Concrete
Concrete is a construction material that consists of cement, commonly
Portland cement, aggregate (generally gravel and sand) and water.
Concrete solidifies and hardens after mixing and placement due to a chemical
process known as hydration. The water reacts with the cement, which bonds the
other components together and eventually creating a stone-like material. It is
used to make pavements, architectural structures, foundations, motorways/roads,
overpasses, parking structures, brick/block walls and footings for gates, fences
and poles.
The Assyrians and Babylonians used clay as cement in their concrete. The
Egyptians used lime and gypsum cement. In the Roman Empire, concrete made from
quicklime, pozzolanic ash / pozzolana and an aggregate made from pumice was very
similar to modern Portland cement concrete. In 1756, the British engineer John
Smeaton pioneered the use of Portland cement in concrete, using pebbles and
powdered brick as aggregate. In modern times the use of recycled materials as
concrete ingredients is gaining popularity because of increasingly stringent
environmental legislation. The most conspicuous of these is fly ash, a by
product of coal fired power plants. This has a significant impact by reducing
the amount of quarrying and landfill space required.
Portland cement is the most common type of cement in general usage. It is a
basic ingredient of concrete, mortar and plaster. English engineer Joseph Aspdin
patented Portland cement in 1824, and it was named after the limestone cliffs on
the Isle of Portland in England because its color is similar to the stone
quarried there. It consists of a mixture of oxides of calcium, silicon and
aluminium. Portland cement and similar materials are made by heating limestone
(a source of calcium) with clay, and grinding this product (called clinker) with
a source of sulfate (most commonly gypsum). When mixed with water, the resulting
powder will become a hydrated solid over time.
High temperature applications, such as masonry ovens and the like, generally
require the use of a refractory cement; concretes based on Portland cement can
be damaged or destroyed by elevated temperatures, but refractory concretes are
better able to withstand such conditions.
The water and cement paste hardens and develops strength over time. In order
to ensure an economical and practical solution, both fine and coarse aggregates
are utilised to make up the bulk of the concrete mixture. Sand, natural gravel
and crushed stone are mainly used for this purpose. However, it is increasingly
common for recycled aggregates (from construction, demolition and excavation
waste) to be used as partial replacements of natural aggregates, whilst a number
of manufactured aggregates, including air-cooled blast furnace slag and bottom
ash are also permitted.
Decorative stones such as quartzite, small river stones or crushed glass are
sometimes added to the surface of concrete for a decorative "exposed aggregate"
finish, popular among landscape designers.
Workability (or consistence, as it is known in Europe) is the ability of a
fresh (plastic) concrete mix to fill the form / mould properly with the desired
work (vibration) and without reducing the concrete's quality. Workability
depends on water content, chemical admixtures, aggregate (shape and size
distribution), cementitious content and age (level of hydration). Raising the
water content or adding chemical admixtures will increase concrete workability.
Excessive water will lead to increased bleeding (surface water) and / or
segregation of aggregates (when the cement and aggregates start to separate),
with the resulting concrete having reduced quality. The use of an aggregate with
an undesirable gradation can result in a very harsh mix design with a very low
slump, which cannot be readily made more workable by addition of reasonable
amounts of water.
Workability can be measured by the "slump test", a simplistic measure of the
plasticity of a fresh batch of concrete following the ASTM C 143 or EN 12350-2
test standards. Slump is normally measured by filling an "Abrams cone" with a
sample from a fresh batch of concrete. The cone is placed with the wide end down
onto a level, non-absorptive surface. When the cone is carefully lifted off, the
enclosed material will slump a certain amount due to gravity. A relatively dry
sample will slump very little, having a slump value of one or two inches (25 or
50 mm). A relatively wet concrete sample may slump as much as six or seven
inches (150 to 175 mm).
Slump can be increased by adding chemical admixtures such as mid-range or
high-range water reducing agents (super-plasticizers) without changing the
water/cement ratio. It is bad practice to add extra water at the concrete mixer.
High flow concrete, like self-consolidating concrete, is tested by other
flow-measuring methods. One of these methods includes placing the cone on the
narrow end and observing how the mix flows through the cone while it is
gradually lifted.
