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Questions and Answers
What principle does the technique of small-scale crystallization mainly rely on?
What principle does the technique of small-scale crystallization mainly rely on?
- Impurities dissolve more easily in hot solvents.
- The solubility of a compound is the same at all temperatures.
- Solutes require more time to crystallize at higher temperatures.
- Saturated solutions can hold more solute at higher temperatures than at lower temperatures. (correct)
Which of the following best describes the first step in small-scale crystallization?
Which of the following best describes the first step in small-scale crystallization?
- Cool the solution to ice temperature.
- Wash the solid with cold solvent.
- Choose a suitable crystallization solvent. (correct)
- Dissolve the solid in an Erlenmeyer flask.
Why is a test tube preferred over an Erlenmeyer flask when performing small-scale crystallization?
Why is a test tube preferred over an Erlenmeyer flask when performing small-scale crystallization?
- Test tubes have a larger capacity for solution.
- Glassware size should match the amount of solution being used. (correct)
- Erlenmeyer flasks do not allow for proper cooling.
- Test tubes prevent contamination more effectively.
What is considered an ideal boiling point range for a crystallization solvent?
What is considered an ideal boiling point range for a crystallization solvent?
Which step is NOT part of the trial process for finding a proper crystallization solvent?
Which step is NOT part of the trial process for finding a proper crystallization solvent?
What is the recommended approach for cooling the solution during small-scale crystallization?
What is the recommended approach for cooling the solution during small-scale crystallization?
What indicates that a solvent is suitable for crystallization when you test it?
What indicates that a solvent is suitable for crystallization when you test it?
What is the purpose of removing the supernatant solution in the crystallization technique?
What is the purpose of removing the supernatant solution in the crystallization technique?
During the solvent trial, if the solid dissolves at room temperature, what does that imply?
During the solvent trial, if the solid dissolves at room temperature, what does that imply?
How long does it typically take to cool a small amount of solution to ice temperature?
How long does it typically take to cool a small amount of solution to ice temperature?
Which of the following is NOT a suggested step when preparing for small-scale crystallization?
Which of the following is NOT a suggested step when preparing for small-scale crystallization?
Why is it important to have a solvent that is insoluble when cold for crystallization?
Why is it important to have a solvent that is insoluble when cold for crystallization?
What can happen if too much hot solvent is used during the initial steps of crystallization?
What can happen if too much hot solvent is used during the initial steps of crystallization?
What should be added to the test tube before heating the mixture to prevent bumping?
What should be added to the test tube before heating the mixture to prevent bumping?
What result shows that a solvent is unsuitable for crystallization after heating?
What result shows that a solvent is unsuitable for crystallization after heating?
Which of the following is a required characteristic of a good crystallization solvent?
Which of the following is a required characteristic of a good crystallization solvent?
What does the melting point of a solid represent?
What does the melting point of a solid represent?
Which statement is true regarding the effects of pressure on melting point?
Which statement is true regarding the effects of pressure on melting point?
What is a characteristic melting point range for a pure organic solid?
What is a characteristic melting point range for a pure organic solid?
How can melting point be used in identifying unknown compounds?
How can melting point be used in identifying unknown compounds?
What is indicated by a broad melting range in a solid?
What is indicated by a broad melting range in a solid?
What is the observed melting point range of crude trimyristin compared to purified trimyristin?
What is the observed melting point range of crude trimyristin compared to purified trimyristin?
What would one expect to find in the melting point report for a pure compound?
What would one expect to find in the melting point report for a pure compound?
Why is the melting point measurement not perfectly accurate?
Why is the melting point measurement not perfectly accurate?
What melting point range indicates that a compound is pure based on the given measurement?
What melting point range indicates that a compound is pure based on the given measurement?
What is the approximate range in which the literature melting point value is expected to lie?
What is the approximate range in which the literature melting point value is expected to lie?
Which type of melting point apparatus uses capillary tubes submerged in an electrically heated oil bath?
Which type of melting point apparatus uses capillary tubes submerged in an electrically heated oil bath?
