• Role of analytical chemistry.

- Fundamentals of Analytical Chemistry - Analytical Chemistry / Instrumentation
• Role of analytical chemistry.

• Introduction

- Analytical chemistry isasold as chemistry. Analytical chemistry acts as a foundation for other branches of chemistry. In fact the science of chemistry came into existence as a result of human inquisitiveness to understand the nature of an extraordinary variety of matter that surrounded him, and this knowledge was obtained by analysing different types of material. A complex material on analysis can give rise to a number of new and simpler constituents. Each of the constituents can be further analysed and if this process is continued, a stage will come when a constituent will not give anything new; such a constituent is called an element. Thus the concept of an element as a substance which cannot be broken down into something simpler by ordinary chemical methods arose from analytical data on different substances.The construction of chemical balance provided a quantitative aspect to chemical analysis. With this development the study of analytical chemistry stimulated quantitative approach to various problems of chemistry. Chemical reactions were studied on the basis of qualitative and also quantitative changes that occurred; this resulted in the discovery of several new compounds and the five laws of chemical combination. In an attempt to theoretically explain these laws, Dalton developed the atomic theory which played a tremendous role in developing chemistry into a quantitative experience and was mainly preparative in nature centred around empirical synthetic methods. The induction of analyticalapproach brought about a revolutionary transformation from magic and alchemy to quantitative scientific chemistry.

• Steps in analytical procedure

Structural information about a complex compound can be obtained either by preparing it from some simple constituents or by breaking it chemically into smaller and simpler units and then identifying them. The former approach involves synthesisof the compound while the latter is termed chemicalanalysis

• Inorganic and Organic Analysis: Chemical analysis is broadly classified as inorganic or organic depending upon the nature of the material under examination. Elementalanalysisdeals with the detection and determination of various elements present in a compound. Functionalgroupanalysis involves the determination of certain groupings of atoms such as carboxyl group (-COOH) or hydroxyl group (-OH) in an organic material.

• Major, Minute and Trace Constituents: A majorconstituentis one whose amount is 1 per cent or more of the sample material. A minorconstituentpresent in quantities smaller than 0.01 percent is called a traceconstituent.

•Complete and Partial Analysis: Chemical analysis is said to be completewhen it involves the determination of all the components detected qualitatively in the sample. The analysis is partial when it aims at determining only one or a few of the components of the sample such as determination of copper in a copper ore.

• Classical Chemical Analysis: Quantitative chemical analysis started with the application of techniques of gravimetricprocedures. In a gravimetric measurement, a known volume of sample solution is treated with an excess of a suitable reagent which quantitatively precipitates the desired constituent present in the sample solution. The precipitate which is of known concentration is filtered, washed, dried and weighed. Knowing the weight of the precipitate, the amount of the desired constituent in the test solution is calculated. For example, an excess of dilute sulphuric acid is added to a given solution containing barium ions. The precipitated of barium sulphate formed is filtered, washed, dried and weighed. From the weight of barium sulphate the quantity of barium in the given solution is calculated. Because such determinations are based on the measurement of weight, these are referred to as gravimetric determinations.

• In electro-gravimetricanalysis, the constituent to be determined is deposited on the electrode by passing electric current through a suitable electrolytic cell. For example, the amount of copper in given copper sulphate solution can be determined by passing current through the solution and weighing the copper deposited at the negative electrode. Here, electric current acts as a precipitating agent.

•Gravimetric determination of a volatile constituent can be done by heating the sample and recording theloss of weight. Gravimetric procedures are quite accurate but are lengthy and tedious.

- Another group of techniques was soon developed in which quantitative analysis was achieved by measuring volume of solutions, hence it was called volumetric analysis. In this type of analysis, to the sample solution of unknown concentration, a reagent solution of known concentration is gradually added till the reaction between them is just complete as shown by some indicator. The volume of the sample and reagent solutions are known, the concentration of the reagent solution is also known so the concentration of the given sample solution can be calculated. For example, a known volume of hydrochloric acid solution whose concentration is to be determined is taken in a conical flask and a few drops of phenolphthalein solution are added as indicator. A solution of sodium hydroxide of known concentration is gradually added through a burette until the solution in the flask becomes just pink. The volume of sodium hydroxide solution added is recorded and from this, the concentration of given hydrochloric acid is calculated. This process is called titration and the determination is termed titrimetric determination.

• Certain substances, under suitable experimental conditions, quantitatively liberate a gas. The measurement of the volume of the liberated gas can be used as a basis for determining the substances producing the gas. Such analytical procedures are called“gasometric methods”. For example, semicarbazide can be decomposed withlead peroxide and the liberated nitrogen can be measured. Knowing the volume of nitrogen, the amount of semicarbazide present in the sample solution can be calculated.

