Experimental techniques in Chemistry

Experimental techniques in Chemistry

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Last edited and updated on: by Sagar Aryal

Analytical Chemistry 

It is the science of chemical characterization.

What are the major steps for qualitative analysis? 

  • Obtaining a sample for analysis
  • Separation of the desired constituent
  • Measurement and calculation of results
  • Drawing conclusion

Experimental techniques in Chemistry

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Filteration

The process used to separate the insoluble particles from the liquid. It can be done by using filter media i.e filter paper and crucibles.

How the rate of filtration can be increased?

The rate of filtration can be increased through conical funnel considerably by using a fluted filter paper. In the case of crucibles, the rate can be increased by placing them in a suction apparatus.

How the process of filtration can run smoothly?

The stem of the funnel should remain continuously full of liquid as long as there is liquid in the conical portion. The stem of the funnel should be several inches long so that is can extend a few centimeters down into the receiving beaker.

How splashing can be avoided during filtration?

The stem of the funnel should be several inches long so that its tip should touch the side of the receiving beaker. In this way, the filtrate runs down the side of the beaker without splashing.

Some important steps for folding filter paper.

  • The paper should be folded twice
  • First fold should be along the diameter
  • In the second fold, edges do not quite match
  • Open the paper on a slightly larger section
  • Insert the paper into 60º degree funnel

What is a fluted filter paper?

It is prepared by arranging the ordinary filter paper in such a way that fan arrangement is obtained. This fan-like arrangement has alternative elevation and depressions. Rate or filtration can be increased by using this paper.

NOTE: From both ordinary filter paper and fluted filter paper, the fluted filter paper is the best way for the filtration.

Difference between Gooch crucible and Glass crucible.

Gooch crucible Glass crucible
It is made up of porcelain. It is made up of glass.
It has a perforated bottom. It has a glass disc sealed into its bottom.
Its perforation are covered with the paper pulp of asbestos. Filter paper of asbestos mat are not used.
Reactive materials can not be filtered easily, that’s why asbestos mat is used. Reactive materials can be filtered easily.
Extra preparation is required. No extra preparation is required.

 Mention the best/convenient or reliable method for the filtration. Why it is preferred over the other methods?

Glass crucible is the best method for filtration. It is preferred over the other methods because it is used for the filtration of reactive materials. No extra preparation is required as in gooch crucible. It does not chemically react with materials.

Crystallization

The process for obtaining crystals of a substance by cooling its hot saturated solution. It is used to purify the crude product.

What is the principle of crystallization?

“Solute should be more soluble in a suitable solvent at high temperature and excess of the solute is thrown out as crystals when it is cooled”.

Name the major steps involved in crystallization.

  1. Choice of solvent
  2. Preparation of saturated solution
  3. Filtration
  4. Cooling
  5. Collection of crystals
  6. Drying of crystals
  7. Decolourization of crystals

Write down the main characteristics of the solvent used for crystallization.

  • It should be cheap
  • It should be safe to use
  • It should be easily removable
  • It does not dissolve impurities
  • It should not chemically react with solute
  • On cooling, it should give well-formed crystals

NOTE: most common solvent used are water, absolute ethanol, diethyl ether, acetone, chloroform, CCl4, and rectified spirit.

NOTE: If a single solvent does not work properly, then we use a combination of two or more solvents.

Why the filtration of the hot saturated solution is necessary during the crystallization of substance?

Filtration of a hot saturated solution is necessary during the crystallization of substance because it avoids premature crystallization of the solute on the filter paper or in the stem of funnel.

What is the effect of cooling on the size of crystals?

  • On quick cooling, small size crystals are obtained
  • On moderate cooling, medium size crystals are obtained
  • On slow cooling, bigger size crystals are obtained

What is filtrate and residue/ precipitate?

Filtrate: the fluid that passes through filter media and reaches into the receiving beaker.

Residue: The insoluble particle left on the filter media

What is Mother liquor?

