EDTA: Titration, Types, Advantages, Disadvantages

EDTA (Ethylene Diamine Tetraacetic Acid) is an organic reagent that is frequently used in the complexometric titration. It is a chelating ligand with two nitrogen and four carboxylic acids that coordinates with metal ions.

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Structure of EDTA


EDTA is made up of four interchangeable hydrogen atoms and forms complexes with the majority of metals in a 1: 1 ratio.

Properties of EDTA

  1. The hexa protic, H6Y2+ form of EDTA is entirely protonated while, tetra protic (H4Y) is electrically neutral. The disodium salt, Na2H2Y, is the type that is typically employed in EDTA titrations. As EDTA is polyprotic, pH affects the forms that are present in the solution.Disodium salt of EDTA (as 2 H+ is replaced by 2 Na in H4Y) is a water soluble chelating ligand frequently utilized in the titrimetric assay at pH <_12.
Disodiun EDTA
Disodium EDTA

2. It has ability to produce metal-EDTA complex with various metal ions.

EDTA titration

EDTA is used to calculate the amount of metal in the metal ion solution. An appropriate indicator, such as Erichrome black T, is employed during the titration. First, at the specified pH of the solution, the indicator forms a complex with metal ions and changes its color.

Metal + In (indicator) → Metal -In complex (red)

Metal -In complex + EDTA → Metal- EDTA + In (blue)

A buffer solution is used to maintain pH. When EDTA is added to the solution, it forms a complex with the metal ion, allowing the indicator to become free and show its original color.

Metal-EDTA complex
Metal-EDTA complex

Stability of metal -ligand complex

Many aspects of metal and ligand ions can have an impact on the stability of metal and ligand complexes. These are mentioned below:

I. Nature of metal ion

small and highly charged metal ions produce strong metal-ligand complexes.

II. Nature of ligand

The ligands that form chelates add additional stability to metal-ligand complex (chelon effect). Ligand with more donor sides produces more stable complexes. For example, in contrast to the combination generated with ammonia, the complex of nickel with a multidentate ligand is more stable.

III. Number of metal chelate rings

The number of chelate rings formed between the ligand and metal ion determines the complex’s stability. The stability increases with the number of these rings.

Role of pH in EDTA titrations

The use of an appropriate pH is related to the stability constant of a metal-EDTA complex. For instance, alkaline pH is necessary for metals with low stability constants. Metals with a high stability constant require a pH between low alkaline and mild acidic. pH also plays an important role in the proper operation of the indicator substance.

Types of EDTA titrations

EDTA has wide applications in inorganic analysis due to its strong complexing effect and commercial availability. Because of its anion’s structure, it can create a chelate ring with less strain and satisfy the coordination number needs by the majority of metal ions.

I. Direct titration

By adding standard EDTA titrant to the sample solution, we may directly determine the presence of a metal ion. This technique can be used to identify several cations. The pH of the buffered solution containing the metal ion is adjusted before it is titrated with the standard EDTA solution. To prevent the precipitation of metal ion hydroxide, additional complexing agents such as tartrate, citrate, and tetraetanol amine can be added.

At the equivalent point, the concentration of metal ion is decreased which is determined by change in the colour of metal indicator.

Metal + In (indicator) → Metal -In complex (red)

Metal -In complex + EDTA → Metal- EDTA + In (blue)

II. Back titration

Due to a variety of factors, many metals cannot be directly titrated using an EDTA solution. They are:

 1. Color is difficult to differentiate.

2. A lack of appropriate indicators.

3. The slow reactivity of metal with EDTA.

4. Precipitation occurs in the pH range required for titration.

In these circumstances, extra EDTA solution is added, and the resulting solution is subsequently buffered to the appropriate pH. The excess EDTA is back-titrated with a standard metal ion solution.

Metal + In (indicator) → Metal -In complex (red)

Metal -In complex + EDTA → Metal- EDTA + In + EDTA(excess) → back titrated with standard metal ion.

Excess EDTA is calculated by titrating the solution with standard metal ion solution and is subtracted from the total volume of the standard EDTA solution to obtain utilized EDTA. It is possible to determine the volume of EDTA metal ions from total volume of EDTA.

ZnCl2, MgCl2, and  MgSO4 are used as standard metal ion solutions. Metal ions like Co+2, Al+3, Pb+2, Hg+2 Ni+2, and Mn+2 can be determined by using the Back titration method.

III. Replacement/ substitition titration

Metal ions that form more stable EDTA complexes than other metals, including Mg 2+ and Zn 2+ are subjected to this type of titration.

