Chapter 4

Diazotization Titrations

INTRODUCTION

The diazotization titration is nothing but the conversion of the primary aromatic amine to a diazonium compound. This process was first discovered in 1853 and was applied to the synthetic dye industry. The reaction mechanism was first proposed by Peter Griessin. In this method, the primary aromatic amine is reacted with the sodium nitrite in acidic medium to form a diazonium salt. This method is first used in the determination of dyes.

PRINCIPLE

The principle involved in this method is that the primary aromatic amine present in the sample reacts with the sodium nitrite in the presence of acid such as hydrochloric acid to obtain a diazonium salt.

R − NH₂ + NaNO₂ +HCl R − N⁺ ≡ N − Cl⁻ + NaCl + H₂O

Sodium nitrite is added to the solution of amine in the presence of acid at 0–5 °C. The amine reacts with the nitrous acid to form nitrosamine, which is followed by the tautomerisation and the water molecule is lost to form the diazonium ion. This diazonium ion is stabilized by the displacement of the positive charge at the ortho and para positions of the ring.

C₆H₅NH₂ + NaNO₂ + HCl C₆H₅N = NCl + NaCl + H₂O

THEORY

When sodium nitrite is reacted with the hydrochloric acid sodium chloride and nitrous acid are formed.

NaNO₂ + HCl NaCl + HNO₂

The obtained nitrous acid is reacted with the primary aromatic amine to form the diazonium salt. The excess of nitrous acid is removed by the addition of ammonium sulphamate solution.

R − NH₂ + HNO₂ R − N = NH + H₂O

The end point is detected by the formation of the blue colour with starch iodide paper. This is prepared by immersing the filter paper in the starch mucilage and potassium iodide solution.

KI + HCl KCl + HI

2HI + 2HNO₂ I₂ + 2NO =2H₂O

I₂ + starch mucilage blue colour end point

PROCEDURE

The general procedure is followed by weighing the sample and transferring it into the standard flask. Then concentrated hydrochloric acid and potassium bromide are added and the rest of the volume is filled with the distilled water. This resulting solution is known as the standard solution.

The appropriate volume of the standard solution is pipetted out and the temperature is maintained at 0-5 °C. Then the solution is titrated with the sodium nitrite solution until the starch iodide paper turns into blue colour.

Another procedure is—after maintaining the conical flask temperature, the pair of platinum electrodes is immersed. Then the electrodes are connected to the potentiometer and slowly titrated with sodium nitrite solution until a permanent deflection is observed at the end point.

END POINT DETECTION

The end point in diazotization titration is detected by the following procedures:

• The excess of nitrous acid is determined by the addition of the starch iodide as an external indicator. After diazotization, one drop of the resulting solution is placed on the starch iodide paper which changes into dark colour.
• Another method for the detection of end point is by immersing the platinum electrodes in the resulting solution and it is also detected by the dead-stop end point method.
• The next method for the detection of the end point in the diazotization titration is by adding the potassium iodide to the nitrous acid with excess acid which liberates the iodine. The liberated iodine is back titrated with the sodium thiosulphate using starch as the external indicator. The end point is detected by appearance of blue colour.

   

  KI + HCl HI + KCl
  HI + 2HNO₂ I₂ + 2NO + 2H₂O

Preparation and Standardization of the Sodium Nitrite Solution

Appropriately weighed sodium nitrite is dissolved in the water and made up to the desired volume.

Standardization of the sodium nitrite is carried out by titrating the previously dried sulphanilamide dissolved in the water and hydrochloric acid solution which is cooled to 15 °C with standard solution of the sodium nitrite.

FACTORS AFFECTING THE DIAZOTIZATION

1. Acid concentration.
2. pH of the NaNO₂.
3. Temperature of the reaction (should be maintained at 0–5 °C): the diazonium compounds are decomposed at elevated temperatures.
4. Reaction time (it takes 10–15 min): the compounds react with nitrous acid at different rates based on the nature of the compound.
5. Slow diazotizable groups: sulpha groups, carboxylic groups and nitrogen oxide group.
6. Fast diazotizing groups: anilide, toluidine and aminophenol.

