(2011-08-07) Basic chemical accounting:
Moles of stuff.
A modern convention that helps put chemical history in perspective.
In chemical equations, a symbol or a formula for a substance is always understood
to denote a definite quantity of it (measured by weight)
technically called a mole of that substance
(symbol: mol ). Thus, when
a non-chemical quantity (most commonly, energy)
appears in a chemical equation,
it's understood to pertain to the implied number of moles.
The name stands for the deprecated term molecule-gram
which was coined when it became known that a chemical species
is normally made of identical units (molecules, ions, etc.).
A mole is merely a particular
number of those things (as many of them as there are
atoms in 12 g of carbon, when only the
dominant isotope is present).
The number of things per mole of stuff is a huge constant,
called Avogadro's number,
known to 7 decimal places:
Na = 6.022141 10 23/ mol
Nevertheless, the convention of using moles uniformly for
all chemical substances doesn't strictly depend on the underlying concept
of atoms and molecules.
It was already made legitimate by the prior
law of definite
proportions (Proust's Law) formulated by the
Frenchman Joseph Proust (1754-1826)
based on the combustion experiments he conducted between 1798 and 1804.
Proust observed that iron (Fe) and "almost every known combustible" may unite with only two
constant proportions of oxygen (by weight).
In modern terms, one example would be CO and CO2
The study of the simple fixed ratio in which moles of various chemicals
combine to form pure chemical compounds is known as
stoichiometry.
Mixtures are different from pure chemical compounds.
Although this is rarely done, if ever, they could be expressed as
linear combinations of the pure chemicals they
consist of. For example, a mole of dry air at sea-level is
approximately
0.78 N2 +
0.21 O2 +
0.01 Ar or, more precisely:
0.7808 N2 +
0.2094 O2 +
0.0094 Ar +
0.0004 CO2
Incidentally, this gives the often-quoted molar weight of air (29 g/mol):
For gases, such molar compositions are often said to be by volume
because of the great nineteenth-century discovery
(Avogadro's law) that equal volumes of
two different gases contain approximately equal numbers of moles
(the lower the pressure, the better the approximation).
For anything but gases, we must use the known molar weights
of the constituents to obtain the molar composition of a mixture from
its weight composition (or vice-versa).
Molar weights were the key to the final
classification of the chemical elements
presented by
Dmitri Mendeleev in 1869
(before the more fundamental notion of atomic number
was made clear, in part as a result of this classification).
The molar weight of a molecule is the sum of the molar weights of the individual
atoms that compose it.
When the utmost in precision is called for (it almost never is)
a relativistic adjustment should be made by
subtracting from that sum a tiny "mass defect" equal to the
binding energy divided by c2.
The modern notion of a chemical element was first proposed,
on empirical grounds,
by Robert Boyle in 1661.
A chemical element is a species that cannot be obtained by combining
other chemicals.
The system of chemical symbols we currently use to
represent every element was devised by
Jacob Berzelius around 1810.
Every symbol consists of one or two latin letters,
only the first one is capitalized:
H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar...
Every element is now known to correspond to one type of atomic
nucleus.
The structure of every neutral atom as a positive nucleus
"orbited" by negative electrons was first proposed by
Ernest Rutherford in 1911
(to explain the results of the notorious Gold
Foil Experiment of 1909).
More precisely, each element is identified by a
unique integer called its atomic number which is usually
denoted by the capitalized letter Z (initial of the
German word Zahl for number ).
The number Z corresponds to the number of elementary positive
charges (one such charge per proton) cointained in every
atomic nucleus of that element.
The atomic nuclei corresponding to a given element
(i.e., a given atomic number Z)
exist in slightly different masses because they
may contain different numbers of neutrons
(a neutron has a mass nearly equal to that of a proton but no
electric charge at all).
The total number of nucleons
(i.e., protons and neutrons) in a nucleus is called
its mass number and is usually denoted by
the letter A.
Nuclei that have the same value of Z but different values of
A are said to be different isotopes of the
same element.
The existence of isotopes explains a fact that puzzled early chemists,
namely that the molar weight of a substance is usually close to an
integer multiple of that of elemental hydrogen (weighing half as
much as hydrogen gas) but is occasionally far from that.
In particular, the molar weight of elemental chlorine (Cl)
is 35.45 g/mol because natural chlorine is essentially
a mixture of
two isotopes:
75.8% of Chlorine-35 (34.9688527 g/mol)
and 24.2% of Chlorine-37 (36.9659026 g/mol)
The discovery of isotopes is usually credited to
J.J. Thomson (1856-1940;
Nobel 1906)
whose
invention
of mass spectrometry, in 1913, established the existence of
two stable isotopes of Neon (Neon-20 and Neon-22).
However, the existence of radioactive isotopes was also established in 1913 by
Frederick Soddy
(1877-1956; Nobel 1921)
who coined the word isotopes in 1914,
from a suggestion made by a distant relative of his,
Dr. Margaret Todd
(1859-1918).
Isotopes have virtually identical chemical properties, except possibly for
the lightest elements (e.g., the different masses of protium and deuterium,
the two stable isotopes of hydrogen, yield measurably
different ionization potentials).
Most elements below Uranium (Z = 92) have several stable
or very long-lived isotopes. Some have only one, some have none.
The lightest element without a naturally-occuring stable isotope is
Technetium (Z = 43) whose
many isotopes
include Technetium-99,
the most significant long-lived fission product
from commercial nuclear reactors.
Tc-99 has a half-life of 211100 years and a
specific radioactivity of 0.62 GBq/g.
The element carbon (C)
corresponds to Z = 6.
Old natural carbon found in mineral deposits
(including carbonates, coal and crude oil) is not radioactive at all,
because it contains only the two stable carbon isotopes:
Nearly 99% of Carbon-12 (6 protons and 6 neutrons, which
serves for the above modern definition of
the mole)
and 1% of Carbon-13 (6 protons, 7 neutrons).
