Brown's Chemistry Advanced A Level Notes - Theoretical–Physical
Chemistry – Equilibria – Chemical Equilibrium Revision Notes PART 4
Ion exchange systems,
cationic and anionic resins, ion exchange theory
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What is an ion exchange resin? How do
cationic and anionic ion exchange materials work? What can we use ion
exchange resins for? The theory of ion exchange and how it is used in
various applications is described and explained.
Part 4 sub–index
Partition between two
Solubility product Ksp
& common ion effect
M = old fashioned shorthand for
4.3 Ion Exchange systems and
cationic/anionic ion exchange resin theory
anion exchange systems
INTRODUCTION to ION EXCHANGE
RESINS - what are they? and how do they work?
materials have the capacity to hold ions in a dynamic equilibrium with
the same ions present in an aqueous solution.
They may be
synthetic polymer resins with immobile negative groups e.g. based on
the sulphonic acid group R–SO2O–H+(s)
acting as a cation exchanger. R represents the molecular
backbone of the polymer resin. (immobile negative group,
exchangeable positive ion)
A resin with
immobile positive groups like R–N(CH3)3+Cl–(s)
can act as an anion exchanger (immobile positive group,
exchangeable negative ion).
occur naturally in the sheet structures of clay minerals in soil which
have excess immobile negative groups based on oxygen (e.g. clay–O–)
which hold cations like H+ or Ca2+.
are ions held on the resin?
charged ions the binding order from strongest to weakest bound is:
charged ions the binding order from strongest to weakest bound is:
For change in cation
charge: Not surprisingly, the general binding order from strongest
to weakest is M3+ > M2+ > M+ as the
increasing charge density of the hydrated ion increases, so will the
attraction of the ion to the immobile negative groups on the resin.
cationic radius and extent of hydration for constant charge:
If you consider
the trends for Group 1 cations (M+) or Group 2 cations (M2+)
things don't seem to add up?
The group trend is for increasing radius
down the group. This produces a decreasing charge density trend which
should result in weaker binding to the negatively charged resin.
However, the radius of the isolated ion does not count here, but what
does matter is the effective radius of the hydrated cation. The
smaller the ion, with its greater charge density, the greater its
attraction for water molecules and the larger the resulting hydrated
Therefore the effective hydrated ionic radius actually decreases
down the group, and the effective surface charge density increases to
give the binding strength order.
Ion exchange case studies
Case study 4.3.1 Ion–exchange
processes are extremely important in soil chemistry
Clay minerals are
based on sheets of linked silicate units.
Here the simple
tetrahedral silicate(IV) ion is SiO44– is linked
together via –O–Si–O– bonds in two dimensions, the resulting silicate
sheets have the general formula (Si2O52–)n
where n is very large number.
These negatively charged siliceous
sheets act as an cation exchange system
negative charge is balanced by various cations e.g. H+, K+,
Mg2+, Ca2+, Al3+ which are adsorbed
on or can fit in between the silicate sheets. The 'equation' below
shows how potassium ions might be exchanged with magnesium ions
One of the many
unfortunate consequences of acid rain from fossil fuel burning, is the
extra hydrogen ions will displace or wash out poisonous aluminium ions
from clay soils which are harmful to plants and animals.
Lime is added to
soil to reduce its acidity. The lime (calcium oxide) forms hydroxide
ions which will neutralise hydrogen ions held on the clay, so
increasing the pH. The hydrogen ions on the clay are replaced by
calcium ions, Ca2+.
minerals act as cation exchangers, anions like chloride and nitrate
are not easily held by these silicate sheets and are readily washed out in rainwater, the latter ion from artificial
ammonium nitrate fertilisers can cause pollution problems like
eutrophication, though the ammonium cation is more likely to be
retained being a positive ion.
cations can be retained for quite some time in soil and only slowly
displaced and dispersed to non–harmful levels. Even now (2006 at the
time of writing) in
Northern England (Cumbria) sheep from a few farms cannot be sold for
meat because of radioactive caesium–137, strontium–90 and iodine–?