CONDITIONS FOR THE DIAZOTIZATION TITRATION

The following conditions are required for the diazotization titration of the amino group containing samples. They are as follows:

1. Rate of titration: Addition of sodium nitrite to the sample solution takes time to react with the amino group present in the sample solution. Different amino compounds react with the nitrous acid at different rates. Based on this, the amino compounds are classified into two main groups. They are as follows:
   1. Slow diazotizable compounds

      Example: Sulphanilic acid and anthranilic acid

   2. Fast diazotizable compounds

      Example: Aniline, aminophenol, and toluidine

      The reaction rate is increased by the addition of the potassium bromide solution.

2. Temperature: Maintenance of the temperature is the main condition for the diazotization titration. The diazonium salts formed are not stable at elevated temperatures. They are readily decomposable at elevated temperatures, therefore, the temperature should be maintained at 0–5 °C.

Types of Diazotization Titrations

There are mainly three types of methods based on the titration procedure. They are as follows:

1. Direct method: The main principle involved in this method is to treat the amino group containing drug with the acid solution. The resulting solution is immersed in the cold water bath or ice water bath by maintaining the temperature at 0–5 °C. Then this solution is titrated with the sodium nitrite solution. The end point is determined by the above-mentioned methods.
2. Indirect method: The principle involved in this method is that the excess nitrous acid is added to the titration sample solution and it is back titrated with the other appropriate titrant. This method is mainly used for the titration of insoluble diazonium salts.
3. Other method: The main principle involved in this method is the formation of the diazo oxide which is more stable than the diazo compounds. For example, the aminophenol is readily oxidized by the nitrous acid and converted to the quinones in the presence of copper sulphate solution and forms the diazo oxide compounds. This readily undergoes the coupling reaction with the nitrous acid.

ADVANTAGES

• Selective for the all types of sulphonamides.
• Sensitive
• Reproducibility

DISADVANTAGES

• Applicable for a very less variety of samples.
• Relatively slow when compared to other methods.
• Temperature conditions to be properly maintained throughout the reaction.
• The end point detection is very difficult.
• The colour produced is not stable.
• Lack of specificity.

APPLICATIONS

• Used in the determination of the sulphonamides.

  Method: An accurately weighed 1 mg sample of sulphonamide is dissolved in the 4 ml of concentrated HCl and in 10 ml of distilled water. Then, this solution is cooled to 15 °C and titrated with the 0.1 M of sodium nitrite solution. The end point is determined by streaking one drop of the titration solution on the starch iodide paper until blue colour is appeared. The percentage amount of the sulpha drug is determined by the following equation:

   

  

   

  where V is the volume of the titrant consumed; M is the molarity of the titrant; EW is the equivalent weight of the drug; W is the weight of the sample.

• Used in the determination of the chloropheneramine.

  Method: The accurately weighed sample is added to the 5 ml of HCl and 50 ml of distilled water. Then the solution is cooled to 15 °C. Then the solution is slowly titrated with the 0.1 N sodium nitrite solution using starch iodide paper as the indicator.

• Used in the determination of the dopamine.
• Used in the determination of the procaine.
• Used in the determination of the amphetamine.
• Used in the determination of the procaine
• Used in the determination of the ephedrine.
• Used in the determination of the P-amino benzoic acid (vitamin B₄).

  Method: The accurately weighed sample is added to the 5 ml of HCl and 50 ml of distilled water. Then the solution is cooled to 15 °C. Then the solution is slowly titrated with the 0.1 N sodium nitrite solution using starch iodide paper as the indicator.

   

  1 ml of 0.1 N sodium nitrite ≡ 0.01371 g of vitamin B₄

REVIEW QUESTIONS

1. What is the principle involved in the diazotization titrimetry?
2. What are the conditions required for the diazotization titrimetry?
3. What are the example drugs assayed by the diazotization titrimetry?
4. What are the advantages of diazotization titrimetry?
5. What are the factors that affect the diazotization end point?
6. What are the different methods used for the end point detection in the diazotization titrimetry?