On the other hand, new carbon in living organisms
is radioactive because of trace amounts of Carbon-14,
a radioactive isotope dubbed
radiocarbon
(6 protons, 8 neutrons)
which is obtained from the carbon dioxide in the air (either
directly by photosynthesis or indirectly by consuming
carbon compounds from other living organisms).
Radiocarbon is constantly formed
cosmogenically
by transmutation of nitrogen in the upper atmosphere.
As radiocarbon decays with a half-life of about
5700 years, the radioactivity of a sample of carbon depends directly
on its age, defined as the time elapsed since the
creature that originally fixed the carbon stopped breathing
(this is the basis for the technique known as carbon dating ).
Mineral carbon is not radioactive at all because either
it lacks any biological origin or
because its biological origin is so ancient that
all traces of radiocarbon have long disappeared.
This modern view of chemical elements has replaced the antiquated doctrine of the
four classical elements (fire, water, air and earth)
which was first proposed by
Empedocles
around 450 BC.
Backed by the great authority of
Aristotle
(384-322 BC) that misguided doctrine hindered the development of both
alchemy and chemistry for two millenia.
(2011-07-18) Alembics and Stills
(3rd century AD)
Purification by evaporation and condensation.
According to Egyptian mythology, Alchemy
was founded by the goddess Isis.
As Alchemy seemed similar to cooking, it was
once considered to be a feminine art, or women's work
(opus mulierum).
This goes a long way toward explaining that one of the earliest alchemist on
record is a woman... She lived in Alexandria in the third century AD.
Her real name was probably Miriam. In English, she's known as
Mary the Jewess.
According to the custom of her day, she concealed her identity by using a
legendary name
as a pseudonym, signing
Miriam the Prophetess, sister of Moses (amusingly, this
caused a lot of confusion among people who took this literally).
Miriam is also known by many other names, including
Maria Prophetissa, Maria Prophetissima,
Mariya al-Qibtyya, Maria the Copt,
Maria the Sage "daughter of the King of Saba",
the Matron Maria Sicula, etc.
Aristotle (384-322 BC)
already knew that fresh water could be obtained by
condensation from evaporated seawater (that's the way Nature
produces rain).
Pliny the Elder
(AD 23-79) describes how distillation was done in his days,
using fibers to absorb condensed vapor in the lid of a covered vessel.
Miriam devised the first true distillation apparatus by letting
the vapor escape in pipes through a modified lid which is now called
a still-head
(the learned term is alembic which is the Arabic name
denoting either that specific part or the whole apparatus).
The still-head and/or the rest of the pipes are cooled by air or water
(wet sponges) to make vapor condense.
Finally, the condensed liquid is collected in receiving vessels.
Fire, heater, oven.
Boiler, cucurbit, still.
Still-head, alembic.
Condenser.
Receiver.
Miriam's original contraption, the tribikos,
called for 3 pipes and 3 receivers.
It is pictured at right, with another simpler design
attributed to her most famous student
Zosimos
of Panapolis. The illustration is from
a famous Alexandrian manuscript written by Zosimos
(third or fourth century AD) which seems to be the oldest
extant alchemical text.
If the condenser operates normally, the apparatus works at constant volume
(no vapor escapes).
Arguably, this key innovation marks the beginning of the slow transition
from ancient alchemy to modern chemistry.
(2011-07-19) Retort (8th century AD)
More than a simplified alembic.
Around 750 AD, Geber invented a simplified
distillation apparatus called a
retort (French: cornue)
as a single piece of glassware adequate for crude distillations into any receiver vessel.
The shape remains one of the most
recognizable symbols for alchemy or chemistry.
Although no longer as popular as it once was, this device remains
a great choice for crude high-temperature distillation
or as a reaction vessel for chemical reactions where a gas is evolved.
(2011-07-18) Production and Distillation of Alcohol
Alcoholic beverages in Prehistory. Hard liquor since the Middle Ages.
For thousands of years, alcohol (C2H5OH)
was of primary importance to human survival,
because it provided safe beverages under unsanitary conditions.
The ancestors of beer and wine had enough ethanol in them
to kill common bacteria and viruses before they infected the drinker.
(In the Orient, the tradition of boiling water to make tea had
similar benefits.)
Even sour wine (vinegar) is fairly safe, because of
the sterilizing effects of the acetic acid that results from the oxidation of alcohol,
mediated by AAB
using oxygen from the air (properly sealed wine won't turn into vinegar):
C2H5OH +
O2
®
CH3COOH + H2O
At first, the inebriating properties of alcohol were just a side-effect that
may or may not have been welcome... However, with only weak alcoholic
beverages available, those who sought that inebriation could not achieve it
without consuming relatively large quantities of liquid fairly rapidly...
At a concentration of about 14% (by volume) alcohol inhibits
the very enzymes (Zymase) that catalyze its production by
anaerobic fermentation:
Under normal atmospheric pressure, ethanol
(aqua vitae, C2H5OH)
cannot be separated from water by distillation alone, because a mixture of
95.629%
alcohol and 4.371% water (by weight)
actually forms what's called an azeotrope (a mixture whose vapor
retains the same composition as the liquid).
As the boiling point of that ethanol-water azeotrope (78.1°C)
is less than the boiling point of either ethanol (78.5°C)
or water (100°C) it tends to evaporate first.
Therefore, the vapor will never contain more than 95.629% of alcohol
by weight
(unless the liquid itself was stronger than that to begin with).
The 190-proof grain alcohol Everclear
made the Guinness Book of World Records in 1979
as the World's most alcoholic beverage.
