COPLEXOMETRIC TITRATION

COPLEXOMETRIC TITRATION

A complexing agent is an electron donating ion or molecule,called a ligand, which by it's ability to form one or more covalent bonds with the metal ion, produces a complex, which has different properties from those of the metal ion.thus the metal may not be precipitated from the complex by the usual metal ion precipitants. The stability of the complex ion varies and greater the stability,       more marked will be the differences in properties, from of the original cations.

Bonding in Complexes :
The bonds are either ordinary covalent bonds, in which both the metal and the ligand contribute one electron each or co-ordinate bonds in which both electrons are contributed by the ligand.

Chelating agents :
Complexes involving simple ligands, that is those forming only one bond, are described as 'co-ordinate compounds'. Ligands having more than one electron donating group are called 'chelating agents'.

Many organic compounds will chelate metals if they contain groups with an easily replaceable proton ( - COOH, phenolic and enolic-OH) or neutral groups offering a lone pair of electrons ( NH₂, CO and alcoholic OH) and the structure of the molecules are such as to permit the formation of stable rings. The greater the number of rings which can be formed; the chelate is likely to be more stable. Most rings formed in chelates involve the highest valency state of a metal, since these are more stable than those involving lower valency states.

The solubility of metal chelates in water depends upon the presence of hydrophilic groups such as COOH, SO₃ H,NH₂ and OH. When both acidic and basic groups are present, the complex will be soluble over a wide range of pH. When hydrophilic groups are absent, the solubilities of both the chelating agent and the metal chelate, will be low; but they will be soluble in organic solvents. The term 'sequestering agent', is generally applied to chelating agents, which form water soluble complexes with bi or polyvalent metal ions. Thus, although the metal remains in solution, they fail to give normal ionic reactions. Ethylenediamine tetra-acetic acid (EDTA) is a typical sequestering agent and can be represented as.

                 HOOC-CH₂                                       CH₂COOH                  

   

         N-CH₂-CH₂-N 

               HOOC-CH₂                                             CH₂COOH

Types of EDTA titrations - 

a) Direct titration - 

    The solution containing the metal ion to be determined is buffered to the desired pH (usually pH-I0) and titrated directly with the standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal ion by the addition of some auxiliary complexing agent such as tartrate, citrate or triethanolamine.

b) Back titraton -




Many metals cannot be titrated directly as they may precipitate from the solution in the necessary pH range or they may form inert complexes or  a suitable metal indicator is not available. In such cases, an excess of standard EDTA is added, the resultant solution is buffered to the desired pH and the excess of EDTA is back titrated with a standard metal ion solution e.g. a solution of Zinc chloride / sulphate or Magnesium chloride / sulphate is often used for this purpose. The end point is detected with the aid of   metal indicator,            which responds to the Zinc or Magnesium ions.

c) Replacement or substitution titration - 
    Substitution titration may be used for metal   ions, that do not react (or react unsatisfactorily) with a metal indicator. The metal cation to be determined may be treated with the Magnesium complex of EDTA, when Magnesium ion is set free equivalent to the cation present and can be titrated with a standard solution of EDTA and a suitable metal indicator.

Metal ion indicators - 

The success of EDTA titration depends upon the precise determination of the end point. The requisites of metal ion indicators for use in visual detection of end point include

a)   The colour reaction must be such that before the end point, when nearly all metal ion is complexed with EDTA, the solution is strongly coloured. 

b)   The colour reaction should be specific or at least selective.

c)   The metal indicator complex must possess sufficient stability, otherwise because of dissociation, a sharp colour change is not obtained. The metal indicator complex must however be less stable then the metal-EDTA complex to ensure that at the end point EDTA removes metal ion from the metal indicator complex. The change in equilibrium from the metal indicator complex to the metal-EDTA complex, should be sharp and rapid.

d)   The colour contrast between the free indicator and the metal-indicator complex, should be such as to be readily observed.

e)   The indicator must be very sensitive to metal ions, so that the colour change occurs as near to the equivalence point as possible.

f)    The above requirements must be fulfilled within the pH range at which the titration is performed. 

Some examples of metal ion indicators -

a)   Murexide -

    This is the Ammonium salt of purpuric acid and it was probably the first metal ion indicator, to be employed in the EDTA titration. Murexide solutions are reddish violet upto pH = 9, violet from pH -9 to 11 and blue above pH - 11. These colour changes are due to progressive displacement of protons from imido groups.

    Murexide forms complexes with many metal ions, only those with Cu, Ni, Co, and Ca are        sufficiently stable to find application in analysis. Their colour in alkaline solution are orange (Cu), yellow (Ni & Co) and red (Ca); the colours vary some what with the pH of the solution.

