2 General Analytical

Dr. Varinder Kaur

Objectives: To study the basics of analytical chemistry and know the following about self generated questions

  1. What is analytical chemistry?
  2. Why is it required?
  3. Where is it used?
  4. How is it used?
  5. How different analytical methods are grouped?
  6. How is it used in daily life?

 

1. Description

It is very difficult to define ‘analytical chemistry’ appropriately, however it may be considered as a “Science of chemical measurements” because it is the science which deals with the detection, identification (recognition), quantitation and separation of chemical components in modern times. In simple words, this branch of chemistry tells about some popular questions related to a substance such as;

  1. Does the given substance contain component A (i.e. detection/qualitative test for A)?
  2. What is the identity of component in the sample or in which form component A is present (Identification/recognition of A)?
  3. How much amount of component A is present in the sample (quantitation or quantification)?
  4. How component A can be separated from other components of the sample (separation)?

 

Example: A contaminated water sample containing traces of some metal ions and pesticide toxicants is given to the analyst for analysis. The following steps will be followed by the analyst for the analysis of sample for each component (Figure 1).

Figure 1 Various steps followed by an analyst to analyze sample for each component

Furthermore, the methodologies used to accomplish the above mentioned steps are modified day to day by the researchers working in this field to improve the routine analytical methods. Therefore, analytical chemists work for the well being of society in order to search solutions for new types of problems, develop new materials and methods to get efficient, selective and cost effective methodologies.

2. Applications of analytical chemistry

Analytical chemistry has extensive applications relative to other sciences. It is an interdisciplinary subject and can be applied in various disciplines such as inorganic chemistry, organic chemistry, physical chemistry, industrial chemistry, biochemistry, clinical chemistry, pharmaceutical chemistry, environmental chemistry, agricultural science, biomedical science, forensic science and space science etc. For instance, various experiments such as qualitative tests to recognize anionic and cationic radicals in a mixture of salts; quantitative tests to measure the amount of a particular element or substance; measurement of quantities on the basis of physical properties of a substance, etc are the common practices used in routine chemistry. In industrial chemistry, testing of raw materials and manufactured products (eg. Cement, steel, paints, drugs etc) is the crucial step and involves analytical procedures. Similarly, monitoring food quality, balanced diet, appropriate amounts of additives etc. can be achieved by analytical chemistry. Moreover, the amounts of various metals in the alloys and components of plastics used in the fabrication of electronics are also optimized by following analytical procedures. Some of the simple examples illustrating contribution of analytical science in interdisciplinary fields are summarized below (Table 1).

Table 1 simple examples illustrating contribution of analytical science in interdisciplinary fields

3. Disciplines of analytical chemistry; Qualitative and Quantitative analysis

The analysis of any component can be done in two different ways; 1) Qualitative analysis, and 2) Quantitative analysis

The qualitative analysis tells about the identity of the component present i.e. it answers “What is present?” and on the other hand quantitative analysis tells about the amount of the component present in the sample or simply it answers “How much is present?”. The main features of the two disciplines are discussed below;

3.1 Qualitative analysis

  1. It identifies or classifies a particular component in a sample.
  2. It tells about the presence of ions, elements, or compounds.
  3. The analysis is done on the basis of chemical and physical properties such as chemical reactivity, solubility, melting point, absorption and emission properties by employing instrumentation techniques. The selection of a technique depends upon the nature of sample. The qualitative analysis may be simple or complex depending upon the physical properties of the component to be determined. To simplify the learning of qualitative analysis, it is divided into two classes; inorganic analysis (which tells about the presence of anions and cations) and organic analysis (which detects the organic compounds giving information about the elements and functional groups present in the sample).

 

3.2 Quantitative analysis

  1. It quantifies (determines amount) a particular component in a sample.
  2. It tells about the amount of the component and expressed as a numerical value.
  3. It is also based upon some physical and chemical properties and the respective methods utilized for the quantitative analysis may be termed as physical and chemical methods. Physical methods involve the measurement of a physical property such as density, refractive index, absorption or polarization of light, electromotive force, magnetic susceptibility etc. In contrast, chemical methods involve various phenomena such as precipitation, neutralization, oxidation, or derivatization (formation of new compound).

3.2.1 Categorization of quantitative analysis

The quantitative methods can be categorized on the basis of amount of the sample taken for measurement (Figure 2).

Figure 2 Categorization of quantitative methods on the basis of amount of the sample taken for measurement

3.2.2 Steps involved in quantitative analysis

Before starting analysis, it is important to follow appropriate steps required for the determination of analyte. Each step in the analysis depends upon several factors, which must be chosen carefully in order to get the accurate results. The important steps for performing an analysis are listed below (Figure 3).

Key question: How to analyze a sample?

Figure 3 Important steps for performing an analysis

1). Defining a problem

The primary step to analyze a sample is to define the problem. This step is associated with various self generated questions related to the analyst, analytical technique or the client who is seeking the information about the analyte. The main queries, which must be cleared before first step of analysis, are as listed below.

