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Test pre-preparation–Why you should evaluate your sample before testing

Posted by Roderick McIntosh on Aug 19, 2020 12:34:47 PM

Contents
Introduction
Literature reviews
Oxidization Balance Checks
References



Introduction

 

After my previous post regarding loading a test cell, I am reminded of the time I had to manually inject caustic soda (sodium hydroxide - NaOH) into hydrogen peroxide (H2O2). I charged the peroxide into the Phi-TEC II test cell and then started a “closed-cell” test. This style of testing allows for heating and temperature control to start before the two reagents are mixed. When the test conditions were ready, I carefully put a known amount of the caustic soda solution into a syringe, and then connected it to the test cell feed line.

As I pressed the syringe barrel down to force in the caustic soda solution, I could feel back pressure starting to build up. Once I had little more than half of it in, I could no longer hold the back pressure on the syringe barrel and had to close the feed line valve and let the mixture react. Within a few seconds, the temperature had increased substantially, and the mixture underwent thermal runaway. The reaction mixture had also given a considerable pressure rise, perhaps not particularly surprising given the presence of H2O2.

Literature reviews

Sometimes when we are planning an experiment, it is worth doing some literature preparation and investigation. Quite often, there will be known chemical incompatibilities, and there are resources where these incompatibilities are recorded. Bretherick’s Handbook of Reactive Chemical Hazards (1), for example, is a fantastic resource. There is a vast number of chemicals listed in this book, along with a few other, perhaps superfluous, entries, including sunspots, “worker at next bench,” and the amusing “tin of beans.”

When I looked, I found that there was indeed an incompatibility listed of caustic soda with hydrogen peroxide. While our test had allowed us to answer the question if the materials were compatible or not, it is an example of where a short time in the library would have saved a day in the lab. Bretherick’s (1) is an extremely useful reference that should be part of any process hazard investigation. Indeed, it is an essential reference for the modern chemist to have in their library.

Oxidization balance checks

Another useful consideration looking at potential hazards is an oxygen balance (OB) calculation (2). This calculation gives an idea of the potential of a molecule breakdown in an explosive manner, or at least a highly energetic decomposition. The closer to zero (or more positive) an OB value is, the more hazardous the compound likely is. .

Let’s say we have a molecule - CaHbOcNd

If this is fully oxidized, the reaction would be;

CaHbOcNd + (a +b/4 -c/2) O2 = aCO2 +b/2 H2O + d/2 N2

The oxygen balance calculation is:

Oxygen balance (OB) = [-1600 ( 2a + b/2 - c )]/ Mr (molecular weight of the molecule)

There are a few considerations needed when completing this calculation:

  • The oxygen balance calculation is intended for molecules with strong oxidizing groups (such as nitro-,       chlorates, peroxy-, etc.)
  • Other oxidizing entities, such as chlorine, are ignored
  • Nitrogen is assumed to remain unoxidized
  • The molecular structure is ignored
  • If the balance is more positive than -200 then take care, the substance may decompose violently
  • Most detonating explosives have an oxygen balance more positive than -100
    • Metallic atoms have not been included in this example but would provide another term in the OB                equation

For example, if we take trinitrotoluene, C6H2.CH3.(NO2)3

The oxygen balance becomes:

-1600 ( 14 + 5/2 - 6 ) / 227 = -74

This is more positive than -100, meaning the molecule is likely to undergo explosive or a detonation decomposition – no great surprise, trinitrotoluene is a known explosive.

It is important to note that the oxygen balance does not apply to every molecule or compound, and should only be applied to compounds with strong oxidizing groups. For example, if the calculation was incorrectly applied to methanol (CH3OH), we would calculate a value of -150. This result suggests that methanol has the potential to decompose violently, whereas, in reality, we know this is not the case.

The oxygen balance calculation is a useful consideration when assessing potential hazards and can indicate when to expect an explosive runaway, but only if applied in the correct circumstances. Most chemists usually know which functional groups are likely to cause problems, by merely looking at the molecule construction – the oxygen balance gives an idea of the potential severity before any tests are run.

 

References

 

  1. Bretherick, L. Bretherick’s Handbook of Reactive Chemical Hazards - 8th edition. s.l. : Elsevier, 2017. ISBN: 9780081009710.
  2.  
  3. John, Barton. Richard, Rogers. Chemical Reaction Hazards. s.l. : Gulf Professional Publishing, 1997. ISBN: 9780884152743.

Topics: process safety, Phi factor, low phi factor test cells, Phi-tec II

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