Other brands of
neutral grain spirits
now include Golden Grain,
Gem Clear and Spirytus
(rectified spirit
from the former Polish state monopoly Polmos).
Some of those brands (including Everclear) are also
sold in lesser grades, because full-strength rectified spirits cannot
legally be sold in several countries or states, including California.
Typically, they are downgraded
to 151-proof spirits that mimick the alcohol content, but not the flavor,
of overproof rum (which has itself been banned in some places).
In the US, "N-proof" denotes a proportion of ethanol x = N/200
(by volume)
corresponding to the following percentage by weight:
y = 79% / (200/N - 0.21)
[ that's 100% for N = 200 ]
Conversely, N = 200 / (0.21 + 0.79/y) = 200 x
For the aforementioned azeotrope (y = 0.95629)
we obtain N = 193.03.
Thus, repeated distillation (rectification)
at normal atmospheric pressure
cannot yield anything stronger than 193 proof
(96.5% by volume).
The Spirytus Luksusowy Polish vodka is labeled
192 proof (96% by volume).
As no ethanol-water azeotrope exists below a pressure of 70 mmHg,
it's possible to obtain nearly pure alcohol by
vacuum distillations
(other chemical methods are used industrially to produce water-free alcohol).
(2003-10-08) Black Powder / Blackpowder / Gunpowder
What is the composition of black powder ?
The French call it either poudre à canon (gunpowder)
or poudre noire (blackpowder).
The loose powder was called serpentine.
The name black powder is of relatively recent origin,
as it appeared only after other explosives were devised which lacked the
black luster of free carbon.
Obviously, the stuff wasn't called gunpowder
before the gun was invented, around 1313.
The invention of the gun is often credited to brother
Berthold Schwarz (Schwartz),
a Franciscan friar from Freiburg
with a bogus last name ("Black" in German) indicating
his interest in alchemy, the black art;
the real name of "Black Bert" was most probably Constantine Anelzin.
He "invented" gunpowder only in the sense that he found a new use for old serpentine
and thus made the new name meaningful.
Black powder was the first explosive ever devised,
and it remained the only one for centuries.
It is composed of the following three solid ingredients:
Saltpeter:
KNO3niter
(or, more rarely, NaNO3Chilean nitrate).
Sulphur: S.
["sulfur" and "sulphur" are equally acceptable spellings]
Carbon: C.
Often as charcoal from wood (willow).
However, simply mixing the ingredients produces only inferior meal powder...
To obtain what's now considered proper black powder,
the ingedients must be "incorporated" in a damp state.
This allows the application of great pressure to form a dense cake,
ultimately broken down into dry grains.
This process is called corning,
and it was first introduced in France in 1429.
Early forms of blackpowder may have existed in China around
AD 700, using crude recipes calling for equal weights of the three components...
Such mixtures would only burn violently without exploding...
Also, explosion cannot occur if raw saltpeter is used,
and the refining of saltpeter is not mentioned before 1240 in a book on
military
technology by the Syrian scholar Hassan Al-Rammah, entitled
al-furusiyya wa al-manasib al-harbiyya.
The first Chinese author to describe an explosive formula was apparently
Huo Lung Ching, in 1412.
In the 6 pages of Liber Ignium (Book of Fires),
Marcus Graecus [an otherwise unknown, possibly fictitious, author]
describes 35 incendiary recipes,
including the one for what became known as English blackpowder:
1 lb of native sulfur,
2 lb of linden or willow charcoal, 6 lb of saltpeter,
which three things are very finely powdered on a marble slab.
The Latin version of the pamphlet didn't appear until 1280 or 1300 and it may
well have been created at that time, although it was claimed to be
an expanded translation by Spaniards of a more ancient Arabic text
(dated AD 848)
and/or a Greek version that did not include the last four formulas.
Roger Bacon (c.1214-1292)
investigated black powder before 1249, when
he devised the recipe he communicated in 1268:
40% more saltpeter than
either sulphur or carbon (7:5:5 formula by weight).
However, the first unmistakable blackpowder explosive composition
is the "German formula" (4:1:1) proposed by Albertus Magnus (c.1200-1280).
The English standard formula, around 1350, called for less sulphur and more charcoal
(6:1:2).
The most commonly quoted modern gunpowder composition seems to date
from around 1800 and calls for 75% saltpeter (niter) oxidizer, with
10% sulfur (S) and 15% charcoal (C) fuel:
Some Historical Formulae for Black Powder (by weight)
Date
Who / What / Where
KNO3
Sulphur
Charcoal
c. 700
Chinese alchemists (?)
1
1
1
1249
Roger Bacon
7
5
5
1275
Albertus Magnus ("German")
4
1
1
c.1300
"English" (Marcus Graecus?)
6
1
2
Swiss "Bernese Powder"
76
10
14
1781
Britain
75
10
15
1794
France
76
9
15
1800
Prussia
75
11.5
13.5
Stoichiometry (see below)
74.8
11.9
13.3
The stoichiometry of the following
simplified
reaction would correspond to about
74.8% niter, 11.9% sulphur and 13.3% carbon (roughly 101:16:18):
2 KNO3
+ 3 C
+ S
®
K2S
+ 3 CO2
+ N2
+ 572 kJ
(505.8 cal/g)
The potassium sulphide
solid residue forms a thick white smoke,
capable of obscuring entire battlefields.
Without sulfur (12.93% carbon) there would be 60% smoke as
potassium carbonate (and 772.6 cal/g):
4 KNO3
+ 5 C
®
2 K2CO3
+ 3 CO2
+ 2 N2
+ 1501.4 kJ
It takes 92.9 g of this mix to release a mole of gas,
whereas only 67.6 g of black powder would suffice
(sulfur prevents wasteful carbonate production).
Newer propellants leave little or no solid residue when properly exploded.
They are thus collectively known as smokeless powders.
