pH in Diazo Coupler


Diazonium Coupling (Diazo Coupling, Azo Coupling)Diazonium Coupling (Diazo Coupling, Azo Coupling):

Due to their positive charge, diazonium cations, which are generated by treatment of aromatic amines with nitrous acid and a stronger mineral acid, may participate in an electrophilic aromatic substitution as an electrophile. The electrophilic reaction center is the terminal nitrogen of the - N=N group. As a result, two aromatic compounds are coupled by a -N=Ngroup. This is known as the azo group (diazo group).

graphThe corresponding reaction is called diazonium coupling (diazo coupling, azo coupling). However, the electrophilicity of diazonium ions is only relatively weak, as their positive charge is delocalized.an2 The unsubstituted benzenediazonium cation may react only with strongly activated aromatic compounds, such as phenolates and amines.

Azo coupling is the most widely used industrial reaction in the production of dyes, lakes and pigments. Aromatic diazonium ions acts as electrophiles in coupling reactions with activated aromatics such as anilines or phenols.

The substitution normally occurs at the para position, except when this position is already occupied, in which case ortho position is favoured. The pH of solution is quite important; it must be mildly acidic or neutral, since no reaction takes place if the pH is too low.

Application Information

Typical process details:

  • Chemical Composition - Coupler + Diazo Coupler – 48 % caustic + Nepthol + Water Diazo -36% HCL + Diecloro Anelene + Water Operating Temperature – 55°C
  • Operating Pressure – 4 kg/cm2


The process is indeed quite aggressive so Glass is best material.

Sensor Selection:

Sensor Option #1: SC21C-AGC55 would be best, if it is possible to pressurize the electrolyte to 5 bar (72PSI).

Features for type SC21C-AGC55

  • Heavy duty pH sensitive glass.
  • Flowing reference system for pollution resistance, and highly stable reference potential.
  • Use in combination with the presurisable electrolyte reservoir to obtain a positive flow towards the process (K1500YA)

Sensor Option #2: SC24V is possible if the cation (salt) content is stable during the pH control step.


The SC24V is a differential pH sensor. This means that the reference is not a (liquid) junction but a glass sensor which does not respond to pH changes (within the applicable range of the sensor). Therefore the sensor is truly maintenance free and the output voltage of the sensor depends only on the salt concentration of the process.

The sensor responds to pH changes rather than analyses the accurate pH value. In that sense it is best to describe the sensor as pH control sensor rather than pH measuring sensor. A pH sensor measures the voltage that the pH membrane measures as function of the pH value of the process sample. This voltage is then compared with the mV output of a reference cell that is independent on the pH value of the sensor.

In most pH control applications the salt concentration is rather constant, so the output of the SC24V differential sensor is only dependent on the pH of the process. A rule of thumb is that a change in salt concentration of +/- 25% has an effect of less than 0.1pH on the pH reading.


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