Complexation Reactions and Titrations Dr. Mohammad Khanfar

The formation of complexes Complex compounds are those formed as a result of a coordinate bond formation between a donor group and acceptor group, to produce a complex which has different properties from those of the free metal ion Donor group (ligand): has at least one pair of unshared electrons available for bond formation. ligands: water, ammonia, and halide ions, carboxyl Acceptor group or atom: has an has one or more of empty orbitals. The acceptor is usually a metal atom (e.g. Mg 2+, Ca 2+, Al 3+,…)

The formation of complexes The number of coordinate bonds that a cation tends to form with electron donors is its coordination number. Typical values for coordination numbers are 2, 4, and 6. The species formed as a result of coordination can be electrically positive, neutral, or negative. For example, copper(II), which has a coordination number of 4, forms a cationic ammine complex. Cu(NH 3 ) 4 2+ ; a neutral complex with glycine, Cu(NH 2 CH 2 COO) 2 and an anionic complex with chloride ion, CuCI 4 2-.

The formation of complexes Chelates, is produced when a metal ion coordinates with two or more donor groups of a single ligand to form a five- or six- member heterocyclic ring. Reaction called chelation reaction. Chelate complexes are more stable than non-chelate complex. The Cu complex of glycine is an example chelation

The formation of complexes A ligand that has a single donor group, such as ammonia, is called unidentate NH 3 -Cu 2+ Diaminoethane, which has two groups available for coordinate bonding, is called bidentate. Multidentate: tridentate, tetradentate, pentadentate, and hexadentate chelating agents are also known. e.g. Ethylenediaminetetraacetic acid (EDTA)

The formation of complexes Complexation reaction are useful in titrimetric analysis as they can be fast and complete However, they are reversible reactions that can be represented by the general equation: Where L is the ligand (donor) and M is the metal The ligand can be charged or neutral The higher the formation constant (K f ) the more complete and spontaneous the reaction would be, the more it would be suitable for titrimetric analysis KfKf

The formation of complexes Complexation reactions occur in a stepwise fashion,

Complexometric titrations Titration curve is usually a plot of pM = - log [M] as a function of the volume of titrant added. Most often, in complexometric titrations the ligand is the titrant and the metal ion the analyte, although occasionally the reverse is true. As titrants, multidentate ligands, particularly those having four or six donor groups, have two advantages over their unidentate counterparts. First, they generally react more completely with cations and thus provide sharper end points. Second, they ordinarily react with metal ions in a single step process (1:1 ratio), whereas complex formation with unidentate ligands usually involves two or more intermediate species

Complexometric titrations Curve A: M having a coordination number of 4 reacts with a tetradentate ligand D to form the complex of MD. Curve B: is for the reaction of M with a bidentate ligand B to give MB 2 in two steps. Curve C: involves a unidentate ligand A that forms MA 4 in four steps

Complexometric titrations Ethylenediaminetetraacetic acid (EDTA), is the most widely used complexometric titrant. The EDTA molecule has six potential sites for bonding a metal ion: the four carboxyl groups and the two amino groups, each of the latter with an unshared pair of electrons. Thus, EDTA is a hexadentate ligand. EDTA is fully protonated (H 4 Y) at pH 10 EDTA is commercially available as H 4 Y and the dihydrate of the sodium salt, Na 2 H 2 y.2H 2 0 EDTA is a valuable titrant because it form stable 1:1 complex with almost all cations except alkali metals

Complexometric titrations This great stability results from the several complexing sites within the molecule that give rise to a cage-like structure, in which the cation is effectively surrounded by and isolated from solvent molecules.

Complexometric titrations EDTA is found to be sparingly soluble in water (0.2% w/v) whereas its corresponding disodium salt is almost 50 times more soluble than the parent compound (solubility 10% w/v). Therefore, it is the disodium salt of EDTA (H 2 Y 2- ) which is normally used in complexometric titrations. Therefore, if a solution is made such that [Y] = [MY], pM = -pK f (or pM = pK` where K` = dissociation constant)

Complexometric titrations Effect of pH on complex formation EDTA has four protonated states, and since the actual complexing species is Y 4-, complexes will form more efficiently and stable in alkaline solution. However, some complexes of divalent and trivalent metals, e.g. copper, lead, nickel, cobalt are stable down to pH 3. Therefore, solutions are usually buffered at a pH at which the complex is most stable and at which the color change of the indicator is most distinct.

