Complexometric Titration

As we all know, the number of diabetes patients is increasing day by day worldwide. Do you know the drugs used for 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 analysis in which the end point is indicated by a colored 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 end point of the titration. 

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

Complexometric Indicator

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

Complexometric Titration using EDTA

You must have heard about hard water problem of Noida. If you want to know 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 experiment for estimation of hardness of water by using complexometric titration. This experiment is also a part of Chemistry Practical Syllabus of coordination chemistry of Class XII CBSE. 

Aim

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

Requirements

Conical flask, burette, pipette, spatula, buffer solution, eriochrome black T indicator, standard EDTA Solution (0.01M), inhibitor. 

Principle

Indicator Eriochrome Black T becomes wine red in color when binds with metal ions while remain blue in color when free from metal ion. While EDTA (which is ethylene diamine tetra acetic acid) is colorless whether its bound to metal ion or not. So, addition of EBT indicator in the sample (water containing metal) makes it wine red in color as eriochrome black 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 remain free in the sample and solution turns blue. 

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We can write complex forming ion of EDTA as H2Y2- which it forms in 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+

Structure of Eriochrome Black – T (Indicator)-

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Reactions including Eriochrome black -T –

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Procedure

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

  2. Now add 1ml of buffer solution.

  3. Now in 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 color of the solution wine red. 

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

  6. End point can be detected by color change from wine red to blue. 

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

Observation Table

S.No.

Volume of water sample (ml)

Burette Reading 

Volume of EDTA added (ml) [Initial - Final]

Initial(ml)

Final(ml)













 

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 – 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 detection of sharp and correct endpoints. 

Types of Complexometric Titration

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 – 

  • Direct Titration- It is the most convenient and simple method of complexometric titration using EDTA. 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. Copper, barium, zinc, mercury, aluminum, lead, bismuth, chromium etc. are metals which 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 excess amount of EDTA. In this excess amount of standard solution of EDTA is added to the metal solution being examined. Then excess amount of EDTA is back titrated by solution of second metal ion. 

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

  • Indirect Titration – Some anions form 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 estimation of amount of total hardness in water. 

  • It is widely used in pharmaceutical industry to determine 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 sample.

  • It is widely used in analytical chemistry. 

If you want to know more about titration then go through the articles Mohr salt titration with KMnO4, precipitation titration, oxalic acid titration with KMnO4 etc. available on Vedantu. You can register yourself on Vedantu or download Vedantu learning app for Class 6-10, IITJEE and NEET for more such articles, NCERT Solutions, study material and mock tests etc. You can also join Vedantu Online classes conducted by our master teachers to clear your doubts related to the topic.