MICROCHEMICAL
REACTIONS
Microchemical identification
methods are not very sensitive - we use solutions of about 1 to 10 grams of
the substance per litre - but only a minute quantity of such a solution is needed.
As one drop equivalent to 30 mg is sufficient, the detection limit is 0.3 to
0.03 mg, in some cases even well below these values! These methods are still
used today for quick screening tests of minerals and non-destructive tests or
examinations for valuables and pictures. Usually a tiny splinter of the sample
is placed on the slide, heated with a drop of acid and covered with a coverslip.
Then a suitable reagent is placed on the sideline of the coverslip and the resulting
crystals are examined . Since the shapes of crytals are most typical this method
is highly selective and meaningful.
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| Potassium perchlorate |
Lead iodide |
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One can even examine
extremely inert minerals. A splinter of the mineral is heated together with
a suitable fusing agent before the blowpipe, using a small loop made of platinum
wire. Since locally temperatures far above 1000 oC are achieved even resistant
minerals react with the fusing agent. The bead is then dissolved on the slide
and microchemically investigated.
Solve a small quantity
of cigarette ash in a drop of hydrochloric acid. After adding some
perchloric acid you get the typical "coffin-shaped crystals" of
potassium perchlorate that sparkle like jewels in polarized light.
Plant ash contains potassium, so we use potassium salts as fertilizer, but
not sodium salts, as sodium is toxic to all land plants.
In the second example some white
pigment was dissolved in nitric acid and mixed with a small amount of potassium
iodide. The yellow six-sided panels are typical of lead iodide - the
white lead pigment was therefore not titanium oxide, which is used today. If
this simple test shows a negetive reaction when applied on the white pigment
of an old picture, then the picture probably is a forgery. And if the white
pigment is identified to be titanium oxide then the forgery is evident!
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