The simplest idea
for a smokeless dark powder is called ammonpulver (AP) and involves
ammonium nitrate (AN) with 10% to 20% charcoal,
although the stoichiometry of the following reactions translates into only
7% to 13% carbon, by weight:
2 NH4NO3
+ C
®
CO2
+ 4 H2O
+ 2 N2
+ 629.6 kJ
(874.4 cal/g)
NH4NO3
+ C
®
CO
+ 2 H2O
+ N2
+ 228.6 kJ
(593.5 cal/g)
Smokeless powders of historical interest
include the following propellants:
Guncotton,
or nitrocellulose (also known as pyropowder, pyrocellulose,
trinitrocellulose and cellulose nitrate) invented in 1845
by the Swiss chemist
Christian Schönbein (1799-1869).
Poudre B
(flakes of nitrocellulose gelatinized with ether and alcohol)
invented in 1884 by Paul Vieille (1854-1934; X1873)
for the 1886 Lebel rifle.
Ballistite
(nitrocellulose & nitroglycerin, with diphenylamine stabilizer)
invented in 1887 by Alfred Nobel (1833-1896).
(2003-11-14) Simple Predictions of Chemical Outcomes
How do we tell what a given initial composition will produce?
This may be tough, since the result of a chemical reaction is
always an equilibrium containing everything that could be produced
(possibly only in minute quantities).
However, for reactions involving chemical explosives, a decent
rule
of thumb is to use the following hierarchy of
fictitious reactions and consider that
each occurs only when the previous ones have been completed
to the fullest possible extent:
Metal + Oxygen
®
Oxide
C + O
®
CO
2H + O
®
H2O
CO +
O
®
CO2
Oxide + CO2
®
Carbonate
N, O, or H
®
½N2, ½O2, or ½H2
C
®
C (black smoke)
This rough approximation of chemical reality is useful, but not foolproof.
(2008-03-22) Thermite
Thermite brings about thermal destruction chemically.
Thermite is a mix of rust
and powdered aluminum
which can be ignited with a strip of magnesium to produce
alumina and
iron.
This popular reaction is able
to deliver molten iron at a very high temperature
(about 2200°C).
Fe2O3 + 2 Al
®
Al2O3 + 2 Fe + 851.5 kJ
(= 3985 J/g)
The precise stoichiometry calls for 2.9 g of ferric oxide for 1 g
of aluminum. An excess of aluminum helps prevent
the formation of hercynite (FeAl2O4 ).
The usual recipe calls for 8 grams of iron oxide for
3 grams of aluminum.
This is the most popular special case of what's known as a
Goldschmidt
reaction (1893) whereby the oxide of a metal (like iron)
is reduced by a more reactive metal (aluminium is the usual choice).
The reaction is initiated either by permanganate and
glycol or by a burning ribbon
of magnesium. When the difference in the reactivities of the two metals
is large, a dangerous explosion may occur.
For example :
3 CuO + 2 Al
®
Al2O3 + 3 Cu + 1203.8 kJ
(= 4114 J/g)
The stoichiometry of that reaction yields the recipe for
copper thermite : Mix
31 g of
cupric oxide
with about
7 g of powdered aluminium
(2003-10-09) Enthalpy of Formation. Hess's Law (1840).
How do we compute the energy balance of a chemical reaction?
The enthalpy
of formation (DH) of a chemical compound is
roughly
the energy required to make it from its constituents
[in their standard forms, as gases, liquids, or crystals].
Once tabulated,
this data can be used to work out the energy balance
in a reaction involving such compounds.
The so-called bond energy is a misguided poor rule-of-thumb
which is unfortunatly still taught ar the introductory level.
In the few cases where it would be applicable (diatomic molecules) it's almost always
incompatible with the standard enthalpy of formation, which refers to formation
from realistic molecules rather than fictitious isolated atoms.
The standard
allotrope
of an element (zero enthalpy of formation)
can be a matter of convention, based on historical considerations.
The table below highlights the case of
phosphorus, which was
first isolated as a waxy solid, in 1669, in the toxic
form of white phosphorus.
A better reference would have been black phosphorus,
the only thermodynamically
stable form below 550°C.
Enthalpies of Formation
( DH f < 0
for exothermic formation )
Substance
(normalized to 298.15 K, 1 atm)
s = solid, l = liquid, g = gas, d = dissolved
For example, the energy released in the combustion of CO is
the difference between the enthalpies of
formation tabulated above for CO and CO2 :
CO + ½ O2
® CO2
+ 282.98 kJ
A positive enthalpy of formation indicates a fairly unstable compound which, like acetylene,
can release energy by reverting back to its elemental components.
On the other hand, a negative enthalpy of formation is no guarantee of stability.
Some such chemicals may even detonate violently into more stable ones,
as does liquid
nitroglycerin
in the following reaction:
Nitroglycerin was invented in 1847 by the Italian chemist
Ascanio Sobrero
(1812-1888) who had been working under the Frenchman
Théophile-Jules
Pelouze (1807-1867) after the discovery of
guncotton (1845).
Sobrero was so frightened by his own discovery that he kept
it secret for more than a year,
describing it as "impossible to handle".
The problem would be solved in 1867 by another student of Pelouze's
Alfred Nobel (1833-1896)
with the invention of dynamite,
a mixture of nitroglycerin with minerals that prevent spontaneous detonation.
That discovery became the source of Nobel's large wealth, which ultimately
allowed the creation of the Nobel Prize...
Of particular theoretical and historical interest is the so-called
heat of neutralization evolved in the aqueous neutralization of a
strong acid and a strong base
(e.g., HCl and NaOH). Remarkably, it doesn't depend on the nature
of the acid or the base, since it boils down to the following reaction:
Like all "complete" chemical reactions, this one actually results in a lopsided equilibrium
where the reactants have nonzero concentrations (in mol/L) verifying the notorious relation:
[ H3O+ ]
[ OH - ] = 10 -14
As Arrhenius first noted in 1884, the very notion of aqueous acidity is based
on that critical equilibrium and the disturbances caused to it by other reactions
that involve either of the two relevant ions.
