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Introduction to Analytical and, Instrumental Methods, ANALYTICAL CHEMISTRY, Analytical chemistry is concerned with the identification of a substance,, the elucidation of its structure and quantitative analysis of its composition. It, is an interdisciplinary branch of science which deals with various disciplines of, chemistry such as inorganic, organic, physical, industrial and biochemistry. It, also finds extensive applications in environmental science, agricultural science,, clinical chemistry, solid state research and electronics, oceanography, forensic, science and space research. The scope of analytical chemistry is very broad and, embraces a wide range of manual, chemical and instrumental techniques., CHEMICAL ANALYSIS, It involves the resolution of a chemical compound into its proximate or, ultimate parts, the determination of its elements or of foreign substances it may, contain. The first requirement in a completely unknown sample is to ascertain, what substances are present in it and which type of impurities are contained, in the sample. The solution of such problems lies within the province of, qualitative analysis. Having ascertained the nature of the constituents of a, given sample, the next step is to determine how much of each (or specified), component is present. Such determinations lie within the realm of, quantitative analysis and a variety of techniques are available to supply the, required information., Types of Chemical Analysis., 1. Proximate analysis, in which the amount of each element in a, sample is determined with no concern as to the actual compounds, present., 2. Partial analysis deals with the determination of selected, constituents in the sample., 3. Trace constituent analysis, a specialised instance of partial, analysis, deals with the determination of specified components present, in very minute quantity., 4. Complete analysis, when the proportion of each component of the, sample is to be determined., Stages of Analysis., A complete chemical analysis, even for a single substance, involves a, series of steps and procedures viz. sampling, preparation of analytical sample,, dissolution of sample, removal of interferences, sample measurement, results, and presentation of data. Each step must be carefully considered and assessed, in order to minimise errors and to maintain accuracy and reproducibility., (3), 1/ 15, Du. Samteen Fanovqui

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4, ANALYTICAL AND STATISTICAL METHODS, Advantages of Chemical Methods., • Chemical methods are based on absolute measurements., • Procedure is simple and accurate., • Equipment required is cheap and readily available., Limitations of Chemical Methods., •Procedure is time consuming., • Accuracy decreases with decreasing concentration., • Lack of specificity and versatility., INSTRUMENTAL METHODS, The methods which measure an electrical property, absorption of radiation or the, intensity of an emission, require the use of a suitable instrument, polarograph,, spectrophotometer etc. and in consequence are referred to as instrumental methods., Advantages of Instrumental Methods., • Instrumental methods are normally applicable at concentrations far too small to be, amenable to determination by chemical methods., • Determination is much faster than purely chemical procedures., • Measurements obtained are reliable. Accuracy is obtained., • Even complex samples can be handled easily., Finds wide application in industry., In most cases a microcomputer can be interfaced to the instrument so that absorption, curves, polarograms, titration curves etc. can be plotted automatically. In fact, by the, incorporation of appropriate servo-mechanisms, the whole analytical process may be, completely automated., Limitations of Instrumental Methods., Despite the advantages possessed by instrumental methods in many directions, their, wide-spread adoption has not rendered the purely chemical or classical methods obsolete, the, situation is influenced by the following factors :, With instrumental methods it is necessary to carry out a calibration operation using a, sample of material of known composition as reference substance., . Many instruments are expensive and their use will only be justified if numerous, samples have to be analysed, or when dealt with the determination of substances, present in minute quantities (trace, subtrace or ultratrace analysis)., Whilst an instrumental method is ideally suited to the performance of a large number, of routine determinations, for an occasional, non-routine analysis it is often simpler to, use a chemical method than to go to the trouble of preparing requisite standards and, carrying out the calibration of an instrument., . The sensitivity and accuracy depends on the instrument., Sizable space is required., Specialised training is needed., ANALYTICAL METHODS ON THE BASIS OF SAMPLE SIZE, 1. Macro involves the analysis of quantities of 0-1 g or more., 2. Meso (Semimicro) measures quantities ranging from 102 g to 10 g., 3. Micro deals with quantities in the range 103, g to 102 g., 2/ 15

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INTRODUCTION TO ANALYTICAL AND INSTRUMENTAL METHODS, 4. Submicro is concerned with samples in the range 10g to 10 g., 5. Ultramicro measures quantities below 104g., A major constituent is one accounting for 1 to 100% of the sample under investigation., A minor constituent is present in the range 0-01 to 1%; a trace constituent is present at, a concentration of less than 0-01%. Further subdivisions include trace corresponds to, 102 - 10 ug g or 10-10 ppm, microtrace to 10-1- 102 pg g or 10- 10 ppm,, nanotrace to 101-102 fg g1, When the sample weight is small (0-1-1-0 mg), the determination of a trace component, at the 0-01% level may be referred to as subtrace analysis. If the trace component is at the, microtrace level, the analysis is termed submicro trace. Determination of a component at, the trace level (0-1 mg) is called ultra trace analysis while with a component at the, microtrace level, the analysis is termed as ultra-microtrace., or 10-10 10-7, ppm., SAMPLING, Sampling is the process of extracting from a large quantity of material a small portion, which is truly representative of the composition of the whole material. The purpose of analysis, is to determine the quality or composition of a material. It is essential to adopt adequate, sampling procedures to know the validity of the analytical results., Sampling Procedure., The reliable method for sampling is one in which portions of the material are selected, based upon statistical probabilities. The result obtained from the given sample may form the, basis of assessing the value of a large consignment of the commodity from which the sample, was drawn., Sampling in Different Physical States., During sampling it is the number of analyte particles which is important rather than the, amount (mass) of the sample. The relationship n 1/R2 applies in all cases where R is the, relative standard deviation and n is the number of particles. The consequence of this, relationship means that different approaches are used for each of the three phases. For gases, and liquids the particle size is very small, e.g., 1 cm of a pure gas at STP contains, 2-5 x 1019 particles and 1 cm of liquid, -, homogeneity depend upon taking sufficient analyte particles to be representative. Since, modern methods of analysis often require considerably less than 1 mg of material in order to, perform a number of analyses, this small amount is obtained by first collecting a gross, sample and then reducing it down to the analytical sample., 2-5 x 1016, but for solids the requirement of, Sampling of Gases., Sampling techniques for gases and vapours involve gas sampling vessels, static sensórs,, entrapment and real-time analysis. A number of instruments (electrochemical sensors) and, techniques (IR, GC-MS) are available for instant analysis., Sampling of Liquids., Several approaches are used to obtain a gross liquid sample viz., small static systems,, flowing contained volumes, open flowing systems. The protocol depends upon whether it is, desirable to obtain a number of discrete samples which will be analysed separately or to, bulk the individual samples taken to give a composite sample. Automatic collectors can, either produce a single composite sample or can collect up to 24 separate samples in individual, 3/ 15

