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Diazonium Chloride Supersaturation Test

1.0 Introduction

1.1 It is vital to ensure that all diazonium chloride solutions are not supersaturated.  If supersaturation occurs the precipitation of dangerously unstable diazonium salts can occur at ant time, on a plant scale.  This is believed to have led to at least one fatality in the process industry.

1.2 Explosions due to drying out of diazonium salts have occurred in the recent past in several processes in the fine chemical and pharmaceutical industries.  Numerous fires and occasional explosions in the ventilation systems connected to diazonium vessels have been due (at least in part) to the formation of diazonium salts.  These fires have occurred on numerous sites around the world.

2.0 Safety

2.1 Diazonium salts are known to be dangerously unstable.  As solids they can detonate due to shock, impact, friction, heat, vibration, or static discharge.  Some diazonium salts are so unstable that in the ambient environment they are only metastable can can detonate unpredictably or spontaneously.

2.2 Where known or suspect materials have to be handled there are two main hazards:

i)   Shock waves and
ii)  Flying debris and
It is important to protect all personnel from these two hazards

2.2.1  Shock Waves

The basis of protection is to limit quantities and disperse or phlegmatise (add inert diluent to any solids produced) and avoid both transport and storage of such materials.  Suitable personal protective equipment must also be worn.
Rule 1. The maximum quantity should be limited to less than 100mg. of total suspect solids in the lab at any one time.
Rule 2. Any solid should, where ever possible, be dispersed in an inert medium.
Rule 3. The material must be made, tested and destroyed in situ in the same day.
Rule 4. The material should not be heated above 40°C
Rule 5. The material must not be ground or crushed.
Rule 6. The material must not be transported outside the laboratory unneccessarily.
Rule 7. The material must not be stored.
Rule 8. Normal precautions against static discharge must be taken
Rule 9. Ear defenders must be worn.

2.2.2  Flying debris

The basis of protection is to avoid confinement, use screens (and/or remote handling equipment where possible) and wear suitable personal protective equipment
Rule 10. Do not handle or store in sealed containers unless this is essential.
Rule 11. Use shatter proof polycarbonate or Perspex screens
Rule 12. Wear leather gloves.
Rule 13. Wear a full face shield and heavy duty PVC or leather apron.

2.3 General Safety - All normal precautions applicable to lab operations must be taken (Risk Assessment - including COSHH, fume-cupboards, disposal, etc.)

3.0 Method for test of supersaturation

3.1 The aim is to produce the minimum quantity of diazonium salt safely and to check the standard plant recipe for supersaturation.

3.2 The principle is to make about 2g of solution in a test tube and to generate a small quantity of solid dispersed (phlegmatised) in filter paper.  The solid is then introduced into the remainder or the solution slightly below the normal operating temperature and checked visually for precipitation.

3.3 Method (not suitable for sulphuric acid based processes)

1. Following normal precautions make a standard plant solution of the diazonium salt.
2. Place in a cleared fume cupboard:-

  1. A vacuum oven set to 20°C.
  2. A bath set to 5°C below normal plant temperature.
  3. A test tube
  4. A length of filter paper, marked at one end, that will fit the test tube.
  5. A teat and Pasteur pipette
  6. A stand and clamp
  7. A screen in front of the bath
3. Transfer about 2ml of the solution to the test tube.
4. Place the test tube in the bath.
5. Pipette one drop of solution onto the unmarked end of the filter paper and place in oven.  Evacuate the oven and leave for one hour.
6. Wear full face shield, ear defenders, leather gloves and apron. 7. Remove filter paper from oven and examine for signs of decomposition. (if any decomposition occurs repeat with oven at RT for 4 hours) and add to the test tube.
8. Gently shake tube. Inspect tube for turbidity or precipitaion.
9. Replace test tube in bath and repeat step 8 every 30 mins for 3h. 10. Any solid precipitate should be dissolved in additional acid
11. Dispose of all solutions in the normal manner.

4.0 Conclusions

The above method ensures that only the minimum of solids are produced (1/20th ml of solution or 25mg of solid).  The solids are dispersed (phlegmatised) in cellulose and any precipitate is limited, phlegmatised in its own solution and easily dissolved.  (Heating is limited, and no unnecessary transport or storage is involved.

It is a criminal offence in the UK to knowingly make dangerous quantities of explosives without a license.  The above procedure is believed to be legal in the UK.

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