(2011-06-21) Hot Ice (the constituent of
reusable heating pads)
The crystallization of sodium acetate trihydrate is exothermic.
Here's the crystallization reaction for the
hot ice
found in the reusable PCMheating pads that have been widely available since 1978
(136.0796 g/mol).
Na+ +
CH3COO- +
3 H2O
®
(NaCH3COO, 3H2O)
+ 38 kJ
The data from the
above table is equivalent to a
latent heat of 280 J/g.
This solidification occurs (below 58°C)
only when nucleation can be initiated by various impurities or, more reliably,
by a little bit of already crystallized sodium acetate trihydrate.
Interestingly, the reaction can also be triggered mechanically by a special
clicker (consisting of a slotted metallic disk) invented in 1978.
That device made possible a fascinating consumer product known as a
reusable heating pad
(also called heat pack or hand warmer
by campers).
The thing consists of a permanently sealed soft transparent pouch containing a clicker and
some hot ice (possibly with a very slight excess of water).
The pack is stored or carried in its liquid form.
When needed, a mere click turns it
into a very warm solid object
(which can later be returned to it metastable
liquid form by heating the pouch in boiling water
until all traces of the crystals have disappeared).
(2007-11-21) Gibbs Function (G): Free Enthalpy (or "free energy").
The sign of DG indicates thermodynamic stability.
A thermodynamically stable compound is indicated by a
negative free energy
of formation DGf
The change in entropy DS
can be large enough to make an endothermic reaction spontaneous.
This is called an entropy driven reaction.
One example is the melting of ice. It's an endothermic reaction
(+6.95 kJ/mol) accompanied by a great increase in
the entropy (disorder) which actually makes
DG negative, so the reaction is indeed
a spontaneous one.
DH and
DG are normally given in kilojoules (kJ)
per mole, whereas DS is usually given in units
of J/K so the product by the absolute temperature (T)
comes out in joules (J).
With such conventions, a conversion factor of 1000 has to be applied
in actual computations.
(2011-08-07) Berthollet's Law of Mass Action
[ Products ] / [ Reactants ] = Equilibrium Constant
Before Berthollet debunked the notion (between 1800 and 1803)
chemists believed in the concept of
elective affinities (Wahlverwandtschaften).
According to that alchemical doctrine, chemical compounds would form or dissociate
in substitution reactions in strict accordance to the
so-called affinities of
pairs of chemical species for each other.
This was thought to occur essentially to the fullest possible extent,
regardless of the respective concentrations of the reactants involved.
(2007-11-21) "Labile" and "unstable" are not quite synonymous.
Kinetics can make a compound not labile in spite of unstability.
Benzene is one compound which is unstable according to its
free energy balance.
Yet, the kinetics involved make the spontaneous decomposition of
benzene into hydrogen and graphite so slow that it's
never observed in practice.
An unstable compound which can decompose fast enough is said to be
labile.
As the example of benzene illustrates, not all unstable compunds are labile.
(2003-10-10) Ink Formulas
What is the composition of traditional inks ?
Natural Ink
Sepia is the most lasting of natural inks, but it's not lightfast.
It is a dark brown liquid
consisting of concentrated melanin,
secreted by Mediterranean cuttlefish and other cephalopods
(it's stored in ink sacs and ejected to confuse attackers).
India Ink (Chinese Ink)
As early as 2500 BC, writing inks were carbon inks
consisting of fine grains of carbon black [from soot] suspended in a liquid.
The Latin name for this was
atramentum librarium and it's now called
India ink or Chinese ink.
On the famous Dead Sea Scrolls of Qumran
(from the third century BC to AD 68),
a red version of this ink is found which uses
cinnabar (HgS) instead of carbon.
The idea is simple:
When the liquid dries out, the solid pigment (C or HgS) remains which
leaves a permanent trace.
Such inks are best used on semi-absorbent stuff, like paper or papyrus (not parchment).
The problem was to keep the grains in suspension long enough to apply the ink.
In plain water, fine grains of carbon black would aggregate under the action of
Van der Waals forces and form flakes
large enough to fall quickly to the bottom of the container.
This flocculation process can be prevented with an hydrophilic additive
which minimizes Van der Waals interactions between the grains
by coating them (as was properly explained only in the 1980s).
Early ink recipes may thus have called for various plant juices
instead of plain water.
It turns out that gum arabic acts this way to stabilize
India ink into a colloidal suspension for days or weeks...
This wonderful invention is at least 4500 years old.
Traditional Chinese ink is not bottled. Instead, ink is produced as needed
by grinding an inkstick on an inkstone after adding a little
water (the inkstone also acts as an inkwell).
Chinese ink-sticks consist of a pigment (usually soot from pine,
oil or lacquer) and a soluble resin which holds the dry stick together
and plays a critical part in the colloidal ink suspension produced by wet grinding.
Nowadays, Chinese ink produced in this traditional way is known by its Japanese
name (Sumi ink) whereas bottled Chinese ink is called India ink.
However, bottled Sumi ink is also available with features that some artists
swear by (see video review by
web comics artist Bryan Christopher Moss).
Iron-Gall Ink, Indelible Ink, Encaustum
In the first century AD, Pliny the Elder
described a basic chemical demonstration
of the principle behind what would become the primary ink of the Middle Ages:
Papyrus soaked in tannin turns black upon contact with a solution of iron salt.
This was not used for actual ink at the time of Pliny,
but "gallarum gummeosque commixtio" is already mentioned as
an established writing ink around AD 420,
in the
encyclopedia of the 7 liberal arts by Martianus Capella.