Concrete has relatively high compressive strength, but significantly lower
tensile strength (about 10% of the compressive strength). As a result, concrete
always fails from tensile stresses � even when loaded in compression. The
practical implication of this is that concrete elements subjected to tensile
stresses must be reinforced. Concrete is most often constructed with the
addition of steel or fiber reinforcement. The reinforcement can be by bars
(rebar), mesh, or fibres, producing reinforced concrete. Concrete can also be
prestressed (reducing tensile stress) using internal steel cables (tendons),
allowing for beams or slabs with a longer span than is practical with reinforced
concrete alone.
The ultimate strength of concrete is influenced by the water-cement ratio
(w/c) [water-cementitious materials ratio (w/cm)], the design constituents, and
the mixing, placement and curing methods employed. All things being equal,
concrete with a lower water-cement (cementitious) ratio makes a stronger
concrete than a higher ratio. The total quantity of cementitious materials
(portland cement, slag cement, pozzolans) can affect strength, water demand,
shrinkage, abrasion resistance and density. As concrete is a liquid which
hydrates to a solid, plastic shrinkage cracks can occur soon after placement;
but if the evaporation rate is high, they often can occur during finishing
operations (for example in hot weather or a breezy day). Aggregate interlock and
steel reinforcement in structural members often negates the effects of plastic
shrinkage cracks, rendering them aesthetic in nature. Properly tooled control
joints in slabs or saw cuts provide a plane of weakness so that cracks occur
unseen inside the joint, making a nice aesthetic presentation. In very high
strength concrete mixtures (greater than 10,000 psi), the strength of the
aggregate can be a limiting factor to the ultimate compressive strength. In lean
concretes (with a high water-cement ratio) the use of coarse aggregate with a
round shape may reduce aggregate interlock.
Experimentation with various mix designs is generally done by specifying
desired "workability" as defined by a given slump and a required 28 day
compressive strength. The characteristics of the coarse and fine aggregates
determine the water demand of the mix in order to achieve the desired
workability. The 28 day compressive strength is obtained by determination of the
correct amount of cementitious to achieve the required water-cement ratio. Only
with very high strength concrete does the strength and shape of the coarse
aggregate become critical in determining ultimate compressive strength.
The internal forces in certain shapes of structure, such as arches and
vaults, are predominantly compressive forces, and therefore concrete is the
preferred construction material for such structures.
Pervious concrete is sometimes specified by engineers and architects when
porosity is required to allow some air movement or to facillitate the drainage
and flow of water through structures. Pervious concrete is referred to as "no
fines" concrete because it is manufactured by leaving out the sand or "fine
aggregate". A pervious concrete mixture contains little or no sand (fines),
creating a substantial void content. Using sufficient paste to coat and bind the
aggregate particles together creates a system of highly permeable,
interconnected voids that drains quickly. Typically, between 15% and 25% voids
are achieved in the hardened concrete, and flow rates for water through pervious
concrete are typically around 480 in./hr (0.34 cm/s, which is 5 gal/ft�/ min or
200 L/m�/min), although they can be much higher. Both the low mortar content and
high porosity also reduce strength compared to conventional concrete mixtures,
but sufficient strength for many applications is readily achieved.
Pervious concrete pavement is a unique and effective means to address
important environmental issues and support sustainable growth. By capturing
rainwater and allowing it to seep into the ground, porous concrete is
instrumental in recharging groundwater, reducing stormwater runoff, and meeting
US Environmental Protection Agency (EPA) stormwater regulations. The use of
pervious concrete is among the Best Management Practices (BMPs) recommended by
the EPA, and by other agencies and geotechnical engineers across the country,
for the management of stormwater runoff on a regional and local basis. This
pavement technology creates more efficient land use by eliminating the need for
retention ponds, swales, and other stormwater management devices. In doing so,
pervious concrete has the ability to lower overall project costs on a first-cost
basis.
Engineers usually specify the required compressive strength of concrete,
which is normally given as the 28 day compressive strength in megapascals (MPa)
or pounds per square inch (psi). Twenty eight days is a long wait to determine
if desired strengths are going to be obtained, so three-day and seven-day
strengths can be useful to predict the ultimate 28-day compressive strength of
the concrete. A 25% strength gain between 7 and 28 days is often observed with
100% OPC (ordinary Portland cement) mixtures, and up to 40% strength gain can be
realized with the inclusion of pozzolans and supplementary cementitious
materials (SCM's) such as fly ash and/or slag cement. As strength gain depends
on the type of mixture, its constituents, the use of standard curing, proper
testing and care of cylinders in transport, etc. it becomes imperative to
equally rely on testing the fundamental properties of concrete in its fresh,
plastic state.