•A particular component of a gaseous mixture can be absorbed in a suitable absorbent. The decrease in the volume of the gaseous mixture gives the volume of constituent absorbed. This method of analysis is called “gas analysis”. For example, a mixture of oxygen and carbon dioxide gases can be passed through a solution of potassium hydroxide when carbon dioxide alone is absorbed. Thus the decrease in the volume of thegaseous mixture will be equal to the volume of carbon dioxide present in the mixture.

•Gravimetric methods along with titrimetric procedures belong to the classical methods of analysis.Titrimetric procedures are much simpler and convenient and hence a large number of titration methods have been worked out for determining a wide variety of inorganic and organic substances. In direct titrimetric methods, sample solution is directly titrated with reagent solution. The indirect procedure consists of adding a known excess of reagent and titrating back the unused reagent.

• Classification of Methods of Quantitative Analysis

The methods of quantitative analysis can be classified from different points of view based on the nature of material under examination, the type of method employed, the amount of desired constituent in sample material and so on.

• The analysis can be termed organic, inorganic, biochemicaletc., depending on the nature of the material analysed.

- Another classification of quantitative analysis is non-instrumentaland instrumental. The former includes gravimetric and titrimetric procedures whereas the latter involves use of instruments such as colorimeter, a conductivity meter or a potentiometer and so on.

- Another classification into classicaland physico-chemicalmethods of analysis is also not very sound theoretically although it is quite useful for the sake of convenience. According to this classification, gravimetric and titrimetric methods are termed chemical methods though they are based on the measurement of physical properties, viz. weight and volume. The methods of analysis based on such physical properties as potential, conductance, current strength, optical rotation, etc., are generally referred to as physio-chemical methods.

• The classical chemical analysis which consists of gravimetric and titrimetric method is also known as wetanalysis. There are certain other procedures which are based on matter-energy interaction such as, a colorimetric determination which involves passage of light a form of energy through solution of the substance, to be determined, which is matter.

• The methods of quantitative analysis can also be classified on the basis of the size of the sample for a determination. The term macro-analysisis used when the determination involves 0.1 g or more of the sample. If the amount of the sample is approximately 0.01 to 0.1 g, the method is called semi-microand for samples weighing 0.001-0.01 g, the term micro-methodis used. Ultra-microanalysis involves samples containing less than 0.001g of material; someauthors have used the term sub-microanalysis also. Certain procedures have been described for analysing quantities smaller than those handled in ultra-micro analysis; these constitute what is known as nanoanalysis.

• According to another classification, macro-methods are those in which sample contains more than 2 milliequivalents (meq) of the material and 0.1N solutions are employed in the determination. In semi-micro methods, the sample contains about 1 meqof substance and 0.05 to 0.1 N solutions are used in its titrimetric determination. When the amount of the substance is around 0.1 meq the method used is called a micro-method; this generally involves the use of 0.01 N reagent solutions.

• Evaluation ofAnalytical Chemistry

A remarkable fact about the evaluationof analytical chemistry is that workers in other branches such as inorganic, organic, biochemistry etc., have also contributed significantly to its growth. The difference in approach is that whereas an analytical chemist have his main interest in the methods and techniques themselves, other workers develop analytical procedures for their own specific problems. For example, a workerin the field of chemical kinetics is not primarily interested in various analytical methods and techniques but may develop a method for quantitatively analysing a substance whose concentration is to be determined in order to obtain the necessary kinetic data. On the other hand main interest of an analytical chemist is in the development and improvement of an analytical procedure itself and in testing its reliability. He also studies the interference caused by the presence of other substances and attempt to modify the procedure so that such an interference can be eliminated. It is necessary to distinguish between an analyticalchemistand an analyst. An analytical chemist carefully chooses a chemical reaction and uses it for developing an analytical procedure taking into account various theoretical considerations. He also studies the effect of different factors that can influence the result of the determination. The job of an analyst is simply to follow the given instructions to perform a determination.

• As already been mentioned, the earlier methods of quantitative analysis were those involving gravimetric procedures. Soon thereafter the technique of volumetric analysis emerged which due to its inherent simplicity and rapidity received preference over gravimetric methods which were lengthy and tedious. One of the earliest volumetric determinationswas developed by Margueritte who titrated ferrous iron with potassium permanganate. Such a procedure is now more appropriately called a titrimetric procedure rather than a volumetric method because the latter is a more general term including gasometric methods and gas analysis which are also based on the measurement of volume. Due to their inherent simplicity titrimetric analyses have found and continue to find extensive applications. Both direct and indirect titrimetric procedures have been worked out forthe quantitative analysis of wide variety of organic as well as inorganic compounds with the modern accent on micro and sub-micro analysis.

