Organic Molecules: Valuable Lab Methods of Identification

Organic molecules are diverse compounds found in living organisms and are essential to life. The laboratory procedures for melting point determination, boiling point determination, and refractive index measurement are important techniques used to identify and characterize organic molecules. By providing valuable physical and chemical information, these methods are valuable tools for identifying and distinguishing between different organic compounds, which is essential for various fields such as pharmaceuticals, food and beverage, and environmental studies.

Melting point determination, boiling point determination, and refractive index measurement are widely used techniques in the analysis of organic molecules due to their simplicity, accuracy, and reliability.

Overall, the application of these techniques has greatly contributed to the understanding and utilization of organic molecules in various fields.

Identification Analysis of Molecules

In these experiments, the samples are prepared and measured using specialized equipment and techniques, and the results are analyzed to determine the physical properties and characteristics of the compounds.

These procedures require careful attention to detail, and a thorough understanding of the underlying principles and techniques involved in the measurement.

Melting Point Determination

Melting point determination is used to determine the purity of a solid compound and identify the compound based on its melting point range.

Materials Required

Melting point apparatus and Melting point capillary tubes.

Any sample of these compounds could be tested:

Pure aspirin (acetylsalicylic acid), pure benzoic acid, pure naphthalene, pure acetaminophen (paracetamol), pure ibuprofen, pure caffeine, pure phthalic acid, and pure terephthalic acid

Procedure

• Fill the melting point apparatus with a suitable heat transfer medium such as silicon oil.
• Take a melting point capillary tube and fill it with a small amount of the compound to be tested.
• Place the capillary tube into the melting point apparatus in such a way that the compound is in contact with the heat transfer medium.
• Heat the sample slowly,  and observe the temperature at which the solid sample starts to melt. Record this temperature as the melting point.
• Repeat the experiment two more times and take the average of the three melting point values obtained.
• Please Note: If the melting point value obtained is within the expected range for the compound, the sample is considered to be pure.
• If the melting point range is broad or lower than the expected range, the sample may be impure or a different compound.
• Report the average melting point value obtained from the three measurements, along with the range of melting point values obtained.
• In addition to the numerical values in your results, include any relevant observations or notes about the sample or the experiment, such as the appearance of the sample, the type of instrument used, or any potential sources of error.

Related Post: Determining Atomic Number using Spectroscopy

Calculation

Determine the purity of a sample using the formula:

purity = (observed melting point / expected melting point) x 100%

where the expected melting point is the melting point of the pure compound and the observed melting point is the melting point obtained from the experimental measurements.

Please note: A sample with a purity of at least 95% is typically considered pure.

Finally, in your results,

• State the objectives of the experiment and the methods used to achieve them.
• Present the results of the melting point determination, including the range of melting points observed for the sample.
• Compare the observed melting point to the literature value or known melting point of the compound, if available.
• Discuss any sources of error or uncertainty in the measurement and suggest ways to improve the accuracy and precision of future measurements.

Safety Precautions

• Wear appropriate personal protective equipment (PPE), such as lab coat, gloves, and safety glasses or goggles.
• Use the melting point apparatus in a fume hood or well-ventilated area to avoid inhalation of fumes or vapors.
• Do not touch the hot melting point capillary or any other hot surfaces with bare hands.
• Use a heat-resistant pad to hold the melting point capillary during the measurement.
• Dispose of any broken or contaminated glassware in designated sharps containers.

Boiling Point Determination

Boiling point determination is used to determine the purity and identity of a liquid compound, and to characterize its properties, such as volatility and intermolecular forces.

Materials Required

Boiling point apparatus (such as a distillation apparatus), Thermometer

Any sample of these compounds could be tested:

Pure ethanol, pure acetone, pure toluene, Pure methanol, Pure 1-propanol, pure 2-butanol, pure ethylene glycol, Pure methylethyl ketone (MEK), Pure methyl isobutyl ketone (MIBK), Pure diethyl ketone, Pure ethylbenzene, Pure o-xylene, & Pure p-xylene.

Procedure

• Set up the boiling point apparatus with the sample in the distillation flask and a condenser to collect the distillate.
• Attach a thermometer to the apparatus and place it in such a way that it is immersed in the distillate.
• Heat the sample slowly and record the temperature at which the first drop of distillate is collected. This temperature is the boiling point of the liquid.
• Record the boiling point temperature at which the majority of the sample is collected. This temperature is known as the true boiling point.
• Repeat the experiment two more times and take the average of the three boiling point values obtained.
• Please Note: If the boiling point value obtained is within the expected range for the compound, the sample is considered to be pure. If the boiling point range is broad or higher than the expected range, the sample may be impure or a different compound.
• Report the average boiling point value obtained from the three measurements, along with the range of boiling point values obtained.
• In addition to the numerical values in your results, include any relevant observations or notes about the sample or the experiment, such as the appearance of the sample, the type of instrument used, or any potential sources of error.