Concrete is typically sampled while being placed, with testing protocols
requiring that test samples be cured under laboratory conditions (standard
cured). Additional samples may be field cured (non-standard) for the purpose of
early 'stripping' strengths, that is, form removal, evaluation of curing, etc.
but the standard cured cylinders comprise acceptance criteria. Concrete tests
can measure the "plastic" (unhydrated) properties of concrete prior to, and
during placement. As these properties affect the hardened compressive strength
and durability of concrete (resistance to freeze-thaw) , the properties of slump
(workability), temperature, density and age are monitored to ensure the
production and placement of 'quality' concrete. Tests are performed per ASTM
International or CSA (Canadian Standards Association) and European methods and
practices. Technicians performing concrete tests MUST be certified. Structural
design and material properties are often specified in accordance with ACI
International code (www.concrete.org) under the "prescription" or "performance"
purchasing options per ASTM C94 (www.astm.org).
Compressive strength tests are conducted using an instrumented hydraulic ram
to compress a cylindrical or cubic sample to failure. Tensile strength tests are
conducted either by three-point bending of a prismatic beam specimen or by
compression along the sides of a cylindrical specimen.
When structures made of concrete are to be demolished, concrete recycling is
a common method of disposing of the rubble. Concrete debris was once routinely
shipped to landfills for disposal, but recycling has a number of benefits that
have made it a more attractive option in this age of greater environmental
awareness, more environmental laws, and the desire to keep construction costs
down.
Pieces of concrete collected from demolition sites are put through a crushing
machine, often along with asphalt, bricks, and rocks. Crushing facilities accept
only uncontaminated concrete, which must be free of trash, wood, paper and other
such materials. Metals such as rebar are accepted, since they can be removed
with magnets and other sorting devices and melted down for recycling elsewhere.
The remaining aggregate chunks are sorted by size. Larger chunks may go through
the crusher again. Smaller pieces of concrete are used as gravel for new
construction projects. Aggregate base gravel is laid down as the lowest layer in
a road, with fresh concrete or asphalt placed over it. Crushed recycled concrete
can sometimes be used as the dry aggregate for brand new concrete if it is free
of contaminants, though the use of recycled concrete limits the strength, and is
not allowed in many jurisdictions.
Recycling concrete provides environmental benefits, as recycling concrete
saves landfill space and using recycled concrete as aggregate reduces the need
for gravel mining.
Bacteria themselves do not have noticeable effect on concrete. However,
anaerobic bacteria in eg. sewage tend to produce hydrogen sulfide, which is then
oxidized by aerobic bacteria present in biofilm on the concrete surface above
the water level to sulfuric acid which dissolves the carbonates in the cured
cement and causes strength loss. Concrete floors laying on ground containing
pyrite are also at risk. Using limestone as the aggregate makes the concrete
more resistant to acids, and the sewage may be pretreated by ways increasing pH
or oxidizing or precipitating the sulphides in order to inhibit the activity of
sulphide utilizing bacteria.
Concrete exposed to sea water is susceptible to its corrosive effects. The
effects are more pronounced above the tidal zone than where the concrete is
permanently submerged. In the submerged zone, magnesium and hydrogen carbonate
ions precipitate about 30 micrometers thick layer of brucite on which a slower
deposition of calcium carbonate as aragonite occurs. These layers somewhat
protect the concrete from other processes, which include attack by magnesium,
chloride and sulfate ions and carbonation. Above the water surface, mechanical
damage may occur by erosion by waves themselves or sand and gravel they carry,
and by crystallization of salts from water soaking into the concrete pores and
then drying up. Pozzolanic cements and cements using more than 60% of slag as
aggregate are more resistant to sea water than pure portland cement.
When some aggregates containing dolomite are used, a dedolomitization
reaction occurs where the magnesium carbonate compound reacts with hydroxyl ions
and yields magnesium hydroxide and a carbonate ion. The resulting expansion may
cause destruction of the material. Other reactions and recrystallizations, e.g.
hydration of clay minerals in some aggregates, may lead to destructive expansion
as well.
Semiconductors
A semiconductor is a solid whose electrical conductivity can be controlled
over a wide range, either permanently or dynamically. Semiconductors are
tremendously important technologically and economically. Silicon is the most
commercially important semiconductor, though dozens of others are important as
well.
Semiconductor devices, electronic components made of semiconductor materials,
are essential in modern electrical devices, from computers to cellular phones to
digital audio players.