However, the latest analyses have disproved dubious reports that this type of ink might
have already been used on the famous Dead Sea Scrolls of Qumran (before AD 68).
Because of the secondary reaction discussed below, which makes it indelible,
iron ink was once known as encaustum
(Latin for "burned in", from the Greek enkauston, meaning
painted in encaustic and fixed with heat).
This is the origin of the English word "ink" itself,
and of its counterparts in a number of other languages:
encre (French),
inchiostro (Italian),
inkt (Dutch),
inkoust (Czech)...
Indelible iron-gall ink is considered the most important ink in the development
of Western civilization, up until the 20th century.
The best iron-gall inks were far superior to most modern inks,
but the corrosiveness of some compositions (discussed below)
regretfully led to the abandonment of all iron-gall inks in favor of
more sophisticated recipes with lesser chemical aggressivity.
Iron-gall ink normally includes what is effectively a
"Chinese ink" component, which provides both body (from gum arabic)
and some initial coloring upon application of the ink.
Otherwise, the main pigmentation of iron-gall ink comes paradoxically from
water-soluble ferrous chemicals with little color of their own:
When the ink dries in air, an oxidation occurs which turns these
ferrous salts into insoluble ferric dark pigments.
In addition, iron-gall ink may react with parchment collagen or paper
cellulose, in a totally indelible way.
Some poorly balanced iron-gall inks have even been observed to
burn holes through paper.
It has been shown
that an excess of ferrous salt in iron-gall ink
leaves permanent traces of active soluble salts
(not properly oxidized into inert pigments) which will catalyze
the slow decomposition of cellulose, especially when acidity is present.
This corrosion is reduced with a proper balance in the composition of the ink.
To prevent deterioration of historical iron-gall ink documents,
the Netherlands Institute of Cultural Heritage (ICN) has introduced an interesting
treatment,
which was first used on a large scale by the conservators of the
Nationaal Archief of the Netherlands:
First, a saturated solution is applied which contains a calcium salt and its acid, namely:
The salt is soluble up to twice the molar concentration of the acid.
This is an oxidation inhibitor which binds the metal ions.
Then, acidity is neutralized with calcium bicarbonate,
which creates an alkaline buffer and also leaves a phytate precipitate in the fibers,
for continued oxidation protection.
Gum Arabic:
True gum Arabic is exuded by the
acacia
senegal tree, which has several other names:
Rudraksha, Gum Acacia, Gum Arabic Tree, Gum Senegal Tree.
Currently, 70% of the World's supply of gum arabic comes from
Sudan.
The related products of other trees of the
Acacia genus are usually considered
inferior substitues for true Gum Arabic.
This includes, most notably, what's known as Indian gum Arabic
which is produced by trees variously called
acacia nilotica,
acacia arabica, babul, Egyptian thorntree
or prickly acacia.
Gum Arabic is a very common thickener and colloidal stabilizer.
Some candies are made from up to 45% gum arabic (E414).
Also called acacia.
[info]
CAS 9000-01-5: Gum acacia;
Arabic gum or
acacia gum
(Indian gum Arabic identifies a lower grade of product).
The natural product is a mixture of the following ingredients:
arabinogalactan oligosaccharides and polysaccharides.
glycoproteins, (proteins with sugars attached).
Ferrous sulfate:
Also known as kankatum, green vitriol or copperas.
(FeSO4, 7 H2O) iron sulphate
in hydrated crystal form (278.01 g/mol).
Tannin: Tannic
(or gallotannic) acid,
extracted by water-saturated ether from crushed gallnuts
( galls, nutgalls, or gall apples ).
It is an anhydrid of gallic acid (next):
COOH.C6H2(OH)2O.COC6H2(OH)3
Gallic acid:
Produced (with glucose)
by the hydrolysis of tannin in acid.
Used in calotypephotography.
C6(COOH)H(OH)3H (170.12 g/mol)
(2003-10-10) Traditional Pigments
Chemicals traditionally used as coloring agents in paints, dyes or inks.
Most of these substances are fairly harmless but
some of them are too toxic for regular use, by modern standards at least...
At left is brazilin, (the expensive
dye behind the lake pigment
used for red velvet) from
brazilwood,
the tree after which the country of Brazil was named.
Pigments:
Carbon Black :
Lampblack, from soot. C (12.01 g/mol)
Manganese Black :
Manganese dioxide. MnO2 (86.937 g/mol)
Cinnabar :
Called vermillion, or Chinese red. HgS (232.66 g/mol)
Red Ochre :
Hematite. Ferric oxide. Fe2O3 (159.69 g/mol)
(2010-10-16) Esters & Waxes. The complexity of natural beeswax.
Waxes are long-chained esters, like myricin :
C15H31COOC30H61
Crude
beeswax (raw beeswax)
is secreted by young female worker bees (6 to 18 days old)
from eight wax glands located on the inner sides of their
sternites,
beneath abominal segments 6, 7, 8 and 9.
Wax is produced in scales weighing about 0.9 mg
(about 3 mm across and 0.13 mm in thickness).
Bees produce wax when the temperature in the hive is between
33°C and 39°C.
For each pound of wax they produce, the bees must consume about 8 pounds of honey.
Beekeepers
will typically harvest one pound of beeswax for 10 pounds of honey.
Refined natural beeswax has a deep gold color. It's available as
yellow beeswax
(Cera Flava, CAS 8012-89-3 or CAS 8033-51-0).
A different product known as
white beeswax
(Cera Alba, CAS 8006-40-4) is actually beeswax
bleached chemically using
nitric or chromic acid
(traditional bleaching involved exposing for weeks thin slices of beeswax
to moist air and sunlight, next to the hives, possibly
remelting several times).