• Over the last 50 years or so there has been a growing tendency to make use of certain instruments to achieve quantitative analysis. For example, instrumental techniques such as potentiometric, conductometric, photometric, amperometric etc., have been applied to locate the end point in a titration or to follow the course of a chemical reaction. It should be noted that in such cases the titrimetric methods are not basically altered from their standard procedures, the instrument simply act as a substitute for an indicator. Recently such methods commonly known as the instrumental methods of analysis have been increasingly used especially in the field of industrial and commercial quantitative analysis due to their rapidity and sensitivity. But it would be a mistake to conclude that the so called chemical methods would be totally eliminated from considerations. There are several reasons in favour of this as discussed below:

- The classical and physicochemical methods of quantitative analysis should be regarded as complementary. Because once an instrument to be used in quantitative analysis has been properly calibrated, it can be used with great advantage to achieve rapid analysis and due to high sensitivity of physicochemical procedures, samples at microgram level can be handled.

- The advantage of microchemical methods are saving of time, labour and material. Much of the work on vitamins, hormones and other natural products could be done due to the development of microanalytical methods because many of the compounds were present in microquantities.

- In about last 50 years there has been increasing sophistication in all areas of chemistry, physics and biological sciences. This created analytical problems which required use of sophisticated instrumentation for their solution. For example, in determining traces of impurities at part per billion level or determining traces of pollutants in the atmosphere of industrial area.

• Analytical Problems and their Solutions

The solutions of all analytical problems, whether qualitative or quantitative, follow the same basic pattern. This may be described under seven general headings:

(1)Selection: The selection of the method of analysis is an importantstep in the solution of an analytical problem. A choice cannot be made until the overall problem is defined, and where possible a decision should be taken by the client and the analyst in consultation. Inevitably, in the method selected, a compromise has to be reached between the sensitivity, precision and accuracy desired of the results and the costs involved. For example, X-ray, fluorescence, spectrometry may provide rapid but rather imprecise quantitative results in a trace element problem. Atomic absorption spectrophotometry, on the other hand, will supply more precise data, but at the expense of more time-consuming chemical manipulations.

(2)Sampling: Correct sampling is the cornerstone ofreliable analysis. The analyst must decide in conjunction with technological colleagues how, where, and when a sample should be taken so as to be truly representative of the parameter that is to be measured.

(3)Preliminary sample treatment: For quantitative analysis, the amount of sample taken is usually measured by mass or volume. When a homogeneous sample already exists, it may be subdivided without further treatment. With many solids such as ores, however, crushing and mixing are prior requirements. The sample often needs additional preparation for analysis, such as drying, ignition and dissolution.

(4)Separation: A large proportion of analytical measurements is subject to interference from other constituents of the sample. Newer methods increasingly employ instrumental techniques to distinguish between analyte and interference signals. However, such distinction is not always possible and sometimes a selective chemicalreaction can be used to mask the interference.

(5)Final measurement: This step is often the quickest and easiest of the seven but can only be as reliable as the preceding stages. The fundamental necessity is a known proportionality between the magnitude of the measurement and the amount of analyte present.

(6)Method validation: It is pointless carrying out the analysis unless the results obtained are known to be meaningful. This can only be ensured by proper validation of the method before use and subsequent monitoring of its performance. The analysis of validated standards is the most satisfactory approach. Validated standards have been extensively analysed by a variety of methods, and an accepted value for the analyte obtained. A standard should be selected with a matrix similar to that of the sample. In order to ensure continued accurate analysis, standards must be reanalysed at regular intervals.

(7)The assessment of results: Results obtained from an analysis must be assessed by the appropriate statistical methods and their meaning considered in the light of the original problem.

• Importance of Analytical Chemistry to Various Branches of Science

• Analytical chemistry finds wide applications not only in different branches of chemistry but also in other physical and biological sciences and in many fields of engineering.

Geologists use analytical procedures for analysing ground water, minerals, rocks, ores etc. In agriculture, chemical analysis is used to determine the composition of soils, in the production of fertilizers, insecticides and weed killers. Medical and biological research programmes depend on chemical analysis which providesuseful information that enhances our understanding of vital processes and helps in developing medicines to cure variousdiseases.

• In order to safeguard public health there is constant checking of foods, drugs, cosmetics, water supplies etc., and this is done in analytical laboratories. Waste disposals and the composition of air in industrial areas are analysed to know the extent of harm they would cause to public health so that necessary preventive steps can be taken.

• There is hardly a branch of national economy which does not make use of analytical techniques. Chemical analysis is important in controlling the quality of raw materials, intermediate and finished products. Hence to produce high quality products it is essential to have analytical control at all the stages of technological processes. The sale of raw materials by suppliers and their purchase by users is by analysis metallurgical products are most essential materials of modern economy. The properties of alloys depend on its composition which is established by analytical methods.

• A very large number of examples can be cited where analytical chemistry finds application. It would be no exaggeration. If it is said that there is hardly a material related to modern living in which analytical chemistry did not play some part. With the development of sophisticated instruments, analytical techniques can now be used to determine traces of impurities at the part per billion levels.

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