Semiconductors are very similar to insulators. The two categories of solids
differ primarily in that insulators have larger band gaps � energies that
electrons must acquire to be free to flow. In semiconductors at room
temperature, just as in insulators, very few electrons gain enough thermal
energy to leap the band gap, which is necessary for conduction. For this reason,
pure semiconductors and insulators, in the absence of applied fields, have
roughly similar electrical properties. The smaller bandgaps of semiconductors,
however, allow for many other means besides temperature to control their
electrical properties.
Semiconductors' intrinsic electrical properties are very often permanently
modified by introducing impurities, in a process known as doping. Usually it is
reasonable to approximate that each impurity atom adds one electron or one
"hole" (a concept to be discussed later) that may flow freely. Upon the addition
of a sufficiently large proportion of dopants, semiconductors conduct
electricity nearly as well as metals. Depending on kind of the impurity, a
region of semiconductor can have more electrons or holes, and then it is called
N-type or P-type semiconductor, respectively. Junctions between regions of N-
and P-type semiconductors have built-in electric fields, which cause electrons
and holes to escape from them, and are critical to semiconductor device
operation. Also, a density difference of impurities produces in the region small
electric field which is used to accelerate non-equilibrium electrons or holes in
it.
In addition to permanent modification through doping, the electrical
properties of semiconductors are often dynamically modified by applying electric
fields. The ability to control conductivity in small and well-defined regions of
semiconductor material, both statically through doping and dynamically through
the application of electric fields, has led to the development of a broad range
of semiconductor devices, like transistors. Semiconductor devices with
dynamically controlled conductivity are the building blocks of integrated
circuits, like the microprocessor. These "active" semiconductor devices are
combined with simpler passive components, such as semiconductor capacitors and
resistors, to produce a variety of electronic devices.
In certain semiconductors, when electrons fall from the conduction band to
the valence band (the energy levels above and below the band gap), they often
emit light. This photoemission process underlies the light-emitting diode (LED)
and the semiconductor laser, both of which are very important commercially.
Conversely, semiconductor absorption of light in photodetectors excites
electrons from the valence band to the conduction band, facilitating reception
of fiber optic communications, and providing the basis for energy from solar
cells.
Semiconductors may be elemental materials such as silicon and germanium, or
compound semiconductors such as gallium arsenide and indium phosphide, or alloys
such as silicon germanium or aluminium gallium arsenide.
Like other solids, the electrons in semiconductors can have energies only
within certain bands between the energy of the ground state, corresponding to
electrons tightly bound to the atomic nuclei of the material, and the free
electron energy, which is the energy required for an electron to escape entirely
from the material. The energy bands each correspond to a large number of
discrete quantum states of the electrons, and most of the states with low energy
are full, up to a particular band called the valence band. Semiconductors and
insulators are distinguished from metals because the valence band in the former
materials is very nearly full under normal conditions.
The ease with which electrons in a semiconductor can be excited from the
valence band to the conduction band depends on the band gap between the bands,
and it is the size of this energy bandgap that serves as an arbitrary dividing
line (roughly 4 eV) between semiconductors and insulators.
The electrons must move between states to conduct electric current, and so
due to the Pauli exclusion principle full bands do not contribute to the
electrical conductivity. However, as the temperature of a semiconductor rises
above absolute zero, the states of the electrons are increasingly randomized, or
smeared out, and some electrons are likely to be found in states of the
conduction band, which is the band immediately above the valence band. The
current-carrying electrons in the conduction band are known as "free electrons",
although they are often simply called "electrons" if context allows this usage
to be clear.
Electrons excited to the conduction band also leave behind electron holes, or
unoccupied states in the valence band. Both the conduction band electrons and
the valence band holes contribute to electrical conductivity. The holes
themselves don't actually move, but a neighbouring electron can move to fill the
hole, leaving a hole at the place it has just come from, and in this way the
holes appear to move, and the holes behave as if they were actual positively
charged particles.
One covalent bond between neighboring atoms in the solid is ten times
stronger than the binding of the single electron to the atom, so freeing the
electron does not imply to destroy the crystal structure.
