General Chemistry Laboratory Techniques for Freshmen

Introduction

General chemistry laboratory courses are a fundamental part of undergraduate science education. These labs provide hands-on experience, allowing students to learn and master essential techniques. This article provides an overview of common laboratory techniques used in freshman general chemistry labs, emphasizing the underlying principles and practical applications of each technique. Understanding these techniques is crucial for conducting experiments accurately and safely, analyzing results effectively, and building a solid foundation for further studies in chemistry and related fields.

Essential Equipment

A well-equipped laboratory is crucial for successful experimentation. Here's an overview of essential equipment commonly found in a general chemistry lab:

Glassware

Glassware is a staple in any chemistry lab. Common types include:

Beakers: Used for mixing, heating, and holding substances.
Flasks: Including Erlenmeyer flasks (for mixing and swirling liquids without spilling) and volumetric flasks (for preparing solutions of precise volumes).
Test tubes: For holding small samples and conducting small-scale reactions.
Graduated cylinders: For measuring approximate volumes of liquids.
Pipettes: Used for transferring precise volumes of liquids. Volumetric glassware, such as pipettes, is capable of measurements of volume that are good to four significant digits. Be particularly careful with the tips of pipets and burets.

Heating Devices

Heating is often required to facilitate chemical reactions. Common heating devices include:

Bunsen burners: Provide a direct, adjustable flame for heating and sterilizing equipment. Flames are never used in the laboratory except in controlled situations (e.g., isolated in fume hoods).
Hot plates: Electrically powered devices that offer controlled heating for reactions requiring precise temperature regulation. Electric hot plates and heating mantles are most commonly used. Be careful not to turn this equipment to its highest setting which can burn it out. It does take several minutes for these instruments to reach the desired temperature.

Balances

Accurate mass measurements are critical for quantitative analysis.

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Balances: Used to measure the mass of solids and liquids with high precision.
Analytical balances: Sensitive instruments capable of measuring mass to four decimal places, ideal for quantitative analysis and preparing precise chemical solutions. For a chemical reaction to be successful, reactants must be added with accurate, specific masses, and products must be accurately weighed at the end of the reaction. Therefore, balances are of immense importance in a chemistry lab.

pH Meters and Electrochemical Instruments

Controlling and monitoring acidity and conductivity are important in many experiments.

pH meters: Used to accurately measure the acidity or alkalinity of solutions.
Electrochemical instruments: Including voltmeters and conductivity meters, utilized for analyzing electrochemical reactions and studying solution conductivity.

Filtration Apparatus

Separating solids from liquids is a common task in chemistry labs.

Filtration equipment: Includes filter paper, funnels, and vacuum filtration setups, employed for separating solids from liquids. This is crucial for purifying substances or isolating precipitates from solution.

Spectrophotometers

Spectrophotometers are used to measure the absorbance or transmittance of light by a sample. This technique is valuable for quantitative analysis and studying the properties of substances in solution.

Stirring and Mixing Devices

Proper mixing is essential for many chemical reactions.

Stirring rods: Manually operated to mix solutions in beakers.
Magnetic stirrers: Use rotating magnetic fields to stir solutions automatically.
Vortex mixers: Used to rapidly mix small volumes of liquid in test tubes or vials.

Basic Techniques

Several core techniques are essential for success in a general chemistry lab:

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Measuring Liquids

Accurate liquid measurement is fundamental.

Graduated Cylinder: Measuring large volumes of liquids requires a graduated cylinder. The graduated cylinder should be rinsed with the solvent you want to measure. To measure the amount of liquid in a graduated cylinder, hold it at eye level and read the meniscus (bottom of the curved liquid surface).
Pipettes: Used to draw out smaller amounts of liquid. To use, press the pipette bulb or pipette filler to draw liquids into the pipette. Dispense liquids slowly and steadily, allowing the liquid to drain completely.
Micropipettes: For precise volume measurements, particularly in biochemical or analytical chemistry experiments.

