Chemistry - Introduction to analytical chemistry

Intrduction : 

Analytical chemistry facilitates investigation chemical composition of substances. It uses the instruments and methods to separate, identify and quantify the matter under study. The analysis thus provides chemical or physical information about a sample. Analysis may be qualitative or quantitative. Qualitative analysis is concerned with the detection of the presence or absence of elements in compounds and of chemical compounds in mixtures. Quantitative analysis deals with the determination of the relative proportions of elements in compounds and of chemical compounds in mixtures.

Analysis : 

Analysis is carried out on a small sample of the material to be tested, and not on the entire bulk. When the amount of a solid or liquid sample is a few grams, the analysis is called semi-microanalysis. It is of two types : qualitative and quantitative. Classical qualitative analysis methods include separations such as precipitation, extraction

and distillation. Identification may be based

on differences in colour, odour, melting point, boiling point, and reactivity. Classical quantitative methods consist of volumetric

analysis, gravimetric analysis, etc.

The branch of chemistry which deals with the study of seperation, identification, qualitative and quantitative determination of the  compositions of different substances, is called analytical chemistry.

Importance of analytical chemistry :

The course of analytical chemistry extends the knowledge acquired by the students in studying general, inorganic and organic chemistry. Chemical analysis is one of the most important methods of monitoring the composition of raw materials, intermediates and finished products, and also the composition of air in streets and premises

of industrial plants. In agriculture, chemical

analysis is used to determine the compostion

of soils and fertilizers; in medicine, to

determine the composition of medicinal

preparations.Analytical chemistry has

applications in forensic science, engineering

and industry. Industrial process as a whole and

the production of new kinds of materials are

closely associated with analytical chemistry.

Analytical chemistry consists of classical, wet

chemical methods and modern instumental

methods

Chemical methods of qualitative analysis: 

Chemical analysis of a sample is carried out mainly in two stages : by the dry method in which the sample under test is not dissolved and by the wet method in which the sample under test is first dissolved and then analyzed to determine its composition. The dry method is usually used as preliminary tests in the qualititative analysis. The semi-micro qualitative analysis is carried out using apparatus such as : test tubes, beakers, evaporating dish, crucible, spot plate, watch glass, wire guaze, water bath, burner, blow pipe, pair of tongues, centrifuge, etc.

The qualitative analysis of organic and

inorganic compounds involves different types

of tests. The majority of organic compounds

are composed of a relatively small number

of elements. The most important ones are :

carbon, hydrogen, oxygen, nitrogen, sulphur,

halogen, phosphorous. Elementary qualitative

analysis is concerned with the detection of the

presence of these elements. The identification

of an organic compound involves tests such as

detection of functional group, determinition

of melting/ boiling point, etc. The qualitative

analysis of simple inorganic compounds

involves detection and confirmation of cationic and anionic species (basic and acidic radical)

in them.

Chemical methods of quantitative analysis: 

Quantitative analysis of organic compounds involves methods such as 

(i) determination of percentage constituent element.

(ii)concentrations of a known compound in the given sample, etc.

Quantitative analysis of simple inorganic

compounds involves methods based on,

(i)decomposition reaction (gravimetric analysis).

(ii) the progress of reaction between two

solutions till its completion (titrametric or

volumetric analysis), etc. 

The quantitative analytical methods involve measurement of quantities such as mass and volume, by mean of some equipment/ apparatus such as weighing machine, burette.

Mathematical operation and error analysis : 

The accuracy of measurement is of a great concern in analytical chemistry. Also there can be intrinsic errors in the analytical measurement. The numerical data, obtained

experimentally, are treated mathematically to

reach some quantitative conclusion. Therefore,

an anlytical chemist has to know how to report

the quantitative analytical data, indicating

the extent of the accuracy of measurement,

perform the mathematical operation and

properly express the quatitative error in the

result. In the following subsection we will

consider these aspects related to measurments

and calculation.