deposited on the soil they graze on. The radioactive contamination
came from rain containing radio–isotopes a few days after the Russian
Chernobyl nuclear reactor explosion in 1986. Caesium is more strongly
bound than most other singly charged cations and some M2+
cations too? All the caesium will eventually end up in the Irish Sea
and very diluted and harmless to aquatic life, but it takes time. The
equation below shows the adsorption of the caesium ions onto an
alumino–silicate sheets in clay by displacing less strongly held
and the strontium
ion Sr2+ is also strongly bound and will also displace
other ions to remain in the soil for some time (see Mg2+...K+
exchange, 1st equation in section 4.3 above). However the radioactive
iodine is likely to end up as the iodide ion. I–, so, being
an anion, is more readily washed out of the soil by rainwater and not
retained by the negatively charged alumino–silicate sheets.
Case study 4.3.2
Removing hardness from water:
- Packs of ion
exchange resins can hold or release ions in an ion exchange process.
- Negative polymer resin columns hold hydrogen ions or sodium ions,
and can act as a cation ion exchange resin.
cations can be replaced by calcium and magnesium ions when hard water passes down the column.
- The more highly charged calcium or magnesium ions are more
strongly held on the negatively charged
resin. The freed or displaced hydrogen or sodium ions do not form a scum with soap
on hard and soft water).
+ Ca2+(aq) [resin]–Ca2+–[resin](s)
+ Mg2+(aq) [resin]–Mg2+–[resin](s)
+ 2Na+(aq) etc.
Case study 4.3.3 Water
- You can also use an anion ion–exchange resin to replace
negative ions by using a positively charged resin initially holding
hydroxide ions (OH–) e.g. to remove chloride (Cl–), nitrate (NO3–
potentially harmful) and sulphate ions (SO42–)
+ Cl–(aq) [resin]+Cl–(s)
+ NO3–(aq) [resin]+NO3–(s)
+ SO42–(aq) [resin]+SO42–[resin]+(s)
+ 2OH–(aq) etc.
- Now, by using both a positive
anion ion–exchange resin (here) and a negatively charged cation ion–exchange resin
(see case study 4.3.2 above), you can completely de–ionise water
because the released hydrogen ions and hydroxide ions combine to form
- The ionic equation for
- However. unfortunately, it will NOT remove
non–ionic substances like organic pesticides etc.
Case study 4.3.4 Separation of trans-uranium actinide elements using
an ion-exchange resin
- Although you can find traces of plutonium (94Pu)
and neptunium (93Np) in uranium ores, the principal sources of elements
with high atomic numbers (Z>92, beyond 92U) come from
bombardment of uranium atoms, (ii) processing used (spent) uranium fuel rods
from nuclear reactors and (iii) you can also produce them in appreciable
quantities (g or kg) in high neutron flux nuclear reactors.
- It is possible to separate out many of these
elements using a solution of them in the +3 oxidation state using a
cation ion-exchange column.
- The solution is initially passed through the
cation exchange resin to absorb the ions. Then a specially buffered
eluent of a complexing agent is then passed through the resin
column. This second solution (the eluent) strips off the M3+(aq) ions
one by one in order of decreasing atomic number.
- An eluent is a liquid/solution that acts as a
mobile carrier phase in this kind of context. It is equivalent to a
carrier gas in gas chromatography or water/butanol liquid in paper
- I've adapted the diagram below from the work
of Seaborg from his book on the "The Chemistry of the Actinide
Elements" published in 1957.
- All these elements are highly radioactive and
their concentration was monitored using a Geiger counter system that measured the
radioactivity of each M3+ fraction as it was eluted from
- So this is a sort of fractionation process or 'ion-exchange
chromatography' with the negatively charged cation resin acting as
the immobile phase and the buffered complexing agent solution acting
as the mobile phase.
- Glenn Seaborg was one of the great chemists of the period studying the chemistry of the 'Actinide Elements' in
the post-WWII 'nuclear' period and has an element named after
himself, element 106, Seaborgium (106Sg).
Data from pre-1957
- The y-axis represents the radioactivity,
which rises and falls as each element (as M3+ ion) is
eluted from the column and the x-axis shows the volume of eluent
coming off the column (both logarithmic scales).
- The radioactivity in the eluent drops is a
measure of the actinide M3+ ion concentration.