What additional feature may be present in advanced versions of the Thomas-Hoover melting point apparatus?
What additional feature may be present in advanced versions of the Thomas-Hoover melting point apparatus?
What is a primary function of the Barnstead Electrothermal Digital Mel-Temp apparatus?
What is a primary function of the Barnstead Electrothermal Digital Mel-Temp apparatus?
What must be done to the solvent after extraction is complete and the extract is dried?
What must be done to the solvent after extraction is complete and the extract is dried?
What device incorporates ramping temperature controls during melting point determination?
What device incorporates ramping temperature controls during melting point determination?
What type of temperature sensor is used in the Barnstead Mel-Temp apparatus?
What type of temperature sensor is used in the Barnstead Mel-Temp apparatus?
What is the primary purpose of a rotary evaporator in a laboratory setting?
What is the primary purpose of a rotary evaporator in a laboratory setting?
Why is a heating bath necessary when using a rotary evaporator?
Why is a heating bath necessary when using a rotary evaporator?
Under what condition does a liquid boil at a lower temperature in the rotary evaporator?
Under what condition does a liquid boil at a lower temperature in the rotary evaporator?
What should be done with the vacuum control stopcock before starting the rotary evaporator?
What should be done with the vacuum control stopcock before starting the rotary evaporator?
What is the role of the condenser in a rotary evaporator?
What is the role of the condenser in a rotary evaporator?
What action should be taken if the receiving flask is too full before using the rotary evaporator?
What action should be taken if the receiving flask is too full before using the rotary evaporator?
What is indicated by the temperature control display on the heating bath?
What is indicated by the temperature control display on the heating bath?
How does the rotary evaporator contribute to cooling during evaporation?
How does the rotary evaporator contribute to cooling during evaporation?
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Study Notes
Small-Scale Crystallization
- Utilizes greater solubility of compounds in hot solvents for purification of organic solids with impurities.
- Crystallization is effective when a saturated solution at high temperature cools, resulting in solid formation.
- For small quantities (usually < 300 mg), specialized methods and smaller containers like test tubes are used.
- Steps include: finding a suitable solvent, dissolving solid in a test tube, cooling the solution, removing supernatant, and washing the solid.
- Effective solvents allow sufficient solubility when hot, but not when cold, typically with moderate boiling points (65-95 °C) for controlled evaporation.
- Solvent trials involve adding solid to test tubes, observing solubility at room temperature, and heating to check for dissolution in hot conditions.
Melting Point Determination
- Defined as the temperature at which a solid transitions to liquid at standard atmospheric pressure.
- Each pure compound has a characteristic melting point, which can vary in literature.
- Melting point is determined by heating a small sample slowly (ideally at 1 °C per minute) to observe a range of temperatures.
- Pure solids have sharp melting points (range ≤ 1 °C); impure ones exhibit broader ranges and depressed melting points.
- Mixed melting points can help confirm identity by comparing melting ranges, as they indicate purity when consistent with literature values.
Melting Point Apparatus
- Various devices exist for melting point measurement, exemplified by the Thomas-Hoover apparatus, which uses heated oil baths and capillary tubes.
- Equipment includes features for improved visibility of the crystal and thermometer.
- The Barnstead Electrothermal Digital Mel-Temp apparatus is widely used, featuring digital displays and microprocessor-controlled temperature settings for precise measurements.
Removing Solvent
- Post-extraction, solvent removal is essential to recover the desired compound from extract solutions.
Büchi Rotary Evaporator
- A sophisticated apparatus designed for rapid and safe removal of volatile solvents through vacuum-induced boiling at lower temperatures.
- Functions include a heating bath to prevent ice formation and ensure efficient evaporation.
- The rotovap setup consists of several components including a vacuum source, temperature controls, and a condenser.
Steps for Using the Rotary Evaporator
- Prepare the heating bath with water and ensure it is operational.
- Fill the condenser with ice for cooling during operation.
- Check and empty the receiving flask if necessary to prevent overflow.
- Ensure vacuum control stopcock is in the "open to air" position before attaching the flask.
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