Scientific notation (exponential notation) :

A chemist has to deal with numbers as large as 602,200,000,000, 000, 000, 000, 000 for the molecules of 2 g of hydrogen gas or as small as 0.00000000000000000000000166g. that is, mass of a H atom. To avoid the writing of so many zeros in mathematical operations, scientific notations i.e. exponential notations are used. Here, any number can be represented into a form N x 10n where 'n' is an exponent having positive or negative values and N can vary between 1 to 10. Thus, we can write the above values as 6.022 x 1023 and 1.66 x 10- 24 g. The number 123.546 becomes 1.23546 x 102, in scientific notation. Note that while writing it, we have moved the decimal to the left by two places and same is the exponent (2) of 10 in the scientific notation. Similarly, 0.00015 can be written as 1.5 x 10-4.

Precision and accuracy of measurement :

Aim of any measurement is to get the

actual value called true value or accepted

value of a quantity. Nearness of the measured

value to the true value is called the accuracy of

measurement. Larger the accuracy smaller the

error. Accuracy depends upon the sensitivity

or least count (the smallest quantity that can

be measured) of the measuring quuipment.

consider, for example, a burette reading of

10.2 mL. For all the three situations in the

Figure, The reading would be noted is 10.2

mL It means that there is an uncertainty about

the digit appearing after the decimal point in

the reading 10.2 mL. This is because the least

count of the burette is 0.1 mL. The meaning

of the reading 10.2 mL is that the true value

of the reading lies between 10.1 mL and 10.3

mL. This is indicated by writing 10.2 ± 0.1

mL. Here, the burette reading has an error of ±

0.1mL.

Errors may be expressed as absolute or

relative error.

Absolute error = Observed value - True value

Relative error is generally a more useful

quantity than absolute error. Relative error is

the ratio of an absolute error to the true value.

It is expressed as a percentage.

Relative error = Absolute error/True value * 100 %

There can be error in a measurement due

to a number of reasons including inefficiency

of the person doing measurement.

Three possibilities of a burette reading 10.2 mL

Multiple readings of the same quantity

are noted to minimize the error. If the

readings match closely, they are said to

have high precision. High percision implies

reproducibility of the readings. High precision

is a prerequisit for high accuracy. Precision

is expressed in terms of deviation. An absolute

deviation is the modulus of the difference

between an observed value and the arithmetic

mean for the set of several measurements made in the same way. It is a measure of absolute error in the repeated observation.

Absolute deviation = Observed value - Mean

Arithmetic mean of all the absolute

deviations is called the mean absolute

deviation in the measurements. The ratio of

mean absolute deviation to its arithmentic

means is called relative deviation.

Significant Figures : 

Uncertainty in measured value leads to uncertainty in calculated result. Uncertainty in a value is indicated by mentioning the number of significant figures in that value. Consider, the column reading 10.2 ± 0.1 mL recorded on a burette having the least count of 0.1 mL. Here it is said that the last digit ‘2’ in the reading is uncertain, its uncertainty is ±0.1 mL. On the other hand, the figure ‘10’ is certain. The significant figures in a measurement or result are the number of digits known with certainty plus one uncertain digit. In a scientific experiment a result is obtained by doing calculation in which values of a number of quantities measured with equipment of different least counts are used. 

Rules for deciding significant figures :

1. All non zero digits are significant; e. g. 

127.34 g contains five significant figures 

which are 1, 2, 7, 3 and 4. 

2. All zeros between two non zero digits are 

significant e. g. 120.007 m contains six 

significant figures.

3. Zeroes on the left of the first non zero digit 

are not significant. Such a zero indicates the 

position of the decimal point. For example, 

0.025 has two significant figures, 0.005 has 

one significant figure.

4. Zeroes at the end of a number are significant if they are on the right side of the decimal point. 

Terminal zeros are not significant if there is no decimal point. (This is beacause the least count 

of an instrument contains decimal point) 

5. In numbers written is scientific notation, all 

digits are significant. For example, 2.035×102 

has four significant figures, and 3.25 × 10-5 has 

three significant figures.