understand the importance of each step to minimise laboratory errors

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PART – II

UNDERSTAND THE IMPORTANCE OF EACH STEP TO MINIMISE ERRORS

Pharma Uptoday

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.

GENERAL INSTRUCTIONS

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.

ERRORS IN WEIGHING

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.

GRADUATED FLASKS

The analyst reads the buret from a position above a line perpendicular to the buret and makes a reading of 12.58 mL.

Reading a buret / pipet

The analyst reads the buret from a position above a line perpendicular to the buret and makes a reading of 12.67 mL.

The analyst reads the buret from a position along a line perpendicular to the buret and makes a reading of 12.62 mL.

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.

Reading a buret / pipete

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 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.

SAMPLE PREPARATION

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.

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.

SAMPLE PREPARATION

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 Associated with Beer’s Law Relationships

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.

Errors in pH Measurement with Glass Electrodes

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.

Titration Errors with Acid/Base Indicators

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.

Sources of error in titrations

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 Titrimetric 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 Gravimetric Analysis

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 Assay

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.

Errors in Related substances

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

MINIMISATION OF ERRORS

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.

References

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