Understand the Importance of Each Step to Minimise Errors in Analytical Laboratory

8/13/2019 Understand the Importance of Each Step to Minimise Errors in Analytical Laboratory

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PART II

UNDERSTAND THE IMPORTANCE

OF EACH STEP TO MINIMISEERRORS

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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 aclean watch glass.

Many chemists cultivate the habit of holding the stopper between thethumb 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 keptscrupulously clean, particularly round the neck or mouth of the bottle.

GENERAL INSTRUCTIONS

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Allow the flask to stand for a while before making the final adjustmentto 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, itshould 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

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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 thebalance 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 isparticularly serious when the relative humidity is low.

ERRORS IN WEIGHING

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Hygroscopic, efflorescent, andvolatile substances must be

weighed in completely closedvessels.

Substances which have beenheated in an air oven or ignited in

a crucible are generally allowed tocool in a desiccator containing asuitable drying agent.

ERRORS IN WEIGHING

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Hygroscopic, efflorescent, and volatile substances must be weighedin completely closed vessels.

Substances which have been heated in an air oven or ignited in acrucible 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 thecrucible 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 desiccatorfor 20-25 minutes, and crucibles of other materials for 30-35 minutesbefore being weighed. It is advisable to cover crucibles and otheropen vessels.

ERRORS IN WEIGHING

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Vessels intended to contain definite volumes of liquid aremarked C or TC or In, while those intended to deliver definitevolumes are marked D or TO or Ex.

The neck is made narrow so that a small change in volume willhave a large effect upon the height of the meniscus: the errorin adjustment of the meniscus is accordingly small.

To read the position of the meniscus, the eye must be at thesame level as the meniscus, in order to a void errors due toparallax.

GRADUATED FLASKS

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The mark extends completely around the neck in order toavoid errors due to parallaxwhen 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 theback of the mark should be seen as a single line.

Parallax is the apparent displacement of a liquid level or of apointer as an observer changes position. Parallax occurs when an

object is viewed from a position that is not at a right angle to theobject.

GRADUATED FLASKS

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The analyst reads the buret from a position above aline perpendicular to the buret and makes a readingof 12.58 mL.

Reading a buret / pipet

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The analyst reads the buret from a position above aline perpendicular to the buret and makes a readingof 12.67 mL.


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The analyst reads the buret from a position along aline perpendicular to the buret and makes a readingof 12.62 mL.


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The errors associated with the use of a volumetric

burette, such as those of drainage, reading, andchange in temperature, are obviated, and weightburettes are especially useful when dealing with non-aqueous solutions or with viscous liquids.

Reading a buret / pipete

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The tips of two styles ofmeasuring pipets.

The Mohr pipet is shown on theleft, and the serological pipet onthe right.

The graduation lines on theMohr pipet stop short of the tip,but on the serological pipet, passthrough the tip.


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The aim of all sample preparation is to provide the analyte ofinterest in the physical form required by the instrument, free ofinterfering substances, and in the concentration range required bythe instrument.

For many instruments, a solution of analyte in organic solvent orwater is required.

Solid samples may need to be crushed or ground, or they may needto 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 berequired.

SAMPLE PREPARATION

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If the physical form of the sample is different from the physicalform required by the analytical instrument, more elaborate samplepreparation is required.

Samples may need to be dissolved to form a solution or pressedinto 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 bemeasured, 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

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Many methods use concentrated acids, flammable solvents, and/orhigh temperatures and high pressures.

Reactions can generate harmful gases.

The potential for runaway reactionsand even explosions existswith preparation of real samples.

The acids commonly used to dissolve or digest samples arehydrochloric 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 acidmust be chosen to match the level of analyte to be determined.

SAMPLE PREPARATION

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Perchloric acid

Specially designed fume hoods are required to preventHClO4 vapors from forming explosive metal perchloratesalts in the hood ducts, and reactions of hot HClO4 withorganic compounds can result in violent explosivedecompositions.