The notion of holes, which was introduced for semiconductors, can also be
applied to metals, where the Fermi level lies within the conduction band. With
most metals the Hall effect reveals electrons to be the charge carriers, but
some metals have a mostly filled conduction band, and the Hall effect reveals
positive charge carriers, which are not the ion-cores, but holes. Contrast this
to some conductors like solutions of salts, or plasma. In the case of a metal,
only a small amount of energy is needed for the electrons to find other
unoccupied states to move into, and hence for current to flow. Sometimes even in
this case it may be said that a hole was left behind, to explain why the
electron does not fall back to lower energies: It cannot find a hole. In the end
in both materials electron-phonon scattering and defects are the dominant causes
for resistance.
The energy distribution of the electrons determines which of the states are
filled and which are empty. This distribution is described by Fermi-Dirac
statistics. The distribution is characterized by the temperature of the
electrons, and the Fermi energy or Fermi level. Under absolute zero conditions
the Fermi energy can be thought of as the energy up to which available electron
states are occupied. At higher temperatures, the Fermi energy is the energy at
which the probability of a state being occupied has fallen to 0.5.
The dependence of the electron energy distribution on temperature also
explains why the conductivity of a semiconductor has a strong temperature
dependency, as a semiconductor operating at lower temperatures will have fewer
available free electrons and holes able to do the work.
When ionizing radiation strikes a semiconductor, it may excite an electron
out of its energy level and consequently leave a hole. This process is known as
electron�hole pair generation. Electron-hole pairs are constantly generated from
thermal energy as well, in the absence of any external energy source.
Electron-hole pairs are also apt to recombine. Conservation of energy demands
that these recombination events, in which an electron loses an amount of energy
larger than the band gap, be accompanied by the emission of thermal energy (in
the form of phonons) or radiation (in the form of photons).
In the steady state, the generation and recombination of electron�hole pairs
are in equipoise. The number of electron-hole pairs in the steady state at a
given temperature is determined by quantum statistical mechanics. The precise
quantum mechanical mechanisms of generation and recombination are governed by
conservation of energy and conservation of momentum.
As probability that electrons and holes meet together is proportional to the
product of their amounts, the product is in steady state nearly constant at a
given temperature, providing that there is no significant electric field (which
might "flush" carriers of both types, or move them from neighbour regions
containing more of them to meet together) or externally driven pair generation.
The product is a function of the temperature, as the probability of getting
enough thermal energy to produce a pair increases with temperature, being
approximately 1/exp(band gap / kT), where k is Boltzmann's constant and T is
absolute temperature.
The probability of meeting is increased by carrier traps � impurities or
dislocations which can trap an electron or hole and hold it until a pair is
completed. Such carrier traps are sometimes purposely added to reduce the time
needed to reach the steady state.
The property of semiconductors that makes them most useful for constructing
electronic devices is that their conductivity may easily be modified by
introducing impurities into their crystal lattice. The process of adding
controlled impurities to a semiconductor is known as doping. The amount of
impurity, or dopant, added to an intrinsic (pure) semiconductor varies its level
of conductivity. Doped semiconductors are often referred to as extrinsic.
The materials chosen as suitable dopants depend on the atomic properties of
both the dopant and the material to be doped. In general, dopants that produce
the desired controlled changes are classified as either electron acceptors or
donors. A donor atom that activates (that is, becomes incorporated into the
crystal lattice) donates weakly-bound valence electrons to the material,
creating excess negative charge carriers. These weakly-bound electrons can move
about in the crystal lattice relatively freely and can facilitate conduction in
the presence of an electric field. (The donor atoms introduce some states under,
but very close to the conduction band edge. Electrons at these states can be
easily excited to conduction band, becoming free electrons, at room
temperature.) Conversely, an activated acceptor produces a hole. Semiconductors
doped with donor impurities are called n-type, while those doped with acceptor
impurities are known as p-type. The n and p type designations indicate which
charge carrier acts as the material's majority carrier. The opposite carrier is
called the minority carrier, which exists due to thermal excitation at a much
lower concentration compared to the majority carrier.
For example, the pure semiconductor silicon has four valence electrons. In
silicon, the most common dopants are IUPAC group 13 (commonly known as column
III) and group 15 (commonly known as column V) elements. Group 13 elements all
contain three valence electrons, causing them to function as acceptors when used
to dope silicon. Group 15 elements have five valence electrons, which allows
them to act as a donor. Therefore, a silicon crystal doped with boron creates a
p-type semiconductor whereas one doped with phosphorus results in an n-type
material.
