Analytical Chemistry

The most common indicator electrode?

platinum electrode

The ion mobility table is on the test:

(1)unequal mobility of Na+ and Cl- (2)The Na+ and Cl- ions will begin to diffuse from the NaCl solution into the water phase. (3)However, Cl- ion has a greater mobility than Na+. (4)That is, Cl- diffuses faster than Na+. (5)As a result, a region rich in Cl- develops at the front.(excessive negative charge) (6)Behind it is a region depleted of Cl- and thus containing excess positive charge. (7)The result is an electric potential difference at the junction of the NaCl and H2O phases.

The junction potential puts a fundamental limitation on the accuracy of direct potentiometric measurements, _____________________________________________________________________________

The junction potential puts a fundamental limitation on the accuracy of direct potentiometric measurements because we usually do not know the contribution of the junction to the measured voltage.

What does the Glass Membrane do?

separates internal solution and external solution

Potentiometric Titrations

you construct a titration curve by measuring the potential at many different volumes of added titrant

Calibrating Glass Electrodes Rules(2)

(1) Following the manufacturer's instructions, adjust the meter reading (usually with a knob labeled"Calibrate") to indicate the pH of the standard buffer. (2)The electrode is then washed, blotted dry, and immersed in a second standard at a different pH(usually pH 4 or 10). (3)If the electrode response were perfectly Nernstian, the voltage would change by 0.05916V per pH unit at 25 °C (4) pH of the second buffer is set on the meter with a knob that may be labeled"Slope" or "Temperature. (5) FINALLY, electrode is dipped in the unknown solution and the voltage is translated directly into a pH reading by the meter.

Advantages of Ion-Selective Electrodes

(1) Linear response to most analyte species over a wide range of concentrations (4 to 6 orders of magnitude). (2)They do not destroy the unknown sample. (3) They introduce negligible contamination. (4)Their response time is usually short (seconds to minutes), so they can be used to monitor flowing samples in industrial applications. (5)They are unaffected by highly coloured or turbid solutions.(6)Specially designed electrodes can be used in otherwise inaccessible locations, such as the interior of living cells.

Disadvantages of Ion-Selective Electrodes

(1) Special care is needed to get reliable results. (2)The precision of ISE measurements is rarely better than 1%, and is usually worse. (3)Electrodes can be fouled by proteins or other organic solutes, leading to sluggish response and drifting potentials.• (4)Certain ionic species interfere with or poison particular electrodes.

Most electrodes fall into one of the following 4 categories:

(1)Glass membranes for H+ and certain monovalent cations (2)Solid-state electrodes based on inorganic salt crystals (3)Liquid-based electrodes using a hydrophobic membrane saturated with a hydrophobic liquid ion exchanger (4)Compound electrodes involving a species-selective electrode enclosed by a region in which that species is separated from other species or produced by a chemical reaction

Even more about Ion Selective Electrode

(1)In general, the potential on an ion-selective electrode is given by E = constant - β(0.05916)log[A[F-] (outside)] (2)Where aX is the activity of the ion being measured, and β is something called the electromotive efficiency. β is close to 1.00(typically > 0.98). Also, if the concentration is not very high, the concentration is equal to the activity.

Selectivity

(1)No electrode responds exclusively to one kind of ion, but the glass electrode is among the most selective. (2)A pH glass electrode responds to Na+ only when [H+] is less than about 10-12 M and [Na+]is greater than about10-2 M.

How does glass membrane work?

(1)The 2 surfaces exposed to aqueous solution absorb some water and become swollen. (2)Most of the metal cations in these hydrated gel regions of the membrane diffuse out of the glass and into solution.

More about Ion Selective Electrode

(1)The electrode responds to the activity of uncomplexed analyte ion. (2) Therefore, potential ligands must be absent or masked. (3)Since we usually want to know concentrations, not activities, an inert salt is often used to bring all the standards and samples to a high and constant ionic strength. (4)If the activity coefficients stay constant, the electrode potential gives concentrations directly.

