Complexometric Titration

Complexometric titration is a volumetric analysis where the endpoint of the analysis or titration is identified by the formation of a coloured complex. It is mainly useful for the determination of a mixture of different metal ions in solution.

As we all know, the number of diabetes patients is increasing day by day worldwide. Do you know the drugs used for the treatment of diabetes contain metals in a specific amount and the metal content in a drug can be determined by complexometric titrations! It is a very useful, simple, and low-cost technique for various medicinal applications in the pharmaceutical field. So, let's discuss this technique in detail.

What is Complexometric Titration? 

Those volumetric titrations or analyses in which the endpoint is indicated by a coloured complex, are known as complexometric titrations. It is also known as chelatometry. In this type of titration, an indicator is used which is capable of producing clear colour change in titration which indicates the endpoint of the titration. 

Complexometric titrations are used for the determination of the concentration of metal ions in the solution. It is a volumetric analysis as the volume of the analyte, titrant, and even indicator play an important role during titration. Indicators such as calcein and Eeriochrome black T etc. are used in the complexometric titration. 

Complexometric Indicator

Complexometric indicators are those indicators that are used in complexometric titrations. These indicators undergo a definite colour change in presence of specific metal ions. These indicators are also known as pM indicators or metallochromic indicators. These indicators are organic molecules that are soluble in water. Calcein, curcumin, Eriochrome Black T, fast sulphon  black, hematoxylin, etc. are some examples of complexometric indicators. 

In a complexometric titration, the detection of the endpoint can be observed by the two methods. These methods are as follows:

1.  Visual Method

It is the most common method because it is accurate and cost efficient. The methods given below are examples of the visual methods used for determining the endpoint of the complexometric titrations.

• Metallochromic or PM indicators

• pH indicators

• Redox indicators

2. Instrumental Method

There is the possibility of human error and inaccuracy in visual methods in determining the endpoint. Therefore, some instrumental techniques are used in endpoint determination. The examples of instrumentation methods are as follows:

• Photometry

• Potentiometry

• Miscellaneous methods.

Complexometric Titration using EDTA

You must have heard about the hard water problem of Noida. If you want to know the estimation of hardness present in water, then complexometric titration is an easy, safe, and cost-effective method to do so. For your better understanding of complexometric titration, we are describing here an experiment for the estimation of the hardness of water by using complexometric titration. This experiment is also a part of the Chemistry Practical Syllabus of the coordination chemistry of Class XII CBSE. 

Aim

To estimate the amount of total hardness of a given water sample by complexometric titration using EDTA. 

Requirements

Conical flask, burette, pipette, spatula, buffer solution, Eeriochrome Bblack T indicator, standard EDTA Solution (0.01M), and inhibitor. 

Principle

Indicator Eriochrome Black T becomes wine red in colour when bound with metal ions while remaining blue in colour when free from metal ions. wWhile EDTA (which is ethylene diamine tetraacetic acid) is colourless whether it’s bound to metal ions or not. So, the addition of the EBT indicator in the sample (water containing metal) makes it wine red in colour as Eeriochrome Bblack T binds with metal ions. Eriochrome Black T binds with metal ions loosely while EDTA binds with metal ions strongly. So, when all metal ions are bound to EDTA, indicator EBT remains free in the sample and the solution turns blue. 

We can write the complex-forming ion of EDTA as H2Y2- which it forms in an aqueous solution. The reactions of it with metal ions can be written as follows – 

M+2 + H2Y2- → MY2- + 2H+

M+3 + H2Y2- → MY- + 2H+

M+4 + H2Y2- → MY + 2H+

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Procedure

1.  Take 25ml of the given sample in a conical flask. 

2. Now add 1ml of buffer solution.

3. Now in the sample and buffer solution, add 1ml of inhibitor as well. 

4. Now add 2 drops of Eriochrome Black – T solution in the mixture which turns the colour of the solution win red. 

5. Fill the burette with an EDTA solution. Now start titrating the mixture with a standard EDTA solution. 

6. An endpoint can be detected by a colour change from wine red to blue. 

7. Repeat the titration three times to obtain 3 concordant readings. 

Observation Table

Calculation 

Total hardness (mg/l) by calcium carbonate = (A×B×1000)/Volume of sample in ml

Where A = Volume of EDTA required by sample or volume of EDTA used in titration

B = 1*(Molarity of EDTA/0.01M)

Result 

The amount of total hardness in the given water sample is ……mg/l. 

Precautions

1. Always rinse the burette and take out the bubbles at the nozzle of the burette. 

2. Always rinse the burette and other flasks with distilled water before using. 

3. No tinge of reddish hue should remain at the endpoint, the solution should be clear blue. 

4. Titration should be performed slowly near the endpoint for the detection of sharp and correct endpoints. 

Types of Complexometric Titration

EDTA can be used as a 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. A few of them are given as follows – 

• Direct Titration – It is the most convenient and simple method of complexometric titration using EDTA. It is similar to the acid–base titration technique. In this titration standard, EDTA solution is added to give sample containing metals using burette till the endpoint is achieved. Copper, barium, zinc, mercury, aluminium, lead, bismuth, chromium, etc. are metals that can be determined by using direct complexometric titration. 

• Back Titration – It is called back titration as in this type of titration, we perform back titration for the excess amount of EDTA. In this, an excess amount of standard solution of EDTA is added to the metal solution being examined. Then the excess amount of EDTA is back titrated by the solution of the second metal ion. 

• Replacement Titration – It is used when direct titration or back titration don’t give sharp endpoints. This analyte (containing metal) is added to the metal–EDTA complex. The metal present in the analyte displaces another metal from the metal–EDTA complex. 

• Indirect Titration – Some anions form a precipitate with metal cations. These anions do not react with EDTA. So, these can be analyzed by indirect titration with EDTA. For example, barium ions can be determined by indirect titration. 

Applications of Complexometric Titrations

• Complexometric titration is used for the estimation of the amount of total hardness in water. 

• It is widely used in the pharmaceutical industry to determine the metal concentration in drugs. 

• Titanium dioxide is used in many cosmetic products. This can be analysed by complexometric titration. 

• It is used to analyse urine samples.

• It is widely used in analytical chemistry. 

Conclusion

Hence, the above article explained the concept of complexometric titration. This article is also useful for the students who will perform this experiment in the laboratory.