Analytical Chemistry: Chapter 15

Nonporous glass plugs

-pores 4-20 nm -less robust -more selective obstructions (allows certain ions to pass but not others) -higher chance of junction potential instability

Storage and care of a glass pH electrode

-store in aqueous solution to prevent dehydration of the glass -if the electrode has dried, recondition it in dilute acid for several hours -if the electrode is to be used above pH 9, soak it in a high-pH buffer

A glass pH electrode must be calibrated:

-with a minimum of two standard buffers (one with higher pH than sample and one with pH lower than sample) -at the same temperature of the unknown; calibrations should be repeated every two hours

Where is the solution that is analyzed in potentiometry?

One end of the salt bridge and the indicator electrode are dipped into the solution that is analyzed

What are the two classes of indicator electrodes?

Metal Electrode and ion-selective electrode

Ion-selective Electrode

selective binding of one type of ion to a membrane generates an electric potential; incorporate an ion selective membrane that enable the ISE to respond selectively to one ion; generate a reliable junction potential; do not involve redox processes

junction potential

voltage difference present at the interface between two dissimilar electrolyte solutions; they develop because oppositely charged ions often have different mobilities; Example: Cl- ions diffuse faster into water than Na+ ions, causing a charge separation at the boundary between a NaCl salt bridge and water

Compound electrodes

with an analyte-selective electrode enclosed by a membrane that separated analytes from other species or that generates analyte in a chemical reaction

Advantages of ion selective electrodes

-Inexpensive compared to other techniques -Linear response to log [analyte] over a wide range -Nondestructive (for gases or solutions, not solids) -Noncontaminating -Short response time -Unaffected by color or turbidity

What are the three most important reference electrodes for potentiometry?

-Standard Hydrogen Electrode (S.H.E) -Saturated Silver-Silver Chloride Electrode (Ag | AgCl) -Saturated Calomel Electrode (S.C.E) all of which use a saturated solution to hold the potential of the reference electrode constant

A typical pH combination electrode consist of what three components?

-inner Ag|AgCl electrode (buffere KCl solution saturated with AgCl) -H+ selective glass membrane -Outer Ag|AgCl electrode (saturated with both KCl and AgCl)

Microporous Glass plugs

-pores 100-3000 nm -best for most measurements (more robust) -fewer selective obstructions -more stable junction potentials

Challenges of Ion-selective electrodes

-precision is usually worse than 1% -Electrodes can be fouled by proteins or other organic solutes -> sluggish, drifting response -Certain ions interfere with or poison particular electrodes -Some electrodes are fragile and have limited shelf life -Reliable concentration measurements require the ionic strength to be held constant in all matrices -Electrodes respond only to uncomplexed ions. Complexed ions are not seen.

Saturated Calomel Electrode (SCE)

A calomel electrode saturated with KCl; based on the half-reaction: 1/2Hg2Cl2 (s) + e- --> Hg(l) + Cl- E = 0.241 V

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; E = 0.197 V

What is the most common metal indicator electrode?

Platinum; it is relatively inert and transmits electrons to or from species in solution; other types: gold, various types of carbon

Response of glass electrode

Potential difference across the glass membrane measured by the inner and outer silver-silver chloride reference electrodes;Depends on chloride concentration in each electrode compartment and potential difference across the glass membrane; Only variable is the pH of the analyte solution outside the membrane

What is the accuracy of a gland electrode?

The most carefully made pH measurements made with a glass electrode have an uncertainty of ± 0.02 pH units at best, which corresponds to ~5% uncertainty in the measured activity, and concentration of H+.

Using standard additions with electrodes

When ion-selective electrodes are used, it is important that the composition of the standard solution closely approximates the composition of the unknown. This can be ensured by using the method of standard additions. In this method, the electrode is immersed in a sample of the unknown and the potential is recorded before and after each small aliquot of standard solution is added. A graphical procedure is used to extrapolate back to the volume of standard (Vs) that would contain the same number of moles of analyte as the unknown.

Solid-state electrodes

based on inorganic crystals or conductive polymers

Repackaged Ag|AgCl

compact and sealed; this enables that entire electrode to be dipped into the solution to be analyzed

Metal electrode

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

indicator (working) electrode

electrode that responds to the analyze activity; the electrical conductor (the Pt wire) that exchanges electrons with the indicator half-cell; Can also refer to an entire half-cell

glass membrane electrode

for H+ and certain monovalent cations

selectivity coefficient

gives the relative response to different species with the same charge; the smaller the selectivity coefficient, the less interference by X when measuring A

liquid based electrodes

hydrophobic polymer membrane saturated with a hydrophobic liquid ion exchanger

What are porous glass plugs in salt bridges used for?

often used to keep most of the salt in the bridge, while still allowing ion currents to flow between the half-cell solutions and the salt bridge

reference electrode

one that maintains a constant potential against which the potential of another half-cell may be measured; half-cell has a fixed composition; the reference electrode is typically held constant using a saturated solution

Carbonate ion-selective electrode plus a Ph electrode used together do what?

provide a better way to measure dissolved CO2

Salt bridge in the Glass pH combination electrode

the outer electrode solution is connected to the analyte solution by a porous salt bridge; Examples: ceramic plug (common), platinum diaphragm (expensive, but less likely to clog)

Saturated Hydrogen Electrode (S.H.E)

the standard reference electrode for tabulated standard reduction potentials; difficult to use because it uses H2 gas and a fresh catalytic Pt surface which is easily poisoned

Glass pH electrode

used to measure pH; the most common ion-selective electrode

Fluoride ISE (solid state electrode)

used to monitor the fluoride content of municipal water supplies; only interfering species is OH-

Potentiometry

uses specifically-configured galvanic cells to measure voltages that provide chemical information about an analyte

Why is junction potential important?

It puts a fundamental limitation on the accuracy of direct potentiometric measurements because we usually do not know the size of the junction potential; E = Eind - Eref + Ejunction

Causes of error in pH measurement

1. Standard buffer pH uncertainty (~0.01 pH unit) 2. Junction potential uncertainty (~0.01 pH unit) 3. Junction potential drift error 4. Concentrated sodium error (glass electrodes respond weakly to Na+) 5. Strong acid error (glass electrodes underestimate the H+ concentration in strong acids) 6. Equilibration time error (when measurements are made too quickly) 7. Hydration of glass initialization error (when a dry electrode is not properly equilibrated prior to first use) 8. Temperature error (Difference between temperatures of standard buffers and sample) 9. Dirty Electrode drift error

Severinghaus electrode

Electrode for carbon dioxide; when CO2 diffuses through the semipermeable membrane, it reacts with water to form carbonic acid. Carbonic acid lowers the pH of the thin layer of electrolyte inside the CO2 permeable membrane. The glass electrode responds to the change in pH, providing a measure of the CO2 concentration outside the electrode.

What is the only analyte that selectively binds to the glass membrane?

H+; all concentrations are constant except those in the analyte solution

What is the sodium (or Alkaline) error?

In highly basic NaOH solutions, where [H+] is very low and [Na+] is high, the weak electrode response to Na+ causes the pH probe to underestimate the true pH of the probe solution.