Why is edta used in complexometric titrations

The major advantage of complexometric titrations is that they are particularly useful for the determination of mixtures of different metal ions in solution. The reaction can be identifying visually at equilibrium or endpoint. Complexometric titrations can also be performed using instruments that give more accurate results.

As it has a simple process, it does not require great expertise, skills, and practice. It is a cost-effective technique since it does not require the use of expensive chemicals, apparatus, and glasswares. The major disadvantage of complexometric titration is that it is a destructive method that often uses relatively large amounts of the substance being analyzed.

Since this is an open system, certain factors such as humidity pH, and temperature can affect the results. This can result in a large amount of chemical waste, which must be disposed of. This requires reactions to occur in a liquid phase, which is generally undesirable due to the chemical of interest. Now start titrating the mixture with standard EDTA solution. End point can be detected by color change from wine red to blue. Repeat the titration for three times to obtain 3 concordant readings.

Volume of water sample ml. Burette Reading. Initial ml. Final ml. Always rinse the burette and take out the bubbles at the nozzle of the burette. Always rinse the burette and other flasks with distilled water before using. No tinge of reddish hue should remain at the endpoint, the solution should be clear blue.

Titration should be performed slowly near the endpoint for detection of sharp and correct endpoints. EDTA can be used as chelating titrant titrant can be defined as the solution whose concentration is known in the titration in many ways. So, EDTA titrations can be performed in many ways. Few of them are given as follows —. It is similar to acid-base titration technique. In this titration standard EDTA solution is added to given sample containing metals using burette till the end point is achieved.

The relative proportions of the complexes will be dependent on the stability constants of the two types of metal- complexing agent complexes. EDTA titration has been traditionally used in quantitating calcium ions in water, in a process referred to as determining water hardness. Water hardness is customarily referred to as concentration of calcium in the form of calcium carbonate. In this method, a colorimetric indicator , [ these are intensely coloured substances in at least one form bound or unbound to the metal and do change colour when the metal-ion analyte binds with it ], is used.

Note that, whereas the equilibrium constants for acids and bases are often tabulated as dissociation constants, equilibrium constants for complex formation are tabulated as formation constants. In the former unit, we learnt that in the titration of a strong acid versus a strong base, a plot of pH against the volume of the solution of the strong base addedyields a point of inflexation at the equivalence point.

The general shapeof a titration curve obtained following the titration of mL of a 0. EDTA is used to titrate many ions. In this subsection, we shall look at the chemistry of EDTA titrations in general. Note that metal complexes of EBT are generally red in colour. Therefore, if a colour change is to be observed with EBT indicator, the pH of the solution must be between 7 and 11 so that the blue form of the indicator dominates when the titrant breaks up the red metal-EBT complex at the end point.

At a pH of 10, the endpoint reaction is:. Red Blue. To avoid this problem of gradual change in colour, a small amount of EDTA:Mg is often added to the titration flask this does not affect the stoichiometry of thetitration reaction because the quantities of EDTA and Mg are equimolar becauseMgIn complex is sufficiently stable that it will not dissociate appreciably priorto attainement of the equivalence point.

In direct titration , the solution containing the metal ion to be determined is buf- fered to the desired pH and titrated directly with a standard EDTA solution. It may be necessary to prevent precipitation of the hydroxide of the metal ion or a basic salt by the addition of an auxillary complexing agent or sometimes called masking agent , since they form stable complexes with potential interference , such as tartarate or citrate.

At the equivalence point the magnitude of the concentration of the metal ion being determined decreases abruptly. This equivalence point is generally determined by the change in color of a metal indicator that responds to changes in pM. Titration of mL of a water sample at pH 13 in the presence of acalcium specific indicator such as Eriochrome Black T required Calculate the hardness of the water sample as CaCO 3 in mg L What was the hardness in mg L -1 of CaCO 3?

This is for the determination of metal ions that cannot be titrated directly with EDTA, say in alkaline solution e. In back titration , an excess known amount of a standard EDTA solution is added to the solution of the analyte.

The resulting solution mixture is then buffered to the desired pH, and the excess EDTA titrated with a standard solution of a second metal ion. The end point is then detected with the aid of an appropriate metal indicator that responds to the second metal ion introduced in the back titration. A g sample of nickel ore was processed to remove interferences and The excess EDTA was titrated with Substitution titration may be used for the metal ions that do not react or react unsatisfactorily with a metal indicator e.

Usually, the determination of the metal ions that form weak complexes with the indicator and the colour change is unclear and vague. The amount of magnesium ion set free is equivalent to the cation present and can be titrated with a standard solution of EDTA. When a solution of Na2H2Y is added to a solution containing metallic ions, com- plexes are formed with the liberation of two equivalents of hydrogen ion, i.

The hydrogen ions thus set free can be titrated with a standard solution of sodium hydroxide using an acid-base indicator or potentiometric end point. Alternatively, an iodateiodide mixture is added as well as the EDTA solution and the librated iodine is titrated with a standard thiosulphate solution.

Note that the solution of the metal to be determined must be accurately neutralized before titration; this is often a difficult matter on account of the hydrolysisof many salts, and constitutes a weak feature of alkalimetric titration. What was the molar concentration of the FeSO 4 solution? This is reaction i. Learning Objectives Define and use the relevant terminologies of complex ion equilibria Compare and contrast between complex ion and Lewis acid-base equilibria Describe and explain the concept of complex equilibria and stepwise equilibrium reactions.

Use the concept of chemical equilibria in complexometric titrations and calculations. Distinguish among the various types of EDTA titrations and their uses. From upload. Thus, if the solution is buffered to about pH What is the role of buffer solution in complexometric titrations? Chemistry Reactions in Solution Buffer Calculations. Ernest Z.