Aqueous solutions of Murexide are unstable and must be prepared every day. Normally, it is better to prepare a mixture of the indicator with pure Sodium chloride in the ratio (1:500) and employ 0.2 to 0.4 g in each titration.

b)   Solochrome black ( eriochrome black T ) - This is the most commonly used metal indicator for EDTA titration and the colour can be observed with the ions of Mg, Mn, Zn, Cd, Hg, Pb, Cu, Al, Fe, Ti, Co, Ni and Pt metals. To maintain the pH constant at the value 10, a buffer mixture is added and most of the above metals must be kept in solution with the aid of a weak complexing agent such as Ammonia or Tartrate. The cations of Cu, Co, Ni, Al, Fe (III), Ti (IV) and Pt, form such stable indicator complexes, that the dyestuff can no longer be liberated by adding EDTA, making direct titration with EDTA impracticable. However, with Cu, Co, Ni, Al, a back titration can be carried out, for the rate of reaction of the EDTA complexes with the indicator is slow and it is possible to titrate the excess of EDTA with standard Zinc or Magnesium ions.

    Cu, Ni, Co, Cr, Fe or Al even in traces, must be absent when conducting a direct titration with other metals; if the metal ion to be titrated does not react with cyanide ion or with triethanolamine; these substances can be used as masking reagents. The addition of 0.5 to 1 ml of 0.001 M o-phenanthroline prior to the EDTA titration eliminates the blocking effect of these metals with Solochrome black and also with Xylenol orange.

c)   Patton and Reeder's indicator - Its main use is in the direct titration of Calcium, particularly in the presence of Magnesium. A sharp colour change from wine red to pure blue is obtained, when Calcium ions are titrated with EDTA at pH values between 12 and 14.

            The dyestuff is thoroughly mixed with 100 times its weight of Sodium sulphate and    1 g of the mixture is used in each titration. The indicator is not very stable in alkaline solution.

d)  Solochrome dark blue or Calcon - This is sometimes referred as Eriochrome blue black RC and is used in the titrations of Calcium in the presence of Magnesium, which must be carried out at a pH 12.3, in order to avoid the interference with Magnesium. Magnesium is precipitated as Magnesium hydroxide and the colour change for Calcium is from pink to pure blue.

e)   Xylenol orange - Direct EDTA titrations of Bi, Th, Zn, Cd, Pb, Co, etc. are readily carried out and the colour change is sharp. By appropriate pH adjustment certain pairs of metal may be titrated, in a single sample solution. Thus Bismuth may be titrated at pH = 1 to 2 and Zinc or lead after adjustment to pH-5 by addition of Hexamine .

                                

f)   Calmagite -An important advantage of this indicator is that its aqueous solution is very stable and the colour change is somewhat clearer and sharper.

g)   Fast sulphon black F -This indicator is virtually specific for Cu ions. The presence of Ammonia or pyridine is required for colour complex with Copper to form.

h)   Bromopyrogallol red -This indicator possesses acid-base indicator properties and is resistant to oxidation. It forms coloured complex with many cations e.g. Bismuth at pH- 2 to 3 in Nitric acid solution, has a colour change from blue to claret red.

i)    Thymolphthalein complexone (Thymolphthalexone) -This indicator contains       a stable lactone ring and reacts only in an alkaline medium. The indicator may be used for Calcium, where the colour change is from blue to colourless. Manganese and Nickel may be determined by adding an excess of standard Calcium chloride solution; the colour change is from very pale blue to deep blue.

j)    Zincon -This indicator is specific for Zinc at pH 9 to 10. It can also be used as         an indicator for the titration of Calcium in the presence of Magnesium.

     

Some practical considerations -The following points must be borne in mind, while carrying out complexometric titrations.

a)   Adjustment of pH - For many EDTA titrations, the pH of the solution is extremely critical; often limits of + 1 unit of pH must be achieved for a successful titration. Whenever a buffer solution is added, it must be ensured that the requisite buffering action is in fact achieved. Hence, it is necessary to make certain that the original solution has first been made almost neutral by cautious addition of Sodium hydroxide or Ammonium hydroxide. When acid solution containing the metallic ion is neutralised by the addition of alkali, care must be taken to ensure that the metal hydroxide is not precipitated.

b)   Concentration of metal ion to be titrated - Most titrations are successful with 0.25 millimole of the metal ion in a volume of 50 to 150 ml of solution. If the concentration of metal ion is too high, then the end point may be difficult to judge.

c)   Amount of indicator - The addition of too much indicator is a fault which must be guarded against; as end point anticipation, which is of great practical value; may be virtually lost if too much of indicator is added; as the colour is too intense. In general a satisfactory colour is obtained by the use of 30 to 50 mg of a solid mixture.

d)   Attainment of the end point - In many EDTA titrations, the colour change in the neighbourhood of the end point is very slow; hence cautious addition of the titrant, coupled with continuous stirring of the solution is recommended.

e)   Detection of colour change - The detection of the end point of the titration is dependent upon the recognition of a specific change in colour and for many observers affected by colour blindness, this may be difficult. This difficulty may be overcome by replacing the human element with a colourimeter or a spectrometer.