Glasses- Redox Reactions

Question 1

What can ions and their redox interactions influence?

Answer 1

Colour, decolourising, UV/blue transparency (food protection), radiation conductivity and heat transport (IR absorption), colour stability, photosensitivity, photocrystallisation, glass staining by silver, refining, bubble formation at refractory interfaces

Question 2

What factors affect redox?

Answer 2

Oxygen partial pressure. Local atmosphere/batch redox. Furnace atmosphere. Temperature when ΔH=0. Time (kinetics of process).

Question 3

Effect of proportion of Fe2+ of overall Fe on overall batch redox

Answer 3

More Fe2+ than Fe3+ means negative redox. As % Fe2+ increases redox becomes more negative. Reverse for when more Fe3+. Batch redox is balance of oxidising/reducing compounds in the batch.

Question 4

Formula for how temperature affects redox state of final glass with iron in

Answer 4

[Fe2+]/[Fe3+]=constant x exp(-ΔH/RT)

Question 5

What does higher temperature mean for redox state of iron?

Answer 5

Generally high temperature means a more reduced state so more Fe2+

Question 6

What happens if a 2nd redox component is added?

Answer 6

Simple answer at melting temperature is nothing

Question 7

Zeroth law of thermodynamics

Answer 7

If A and B are in thermal equilibrium with each other and A and C are in thermal equilibrium with each other, then Band C are in thermal equilibrium with each other.

Question 8

Zeroth law applied to oxygen partial pressures and redox pairs

Answer 8

If Fe2+/Fe3+ is in equilibrium with pO2 in atmosphere and so is Ce3+/Ce4+, then Fe2+/Fe3+ and Ce3+/Ce4+ are in equilibrium with each other.

Question 9

What happens to each redox species on cooling?

Answer 9

Each redox species moves out of equilibrium with furnace atmosphere/batch atmosphere so they are are out of equilibrium with each other.

Question 10

Example times for oxygen diffusion distances

Answer 10

Takes 100h for a glass a few cm deep in a crucible at 1450C to reach equilibrium. At these conditions oxygen diffusion distances about 2cm. If quench glass melt in 1min then diffusion distance for oxygen reduced by factor (100x60/1)^1/2 (70) so down to 0.3mm which is much less than dimensions of glass melt.

Question 11

Equation for interaction between Fe and ace ions

Answer 11

Fe2+ + Ce4+ Fe3+ + Ce3+
No assumption about direction of reaction
This is defined by appropriate Gibbs free energy change with T on cooling from the melting T.

Question 12

Barton data

Answer 12

One way to express direction of redox reactions with two different species (like iron and cerium). In table the species at the top will tend to oxidise one below on cooling. Means Ce will tend to be reduced and oxidise Fe.

Question 13

Assumption for how far redox reaction with two species go

Answer 13

Simple model is to assume reactions go to completion (stoichiometric calculation)

Question 14

What happens on quenching of melt with iron in?

Answer 14

Reaction of Fe2+ and O2 tries to move to make more Fe3+ but can’t because diffusion of oxygen in the melt is very slow. Instead the effective oxygen partial pressure in the melt (pO2) changes with the Fe2+/Fe3+ ratio staying the same. Can write an equilibrium constant for this equal to molar concentrations of RHS over that of LHS with appropriate power once equation divided through by 4.

Question 15

When does a system reach equilibrium?

Answer 15

When all the redox pairs correspond to the same effective pO2

Question 16

Improved model compared to using stoichiometric calculations

Answer 16

Use equations for equilibrium reached when redox pairs correspond to same effective pO2. Also know total amount of redox species (e.g [Fe2+]+[Fe3+]) must remain constant. Charge balance must be retained as 0. Fe2+ doesn’t go quite to 0 in presence of Ce.

Question 17

What is Ce-Fe system used for?

Answer 17

Glass decolourisation
Reduce UV transmission and food degradation
Solar panels (increase photon transmission using low iron/low Fe2+ glasses)
Fe3+/solarisation

Question 18

What is needed for colour measurement?

Answer 18

Well polished homogeneous sample. Must correct for reflection losses. Can often assume that ε (think emissivity) is independent of matrix composition and use measurement of absorption to give concentration

Question 19

What does absorption depend on?

Answer 19

Sample thickness, colouring agent concentration, extinction coefficient of absorbing species (=f(λ)).

Question 20

Eye’s perception of colour

Answer 20

The eye perceives incoming light in terms of the response of 3 detectors and their relative balance. Can calculate these responses as 3 terms of the form:
X=ΣR(λ)*xbar(λ)
Where first term is for light and second is for eye’s response. These coordinates allow for a measure of colour