Junction potentials

(1)They cancel out the preferable Ion mobility of each ion (2)A junction potential exists at each end of a salt bridge (3)The junction potentials at each end of a salt bridge often partially cancel each other (4)Although a salt bridge necessarily contributes some unknown net potential to a galvanic cell, the contribution is small (a few millivolts)

Calibrating Glass Electrode Rules

(1)calibration procedure is different for each model of pH meter (2)before calibrating, wash it with deionized water and BLOT. DO NOT WIPE (3)dip the electrode in a standard buffer (pH near 7) and let the electrode equilibrate for at least a minute (4)all solutions for calibration and measurement should be stirred continuously during the measurement

Glass membrane :How it works?

(1)the species that migrate across the membrane is not the same as the species selectively adsorbed at each membrane surface. (2)The resistance of the glass membrane is typically 10^8 ohms, so very little current actually flows across it.

Definition: Junction Potentials

*voltage that exists at the interface between the salt bridge and each half-cell **This voltage is usually small, but it is almost always of unknown magnitude.

Why is platinum used in an electrode?

Platinum is used because it is relatively inert - it doesn't participate in many chemical reactions.

Potentiometry

The measurement of cell voltages to get chemical information

How can we turn the unknown into a half-cell?

We can turn the unknown into a half-cell by inserting an electrode (such as a Pt wire) into the solution to transfer electrons to or from the species of interest.

The last of Ion Selective Electrode

When is it "+", and when is it "-"? The sign is positive if X is a cation and it's negative if X is an anion. Example 29, 30, 31

What's the purpose of the platinum electrode?

When it is used as an electrode, its purpose is simply to transmit electrons to or from are active species in solution.

Where no metallic substance is indicated, the potential is established on __________________, such as platinum.

Where no metallic substance is indicated, the potential is established on an inert metallic electrode, such as platinum.

Calomel electrode

a type of reference electrode

Silver-Silver Chloride Electrode

a type of reference electrode

Doping

adding a small amount of Eu(II) in place of La(III)

Direct Potentiometry

an electrode is calibrated by measuring its potential at different concentrations of analyte and then creating a calibration plot (or finding the equation that relates potentially to concentration).

Combination electrode

contains both glass and reference electrodes in one body

Indicator Electrode

electrode responds directly to the analyte

Fluoride Ion-Selective Electrodes

the fluoride electrode, using a crystal of LaF3 doped with Eu(II). *slowly replacing Fluoride ion from the solution is selectively adsorbed on each surface of the crystal **F- can migrate from one side to the other and establish a potential difference across the crystal necessary for the electrode to work. ***E = constant- beta .... (equation)

Reference Electrodes

the second half-cell (with a constant potential)

Glass electrode for pH

type of ion selective electrode

ionic species are present in aqueous solution at ________ and ________

unit activity (about 1 M) gases are at 1 bar pressure (about 1 atm).

electroactive species

An electroactive species is one that can donate or accept electrons to or from an electrode.

Ion-selective electrode

Any electrode that predominantly responds to one species *producing electrode sensitive to one species

Junction Potentials: How it works

Any time 2 dissimilar electrolyte solutions are placed in contact, an electric voltage develops at their interface.

Calibrating Glass Electrode: Why do we buffer before calibration?

Before using a pH electrode, you must calibrate it using 2 or more standard buffers selected so that the pH of the unknown lies within the range of the pH values of the buffers.

Electrode Potentials

Each half reaction (oxidation or reduction)

Standard Electrode Potential

E°, measures the tendency for a reduction process to occur at an electrode

ion-exchange equilibrium

H+ replaces metallic cations in the glass (the ion ion is the catalyst)

What happens when Pt reacts with the electrolyte solution?

In cases where Pt reacts with the electrolyte solution, a gold electrode can usually be used.