Pharma Analysis

Explain ATR phenomenon and its applications

(a) Explain ATR phenomenon and its applications
Ans.2.(a) ATRInternal reflection spectroscopy is a technique for obtaining IR spectra of samples that are difficult to deal with, such as solids of limited solubility, films, threads, pastes, adhesives, and powders. Principle when a beam of radiation passes from a denser to a less dense medium, reflection occurs. The fraction of the incident beam reflected increases as the angle of incidence becomes larger; beyond a certain critical angle, reflection is complete. It has been shown both theoretically and experimentally that during the reflection process the beam penetrate a small distance into the less dense medium before reflection occurs. 

Discuss how certain compounds produce fluorescence or phosphorescence when exposed to electromagnetic radiation. How this property is exhibited only by organic compounds with certain structural specifity?

  • Posted on: 13 October 2016
  • By: admin

Q.1. (c) Discuss how certain compounds produce fluorescence or phosphorescence when exposed to electromagnetic radiation. How this property is exhibited only by organic compounds with certain structural specifity? Explain
Ans.1.(c) When electromagnetic radiation passes through matter, a variety of phenomena may occur. One of them is; in some cases, the molecules after absorbing radiation become excited but they do not lose energy very quickly but with some delay. In such cases, the energy is re-emitted as radiation, usually of longer wavelength: than was originally absorbed. This phenomenon is termed as fluorescence. If in some cases, there is a detectable time delay in re-emission, the phenomenon is termed as phosphorescence.

Give principle of UV-visible spectroscopy

Q.1.(b)Give principle of UV-visible spectroscopy, Explain why cellulose is white in colour, picric acid is yellow and aromatic diazotization and coupling produces different shades.
Ans.1. (b)The wavelength range of UV radiation starts at blue end of the visible light (above 4000 A°) and ends at 2000 A°. However, the wavelength of visible radiation starts at 8000 A° and ends at 4000 A°. Spectroscopically, visible light acts in the same way as UV light.

Define and differentiate between auxochromes and chromophores. Exemplify and explain how auxochrome shifts

Ans.1.
(a) CHROMOPHORE: The term chromophore was previously used to denote a functional group of some other structural feature of which gives a color to compound. For example- Nitro group is a chromophore because its presence in a compound gives yellow color to the compound. But these days the term chromophore is used in a much broader sense which may be defined as “any group which exhibit absorption of electromagnetic radiation in a visible or ultra-visible region “It may or may not impart any color to the compound. Some of the important chromophores are: ethylene, acetylene, carbonyls, acids, esters and nitrile groups etc.

How will you ascertain purity of sample by DSC? Discuss its principle

Q.5. (b)How will you ascertain purity of sample by DSC? Discuss its principle.  
Ans.5. (b) Principle

In DSC the heat flow is measure and plotted against temperature of furnace or time to get a thermo gram. This is the basis of Differential Scanning Calorimetry (DSC). The curve obtained in DSC is between dH/dtinmJ s-1 ormcal s-1 as a function of time or temperature. A typical DSC curve is shown in Fig. 1.19. The deviation observed above the base (zero) line is called exothermic transition and below is called endothermic transition. The area under the peak is directly proportional to the heat evolved or absorbed by the reaction, and the height of the curve is directly proportional to the rate of reaction. Therefore Eq. 11.1 is equally valid for DSC scheme also. The only difference is the calibration factor K in case of DSC is independent of temperature. This is a major advantage of DSC over DTA.
Peak area (A) = ± _ H m K …(1.7)

Discuss factors affecting DTA results

Q.5. (a) Draw diagram of apparatus and give methodology of DTA. Discuss factors affecting DTA results.
Ans.5. (a) Differential Thermal Analysis

In differential thermal analysis (DTA), a sample and a thermally inert reference material differencebetween the sample and the reference is measured as a function of the temperature applied. Because, during transition, the sample may either absorb or evolve heat, the difference in the temperature between the sample and the standard is equivalent to the temperature of transition and can indicate if the transition is endothermic or exothermic. Usually, ΔT is plotted against the temperature, T, or as a function of time (t).
DTA data are probably the most accurate of all thermal techniques, because the thermocouple is insert     into the sample; however, only the temperature of a transition and not the amount of heat can be measured from a DTA curve, as the area under the peak is not proportional to the amount of energy transferred into or out of the sample.
Factors affecting the DTA curve

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