PART – II
UNDERSTAND THE IMPORTANCE
OF EACH STEP TO MINIMISE
ERRORS
∗ For analytical reagents
∗ no bottle is to be opened for a longer time than is absolutely necessary,
∗ no reagent is to be returned to the bottle after it has been removed, the likelihood of
any errors arising from some of the above possible causes is considerably reduced.
∗ Liquid reagents should be poured from the bottle; a pipette should never
be inserted into the reagent bottle.
∗ Particular care should be taken to avoid contamination of the stopper of
the reagent bottle.
∗ When a liquid is poured from a bottle, the stopper should never be
placed on the shelf or on the working bench; it may be placed upon a clean watch glass.
∗ Many chemists cultivate the habit of holding the stopper between the
thumb and fingers of one hand.
∗ The stopper should be returned to the bottle immediately after the
reagent has been removed, and all reagent bottles should be kept
∗ Allow the flask to stand for a while before making the final adjustment
to the mark to ensure that the solution is at room temperature.
∗ It should be noted, however, that for some solutions as, for example,
iodine and silver nitrate, glass containers only may be used, and
∗ in both these cases the bottle should be made of dark (brown) glass:
solutions of EDTA are best stored in polythene containers.
∗ Immediately after the solution has been transferred to the flask, it
should be labelled with:
(1) the name of the solution; (2) its concentration (if any);
The chief sources of error are the following:
∗ Change in the condition of the containing vessel or of the
substance between successive weighings.
∗ by absorption or loss of moisture,
∗ by electrification of the surface caused by rubbing,
∗ by its temperature being different from that of the balance case.
∗ Effect of the buoyancy of the air upon the object and the weights.
∗ A buoyancy error is the weighing error that develops when the object
being weighed has a significantly different density than the masses ∗ Errors in recording the weights. The correct reading of weights is
best achieved by checking weights as they are added to the balance and as they are removed from the balance.
∗
Hygroscopic,
efflorescent,
and
volatile
substances
must
be
weighed in completely closed
vessels.
∗
Substances
which
have
been
heated in an air oven or ignited in
∗ Hygroscopic, efflorescent, and volatile substances must be weighed
in completely closed vessels.
∗ Substances which have been heated in an air oven or ignited in a
crucible are generally allowed to cool in a desiccator containing a suitable drying agent.
∗ The time of cooling in a desiccator cannot be exactly specified, since
it will depend upon the temperature and upon the size of the crucible as well as upon the material of which it is composed.
∗ Platinum vessels require a shorter time than those of porcelain,
glass, or silica.
∗ It has been customary to leave platinum crucibles in the desiccator
for 20-25 minutes, and crucibles of other materials for 30-35 minutes before being weighed. It is advisable to cover crucibles and other
∗
Vessels intended to contain definite volumes of liquid are
marked C or TC or In, while those intended to deliver definite
volumes are marked D or TO or Ex.
∗
The neck is made narrow so that a small change in volume will
have a large effect upon the height of the meniscus: the error
in adjustment of the meniscus is accordingly small.
∗
The mark extends completely around the neck in order to
avoid errors due to
parallax
when making the final adjustment;
the lower edge of the meniscus of the liquid should be
tangential to the graduation mark, and both the front and the
back of the mark should be seen as a single line.
∗ Parallax is the apparent displacement of a liquid level or of a
pointer as an observer changes position. Parallax occurs when an object is viewed from a position that is not at a right angle to the object.
∗
The analyst reads the buret from a position above a
line perpendicular to the buret and makes a reading
of 12.58 mL.
∗
The analyst reads the buret from a position above a
line perpendicular to the buret and makes a reading
of 12.67 mL.
∗
The analyst reads the buret from a position along a
line perpendicular to the buret and makes a reading
of 12.62 mL.
X
X
∗
The errors associated with the use of a volumetric
burette, such as those of drainage, reading, and
change in temperature, are obviated, and weight
burettes are especially useful when dealing with
non-aqueous solutions or with viscous liquids.
∗
The
tips
of
two
styles
of
measuring pipets.
∗
The Mohr pipet is shown on the
left, and the serological pipet on
the right.
∗
The graduation lines on the
Mohr pipet stop short of the tip,
but on the serological pipet, pass
through the tip.
∗ The aim of all sample preparation is to provide the analyte of
interest in the physical form required by the instrument, free of interfering substances, and in the concentration range required by the instrument.
∗ For many instruments, a solution of analyte in organic solvent or
water is required.
∗ Solid samples may need to be crushed or ground, or they may need
to be washed with water, acid, or solvent to remove surface
∗ If the physical form of the sample is different from the physical
form required by the analytical instrument, more elaborate sample preparation is required.
∗ Samples may need to be dissolved to form a solution or pressed
into pellets or cast into thin films or cut and polished smooth.
∗ The type of sample preparation needed depends on the nature of
the sample, the analytical technique chosen, the analyte to be measured, and the problem to be solved.
∗ Most samples are not homogeneous.
∗ Many samples contain components that interfere with the
determination of the analyte.
∗ A wide variety of approaches to sample preparation has been
∗ Many methods use concentrated acids, flammable solvents, and/or
high temperatures and high pressures.
∗ Reactions can generate harmful gases.
∗ The potential for “runaway reactions” and even explosions exists
with preparation of real samples.
∗ The acids commonly used to dissolve or digest samples are
hydrochloric acid (HCl), nitric acid (HNO3), and sulfuric acid
Perchloric acid
∗
Specially designed fume hoods are required to prevent
HClO4 vapors from forming explosive metal perchlorate
salts in the hood ducts, and reactions of hot HClO4 with
organic compounds can result in violent explosive
decompositions.
