Conductance titrations, acid-base complexation

In principle, any type of titration can be carried out conductometrically provided that during the titration a substantial change in conductance takes place before and/or after the equivalence point. This condition can be easily fulfilled in acid-base, precipitation and complex-formation titrations and also the corresponding displacement titrations, e.g., a salt of a weak acid reacting with a strong acid or a metal in a fairly stable complex reacting with an anion to yield a very stable complex. However, for redox titrations such a condition is rarely met. 

Useful exploratory studies of acid-base behaviour in solvents of low dielectric constant have been made by conductance " and potentio-metric " titrations. Association constants are usually obtained from spectrophotometric measurements. The strengths of various bases can be compared by means of their association with an indicator acid like 2,4-dinitrophenol. If both acid and base are colourless, a competition for the base can be established between the acid and an indicator acid like bromophthalein magenta In solvents like benzene, other reactions than simple 1 1 association between B and RX may occur. Self-association of the acid or base is one such auxiliary reaction. A classic example is the dimerisation of carboxylic acids in benzene. If allowance is not made for this, constant values of the quotient [BRX]j[B][RX] will not be obtained. (Variations in the quotient cannot be attributed to interionic forces or other nonideal behaviour BRX is scarcely dissociated into ions at all and in spectrophotometric work very low concentrations of B and RX can be used.) Evidence for association ratios other than 1 1 can be obtained from indicator studies. The method developed by Kolthoff and Bruckenstein for studying reactions in anhydrous acetic acid fails for reactions in benzene and similar solvents because more than one acid molecule reacts with the indicator to give complexes of the form/w J r"(HX)yi. In such studies it is generally a good approximation... 

When ethylenediamme is. aided to a solution ofcobaltfll) chloride hexahydraic in concentrated hydrochloric acid, a Hue crystalline solid is obtained in 80% yield. Analysis of this compound shows ihal it contains 14.16% N. 12.13% C. 5.09% H. and 53.70% Cl. The effective magnetic moment is measured as 4.6 BM. The blue complex dissolves in water to give a pink solution, the conductivity of which is 852 ohm 1 cm mot"1 at 25 °C. The visible spectrum of a dmso solution of the complex his bands centered at 3217.5610. and 15,150 cm" (molar absorptivity = 590 mol-1 Lem-1), but for a water solution, the absorptions occur et 8000. —16.00031x119.400cm-I(nx)lar absorptivity = 5 mol-1 Lem-1). In u titration with sodium hydroxide, each mole of Ihe complex neutralizes four moles of base. Determine the formula and structure of the complex. Account for dll reactions and observations. 

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