Lab techniques: Recrystallization and Chromatography

• The diameter of the amino acid spot should not exceed 20 mm to ensure proper separation and prevent overlapping of spots during chromatography.
• If the solvent front is not marked before it evaporates, the Rf values cannot be accurately calculated.
• Percent recovery of recrystallized benzoic acid is calculated as (weight of purified crystals / weight of impure crystals) * 100.
• The amino acid with the lowest Rf value (0.45) is the most polar.
• Rf value is calculated as (distance traveled by solute / distance traveled by solvent front).
• Possible reasons for no crystal formation after cooling include: too much solvent was used, the solution was not concentrated enough, or the solution was not cooled sufficiently.
• Using too much activated charcoal during recrystallization can adsorb the desired product, reducing the yield.
• The retention factor (Rf) of the solute is 0.5.
• The primary purpose of recrystallization is to purify a solid compound.
• Adding charcoal to a superheated solution may cause rapid boiling and eruption.
• Adding excess activated charcoal can reduce the yield by adsorbing the product.
• A major consequence of superheating during distillation is bumping or violent boiling.
• Uneven heating contributes the most to superheating in distillation.
• The percentage of benzoic acid remaining in solution is calculated from the difference between the initial and crystallized amounts.
• The purity percentage before recrystallization is calculated as (weight of pure substance / total weight of impure substance) * 100.
• Boiling chips must be added before heating to promote even boiling and prevent bumping.
• Distilling to complete dryness can lead to the formation of explosive peroxides, especially with certain solvents.

Percent Recovery Calculation Example

• Initial dissolved: 2.0 g
• Remaining dissolved at room temperature: 0.2 g
• Recovered upon cooling: 1.8 g
• Percent recovery: 90%.
• Excess water in recrystallization reduces the yield because more product remains dissolved.
• The starting mixture should be smelled before distillation to identify the components present.
• The expected trend in a temperature vs. time distillation curve is a gradual increase with a plateau at each component's boiling point.
• Solvent polarity affects the Rf value of a solute in chromatography.
• Increased solvent polarity increases Rf values in paper chromatography.
• In thin-layer chromatography, a liquid solvent or solvent mixture serves as the mobile phase.
• Flammable materials should be removed to prevent fire hazards during distillate flammability testing.
• Gradually increasing the flame during distillation ensures controlled heating and prevents bumping or overheating.
• Benzoic acid crystallizing too quickly results in smaller, less pure crystals with trapped impurities.
• Washing crystals with too much solvent dissolves some of the purified product, reducing the yield.
• Glacial acetic acid is used in amino acid chromatography to adjust the pH of the mobile phase and improve separation.
• Distilled water's polarity makes it a useful component of the mobile phase in chromatography, aiding in the separation of polar compounds.
• The ratio of glacial acetic acid to distilled water in the solvent affects the migration of amino acids based on their polarity and interaction with the mobile and stationary phases.
• The percent of the original benzoic acid sample that was impurities can be calculated from the initial purity percentage.
• Air bubbles trapped inside the condenser reduce its efficiency, causing poor condensation and loss of distillate.
• Without a thermometer during distillation, one cannot monitor the temperature and thus cannot effectively separate compounds based on boiling points.
• The primary purpose of cooling the distillate in a water trough is to ensure efficient condensation and collection of the distillate.
• Discarding the first few drops of distillate removes any low-boiling impurities or residual solvents from the apparatus.
• Exposure to ninhydrin can be minimized by using gloves and working in a well-ventilated area.
• Filter paper should not be touched with bare hands to prevent contamination with oils and dirt from the skin.
• The baseline should be drawn with a pencil and not ink to prevent the ink from dissolving in the solvent and interfering with the chromatography.
• A low percent recovery in recrystallization is likely due to loss of product during transfers, incomplete precipitation, or over-washing.
• Weight of recovered benzoic acid is the mass of the purified crystals obtained after recrystallization.
• Using an analytical balance ensures accurate measurements of benzoic acid before and after purification, critical for calculating percent recovery.
• A high percent recovery (close to 100%) indicates minimal loss of product during the recrystallization process.
• More polar amino acids move slower in chromatography.
• The purpose of using ninhydrin in amino acid chromatography is to visualize the amino acids as purple spots.
• If the condenser isn't cooled properly, vapors will not condense efficiently, leading to loss of product.
• No distillate collected could be due to leaks in the apparatus, insufficient heating, or a very high-boiling substance.
• Collecting distillates in different flasks allows for the separation of different fractions based on boiling points.
• A strong alcohol smell indicates the presence of an alcohol in that fraction.
• Chromatography paper must be handled with gloves to prevent contamination from skin oils.
• The chromatography chamber left open leads to solvent evaporation, affecting the accuracy of Rf values.
• Rf values indicate the relative affinity of a compound for the stationary and mobile phases.
• A good recrystallization solvent should dissolve the compound when hot and not dissolve or only slightly dissolve the compound when cold, not react with the compound, have a boiling point lower than the compound's melting point, and have a low boiling point so it can be easily removed from the crystals.
• Slow cooling to form crystals helps improve purity the most.
• Recrystallization does not give a 100% yield because some product remains dissolved in the solvent or is lost during transfer.
• Using too much solvent increases solubility and reduces yield of recrystallization.
• If benzoic acid doesn't crystallize after cooling, try scratching the flask, seeding with a pure crystal, or further cooling.
• Washing crystals with hot solvent dissolves more of the product, reducing yield.
• The high-water temperature increases solubility of benzoic acid.
• Fluted filter paper increases the surface area and speeds up hot filtration.
• Decolorization is the process of removing colored impurities from a solution.
• Solution recrystallization involves dissolving a solid in a solvent then allowing it to recrystallize.
• Drying recrystallized crystals prevents inaccurate weight measurement due to residual solvent.
• Too much activated charcoal can adsorb the desired compound, lowering the yield.
• Evaporate some of the solvent to concentrate benzoic acid mixture, if too much was added.
• Benzoic acid that crystallizes too quickly will likely be impure because it traps impurities.
• Washing crystals with too much solvent results in loss of product.
• No crystals can form if too much solvent was used.
• Boiling Point determines which liquid will get collected first during distillation.
• Excessive boiling chips can lead to excessive bumping.
• Temperature rises gradually, plateaus at boiling points describes temperature vs time distillation curve.
• Air bubbles reduce its efficiency.
• Distillation must be dry to ensure no additional unwanted products.
• First few drops removes any possible products.

A brief overview of calculating Rf values in chromatography, understanding recrystallization, and percent recovery. Important lab techniques. Understand the importance of solvent use; too much solvent can prevent crystal formation upon cooling.