of one type of ion across the electrode membrate generates an electric potential.

B/c the rates of many redox rxns on carbon surfaces are fast.

-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

-pH measurement cannot be more accurate than our standards, which are typically +/-0.01 pH unit.

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.

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.

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.

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.

It takes time to equilibrate with a buffer; a well buffered soln. takes about 30 secs.

A dry electrode requires several hours of soaking before it responds to H+ correctly.

A pH meter should be calibrated at the same temp. at which the measurement will be made.

(1) Standards
(2) Junction potential
(3) Junction potential drift
(4) Sodium Error
(5) Acid Error
(6) Equilibrium time
(7) Hydration of glass
(8) Temperature

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.