The mother liquor is the part of a solution that is left over after the crystallization. it is removed by heating or drying.

OR

It is the liquid obtained by filtering the crystals by filtration.

Mention the methods used for the drying of crystals.

  • Pressing the crystals between the folds of filter paper
  • By using oven
  • By using vacuum desiccator

What are the disadvantages of drying the crystals by filter paper?

  • Crystals may crush during the pressing them between the folds of filter paper
  • The fiber of paper often contaminate the product

What is vacuum desiccators? How it dries the crystals?

It is an equipment used for the drying of crystals. In this, crystals are spread on the watch glass and kept in the vacuum desiccator for several hours. The drying used in the desiccator are calcium chloride( CaCl2), Silica gel (SiO2), and phosphorus pentaoxide (P2O5).

Mention the best and reliable method for the drying of crystals.

Vacuum desiccator is the best and reliable method for the drying of crystals. It is an equipment used for the drying of crystals. In this, crystals are spread on the watch glass and kept in the vacuum desiccator for several hours. The drying used in the desiccator are calcium chloride( CaCl2), Silica gel(SiO2), and phosphorus pentaoxide(P2O5).

How the decolorization of undesirable colors and dehydration is carried out for the freshly prepared crystalline substances?

OR

How undesirable colors are removed from the crystals?

Impurities or undesirable colors are removed by heating the substance in the solvent with a sufficient quantity of finely divided powdered animal charcoal and filtering the hot saturated solution. In the way, colored impurities are adsorbed by animal charcoal.

NOTE: Animal charcoal is an adsorbent.

Why there is a need to crystallize the crude product?

Impurities are mostly present in crude products. Insoluble impurities are separated by filtration but soluble impurities are removed by crystallization. that’s why a crude product is crystallized to remove impurities.

Sublimation

The process of conversion of solid directly into the vapor phase without passing through the liquid phase. Benzoic acid, ammonium chloride(NH4Cl), Iodine(I2), and naphthalene.

Define sublimand and sublimate.

Sublimand: Substance that shows the sublimation process.

Sublimate The pure solid deposits on the inner side of the funnel after the sublimation.

NOTE:

Evaporation/ Vaporization: It is the conversion of liquid into the vapor phase.

Condensation: It is the conversion of gas into the liquid phase

Solvent Extraction

What is distribution law/partition law?

“A solute distributes itself between two immiscible liquid in a constant ratio of concentration irrespective of the amount of solute added”.

What are the application of distribution law?

  • Solvent extraction
  • Partition chromatography

Define solvent extraction or liquid-liquid extraction.

It is a technique in which solute is separated from the solution by shaking the solution with a suitable solvent in which solute is more soluble and added solvent does not mix in the solution

What is the distribution coefficient?

It is the ratio of the amount of solute dissolved in immiscible liquids at equilibrium.

KD = Con. of solute in the organic phase/ Con. of solute in the aqueous phase

What is ether extraction?

It is the most common laboratory technique. It is used to separate the organic compounds from the water. In this technique, ether is added to the water that has organic products. Organic compound moves from water to ether due same nature. The inorganic impurities remain in the water. The ether layer is then separated and organic compound is obtained by evaporating the ether.

Why I2 is less soluble in water but more soluble in CCl4?

Iodine is less soluble in water because water is polar and iodine is non-polar. Iodine is soluble in CCl4 because both are alike means both are non-polar in nature.

NOTE: Solubility of iodine is increased by adding KI in water.

Chromatography

It is derived from the Greek word “khromatos” which means color writing. It is a separation technique.

What is the principle of chromatography?

“Components of mixtures are separated on the basis of their relative affinities for stationary and mobile phase”.

Difference between the mobile phase and the stationary phase.