  • What type of analysis is required i.e qualitative or quantitative?
  • What is the requirement of client or to which field client belongs?
  • For what purpose and when the information will be used?
  • What should be the accuracy or precision as per the need of the client?
  • How much financial support is available for the analysis?

2). Selection of method of analysis

The second step involves selection of appropriate method for the analysis. The analytical method for the analysis of a component may be selected on the basis of the chemical and physical properties associated with the analyte. The detailed discussion of the analytical methods is given in next section. Before choosing a best analytical method, following points must be clear to the analyst.

  • What is the type of sample? (i.e gas, liquid, solid, metallic, organic, mixed inorganic-organic etc).
  • What is the size of the sample? (i.e. macroanalysis, microanalysis, semi-microanalysis, sub-microanalysis etc.)
  • What type of sample preparation is required?
  • How much sensitivity, precision and accuracy are needed?
  • What type of instruments is available for the analysis as per the chemical and physical properties of the analyte?
  • How much cost, time and expertise are associated with the type of analytical method to be chosen?
  • Is there any similar report in the literature or standards available?

3). Sampling:

This step involves the procurement of sample. The sample must represent the bulk of material from which sample is collected. In order to fulfill this requirement, material should be homogenous or reasonably pure. Secondly, sampling must be error-free.

4). Sample preparation (including separation of the analyte from the matrix if any)

In this step, sample is prepared before analysis. It means analyte (desired substituent) is converted into a form which can be used for analysis directly with a particular instrument. The main steps involved are listed below.

  • This step mainly involves dissolution, digestion, burning etc.
  • Derivatization: It means conversion of analyte into another molecule by some suitable reagent. The derivatization imparts a characteristic feature to the analyte so that it can be traced with a suitable technique.
  • Pre-concentration: This means transformation of an analyte from bulk to small volume using liquid-liquid extraction, solid phase extraction etc.
  • Separation: If the sample consists of mixture of components, separation of analyte from the bulk is required for its measurement and the separation method can be selected on the basis of the nature of the analyte and the matrix. It involves distillation, precipitation, solvent extraction, chromatographic separation and electrophoretic separation etc.

 5). Measurement and data interpretation

4. Classification of analytical methods/techniques

Figure 4 Categorization of analytical methods on the basis of property used for the analysis

Both the qualitative and quantitative analysis is based upon the physical and chemical properties of the analyte. Therefore, modern analytical methods can be classified broadly into three groups on the basis of the property used to analyse the sample.

1) Chemical methods, 2) physical methods, 3) Physico-chemical methods

The chemical methods involve use of chemical reactions for the qualitative or quantitative analysis of an analyte. For example, evolution of gas, formation of precipitates, appearance of color etc. can be used for the qualitative analysis while stoichiometry of reagent used, weight of the precipitates formed etc can be used for the quantitative analysis. The physical methods involve measurement of a particular physical property such as density, viscosity, refractive index etc for the determination of an analyte. The physico-chemical methods involve use of chemical reactions in order to get desirable physical property of an analyte. For instance pH variation, conductance variation, potentiometric determination etc are used for quantitative analysis in which a physical property is directly related to the concentration of the analyte. The physical and physico-chemical methods require instruments for the determination of analyte, therefore, they are grouped together as instrumental methods. All the analytical methods can be categorized as given below (Figure 4).

 

Gravimetric methods involve determination of mass of the analyte or its derivative formed. Volumetric method involves measurement of the volume of reagent solution used for the complete reaction of analyte. Electroanalytical methods involve measurement of electrical properties such as potential, current, resistance, and quantity of electrical charge etc. Spectroscopic methods are based upon the interaction between electromagnetic radiation and analyte or the emission of radiation by analytes. Mass spectrometry measures mass-to-charge ratio of ions. Some other techniques measure rate of radioactive decay, heat of reaction, rate of reaction, sample thermal conductivity, optical activity, and refractive index etc.

5. Real life applications

Example 1 A milk sample can be tested for the detection (qualitative analysis) of urea as well as to know the quantity of urea added to the milk (quantitative analysis) using p-dimethylaminobenzaldehyde. Although reagent for qualitative and quantitative analysis is same but the procedures adopted are different.

Example 2 Testing of blood samples for sugar, oxygen and carbon dioxide, ionized calcium etc.

Example 3 Testing of food additives in the manufacturing units

Example 4 Standardization of components in a drug in pharmaceutical industries

Example 5 Water analysis for the presence of contaminants

Example 6 Testing of hydrocarbons, nitrogen oxides, and carbon monoxide present in automobile exhaust gases for pollution check

Example 7 Analysis of metal composition in steel during production units to achieve a desired strength, hardness, corrosion resistance, and ductility

Bibliography

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  • A brief book that succinately discusses what analytical chemists do and how they do it.
  • R.W. Murray, Analytical Chemistry is what analytical chemists do, Editorial, Anal. Chem., 66 (1994) 682A.
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