White beeswax is cream-colored.
The wax made by bees is a complex mixture
(of at least 284 distinct compounds) whose composition
varies substantially from one batch to the next.
In 1848,
Sir Benjamin Collins Brodie,
Jr. (1817-1880) separated beeswax by means of alcohol into three main
constituents, found in varying proportions, which he called Myricin,
Cerin and Cerolein.
Those constituents are mixtures, rather than pure chemical compounds.
However, Myricin and Cerin
are routinely identified with their dominant compounds
(melissyl palmitate and cerotic acid respectively).
Thus, here's how natural beeswax may be
approximately described:
About 70% of Myricin (insoluble in boiling alcohol)
which is chiefly a long-chain ester melting at 72°C (see below).
It's formally calledmyricyl palmitate or melissyl palmitate
C15H31COOC30H61
About 25% of Cerin, similar to cerotic acid
(dissolved by boiling alcohol) which melts at 79°C.
It was totally absent from one of the samples
(originating from Ceylon) analyzed by Brodie.
H(CH2)25COOH
About 5% ofCerolein (dissolved by cold
alcohol or ether) which melts at 23°C.
It is
cerolein which gives beeswax most of its odor and color.
Pure myricin is identified as
Triacontanyl palmitate or Melissyl palmitate
which is the long-chain fatty ester
formed by palmitic acid and the
long-chain saturated alcohol variously called
triacontanol, myricyl alcohol,
melissyl alcohol or melissin.
(2010-10-18) Pine Tar Pitch (brewer's pitch)
vs. Cedar Pitch
Pine tar pitch can be obtained by
dry distillation
of resinous wood.
It's a mixture of
resin acids,
similar to the so-called pyroabietic acid, obtained by heating
abietic acid
between 250°C and 350°C
(abietic acid is the main constituent of
rosin;
it's also known as abietinic acid or sylvic acid).
Such products are also found in
tall oil.
The principal compounds so obtained are:
Dehydroabietic acid, or DHA
(CAS 1740-19-8)
C20H28O2
Abietic acid
C20H30O2 (rosin)
Dihydroabietic acid C20H32O2
Tetrahydroabietic acid
C20H34O2
Also involved is
pimaric acid, a close relative
of abietic acid itself.
Cedar Tar Pitch :
The chemistry of Cedar pitch is not the same as that of pine pitch...
It involves a totally different type of resin acid:
plicatic acid
C20H22O10.
(2010-10-11) Gum Arabic: A great ancient commodity.
The magic bullet of ancient chemistry is not just for candy or ink.
Jerome A. Samounce is a minister in North Carolina
who tries to bring scripture to life by reproducing Biblical artefacts using
ancient technology.
On 2010-01-06, he approached me with a few technical questions
about his latest project:
Reproducing an authentic 3-cubit Judean javelin from the
Davidic Dynasty...
The shaft of such a javelin was made of
ash wood
(finished with linseed
oil) 1" thick in
the middle (and ½" at either end).
At one end, it was split and carved to accomodate a bronze tip.
The two halves were then glued back together.
That was the main problem:
What could this weapon-grade Biblical glue be?
It had been merely described as "a glue based on cedar
pitch".
Jerome had also found that archeological reports consistently mention
two other ingredients besides cedar or pine pitch: Beeswax and ground ash powder.
(the presence of some inert powder should come as no surprise
to whoever has ever tried to optimize the mechanical properties of
thick layers of modern epoxy glue).
By themselves, those three ingredients don't mix and yield disappointing results.
On a hunch, I suggested that ancient craftsmen would almost certainly have tried
Gum Arabic as a key additive
(I even suggested that experimentation might start
with 1%, 2% and 4% of Gum Arabic ). Bingo!
The immediate result was an excellent Biblical glue.
Here is the recipe (by weight) obtained in the subsequent
backyard experiments
performed by Jerome Samounce et al
(see full
report).
50 parts of pine tar pitch (cedar pitch would be more authentic).
15 to 20 parts of beeswax (the more beeswax, the more flexibility).
10 parts of inert powder (finely ground sawdust, or ash).
3 parts of Gum Arabic.
At first, I had thought that gum Arabic would merely help the
mix form a water-free colloid which would freeze solid upon cooling
(compare that to frozen mayonnaise if you must).
However, the experiments of Samounce seem to indicate that
gum Arabic induces a decomposition of hot beeswax
(with emission of an unidentified gas which might be carbon dioxide).
This yields a compound that appears to act as a hardener of natural resin
(just like the hardener coompound in modern two-part epoxy glue).
We're still pondering what the actual chemical reactions might be... Stay tuned.
(2011-08-02) Ancient Acids
From vinegar to vitriolic and muriatic acid.
Acetic Acid, Ethanoic Acid, CH3COOH :
This is what gives vinegar its acidity.
It results from the oxidation of alcohol in the air, induced by bacteria.
Sulfuric Acid, Vitrolic Acid, Oil of Vitriol, H2SO4 :
Pure sulfuric acid (H2SO4 )
is an oily substance formerly known as oil of vitriol.
The purified form (which is colorless) was probably obtained shortly after
the introduction of the copper still for alchemical research by
Mary the Jewess
(third century AD). Vitriolic acid doesn't attack copper.
Sulfuric acid is a dangerous substance with a high boiling point
(337°C) which makes its distillation very hazardous.
Above a concentration of 80% or so, the vapor contains a substantial
amount of acid and is highly toxic.
Without distillation, vitriolic acid can be concentrated by boiling it partially
(which is itself dangerous enough, as previously noted).
As this ancient method also concentrates impurities, it makes the stronger
grades of vitriolic acid appear darker and the Sumerians were trading
different grades according to their colors...