Hydrofluoric acid:
∗ Concentrated HF is used for dissolving silica-based glass and many
refractory metals such as tungsten, but it is extremely dangerous to work with.
∗ It causes severe and extremely painful deep tissue burns that do
not hurt immediately upon exposure. However, delay in treatment for HF burns can result in serious medical problems and
Hydrochloric acid:
∗ HCl is the most commonly used non-oxidizing acid for dissolving
metals, alloys, and many inorganic materials. HCl dissolves many materials by forming stable chloride complexes with the dissolving cations.
∗ There are two major limitations to the universal use of HCl for
dissolution.
∗ Some elements may be lost as volatile chlorides ∗ Some chlorides are not soluble in water.
∗ A 3:1 mixture of HCl and HNO3 is called aqua regia, and has the
Nitric acid:
∗
HNO3 is an
oxidizing acid
; it has the ability to convert the
solutes to higher oxidation states. It can be used alone for
dissolving a number of elements, including nickel, copper,
silver, and zinc.
∗
This use of acids to destroy organic matter is called wet
ashing or digestion, as has been noted. H2SO4 is a strong
oxidizing acid and is very useful in the digestion of organic
samples.
∗
Its main drawback is that it forms a number of insoluble or
∗
Some bases, such as sodium hydroxide and tetramethyl
ammonium hydroxide, are used for sample dissolution, as
are some reagents that are not acids or bases, like
hydrogen peroxide.
PRECAUTIONS:
∗
Sample preparation should be performed in a laboratory
fume hood for safety. Goggles, lab coats or aprons, and
gloves resistant to the chemicals in use should be worn at
all times in the laboratory.
∗ All spectrometric measurements are subject to indeterminate
(random) error, which will affect the accuracy and precision of the concentrations determined using spectrometric methods.
∗ A very common source of random error in spectrometric analysis is
instrumental “noise”.
∗ Noise can be due to instability in the light source of the instrument,
instability in the detector, variation in placement of the sample in the light path, and is often a combination of all these sources of
∗
When single-beam optics are used, any variation in the
intensity of the source while measurements are being made
may lead to analytical errors.
∗
Slow variation in the average signal (not noise) with time is
called drift,
∗
Drift can cause a direct error in the results obtained.
∗
There are several sources of error in the routine measurement
of pH.
∗
One source of error that may occur with any pH probe, not just
glass electrodes, is in the preparation of the calibration buffer
or buffers.
∗
Any error in making the buffer or any change in composition
∗ Glass electrodes become sensitive to alkali metal ions in basic
solution (pH . 11) and respond to Hþ and Naþ, Kþ, and so on. This results in the measured pH being lower than the true pH.
∗ The magnitude of the alkaline error depends on the composition of
the glass membrane and the cation interfering. This error is called the alkaline error.
∗ Special glass compositions are made for electrodes that are used in
highly alkaline solutions to minimize the response to non-Hþ ions.
∗ Glass electrodes also show an error in extremely acidic solutions
(pH , 0.5).
∗ The acid error is in the opposite direction to the alkaline error; the
Errors in pH Measurement with Glass
We find two types of titration errors in acid/base titrations.
∗ The first is a determinate error that occurs when the pH at which
the indicator changes color differs from the pH at the equivalence point.
∗ This type of error can usually be minimized by choosing the
indicator carefully or by making a blank correction.
∗ The second type is an indeterminate error that originates from the
∗
Two important sources of error in titrations involving iodine
are:
∗ loss of iodine owing to its appreciable volatility;
∗ acid solutions of iodide are oxidised by oxygen from the air.
∗
Failure of reactions to proceed to completion,
∗
Involvement of either induced or side reactions,
∗
Reactions due to substances other than the one being
assayed, and
∗
A noticeable difference occurring between the stoichiometric
∗
Significant solubility of precipitates,
∗
Co-precipitation and post-precipitation,
∗
Decomposition,
∗
Volatalization of weighing forms on ignition,
∗
Precipitation of constituents other than the desired ones.
∗ Incorrect weighing & transfer of analytes & standards.
∗ Insufficient extraction of the analyte from the matrix e.g. tablets ∗ Incorrect use of pipettes, burettes, volumetric flasks for volume
measurement.
∗ Measurement carried out using improperly calibrated
instrumentation.
∗
Particulate matter from the atmosphere, machines, devices
from containers.
∗
Cross contamination from the other samples or other
products or solutions.
∗
Microbiological contamination.
∗
Instruments with low sensitivity.
∗ Systematic errors can often be materially reduced by one of the
following methods.
∗ Calibration of apparatus and application of corrections ∗ Running a blank determination
∗ Running a control determination
∗ Use of independent methods of analysis
∗ Text book of Quantitative Chemical Analysis- 5th Edition –Vogel. ∗ Pharmaceutical Analysis : A Textbook for Pharmacy Students &
Pharmaceutical Chemists – David G. Watson
∗ Handbook of instrumental techniques for analytical chemistry –
Frank Settle.
∗ Instant Notes in Analytical Chemistry – D. Kealey & P.J. Haines.
∗ Analytical Chemistry for Technicians 3rd edition (CRC, 2003) –
Kenkel.
∗ pharmaceutical-drug-analysis book 2nd edition – Ashutoshkar.
∗ Fundamentals of Analytical Chemistry 8th edition HQ (Thomson,
2004) – Douglas A. Skoog.