Complexometric titrations Indicators for EDTA Titrations The indicator is a dye which is capable of acting as a chelating agent to give a dye-metal complex. The latter is different in color from the dye itself and also has a lower K f constant than the EDTA-metal complex. The color of the solution, therefore, remains that of the dye complex until the end point, when an equivalent amount of sodium edetate has been added. As soon as there is the slightest excess of edetate, the indicator is displaced and the metal-indicator complex decomposes to produce free indicator; this is accomplished by a change in color.

Complexometric titrations Eriochrome Black T is a typical metal ion indicator that is used in the titration of several common cations. Its behavior as a weak acid is described by the equations: It has blue color in the pH range (7-11) in the uncomplexed form The metal complexes of Eriochrome Black T are generally red,, therefore, it is necessary to adjust the pH to 7 or above so that the blue form of the species, HIn 2-, predominates in the absence of a metal ion. pKa 6.3 pKa 11.5

Complexometric titrations Until the equivalence point in a titration, the indicator complexes the excess metal ion so that the solution is red. With the first slight excess of EDTA, the solution turns blue as the free uncomplexed indicator accumulated Other indicators: Alizarine flurine blue Calcon Catechol violet Diphenylcarbazone Methyl thymol blue Tiron Murexuide

General principles involved in disodium edetate titrations Direct Titrations: A suitable buffer solution and indicator are added to the metal ion solution and the solution titrated with standard disodium edetat until the indicator just changes color. A blank titration may be performed, omitting the analyte as a check on the presence of traces of metallic impurities in the reagents. Example: 0.3 g sample of magnesium sulphate (MgSO 4, g/mol) was dissolved in water and buffered with ammonia at pH 10, mixture of Eriochrome Black T and sodium chloride (1:99) was added as indicator. The mixture was titrated with 45 ml of 0.05 M disodium edetate until the solution becomes full blue. 0.3 ml was consumed in blank titration. Calculate the %w/w of MgSO 4. Answer: 89.7%w/w

General principles involved in disodium edetate titrations Back titartion Back-titration procedures are used when no suitable indicator is available. when the reaction between analyte and EDTA is slow, or when the analyte forms precipitates at the pH required for its titration. The sample is heated with measured excess of disodium edetate to form the soluble complex with the analyte (metal), and then the excess EDETA is back-titrated with Mg 2+ or Zn 2+ of known concentration using a suitable indicator.

General principles involved in disodium edetate titrations Example: Determination of the % of Ca 3 (PO 4 ) 2, since tricalcium phosphate is insoluble, the sample is dissolved with the aid of heat in excess HCl. One mole of tricalcium phosphate react with three moles of EDTA. 0.7 g sample of tricalcium phosphate ( g/mol) was dissolved and heated in 250 ml of diluted HCl solution. After 15 min, 50 ml aliquot was treated with 55mL of 0.05M disodium edetate. Ammonia buffer was added to bring the pH to 10 and the disodium edetate not required by the sample is back titrated with 32 mL of 0.05M zinc chloride using mordant black II as indicator to red color (complexed indicator). Calculate %w/w of Ca 3 (PO 4 ) 2. Answer: 84.9%

General principles involved in disodium edetate titrations Water Hardnes Water "hardness" was defined in terms of the capacity of cations (Ca, Mg, and other heavy metals) in the water to replace the sodium or potassium ions in soaps and to form precipitated products that cause "scum" in the sink or bathtub. The determination of hardness is a useful analytical test that provides a measure of the quality of water for household and industrial uses. The test is important to industry because hard water, on being heated, precipitates calcium carbonate, which clogs boilers and pipes. To determine Ca 2+ hardness only, murexide is used as indicator in strongly alkaline solution (pH 12) since, under these conditions, it chelates with Ca 2+ only and Mg 2+ precipitates as Mg(OH) 2.

Titration of Ca 2+ and Mg 2+ in a 50.00mL sample of hard water required mL of M EDTA. A second 50.00mL aliquot was made strongly basic with NaOH to precipitate Mg 2+ as Mg(OH) 2. The supernatant liquid was titrated with mL of the EDTA solution. Calculate: (a) the total hardness of the water sample, expressed as ppm. (b) the concentration in ppm of CaC0 3 in the sample. (c) the concentration in ppm of MgCO 3 in the sample.

How to convert molarity to normality The general formula to convert molarity to normality: N = M x F F depends on the reaction type. For acid-base reaction, F is the number of accepted or donated protons per molecule for basic and acidic substances respectively. It’s more complex for complexation reaction. For the general equation of complexation reaction : YL + XM N+ LM Where L is the ligand (donor group), M is the metal (acceptor group or atom), and N is the charge of the cation. For the ligand (L), F = X/Y N For the metal (M), F = N

How to convert molarity to normality What is the normality of 0.05 M of disodium edetate and 0.1 M solution of Ca 3 (PO 4 ) 2.