Calculation

The boiling point can be corrected for atmospheric pressure using the Clausius-Clapeyron equation:

ln(P₁/P₂) = ΔH_vap/R [1/T₂ – 1/T₁]

where P₁ and P₂ are the vapour pressures at temperatures T₁ and T₂, respectively; ΔH_vap is the enthalpy of vaporization; R is the gas constant; and ln is the natural logarithm.

The boiling point can then be calculated using the vapor pressure and temperature data obtained experimentally.

Assignment

Use LabVIEW or HYSYS to model and simulate the above distillation process.

Safety Precautions

Wear appropriate PPE, such as lab coat, gloves, and safety glasses or goggles.

Use the distillation apparatus in a fume hood or well-ventilated area to avoid inhalation of fumes or vapors.

Be cautious when handling the hot distillation flask or condenser.

Use a heat-resistant pad to hold the distillation flask during the measurement.

Dispose of any broken or contaminated glassware in designated sharps containers.

In this work,

State the objectives of the experiment and the methods used to achieve them.

Present the results of the boiling point determination, including the temperature range observed for the sample and the identification of any impurities or deviations from the expected boiling point.

Compare the observed boiling point to the literature value or known boiling point of the compound, if available.

Discuss any sources of error or uncertainty in the measurement and suggest ways to improve the accuracy and precision of future measurements.

Refractive Index Measurement

Refractive index measurement is used to determine the concentration of a solution, and to identify or differentiate compounds based on their refractive index values.

Materials Required

Refractometer

Any sample of these compounds could be tested:

Pure water, pure ethanol, pure glycerol, pure propylene glycol, Pure 1,3-butanediol, pure methanol, pure 1-propanol, pure 2-butanol, and pure ethylene glycol.

Procedure

• Set up the refractometer according to the manufacturer’s instructions.
• Clean the prism surface of the refractometer with a suitable solvent such as ethanol.
• Place a small amount of the sample on the prism surface and close the refractometer cover.
• Look through the eyepiece of the refractometer and observe the position of the boundary line between the illuminated and dark areas. This line is known as the critical angle.
• Record the critical angle and use it to calculate the refractive index of the sample using the appropriate equation.
• Repeat the experiment two more times and take the average of the three refractive index values obtained.
  • Note: The refractive index of a substance can be affected by the wavelength of the light used. Therefore, it is important to specify the wavelength of the light used in the measurement.

If the refractive index of the sample is significantly different from the expected value, the sample may be contaminated, or there may be errors in the measurement. In this case, the measurement should be repeated, and the sample should be purified if necessary.

Calculation

The refractive index measurement can also be used to determine the concentration of a solution. In this case, a series of solutions with known concentrations of the compound are prepared, and the refractive index of each solution is measured.

A calibration curve is then constructed by plotting the refractive index versus the concentration. The concentration of an unknown solution can then be determined by measuring its refractive index and using the calibration curve to find the corresponding concentration.

If a refractive index measurement is being used to determine the concentration of a solution, a calibration curve can be constructed by plotting the refractive index versus the known concentrations of the compound. In this case, a graph would be necessary to display the relationship between the refractive index and concentration of the compound.

Report the average refractive index value obtained from the three measurements, along with the range of refractive index values obtained.

If the refractive index value obtained is within the expected range for the compound, the sample is considered to be pure. If the refractive index value is significantly different from the expected range, the sample may be impure or contaminated.

In addition to the numerical values in your results, include any relevant observations or notes about the sample or the experiment, such as the appearance of the sample, the type of instrument used, or any potential sources of error.

Related Post: Determining the Mass Number of an Isotope

Preparation of Calibration Curve

Prepare a series of standard solutions with known concentrations of the compound. The concentrations should cover a range that is relevant to the samples being analyzed.

Measure the refractive index of each standard solution using a refractometer.

Repeat the measurement three times for each standard solution to obtain a more accurate average value.

Use Manual, OriginLab or Prism to plot and analyze calibration curves for refractive index measurements.

Plot a graph of refractive index versus concentration for the standard solutions. The graph should be a straight line that passes through the origin (i.e., a linear calibration curve). If the graph is not linear, the instrument may need to be calibrated or the samples may need to be re-measured.

Use the calibration curve to determine the concentration of an unknown sample based on its refractive index. This can be done by measuring the refractive index of the unknown sample, and then using the calibration curve to interpolate the corresponding concentration.

Discuss any sources of error or uncertainty in the measurement and suggest ways to improve the accuracy and precision of future measurements.

Safety Precautions

Wear appropriate PPE, such as a lab coat, gloves, and safety glasses or goggles.

Avoid direct contact with the sample or any hazardous chemicals used in the measurement.

Keep the refractometer and its components clean and free of any contaminants or spills.

Dispose of any broken or contaminated glassware in designated sharps containers.

Simple Takeaway Quiz

In conclusion, lab methods for organic molecule identification play a crucial role in determining the chemical composition and structure of organic molecules. The success of organic molecule identification relies on the expertise and experience of the analyst, as well as the accuracy and precision of the instruments and methods employed.