Mixing Liquids

Effective mixing ensures homogeneity.

Pour liquids into a larger container to mix. Hold the container with a steady hand, ensuring a smooth and controlled pour. Use a funnel when transferring liquids into narrow-necked containers to prevent spills.
Use gentle swirling motions to mix solutions without causing splashing or spilling. Avoid excessive agitation, especially with volatile or reactive substances.
Use a glass stirring rod to mix solutions that require more thorough blending. Rinse the glass rod with distilled water between uses to prevent contamination.

Filtration

Separating solids from liquids is a common procedure.

Fold filter paper into the appropriate shape and place it in a filtration funnel. Ensure the funnel is securely placed in a ring stand or holder. Carefully pour the mixture to be filtered into the funnel. Allow the liquid to pass through the filter paper while retaining the solid.

Handling Solids

Proper techniques ensure accurate measurements and transfers.

Use a weighing boat or paper to weigh solids on a balance. Tare the balance to account for the weight of the container, effectively “zeroing out” the balance.
Use a spatula or scoopula to transfer solids between containers or to weigh boats.

Titration

Titration is the slow addition of one solution of a known concentration (called a titrant) to a known volume of another solution of unknown concentration until the reaction reaches neutralization, which is often indicated by a color change. The solution called the titrant must satisfy the necessary requirements to be a primary or secondary standard. In a broad sense, titration is a technique to determine the concentration of an unknown solution.

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Prepare standard solutions by accurately measuring a known quantity of solute and adding solvent to a volumetric flask to the calibration mark. Use a wash bottle to rinse any remaining solute from the sides of the flask into the solution.
Set up the burette, which is a long graduated glass tube used to dispense small amounts of liquids via a stopcock. Rinse the burette with the titrant solution to remove any impurities. Fill the burette with the titrant, ensuring no air bubbles are trapped.
Place the analyte solution (the solution to be titrated) in a clean Erlenmeyer flask. Add a few drops of an appropriate indicator (e.g., phenolphthalein or methyl orange) to the analyte solution.
Slowly add the titrant from the burette into the analyte solution while swirling the flask gently. As the endpoint approaches, add the titrant drop by drop to avoid overshooting the endpoint. Observe the color change of the indicator to determine the endpoint of the titration.

Distillation

Distillation of compounds is a method of separation which exploits the differences in boiling point of a crude mixture. Several methods exist. Distillation is a method of purifying organic compounds. It takes advantage of the fact that two different compounds probably have two different boiling points. Suppose two different liquids are present in a homogeneous mixture (they are completely miscible, or they mix completely together, like water and alcohol). If they have two different boiling points, one of the compounds will evaporate before the other one does.

Liquid-Liquid Extraction

Liquid-Liquid extraction is a method by which a compound is pulled from solvent A to solvent B where solvents A and B are not miscible. The most common method of liquid-liquid extraction is performed using a separatory funnel. Solvent Partitioning (Liquid - Liquid Extraction). Work with this equipment in a proper fashion and it will perform remarkably well. First, check that the stopper fits and that the stopcock works properly. Fill the funnel with the two solvents. Stopper. Pour out the upper layer if necessary. Save both upper and lower layers until you are certain that you have the compound you want. If you do not throw it away, it is not lost.

Acid-Base Extraction

An acid-base extraction is a type of liquid-liquid extraction. It typically involves different solubility levels in water and an organic solvent. The organic solvent may be any carbon-based liquid that does not dissolve very well in water; common ones are ether, ethyl acetate, or dichloromethane. Acid-base extraction is typically used to separate organic compounds from each other based on their acid-base properties.