Semiconductors with predictable, reliable electronic properties are necessary
for mass production. The level of chemical purity needed is extremely high
because the presence of impurities even in very small proportions can have large
effects on the properties of the material. A high degree of crystalline
perfection is also required, since faults in crystal structure (such as
dislocations, twins, and stacking faults) interfere with the semiconducting
properties of the material. Crystalline faults are a major cause of defective
semiconductor devices. The larger the crystal, the more difficult it is to
achieve the necessary perfection. Current mass production processes use crystal
ingots between four and twelve inches (300 mm) in diameter which are grown as
cylinders and sliced into wafers.
Because of the required level of chemical purity and the perfection of the
crystal structure which are needed to make semiconductor devices, special
methods have been developed to produce the initial semiconductor material. A
technique for achieving high purity includes growing the crystal using the
Czochralski process. An additional step that can be used to further increase
purity is known as zone refining. In zone refining, part of a solid crystal is
melted. The impurities tend to concentrate in the melted region, while the
desired material recrystalizes leaving the solid material more pure and with
fewer crystalline faults.
In manufacturing semiconductor devices involving heterojunctions between
different semiconductor materials, the lattice constant, which is the length of
the repeating element of the crystal structure, is important for determining the
compatibility of materials.
Analytical chemistry
Analytical chemistry is the analysis of material samples to gain an
understanding of their chemical composition, structure and function.
Analytical chemistry is a sub discipline of chemistry that has the broad
mission of understanding the chemical nature of all matter. This differs from
other sub disciplines of chemistry in that it is not intended to understand the
physical basis for the observed chemistry as with physical chemistry and it is
not intended to control or direct chemistry as is often the case in organic
chemistry. Analytical chemistry generally does not attempt to use chemistry or
understand its basis; however, these are common outgrowths of analytical
chemistry research. Analytical chemistry has significant overlap with other
branches of chemistry, especially those that are focused on a certain broad
class of chemicals, such as organic chemistry, inorganic chemistry or
biochemistry, as opposed to a particular way of understanding chemistry, such as
theoretical chemistry. For example the field of bioanalytical chemistry is a
growing area of analytical chemistry that addresses all analytical questions in
biochemistry, (the chemistry of life). Analytical chemistry and experimental
physical chemistry, however, have a unique relationship in that they are very
unrelated in their mission but often share the most in common in the tools used
in experiments.
Analytical chemistry is particularly concerned with the questions of "what
chemicals are present, what are their characteristics and in what quantities are
they present?" These questions are often involved in questions that are more
dynamic such as what chemical reaction an enzyme catalyzes or how fast it does
it, or even more dynamic such as what is the transition state of the reaction.
Although analytical chemistry addresses these types of questions it stops after
they are answered. The logical next steps of understanding what it means, how it
fits into a larger system, how can this result be generalized into theory or how
it can be used are not analytical chemistry. Since analytical chemistry is based
on firm experimental evidence and limits itself to some fairly simple questions
to the general public it is most closely associated with hard numbers such as
how much lead is in drinking water.
Modern analytical chemistry is dominated by instrumental analysis. There are
so many different types of instruments today that it can seem like a confusing
array of acronyms rather than a unified field of study. Many analytical chemists
focus on a single type of instrument. Academics tend to either focus on new
applications and discoveries or on new methods of analysis. The discovery of a
chemical present in blood that increases the risk of cancer would be a discovery
that an analytical chemist might be involved in. An effort to develop a new
method might involve the use of a tunable laser to increase the specificity and
sensitivity of a spectrometric method. Many methods, once developed, are kept
purposely static so that data can be compared over long periods of time. This is
particularly true in industrial quality assurance (QA), forensic and
environmental applications. Analytical chemistry plays an increasingly important
role in the pharmaceutical industry where, aside from QA, it is used in
discovery of new drug candidates and in clinical applications where
understanding the interactions between the drug and the patient are critical.
Analytical methods rely on scrupulous attention to cleanliness, sample
preparation, accuracy and precision.
Many practitioners will keep all their glassware in acid to prevent
contamination, samples will be re-run many times over, and equipment will be
washed in specially pure solvents.
A standard method for analysis of concentration involves the creation of a
calibration curve.
If the concentration of element or compound in a sample is too high for the
detection range of the technique, it can simply be diluted in a pure solvent. If
the amount in the sample is below an instrument's range of measurement, the
method of addition can be used. In this method a known quantity of the element
or compound under study is added, and the difference between the concentration
added, and the concentration observed is the amount actually in the sample.
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