In this technique, a weak metal EDTA complex is introduced to the metal ion solution. Complexes  like Mg – EDTA and Zn – EDTA are commonly used as weak EDTA complexes During the titration, a metal ion in the weaker metal EDTA complex is replaced with another metal to generate the strong metal EDTA complex.

When Mg – EDTA is titrated with another metal ion solution, the Mg2+ is replaced by the metal ion in the solution. Which is then titrated with EDTA in presence of metal ion indicator. This allows for the calculation of the amount of Mg2+, which is then used to determine the number of metal ions in solution.

 For example : Solochrome black gives poor end point in the determination of Ca  by direct titration , But when it is subjected to the replacement titration it  gives sharp end point.

IV. Indirect titration

This method is used to determine amount of  silver, gold and ions such as halides, phospates, and sulphates that do not form complex with EDTA.


a. As gold and silver cannot be titreated directly with EDTA, they are first replaced with [Ni(CN)4] 2- ion, from which Nickel ion is liberated, and the corresponding amount of nickel liberated is measured by EDTA.

[Ni(CN)4] 2-  + 2 Ag+ ⇌ 2 [Ag ( CN)2 ] + Ni 2+

b. Phosphate can be measured by precipitating it as Mg (NH4) PO4. 6H2O, dissolving it in dilute HCl, adding extra standard EDTA solution, buffering it at pH=10, and then back titrating with standard Mg2+ ion solution while solochrome black is present.

V. Alkalimetric titration

When disodium EDTA solution is introduced to a solution containing metallic ions, complexes form due to the liberation of hydrogen ions. This approach relies on the concept that free H+ ions are released during complexation.

M+n + H2Y-2  → MY(n-4)+ + 2H+

In presence of an appropriate acid-base indicator, the liberated H+ ions are titrated with a standard solution of alkalilike NaOH.

Titration of the mixture

When the mixture contains more than one metal ion then titration of such solution is carried out by  different methods they are as follows:

1. By suitable control of the pH of the solution

This approach is employed when distinct metal-EDTA complexes have different pH stability and the titration is performed with different indicators.

2.  Use of Masking agent

Masking is the process of transforming a substance so that it does not enter a specific reaction without physically separating it or its reaction products. The cyanide ion acts as a potent masking agent. It is possible to identify cations like Ca2+, Mg2+, Pb2+, and Mn2+ by masking them with an excess of potassium or sodium cyanide.

3. By separation

If the ions are not tedious, this procedure is used. Metal first forms ppt with a complexing agent that is dissolved in the solution. The cation is then complexometrically titrated.

4. Kinetic masking

Due to their kinetic inertness, some metal ions do not efficiently enter the complexation reaction. In such a situation, Kinetic masking is applied.

Advantages of EDTA titration

  1. ETDA can form stable complexes with different metal ions.
  2. As the complexation takes place in a single step, the titration of the metal results in a significant change in the concentration of metal ions at the equivalence point.
  3. Since all of the Metal-EDTA complexes are soluble in water, aqueous media can be used for all experiments.


  1. Alkaline pH can lead to the formation of insoluble hydroxides or basic salts in several ETDA titrations. These salts might hinder complexation by competing with it.
  2. As EDTA forms stable complexes with the majority of metal ions, it lacks selectivity when used to determine the presence of a single metal cation in a solution containing a variety of metal ions.

Suggested video



  1. https://www.vedantu.com/chemistry/complexometric-titration.
  2. http://webhost.bridgew.edu/c2king/CH241/Lec8_%20EDTA%20Titrations_Condensed.pdf.
  3. https://www.shivajicollege.ac.in/sPanel/uploads/econtent/eeaf5776029778ffb513f37a93659ebd.pdf
  4. https://chem.libretexts.org/Bookshelves/Analytical_Chemistry/Supplemental_Modules_(Analytical_Chemistry)/Quantifying_Nature/Volumetric_Chemical_Analysis_(Shiundu)/14.4%3A_Complex_ion_Equilibria_and_Complexometric_Titrations.
  5. https://collegedunia.com/exams/complexometric-titration-chemistry-articleid-6319.
  6. https://byjus.com/chemistry/complexometric-titration/

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Kabita Sharma

Kabita Sharma, a Central Department of Chemistry graduate, is a young enthusiast interested in exploring nature's intricate chemistry. Her focus areas include organic chemistry, drug design, chemical biology, computational chemistry, and natural products. Her goal is to improve the comprehension of chemistry among a diverse audience through writing.

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