Spirit of Salt, Muriatic Acid, Hydrochloric Acid, HCl :
Many modern accounts advocate a fairly recent discovery of
muriatic acid (hydrochloric acid, HCl)
which is plain silly.
Dropping a pinch of ordinary table salt (NaCl)
into sulfuric acid will
evolve a gas with the unmistakable corrosive smell of HCl
(I just did that with some drain opener labeled
CAS 7664-93-9,
just to check how obvious this really is).
H2SO4 + NaCl
®
NaHSO4 + HCl
Unavoidably, some of the HCl remains in the solution,
giving it a smell that wasn't there before.
It's impossible that an experimenter of the caliber of
Mary the Jewess
could have missed that with the means at her disposal.
Ferdinand Hoefer
(1811-1878)
rightly attributes to her the discovery of muriatic acid.
This is legitimate in spite of the usual
fact that the original discoverer could have been some earlier anonymous soul
(with access to vitriolic acid) who did the same experiment before
Miriam
but didn't follow-up the way she (undoubtedly) did.
(2003-11-01) Gold Chemistry
Aqua regia, the "Royal Water" which dissolves gold and platinum.
Like silver, gold is impervious to strong acids like
hydrochloric acid
(formerly called muriatic acid, "marine acid" or "spirit of salt").
Unlike silver, gold cannot be oxidized by nitric acid
(aqua fortis)...
However, early alchemists did discover that a mixture of nitric and
hydrochloric acids was able to dissolve gold, the so-called royal metal.
They dubbed the potent mixture "Royal Water", aqua regis or
aqua regia.
Aqua regia is already mentioned in the world's
first
encyclopedia,
published in AD 77 by Pliny the Elder
(AD 23-79).
The alchemical works of
Ramon Llull (c.1232-1316)
contain prominently the traditional preparations of
aqua fortis (nitric acid)
and aqua regia.
Aqua regia is a mixture of at least 3 moles of
hydrochloric acid per mole of nitric acid
(too much hydrochloric acid is better than too little).
It's used hot and concentrated for best efficiency.
Aqua regia is also called chloroazotic, chloronitric,
nitromuriatic, or nitrohydrochloric acid
("eau régale" in French).
Nitrosyl chloride
and chlorine fumes are evolved upon mixing:
HNO3 + 3 HCl
®
NOCl + Cl2 + 2 H2O
The chemical equilibrium for the oxidation of gold by the nitrate ions in nitric
acid would only result in a minute concentration of auric cations
[Au+++], but in aqua regia
the concentration of auric ions is constantly depleted because
auric cations combine quickly with chlorine anions to form complex
chloroaurate ions:
Au+++ + 4 Cl -
®
AuCl4-
The speed of the overall reaction is limited by the [Au+++ ]
concentration in the relevant redox equilibrium.
As this improves with temperature, aqua regia may be used
at 100°C or more (in a bath of boiling salty water).
A more exotic compound (interhalogen compound) which readily dissolves gold by
forming gold trichoride is
iodine monochloride (ICl).
Gold forms compounds in two oxidation states: +1 (aurous)
and +3 (auric):
Byproducts or reactants in the electrolytic refining of gold:
CAS 10294-29-8:
Aurous chloride / Gold monochloride (AuCl).
CAS 13453-07-1:
Auric chloride / Gold trichloride (AuCl3).
CAS 16903-35-8:
Chloroauric acid (HAuCl4).
CAS 16961-25-4:
--- trihydrated crystals
(HAuCl4, 3 H2O).
Note: The term "gold chloride"
is unfortunately used for any of the above!
CAS 13967-50-5:
Potassium dicyanoaurate K[Au(CN)2].
Rheumatoid arthritis medicine:
CAS 15189-51-2:
Sodium aurichloride (NaAuCl4, 2 H2O).
The combination of gold trichloride with the chloride of
another metal is called an aurochloride, aurichloride,
chloraurate or [best] chloroaurate.
Fulminating Gold, the First High Explosive:
Since gold is so difficult to combine with other elements,
all gold compounds are fairly unstable.
Some much more so than others, though:
In 1659, Thomas Willis
and Robert Hooke demonstrated
that a powder of gold hydrazide explodes on a mere concussion,
without the need for air or sparks (which were once thought to be required
for any kind of ignition).
Gold hydrazide (also known as aurodiamine) is a water-soluble substance
obtained by letting an ammoniacal solution react with an auric hydroxide precipitate
(itself obtained from a gold solution prepared with aqua regia).
Gold hydrazide has a dirty olive-green color (AuHNNH2 ).
Gold hydrazide is apparently only one of several explosive compounds which have been called
fulminating gold (aurum fulminans).
Around 1603, another kind of fulminating gold ("Goldkalck" or "Gold Calx")
was described as the precipitate of gold by potassium carbonate.
These kinds of "fulminating gold" are distinct from "gold fulminate",
the gold salt of fulminic acid (CNOH), another expensive explosive...
In spite of its price, fulminating gold is said to have been used militarily in 1628.
The discovery of fulminating gold has been attributed to the legendary
alchemist Basil Valentine
(Basilius Valentinus) a benedictine monk born in 1394 whose
persona was also used by other early chemists, possibly including
Johann Thölde (c.1565-1624).
His main work (The Twelve Keys of Basil Valentine) was first published in 1599.
Basil Valentine has been regarded
as the father of modern chemistry (see next section).
Arguably, chemistry became a science when
Antoine Lavoisier established that
mass is conserved in any chemical reaction,
about which he stated:
Rien ne se perd, rien ne se crée, tout se transforme.
It's only with the advent of Relativity Theory
that this fundamental conservation law would be proved to be only a first
approximation, albeit an excellent one:
Unlike what happens in nuclear reactions,
the relative variation of mass involved in chemical reactions is so minute
that it can't be measured directly.
(2003-10-10) Ink Formulas