SAMPLE PREPARATION

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Hydrofluoric acid:

Concentrated HF is used for dissolving silica-based glass and many

refractory metals such as tungsten, but it is extremely dangerousto work with.

It causes severe and extremely painful deep tissue burns that donot hurt immediately upon exposure. However, delay intreatment 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

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Hydrochloric acid:

HCl is the most commonly used non-oxidizing acid for dissolving

metals, alloys, and many inorganic materials. HCl dissolves manymaterials by forming stable chloride complexes with thedissolving cations.

There are two major limitations to the universal use of HCl fordissolution.

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 theability to dissolve gold, platinum, and palladium.

SAMPLE PREPARATION

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Nitric acid:

HNO3 is an oxidizing acid; it has the ability to convert thesolutes to higher oxidation states. It can be used alone fordissolving a number of elements, including nickel, copper,silver, and zinc.

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

SAMPLE PREPARATION

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This use of acids to destroy organic matter is called wetashing or digestion, as has been noted. H2SO4 is a strongoxidizing acid and is very useful in the digestion of organicsamples.

Its main drawback is that it forms a number of insoluble orsparingly soluble sulfate salts.

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Some bases, such as sodium hydroxide and tetramethyl

ammonium hydroxide, are used for sample dissolution, asare some reagents that are not acids or bases, likehydrogen peroxide.

The chemical literature contains sample dissolutionprocedures for virtually every type of material known andshould be consulted.

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PRECAUTIONS:

Sample preparation should be performed in a laboratoryfume hood for safety. Goggles, lab coats or aprons, andgloves resistant to the chemicals in use should be worn atall times in the laboratory.

SAMPLE PREPARATION

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All spectrometric measurements are subject to indeterminate(random) error, which will affect the accuracy and precision of theconcentrations determined using spectrometric methods.

A very common source of random error in spectrometric analysis isinstrumental noise.

Noise can be due to instabilityin the light source of the instrument,instability in the detector, variation in placement of the sample inthe light path, and is often a combination of all these sources of

noise and more. Because these errors are random, they cannot beeliminated. Errors in measurement of radiation intensity lead directly to errors

in measurement of concentration when using calibration curves andBeersLaw.

Errors Associated with Beers Law

Relationships

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When single-beam optics are used, any variation in the

intensity of the source while measurements are being mademay lead to analytical errors.

Slow variation in the average signal (not noise) with time iscalled drift,

Drift can cause a direct error in the results obtained.

Errors Associated with Beers Law

Relationships

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There are several sources of error in the routine measurementof pH.

One source of error that may occur with any pH probe, not justglass electrodes, is in the preparation of the calibration bufferor 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 byvery basic buffers (thereby making them less basic).

Errors in pH Measurement with GlassElectrodes

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Glass electrodes become sensitive to alkali metal ions in basicsolution (pH . 11) and respond to H and Na, K, 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 calledthe alkaline error.

Special glass compositions are made for electrodes that are used inhighly alkaline solutions to minimize the response to non-H 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; themeasured pH values are too high.

Errors in pH Measurement with GlassElectrodes

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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 equivalencepoint.

This type of error can usually be minimized by choosing theindicator carefully or by making a blank correction.

The second type is anindeterminate errorthat originates from the

limited ability of the eye to distinguish reproducibly theintermediate 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 concentrationof the indicator, and on the sensitivity of the eye to the twoindicator colors.

Titration Errors with Acid/BaseIndicators

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Two important sources of error in titrations involving iodineare:

loss of iodine owing to its appreciable volatility;

acid solutions of iodide are oxidised by oxygen from the air.

Sources of error in titrations

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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

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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

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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

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Systematic errors can often be materially reduced by one of thefollowing 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

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Text book of Quantitative Chemical Analysis- 5thEdition 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 2ndedition 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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Understand the Importance of Each Step to Minimise Errors in Analytical Laboratory