Mobile phase Stationary phase
The phase that moves over the stationary phase. The phase over which mobile phase flows.
It is always a gas of liquid. It is maybe liquid or solid.
Commonly mobile phases are water, ethanol, and acetone Stationary phases are solid glass rod coated with silica gel and alumina, filter paper

 Difference between adsorption and partition chromatography.

Adsorption chromatography Partition chromatography
In this chromatography, the stationary phase is solid. In this chromatography, the stationary phase is liquid.
Solute leaves the mobile phase and adsorbed on the surface of the stationary phase Solute distributes itself between the mobile phase and stationary phase
Example: TLC (Thin layer chromatography), GC (Gas chromatography) Example: Paper chromatography

 NOTE: Paper chromatography can be done in three following ways.

  1. Ascending
  2. Descending
  3. Radial/Circular

What is the Rf value? why is has no unit?/ why Rf is always less than 1.0?

It is the ratio of the distance covered by the solute from the original spot to the distance covered by the solvent from the original spot.

Rf =    distance covered by solute/ distance covered by solvent

It has no unit because it is a ratio and ratio has no unit.

Its value is always less than unity because the distance traveled by the solvent in denominator and distance traveled by solute is in the numerator.

What are the applications/uses of chromatography?

  • It is useful for the organic synthesis
  • It is used for the isolation, purification, and separation of substances from mixtures
  • It is used for both quantitative and qualitative analysis
  • Used to determine the purity of the substance

What is the solvent front?

It is the distance covered by the solvent from the baseline over the stationary phase.

What is a chromatogram?

It is a paper that represents the pattern of components of a mixture of drying.

References

  1. Pirbazari, M., Ravindran, V., Badriyha, B. N., & Kim, S. H. (1996). Hybrid membrane filtration process for leachate treatment. Water Research30(11), 2691-2706.
  2. Dore, J. E. (1978). U.S. Patent No. 4,124,506. Washington, DC: U.S. Patent and Trademark Office.
  3. Ruth, B. F. (1935). Studies in Filtration… IV Nature of Fluid Flow through filter septa and Its Importance in the Filtration Equation. Industrial & Engineering Chemistry27(7), 806-816.
  4. Jones, A. G. (2002). Crystallization process systems. Elsevier.
  5. Tavare, N. S. (2013). Industrial crystallization: process simulation analysis and design. Springer Science & Business Media.
  6. Kashchiev, D., Borissova, A., Hammond, R. B., & Roberts, K. J. (2010). Effect of cooling rate on the critical undercooling for crystallization. Journal of Crystal Growth312(5), 698-704.
  7. Hale, N. (1993). U.S. Patent No. 5,246,518. Washington, DC: U.S. Patent and Trademark Office.
  8. Holden, C. A., & Bryant, H. S. (1969). Purification by sublimation. Separation Science and Technology4(1), 1-13.
  9. Huddleston, J. G., Willauer, H. D., Swatloski, R. P., Visser, A. E., & Rogers, R. D. (1998). Room temperature ionic liquids as novel media for ‘clean’liquid–liquid extraction. Chemical Communications, (16), 1765-1766.
  10. Wei, G. T., Yang, Z., & Chen, C. J. (2003). Room temperature ionic liquid as a novel medium for liquid/liquid extraction of metal ions. Analytica Chimica Acta488(2), 183-192.
  11. Kubota, F., & Goto, M. (2006). Application of ionic liquids to solvent extraction. Solvent Extraction Research and Development13, 23-36.
  12. Miller, J. M. (2003). Chromatography. digital Encyclopedia of Applied Physics, 1055-1102.
  13. Smith, I. (Ed.). (2013). Chromatography. Elsevier.
  14. Pauling, L., Robinson, A. B., Teranishi, R., & Cary, P. (1971). Quantitative analysis of urine vapor and breath by gas-liquid partition chromatography. Proceedings of the National Academy of Sciences68(10), 2374-2376.
  15. Kiselev, A. V., & Yashin, Y. I. (2013). Gas-adsorption chromatography. Springer.

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