Chapter 17 Complexometric Titrations & Precipitation Titrations PDF

Modified by Dr. Mohammed Rasheed Chapter 17 Complexometric titrations & Precipitation titrations ✓ Formation of complex ✓ Many metal ions can accept unshared pairs of electrons from an anion or molecule to form coordinate covalent bonds. ✓ The molecule or ion species containing atom which donat...

Modified by Dr. Mohammed Rasheed Chapter 17 Complexometric titrations & Precipitation titrations ✓ Formation of complex ✓ Many metal ions can accept unshared pairs of electrons from an anion or molecule to form coordinate covalent bonds. ✓ The molecule or ion species containing atom which donates the electrons is called a ligand or complexing agent. ✓ The ion which accepts the donated electrons is called the central ion or central atom. And the product resulting from a reaction between a metal ion and a ligand is referred to as a coordination compound or complex ion. Water, ammonia and halide ions are common inorganic ligand. Coordination number: the no. of covalent bonds it tends to form with electron donor species. Chelate: formed when a metal ion coordinates with two or more donor group of a single ligand to form five or six-member hete rocyclic ring  The coordination number is the maximum of atoms or groups that can combine, in the coordination sphere, with central atom. Ligands containing a single donor atom are called monodentate; those which shares more than one pair of electrons are said to be bi-, tri-, poly-dentate. The complex can have either a positive or a negative charge, or it can be neutral. Ex. Monodentate: NH3 is mono dentate [Cu(H2O)4]2+ + 4 NH3 → [Cu(NH3)4]2+ + 4 H2O  Bidentate Ligand A complex can contain more than one central metal ion. In such a case a ligand attached to two or more metal ions. A mononuclear complex contains a single metal ion ; a binuclear complex contains two central metal ions, and so forth. Titrations with Inorganic Complexing Agents Complexometric titrations: a metal ion reacts with a suitable ligand to form a com plex, and the equivalence point is determined by an indicator. The formation of sol uble inorganic complexes is not widely used for titrations, but the formation of precipitates, particularly with silver nitrate (AgNO3) as the titrant, is the basis for important determinations. Complexometric titrations a plot of pM = -log [M] as a function of the volume of titrant added. In the titrations, the ligand is usually the titrant, and the metal ion is the analyte. Most simple inorganic ligands are unidentate, which can lead to low complex stability and indistinct titration end points. As titrants, multidentate ligands, those having four or six donor groups, have two advantages. ✓ 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, whereas complex formation with unidentate ligands usually involves two or more intermediate species. Titrations with Inorganic Complexing Agents Curve A is computed for a reaction in which a metal ion 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 MB2 in two steps. The formation constant for the first step is 1012 and for he second 108. Curve C involves a unidentate ligand A that forms MA4 in four steps with s uccessive formation constants of 108, 106,104, and 102. ❖ These curves demonstrate that a much sharper end point is obtained with a reaction that takes place in a single step. For this reason, multi dentate ligands are ordinarily preferred for complexometric titrations.  Organic complexing agent is useful in precipitating metals to prevent interferences Many organic reagents are used to convert metal ions into forms that can be readily extracted from water into an immiscible organic phase. Masking Agent: is the formation of stable complexes that bind a metal and prevent it from interfering in a determination of another metal. 17D Aminocarboxylic acid titrations Tertiary amines that also contain carboxylic acid groups form remarkably stable chelates with many metal ions. Ethylenediaminetetraacetic Acid (EDTA) is the most widely used complexometric titrant. It has six potential sites for bonding a metal ion: the four carboxyl groups and the two amino groups. EDTA is a hexadentate ligand. Hexadentate ligand CEDTA = [H4Y]+[H3Y−]+[H2Y2−]+[HY3−]+[Y4−]  The component distribution of the acid (%) depends on pH      ❖ The Fully protonated form H4Y is only a major component in very acidic solution pH10)  EDTA Complex EDTA combines with metal ions in a 1:1 ratio regardless of the charge on the cation. It forms chelates with all cations (except alkali metals); most of these chelates are sufficiently stable for titrations. EDTA behaves as a hexa-dentate ligand. The equilibrium constant for the reaction of a metal with a ligand is called the formation constant, Kf, or the stability constant Formation constant: ❖ Note that Kf for EDTA is defined in terms of the species Y4- reacting with the metal ion. The equilibrium constant could have been defined for any of the other six forms of EDTA in the solution. ❖ Note the large Kf for most metals ❖ Y4- is hexadentate ligand forms very stable complexes (usually octahedral structures) with most of the transition metals. The donor atoms in EDTA4- are the two N atoms, and the four, negatively charged O atoms. 17D-3 Equilibrium calculations involving EDTA Calculate Y-4 to calculate Mn+ in the equation Substitute for Y4- See Fig 17-2 (When pH > 10 NiY2- Ni2+ + Y4- K = 2.4 x 10-19 The complex is the only source of both Ni+2 and EDTA Species K'NiY = 4 KNiY given given Influence of pH on the titration of 0.0100M Ca2+ with 0.0100M EDTA. Note that the end point becomes less sharp as the pH decreases because the complex formation reaction is less complete under these conditions. At high pH 4 → 1 ❖ Titration curves for 50.0 ml 0f 0.0100 M solutions of various cations at pH 6.0. 17D-6 Indicator for EDTA titration The most common indicators are those that form colored chelates with m etal ion in a pM range that is speciall for the particulate cation and dye. Diprotic acid indicator ❖ Its behavior as a weak acid is shown below (can be used as acid/base indicator Metal complex is red, after the end point the color is blue  EDTA titration Curve Because of the large formation constant, the reaction of cal cium with EDTA is more comp lete, and a large change occurs in the equivalence point regio n, the shaded area shows the transition range for the indicat or Eriochrome Black T A typical titration is illustrated by the reaction of Mg2+ with EDTA, using Eriochrome black T as the indicator. MgIn + EDTA → MgEDTA + In (red) (colorless) (colorless) (blue) Ex. A 50.00-mL water sample requires 12.00 mL of 0.0100 M EDTA. Calculate the hardness of this sample as ppm CaCO3 (100.1 g/mol). From titn. (1:1 reaction); mole EDTA = mole Ca2+. g CaCO3 = 1. 201 x 10-2 g ppm CaCO3 = 12.01 mg/0.0500 L = 240.2 ppm Ammonia is an auxiliary complexing agent The concentration of the auxiliary complexing agent should be minimum to lower the impact on the end point sharpness. Note that the aux. agent does not affect beyond the equivalence point