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ANALYTICAL AND STATISTICAL METHODS, containers. The separate aliquots of liquids can be analysed individually and the results, combined or the aliquots can be combined into one gross sample and replicated analysis, performed., Sampling of Solids., For gases and liquids, although both spatial and chronological variations may occur, at, a particular location and time most gas and liquid samples are homogeneous and contain a, large number of particles. This is not true in solid sampling where inhomogeneity of the, material, small variations in positions and particle size can be highly significant. Generally, sampling of solids requires an even more rigorous and systematic approach than for gases and, liquids. It is found that sampling errors are inversely proportional to the number of particles, taken. The easiest way to sample a homogeneous material is the grab sample. The whole of, the material to be analysed is known as the population or the lot. This may be a single, more, or less homogeneous mass or it may contain a number of individual units. From this is drawn, the gross sample or bulk sample. This is obtained by taking increments from the lot in such, a way that, when combined, they should be representative of the lot. The analytical sample is, taken from this prior to analysis., Note that :, • Correct sampling of materials is important to (i) obtain a representative portion of the, material for analysis and (ii) prevent the occurrence of accidents with sampling, hazardous materials., • Sampling variance, V is inversely proportional to the number of sampling increments, (n), i.e., V= kln where k is a constant dependent on the size of the increment and, variation within the bulk material., • Sample types are : (i) Bulk samples, (ii) stratified (segregated) samples and, (iii) discrete units (packets, bottles etc.). Bulk samples are those which may contain, several components and particle sizes but which do not have an observable boundary, between one part of the sample and another., SAMPLING STATISTICS, Errors in measurement are classified as determinate (systematic) errors, which give a, measure of accuracy and indeterminate (random) errors, which are a measure of precision., For normal statistical manipulations it is often assumed that the determinate errors are, reduced at a low value and the remaining uncertainty in the result is due to random error, that are quantified by calculating the standard deviation. However, the standard deviation of, a method is composed of the standard deviation of each step in the analysis. Hence while, analysing real samples, the overall standard deviation S, is given by, S, = VS + S or V. = Så + S or V.= (VA + Vs), ...(1), where S is the analytical standard deviation and Ss the sampling standard deviation., S, can be separated into two discrete components:, 1. Same-sample variation i.e., variation in instrument or signal response for, repetitive measurements of the same sample., 2. Different-sample variation is the within-lot variance and it depends on the, relative composition of the sample and the homogeneity of the particles., The sampling variation Ss or between-sample variation measures the variability, between different parts of the population., 4/ 15

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INTRODUCTION TO ANALYTICAL AND INSTRUMENTAL METHODS, 7, Note that it is better to take a large number of samples (to improve sampling precision), and to analyse them with a method of moderate precision than to use a high-precision, technique on a small number of samples. Composite samples can also greatly increase the, overall precision of the experiment under suitable circumstances., Sampling plan should be prepared by considering, size of the sample,, (i), (ii) number of samples to be taken and, (iii) individual or composite sample., Exercise. A particular analytical technique can give a precision of 1% and the sampling error, is 6% (Ss 0-06). What is the overall precision and is it worth considering a slower technique, which can give a precision of 0-2%?, Solution. From equation (1), S, = VS+ S = V(0-06)2 + (0-01), %3D, = 0-0608 = 6-1%, For 0-2% analytical precision Sg= V(0-06)² + (0-002)² = 0-060, It is not worth using a technique of higher precision., Exercise. Express confidence limit., Solution. Confidence limit is given by the relationship :, H =It ts, / Vn, where S, is the standard deviation of the individual sample, n is the number of samples, taken, i is the mean of the analytical results and serves as an estimate of the true mean u., Exercise. An estimate of the variability of iron in a consignment of an ore, based on 16, measurements was found to be t 1-5%. How many samples should be taken to give (at the 95%, confidence level) a sampling error of less than 0-5% iron., Solution. Apply the equation u=tts / Vn. In this problem the value 0-5% is the, difference between the sample mean i and the actual value u. If this value is indicated by E, then the above equation may be written as, E = ts / Vn. So, n=, E, However, the value of t for (n - 1) i.e., 15 degrees of freedom at 95% confidence level is, 2-13., 2., 2-13 x 1-5, n 3=, = 41, 0-5, This test shows that at least 41 samples are required if the specifications in this example, are to be satisfied., 5/ 15