Doc Brown's Chemistry Advanced A Level Notes - Theoretical–Physical Advanced Level Chemistry – Equilibria – Chemical Equilibrium Revision Notes PART 4

Part 4.3 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

4.1 Partition between two phases

4.2 Solubility product K_sp & common ion effect

4.3 Ion–exchange systems (this page)

M = old fashioned shorthand for mol dm^–3

4.3 Ion Exchange systems and cationic/anionic ion exchange resin theory

cation and anion exchange systems

• INTRODUCTION to ION EXCHANGE RESINS - what are they? and how do they work?

• Ion–exchange 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)

    • The function of a cation exchange resin.

  • 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).

    • The function of an anion exchange resin.

  • Cation exchangers 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⁺.

• How strongly are ions held on the resin?

  • For singly charged ions the binding order from strongest to weakest bound is:

    • Cs⁺ > Rb⁺ > K⁺ > NH₄⁺ > Na⁺ > Li⁺

  • For doubly charged ions the binding order from strongest to weakest bound is:

    • Ba²⁺ > Sr²⁺ > Ca²⁺ > Cu²⁺ > Zn²⁺ > Mg²⁺

  • 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.

  • Effect of 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 ion.

    • 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 SiO₄^4– is linked together via –O–Si–O– bonds in two dimensions, the resulting silicate sheets have the general formula (Si₂O₅^2–)_n where n is very large number.

  • These negatively charged siliceous sheets act as an cation exchange system

  • The excess 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

    • 2[clay–O]^–K⁺_(s) + Mg²⁺_(aq) [clay–O]^–Mg^2+–[O–clay]_(s) + 2K⁺_(aq)

  • 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.

    • [(clay–O)₃]^3–Al³⁺_(s) + 3H⁺_(aq) 3[clay–O]^–H⁺_(s) + Al³⁺_(aq)

  • 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⁺.

    • The overall neutralisation and ion exchange can be summarised as ...

    • 2[clay–O]^–H⁺_(s) + Ca²⁺_(aq) + 2OH^–_(aq) [clay–O]^–Ca^2+–[O–clay]_(s) + 2H₂O_(l)

  • Since clay 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.

  • Radioactive 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 potassium ions,

    • [clay–O]^–K⁺_(s) + Cs⁺_(aq) [clay–O]^–Cs⁺_(s) + K⁺_(aq)

    • 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.
  • These 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 (see GCSE/IGCSE notes on hard and soft water).
  • e.g. 2[resin]^–H⁺_(s) + Ca²⁺_(aq) [resin]^–Ca^2+–[resin]_(s) + 2H⁺_(aq)
  •  or 2[resin]^–Na⁺_(s) + Mg²⁺_(aq) [resin]^–Mg^2+–[resin]_(s) + 2Na⁺_(aq) etc.
• Case study 4.3.3 Water purification:
  • 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^– and potentially harmful) and sulphate ions (SO₄^2–) etc.
    • [resin]⁺OH^–_(s) + Cl^–_(aq) [resin]⁺Cl^–_(s) + OH^–_(aq)
    • [resin]⁺OH^–_(s) + NO₃^–_(aq) [resin]⁺NO₃^–_(s) + OH^–_(aq)
    • 2[resin]⁺OH^–_(s) + SO₄^2–_(aq) [resin]⁺SO₄^2–[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 very pure water.
    • H⁺_(aq) + OH^–_(aq) ==> H₂O_(l) 
    • The ionic equation for neutralisation.
  • 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 (i) 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 chromatography.
    • 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 the column.
      • 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 sequence.
  • Pretty good using a humble ion-exchange column!
• OTHER USES OF ION EXCHANGE RESINS in the chemical and pharmaceutical industries
  • 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 materials.
  • 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 regenerated).
  • 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 environment.
    • 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 atoms.
  • Chemical Processing – Catalysis :
    • Ion exchange resins are solid and 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 hydrolysis, esterification, hydration or dehydration and polymerization processes.
    • The advantages of ion exchange resin catalysts is that (i) its easy to separate the catalyst from the products of reaction, (ii) 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:
    • 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 concentration.
    • 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.
    • Other specific chelating resins can be used for metals recovery such as copper, nickel, cobalt and precious metals.
  • Pharmaceuticals and Fermentation:
    • Ion exchange resins can be used as carriers for medicinal materials and in slow release medical applications.
    • 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 cholesterol.
    • 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.

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WHAT NEXT?

Advanced Equilibrium Chemistry Notes Part 1. Equilibrium, Le Chatelier's Principle–rules * Part 2. K_c and K_p equilibrium expressions and calculations * Part 3. Equilibria and industrial processes * Part 4 Partition between two phases, solubility product K_sp, 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 * Part 8. Phase equilibria–vapour pressure, boiling point and intermolecular forces watch out for sub-indexes to multiple sections or pages

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