Drying Solvents

These days many laboratories will use a commercially available solvent purification system, others will distil solvents using more traditional techniques. Tetrahydrofuran, dichloromethane, dimethylformamide, chloroform, acetonitrile, methanol, diethyl ether and toluene are all commonly used solvents, and in many cases they are required in anhydrous form. In some cases there are multiple ways to dry a given solvent. When an organic solvent has been exposed to aqueous solutions it will contain a small amount of water, the amount depending on the solubility of water in the solvent. To prepare a pure product, it is necessary to dry the solution using an appropriate drying agent. A drying agent is usually an anhydrous inorganic salt which reacts with the water present to form a hydrate. Anhydrous MgSO4, for example, reacts with water to form the heptahydrate MgSO4? 7H2O. Some typical drying agents are listed below. Magnesium Sulfate MgSO4? Sodium Sulfate Na2SO4? Calcium Sulfate CaSO4? The organic solution may appear milky when wet and clears up when the drying agent is added. Anhydrous magnesium sulfate is a free-flowing powder which cakes and sticks to the bottom of the flask as it becomes hydrated. Add just enough so when you swirl the flask a few crystals will circulate. Never add a large amount of drying agent because your product can become absorbed on the surface, reducing the yield.

Thin Layer Chromatography

Thin layer chromatography (TLC) is a chromatographic technique used to separate the components of a mixture using a thin stationary phase supported by an inert backing. It may be performed on the analytical scale as a means of monitoring the progress of a reaction, or on the preparative scale to purify small amounts of a compound. TLC is an analytical tool widely used because of its simplicity, relative low cost, high sensitivity, and speed of separation. Chromatography Columns, Chromatography I: TLC.

Recrystallization

Crystallization is used to purify a solid. The process requires a suitable solvent. A suitable solvent is one which readily dissolves the solid (solute) when the solvent is hot but not when it is cold. The best solvents exhibit a large difference in solubility over a reasonable range of temperatures. This technique is no longer as widely used as it was before the advent of flash chromatography, but it's still quite useful! A particular advantage is that compounds can be recrystallized in amounts that are somewhere between streaky and impossible to column.

dissolve solid in minimum amount of boiling solvent - add solvent in small amounts. For example, if you add 5 mL and approx. half of the solid dissolves, it should take only another 5 mL to dissolve the remaining half.
to determine this, add ca. 10% more hot solvent.
to "decolorize", use a small amount of charcoal and filter with "filter aid".

Fractional crystallization

Fractional crystallization is a method of refining substances based on differences in solubility. It fractionates via differences in crystallization (forming of crystals). If a mixture of two or more substances in solution are allowed to crystallize, for example by allowing the temperature of the solution to decrease, the precipitate will contain more of the least soluble substance. The proportion of components in the precipitate will depend on their solubility products.

Rotary Evaporation

Rotary evaporation is the process of reducing the volume of a solvent by distributing it as a thin film across the interior of a vessel at elevated temperature and reduced pressure. This promotes the rapid removal of excess solvent from less volatile samples. Most rotary evaporators have four major components: heat bath, rotor, condenser, and solvent trap. An aspirator or vacuum pump needs to be attached, as well as a bump trap and round bottom flask containing the concentrated sample. If you used reduced pressure to concentrate solution, use the water aspirator with a TRAP in the line. DO NOT turn off the water until the pressure is released. Do NOT use ebullation if using the Roto-Evaporator.

Reflux

Reflux is a technique involving the condensation of vapors and the return of this condensate to the system from which it originated. It is used in industrial and laboratory distillations. It is also used in chemistry to supply heat to reactions over a long period of time.

Proper Use of Balances

For a chemical reaction to be successful, reactants must be added with accurate, specific masses, and products must be accurately weighed at the end of the reaction. Therefore, balances are of immense importance in a chemistry lab.

Proper Use of a Desiccator

A desiccator is an airtight container which maintains an atmosphere of low humidity through the use of a suitable drying agent which occupies the bottom part of the desiccator. It is used both for the cooling of heated objects and for the storage of dry objects that must not be exposed to the moisture normally present in the atmosphere.

