Understand the importance of each step to minimise Laboratory errors

PART – II
UNDERSTAND THE IMPORTANCE
OF EACH STEP TO MINIMISE
ERRORS
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GENERAL INSTRUCTIONS
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
scrupulously clean, particularly round the neck or mouth of the bottle.

GENERAL INSTRUCTIONS
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);
(3) the data of preparation; and
(4) the initials of the person who prepared the solution, together
with any other relevant data.

ERRORS IN WEIGHING
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.
A porcelain or glass object will occasionally acquire a static charge
sufficient to cause a balance to perform erratically; this problem is
particularly serious when the relative humidity is low.

ERRORS IN WEIGHING
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.

ERRORS IN WEIGHING
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
open vessels.

GRADUATED FLASKS
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.
To read the position of the meniscus, the eye must be at the
same level as the meniscus, in order to a void errors due to
parallax.

GRADUATED FLASKS
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.


Reading a buret / pipet
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
of 12.67 mL.


The analyst reads the buret from a position along a
of 12.62 mL.


X
X
√

Reading a buret / pipete

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 nonaqueous 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.


SAMPLE PREPARATION
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
contamination.
Liquid samples with more than one phase may need to be
extracted or separated. Filtration or centrifugation may be
required.

SAMPLE PREPARATION
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
developed to deal with these problems in real samples.

SAMPLE PREPARATION
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
(H2SO4). These acids may be used alone or in combination.
The choice of acid or acid mix depends on the sample to be
dissolved and the analytes to be measured. The purity of the acid
must be chosen to match the level of analyte to be determined.

SAMPLE PREPARATION
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.


SAMPLE PREPARATION
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
even death from contact with relatively small amounts of acid.
Glass beakers and flasks cannot be used to hold or store even
dilute HF.
Teflon or other polymer labware and bottles are required.

SAMPLE PREPARATION
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
ability to dissolve gold, platinum, and palladium.

SAMPLE PREPARATION
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.
The problem with the use of HNO3 by itself is that it often
forms an insoluble oxide layer on the surface of the sample
that prevents continued dissolution. For this reason, it is
often used in combination with HCl, H2SO4 , or HF.

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
sparingly soluble sulfate salts.


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.
The chemical literature contains sample dissolution
procedures for virtually every type of material known and
should be consulted.


SAMPLE PREPARATION
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.


Errors Associated with Beer’s Law
Relationships
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
noise and more. Because these errors are random, they cannot be
eliminated.
Errors in measurement of radiation intensity lead directly to errors
in measurement of concentration when using calibration curves and
Beer’s Law.

Errors Associated with Beer’s Law
Relationships
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.


Errors in pH Measurement with Glass
Electrodes
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
on storage of the buffer will result in error in the pH measured.
Common problems with buffers are bacterial growth or mold
growth in organic buffers, and absorption of CO2 from air by
very basic buffers (thereby making them less basic).

Errors in pH Measurement with Glass
Electrodes
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
measured pH values are too high.

Titration Errors with Acid/Base
Indicators
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
limited ability of the eye to distinguish reproducibly the
intermediate color of the indicator.
The magnitude of this error depends on the change in pH per
milliliter of reagent at the equivalence point, on the concentration
of the indicator, and on the sensitivity of the eye to the two
indicator colors.

Sources of error in titrations
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.


Errors in Titrimetric Analysis
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
equivalence point of a reaction and the observed end-point.


Errors in Gravimetric Analysis
Significant solubility of precipitates,
Co-precipitation and post-precipitation,
Decomposition,
Volatalization of weighing forms on ignition,
Precipitation of constituents other than the desired ones.


Errors in Assay
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.
Failure use an analytical blank.
Selection of assay conditions that cause degradation of the analyte.
Failure to allow for or to remove interference by excipients in the
measurement of an analyte.

Errors in Related substances
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.


MINIMISATION OF ERRORS
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
Running parallel determinations
Standard addition
Internal standards
Amplification methods
Isotopic dilution

References
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.
Undergraduate instrumental analysis 6th edition – James W.
Robinson.

Understand the importance of each step to minimise Laboratory errors

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