- At the time of this pioneering work, the
elements nobelium (102No) and lawrencium (103Lr)
were not recognised but there position predicted!
- I've added the atomic numbers for the
elements that were definitely known at the time in the elution
- Pretty good using a humble ion-exchange
OTHER USES OF ION EXCHANGE RESINS in the chemical and pharmaceutical
- There are many uses of ion exchange resins in
the chemical industry and applications in the pharmaceutical
industry and the use of direct treatments with ion exchange
- Nitrate Removal:
Ion exchange is
used for the removal of nitrates from
nitrate polluted waters
e.g. from farmland using a strong base
anion exchange resin operating in
the chloride ion form (salt solution
- Specialised Waste Treatments:
- e.g. radioactive waste systems in nuclear
power plants include ion exchange
systems for the removal of trace
quantities of radioactive nuclides from
water that will be released to the
- Cation ion exchange resins were used, and still
are, to separate metallic element ions from nuclear reactor waste.
Historically, this the most important method of separating and
identifying the products of nuclear fission and elements like
plutonium and americium formed by neutron bombardment of lighter
- Chemical Processing – Catalysis :
- Ion exchange resins are solid
insoluble but are reactive and can act as acids, bases,
or salts. Therefore ion exchange resins can replace alkalis,
acids and metal ion catalysts in
hydration or dehydration and polymerization processes.
- The advantages of ion
catalysts is that (i) its easy to separate the catalyst from the products of reaction,
repeated reuse, (iii) reduction of side
reactions and (iv) lack of need for special
alloys or lining of chemical plant equipment -eg reactor vessels.
- Purification by ion
exchange can be used to remove contaminating acids, alkalis, salts
from non-ionised or slightly ionised organic or inorganic
substances. Ion exchange resins can be used in metal extraction by a
process of separation and
- In aqueous
mixtures containing large amounts of
contaminants and only small amounts of a
desired ionic solute, ion exchange resins
can be used to selectively isolate and
concentrate the desired solute, for
example, the recovery of uranium
from sulfuric acid leach solution with
strong base anion resins.
specific chelating resins can be used
for metals recovery such as copper,
nickel, cobalt and precious metals.
- Pharmaceuticals and
- Ion exchange resins
can be used as carriers for medicinal
materials and in slow release medical
- In some cases, the ion
exchange resin has the medicinal
affect desired, for example, Cholestyramine, a dried and ground
strong base anion resin is used to bind
bile acids for reducing blood
- Ion exchange resins are used in a variety of fermentation
and biotechnology applications, using processes to isolate and purify
lysine, streptomycin and neomycin
and other similar antibiotics.
uses of ion exchange resins
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chemistry notes on theory and uses of ion exchange resins for university entrance examinations key phrases: what does an
ion exchange resin do? what is a cation exchange resin? what
is an ion exchange resin? how can you remove the hardness in
water using an ion exchange resin? how can you deionise
water with ion exchange resin? what is the binding order
trend of ions to ion exchange resins? how can soil act as an
ion exchange system? how can you purify water using ion
exchange resins? How does a kitchen water filter work to
purify water by removing ions? what do we mean by ion
exchange? how can you use ion exchange resins to remove
nitrate from contaminated water? give an example of how ion
exchange resins can be used in catalysis explain how ion
exchange resins can be used to purify substances how can ion
exchange resins be used to extract metals? how can the
products of nuclear fission be separated by ion exchange
resins? give examples of medical uses of ion exchange
resins. using cation ion-exchange column to separate out
ions of the actinide trans-uranium elements
Advanced Equilibrium Chemistry Notes Part 1. Equilibrium,
Le Chatelier's Principle–rules
* Part 2. Kc and Kp equilibrium expressions and
calculations * Part 3.
Equilibria and industrial processes * Part 4
Partition between two
phases, solubility product Ksp, common ion effect,
ion–exchange systems *
Part 5. pH, weak–strong acid–base theory and
calculations * Part 6. Salt hydrolysis,
acid–base titrations–indicators, pH curves and buffers * Part 7.
Redox equilibria, half–cell electrode potentials,
electrolysis and electrochemical series
pressure, boiling point and intermolecular forces watch out for sub-indexes
to multiple sections or pages
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