Proper Use of a Buret

The volumetric analysis exercises will make use of a 50 mL buret. Calibration of a Buret: To carry out this procedure you will require, in addition to a volumetric buret, two clean, dry 125 mL Erlenmeyer flasks and one #5 rubber stopper.

Use of a Volumetric Pipet

Volumetric glassware is capable of measurements of volume that are good to four significant digits and is consequently expensive. You should be careful in handling this type of equipment so that breakage losses are minimized. Be particularly careful with the tips of pipets and burets.

Vacuum Filtration

Suction filtration is a chemistry laboratory technique which allows for a greater rate of filtration. Whereas in normal filtration gravity provides the force which draws the liquid through the filter paper, in suction filtration a pressure gradient performs this function. TRAP: For suction filtration, you want a clean glass trap in between your filter flask and the suction source. One reason is obvious--in the event that your filtrate is sucked out of the filter flask, it can be trapped and recovered before it goes down the drain or into the house vacuum line. Another reason is that a changing flow of water affects the pressure in the water aspirator so that water can back up and flow towards your filter flask.

Quenching Reactions

Quenching a reaction refers to the deactivate any unreacted reagents. Quenching Reactions: Grignards, Quenching Reactions: Lithium Aluminium Hydride.

Condensing Volatile Gases

Ever had to run a reaction with a volatile gas? It's not a very common thing to have to do, but every once in a while, it needs to be done.

Cooling baths

Cooling baths are used extensively in organic chemistry for a variety of reasons. The low temperature of these baths is determined both by the appropriate use of solvent as well as a cryogenic agent such as liquid nitrogen, dry ice or ice. Temperatures between -20 and -80° can be obtained using varied mixtures of ethylene glycol and ethanol over dry ice.

Vacuum Equipment

Vacuum equipment is used to generate, maintain, and manipulate pressures below that of the ambient atmosphere. Many common lab procedures require vacuum conditions, such as inert gas purging, cannulation, and solvent evaporation. Vacuum equipment often requires special care to maintain.

Heating a Crucible to Constant Weight

Your first exercise teaches you some skills on the proper use of the laboratory burner (in this case called a Tirill Burner), the adjustment of the flame and the proper placement of a crucible which is to be heated to constant weight.

Error Analysis

Error analysis is a fundamental aspect of experimental chemistry. It involves identifying and quantifying sources of uncertainty to ensure the accuracy and reliability of experimental results. Error analysis is a fundamental process in general chemistry labs, and a high-yield DAT topic. Error analysis involves identifying and quantifying sources of uncertainty to ensure the accuracy and reliability of experimental results.

Types of Errors

Random errors: Caused by unpredictable fluctuations during measurements. They can be mitigated by taking multiple readings and calculating averages to minimize the impact of outliers.
Systematic errors: Stem from consistent biases in experimental setup or measurement techniques, often requiring adjustments to equipment calibration or procedures for correction.

Percent Error

Percent error is a measure of the accuracy of an experimental result compared to a known or accepted value. It quantifies the discrepancy between the observed value and the true value, expressed as a percentage of the true value.

Percent Theoretical Yield: Although you may have obtained the product you desired, the amount of material you obtained (in grams), compared to the amount you could have obtained (in grams), is a valuable piece of information. It can suggest that you did or did not run the reaction efficiently or that other products may have been formed via other reactions. To calculate % Theoretical Yield you must first calculate the theoretical yield. You then take actual yield/theoretical yield x 100 = % theoretical yield.

Precision and Accuracy

Precision and accuracy are two essential concepts that define the reliability of experimental measurements. Precision refers to the consistency or repeatability of results, indicating how close multiple measurements of the same quantity are to each other. A highly precise measurement produces values that cluster tightly around a central value. On the other hand, accuracy refers to how close a measured value is to the true or accepted value of a quantity. An accurate measurement reflects minimal systematic error and is in close agreement with the true value. While precision highlights the reproducibility of results, accuracy underscores their correctness.

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