Download DEPARTMENT OF CHEMISTRY LECTURE NOTES

DEPARTMENT OF CHEMISTRY
LECTURE NOTES- M.Sc ORGANIC CHEMISTRY
SUB: ANALYTICAL CHEMISTRY
CODE NO: CHET 401
AUTHOR: Dr. P. Kamaraj
LECTURE: 1
Analytical Chemistry is a measurement science consisting of a set of powerful ideas and
methods that are useful in all fields of science and medicine.
Analytical Chemistry is related to all fields of Science.
Chemical analysis has two aspects:
Qualitative and Quantitative.
Qualitative analysis:
Aims at identifying constituents of a given system such as presence of different compounds
in a mixture or of different elements in a compound.
Quantitative analysis:
Is concerned with the determination of amounts of different constituents present in a
system.(usually the given material is first analysed qualitatively and this is followed by
quantitative analysis).
Quantitative analysis methods:
1. Gravimetric methods determine the mass of the analyte.
2. Volumetric method determines the volume of a solution containing sufficient reagent to
react completely with the analyte is measured.
3. Electroanalytical methods involve the measurement of electrical properties such as
potential, current, resistance and quantity of the electrical charge.
4. Spectroscopic methods are based on the measurement of the interaction between
electromagnetic radiation and analyte atoms or molecules.
5. Miscellaneous methods include the measurement of the quantities such as mass to charge
ratio by mass spectrometry, rate of radioactive decay, heat of reaction, heat of reaction,
rate of reaction, thermal conductivity, optical method and refractive index.
FLOW DIAGRAM OF A TYPICAL QUANTITATIVE ANALYSIS Select method Acquire sample Process sample No Is the sample soluble?
yes No Change chemical form Measurable property Yes
Eliminate interferences
Measure property
Calculate results Estimate the reliability of the results carry out chemical ssolution Analytical methods are broadly classified into two types 1. Those based on matter‐matter interaction. Example the reaction between a substance being determined and another substance which acts as a reagent. Eg. Titrimetric determination of HCl with NaOH. The analysis is based on the reaction of one substance (HCl) with another substance(NaOH). 2. Those based on matter‐energy interaction. Example when light is passed through a solution of a substance A, the amount of light absorbed is proportional to the quantity of A present. This forms the basis for the determination of A. It would involve the measurement of light which is a form of radiant energy. Some of the analytical techniques developed involving the measurement of radiant energy. i.
Colorimetry ii.
Turbidity and Nephelometry iii.
Spectrophotometry iv.
Flame photometry v.
Emission spectrography vi.
Fluorimetry vii.
Atomic Absorption spectroscopy and viii.
Polarimetry. LECTURE:2 Colorimetry Consider the solutions of K2Cr2O7 of concentration 1%(solution A) and 5%(solution B) placed into similar glass tubes. The colour of the solution A will appear to be light yellow , while that of solution B will be darker yellow. From this we know that solution B is more concentrated than solution A. i.e. the intensity of the colour is related to the concentration of the substance producing the colour.. This forms the basis of colorimetric analysis. If the substance to be determined is not coloured, a suitable reagent which produced a coloured compound is added. Eg. Fe 3+ ions form a red coloured complex when treated with an excess of KCNS. The intensity of the red colour is proportional to the concentration of Fe3+ ions. THEORY OF COLORIMETRIC ANALYSIS: When a beam of light falls upon a homogeneous medium, a part of the incident light is not reflected, a part of it is absorbed and the remainder is transmitted. Io = Ir+ Ia +It where Io = intensity of incident beam Ir = intensity of reflected light Ia = intensity of absorbed light It= = intensity of transmitted light. BEER LAMBERT’S LAW: Colorimetric methods: These methods are based on comparison of colour intensity of the unknown solution with that of a solution of known concentration. Some methods of comparison involve visual comparison while other methods use a photocell for measuring intensity of transmitted light. STANDARD SERIES METHOD AND BALANCING METHOD‐ constitute visual colorimetric methods STANDARD SERIES METHOD: A series of known solutions of the substance to be determined are prepared and their colour is compared visually with that of the given unknown solution. The concentration of the unknown solution will be equal to the concentration of that known solution whose colour exactly matches with that of the unknown solution. This method may give 3‐8% error. LECTURE 13 ELECTROANALYTICAL TECHNIQUES Equilibrium potential: When a metal electrode is dipped in a solution, if the rate of formation of ions is equal to the rate of their discharge, the electrode is said to be in equilibrium. The potential acquired by the electrode under such a condition is called the reversible or equilibrium potential. Polarisation: When the electrode is in equilibrium, there is no flow of current through the electrode. If such conditions are created that there is an actual passage of current through electrode, then it will be disturbed from its equilibrium condition. This disturbance to the equilibrium associated with the flow of current is called electrolytic polarization. The minimum electrolytic voltage to be applied in order to bring about continuous electrolysis is called decomposition voltage. OVERVOLTAGE: Liberation of H2 at the cathode. Liberation of O2 at the anode. H2 deposits at Pt. H+ ions are present. H2 electrode is set up. Similarly O2 electrode is set up at Pt anode. These electrodes (H2/ O2) set up a Hydrogen‐oxygen cell whose emf (1.23V) opposes the applied emf. This is responsible for the current not being increased in the beginning. The emf of the H2‐O2 cell is called back or counter emf. Therefore the decomposition voltage should be 1.23V. However the observed value is 1.7 V. This difference is called over voltage. (1.7‐1.23 = 0.47 V).