spot tests for explosives

Question 1

What is fire?

Answer 1

conversion of solid matter to its gaseous form followed by

• rapid oxidation of a gaseous fuel
• rapid release of energy from the exothermic reaction

the heat released in the reaction is called “heat of combustion”

gases are heated to a high temp

gases are soot emit light and heat

Question 2

Fire triangle

Answer 2

energy, oxidiser, fuel

Question 3

What is flash point?

Answer 3

lowest temp at which there is sufficient vapour pressure to cause a flammable mixture if ignited by spark or flame

Question 4

What is fire point?

Answer 4

just above flash point. lowest temp at which a substance continues to burn

Question 5

What is ignition temp?

Answer 5

minimum temp at which fuel will spontaneously ignite

spark or flame free event

the presence of oxygen is always a requirement

Question 6

What is an explosive?

Answer 6

common term is a bomb

bomb is any explosive device that typically relies on the exothermic reaction of an explosive substance which will produce an extremely sudden and violent release of energy producing immense quantities of gas

the build up of gas pressure, a confined space is the actual “explosion”. the damage is caused by rapidly escaping gases from confinement.

the ignition of an explosive is called detonation

combustion of explosives occurs so rapidly that there isn’t enough time for the oxygen in the surrounding atmosphere to combine with the fuel

therefore many explosives must have their own source of oxygen or oxidising agents

Question 7

Types of explosives

Answer 7

explosives are classified as high and low explosives based on their speed of decomposition

in a low explosive, this speed is called the speed of deflagration

in high explosive, it is called the speed of detonation

Question 8

Is black powder a high or low explosive?

Answer 8

low

Question 9

What is black powder comprised of?

Answer 9

75% potassium nitrate
15% charcoal
10% sulphur

Question 10

What is the oxidising agent in black powder?

Answer 10

KNO3

Question 11

Black powder

Answer 11

volume expansion = 5100x
burns when confined
becomes lethal and explosive when confines

heat applied to powder, oxygen from KNO3 is liberated
combines with the carbon and sulphur
combustion of charcoal and sulphur produces 2 gases (CO2 and SO2)

Question 12

Are organic explosives high or low explosives?

Answer 12

high

Question 13

what is the most common compound used in organic explosives?

Answer 13

nitro-compounds

Question 14

What is nitro-glycerine?

Answer 14

organic explosive

viscous liquid at room temp that explodes violently on agitation

Question 15

What is dynamite?

Answer 15

nitro-glycerine absorbed into a sol support

Question 16

Initial extraction of explosive residues

Answer 16

should be done with acetone
extracts both inorganic and organic compounds to a sufficient extent to enable spot tests to be performed
although nitrocellulose and nitrostarch are not truly soluble they form a colloidal solution which can be used

Question 17

What is used as a spot test for ammonium ions?

Answer 17

nessler reagent

solution made from a mixture of potassium tetraiodomercuate in KOH

Question 18

what is the positive result for ammonium ions using nessler reagent?

Answer 18

orange/brown precipitate

Question 19

What is used as a spot test for nitrates?

Answer 19

diphenylamine
diphenylamine reaction of nitric acid proceeds via the oxidation of diphenylamine to the colourless N,N’-diphenylbenzidine

Question 20

What colour shows a positive result for nitrates when using diphenylamine?

Answer 20

dark blue

Question 21

what is nitrocellulose?

Answer 21

a constituent of smokeless powder and some dynamites. The ONO2 group of the nitrocellulose can be detected by fusion with benzoin. When heated to 130 degrees vapours formed are tested with paper impregnated with Griess reagent. A positive red stain develops on the paper

Question 22

What is nitroglycerin?

Answer 22

can be detected using the DPA reagent and H2SO4. a deep violet semiquinoid compound is formed. Selective detection of nitroglycerin is achieved by exploiting its relatively high vapour pressure

Question 23

Zinc

Answer 23

homemade explosives

diphenylthiocarbazone in an alkaline solution forms insoluble, coloured inner complex salts with many metal ions

Question 24

magnesium

Answer 24

explosive devices often use magnesium filings or powder.

magnesium salt give a blue precipitate in alkaline solutions of quinalizarine.

Question 25

What is 2-4-Dinitrotoluene (DNT)?

Answer 25

frequently used as a plasticiser, deterrent coating and burn rate modifier in smokeless gunpowder

Question 26

What is 2,4,6-trinitrotoluene (TNT)?

Answer 26

military explosive

Question 27

DNT and TNT

Answer 27

characteristic red to violet colours develop when DNT and TNT compounds are treated in alkaline solutions.
a new glowing spray on film rapidly indicates the presence of nitrogen-based explosive residues via fluorescent polymers

Question 28

Chlorates

Answer 28

used in many explosives and primarily homemade devices
in strong phosphoric acid solution, chlorates warmed with MnSO4 form violet coloured manganese III phosphate ions. This is an extremely robust test for chlorates.

Question 29

Perchlorates

Answer 29

present in most flash powders and form oxidising agents in many homemade devices
a relatively selective test for perchlorates is based on the formation of violet precipitates in methylene blue solution