Solvents provide a medium for dissolved reactants to interact and facilitate chemical reactions, and their elements make your selection an important consideration while preparing a solution. When a solute and solvent are properly compatible, they will produce the right traits required for analysis. By understanding the impacts of your solvent composition and the reactions during analysis, you can accurately control the efficacy of your solution and produce the desired concentrations specific to your testing goals.  

Whether you are preparing solutions for laboratory experiments or formulating products, the process involves careful calculations, precise measurements, and understanding chemical interactions. It may also require some practice and possibly some trial and error. With the correct knowledge, skills, and tools needed to mix solvents with different samples, your compositions become more accurate, consistent, and reliable.  

Understanding Chemical Solutions and Solvents

Solvents create solutions in liquid form. When they dissolve solutes uniformly, a desired homogeneous mixture is produced. Chemical interactivity and solution compatibility depend on a solvent’s inherent properties, especially polarity, boiling point, and solubility.

The molecular charge that polar and non-polar molecules produce either attracts or repels interactivity between two liquid relationships. The “like dissolves like” rule of thumb is a basic polarity principle to keep in mind: polar solvents dissolve ionic and other polar compounds, whereas non-polar solvents are better for non-polar molecules. The polarity index electronegativity differences are based on atomic structures that produce a partial negative charge (δ-) and partial positive charge (δ+) at respective poles. Electronegativity between 0.4 – 1.7 is generally considered polar, <0.4 is non-polar, and >1.7 is very polar or ionic.

The boiling point of a solvent is also a decisive factor while preparing solutions. This indicates temperature parameters and how strong or weak intermolecular forces may be. Higher boiling points are a component of stronger solubility while creating a wider range of temperatures for mixing and reaction needs before evaporation. This also controls distillation concentrations, vapor pressure, and volatility.  

Below, we’ve listed some polarity values and boiling points for high-purity solvents available at Birch Biotech, where you can see the direct relationship between the two: 

Solvent

Polarity

Polarity Index

Boiling Point (C)

Hexane

non-polar

0.1

36.1

Dichloromethane

polar

2.1

39.7

Isopropyl Alcohol

polar

3.9

82.3

Tetrahydrofuran

polar

4.0

66.0

Methanol

polar

5.1

64.7

Acetonitrile

polar

5.8

81.6

Water

polar

10.2

100.0

 Although polarity can directly influence solubility, they are not necessarily the same. Solubility strictly refers to a solute’s ability to dissolve into a solvent. While polarity attractions and temperature are some determinants, other characteristics like pressure, molecular structure, intermolecular forces, hydrolysis reactivity, and hydrophilicity/hydrophobicity all become constituents for solubility.

Aqueous Solutions

Aqueous solutions are mixtures where water acts as a solvent or a component in a solution. Due to its high polarity, high dielectric constant, stability, and reaction predictability, water’s unique properties allow it to dissolve a wide range of ionic and polar compounds. The concentration of solutes in an aqueous solution affects its properties in the following ways:

  • Boiling point elevation: Higher solute concentrations raise the boiling point.

  • Freezing point depression: Higher solute concentrations lower the freezing point.

  • Osmotic pressure: Solute concentration drives the flow of solvent across a semipermeable membrane.

  • Vapor pressure: Higher solute concentration decreases solvent vapor pressure.

Preparing the Stock Solution

Stock solutions are concentrated mixtures that serve as the foundation for many laboratory experiments. Proper preparation ensures accurate results and reproducibility in your work.

Selecting the Correct Volumetric Flask

Choose a volumetric flask that matches your desired stock solution volume. Flasks come in various sizes, typically ranging from 10 mL to 2 L. Consider the solubility of your solute when selecting your solvent.

Ensure the flask is clean and dry before use. Rinse it with a small amount of your solvent to remove any residual contaminants. Label your flask clearly with the solution name, concentration, and date of preparation. This practice helps maintain organization and prevents mix-ups in busy lab environments.

Accuracy in Measuring Solutes and Solvents

Precise measurements are detrimental in creating accurate stock solutions. Use an analytical balance to weigh any solid solute, ensuring at least four decimal places of precision for small quantities. Calibrated micropipettes can be used to accurately measure small quantities of concentrated liquid solutes.

When measuring liquids, use the appropriate glassware. Pipettes offer high accuracy for small volumes, while graduated cylinders work well for larger amounts.

Dissolve your solute in a small amount of solvent in a separate beaker. Stir thoroughly until completely dissolved. Transfer this mixture to your volumetric flask, rinsing the beaker several times with small portions of solvent to ensure complete transfer.

Fill the flask to just below the calibration mark with your solvent. Use a dropper for the final adjustments, aligning the bottom of the meniscus with the desired graduation mark for precise volume measurement.

Calculating Composition for Desired Concentration

Determining the correct composition for a solution with a specific concentration requires precise calculations, so consider the desired concentration, molecular weight of the solute, and the amount of solvent required.

Molar Concentrations 

Molar concentration is a systematic way for researchers and chemists to calculate the exact abundance of certain solvents, solutes, and other compounds needed for high-precision testing:

Moles (mol)

the number of particles present, typically atoms or molecules; 1 mol = 5,022 x 10^23 particles (Avogadro’s Number)

Molarity

Moles of solute per liter of solution

Molality

Moles of solute per kilogram of solvent

Mass percent

Mass of solute per mass of solution

Parts per million (ppm)

Mass of solute per million parts of solution (mg/L in water)

 

Molarity measures concentrations of compounds in moles for precise volume-based measurements commonly used in research and industrial settings. Molality is often used to determine the mass of solutes for calculating colligative properties like boiling point elevation and freezing point depression. Mass percent applies concentrations important for formulating industrial and consumer products. Given the specificity needed for systems like HPLC, parts per million (ppm) and moles are units of measurement that can be used to find trace impurities in the pharmaceutical, environmental, and toxicology fields.

Determining Moles and Molecular Weight

To calculate the composition for a desired concentration, start by determining the number of moles of solute needed. Use the formula where one mole (mol) equates to 6.02 x 10^23 atoms, molecules, or other particles:

moles = desired concentration (moles/volume) × volume of solution

Next, find the molecular weight of your solute. This information is typically available on chemical data sheets or in reference books or can be calculated by adding together the molecular weights of all atoms in the molecule. To calculate the mass of solute needed, multiply the number of moles by the molecular weight:

mass of solute = moles × molecular weight (grams/mol)

This gives you the amount in grams of solute to add to your solvent.

Working with Different Types of Solutions

For molecular solutions, solutes dissolve without breaking into ions, meaning you'll work directly with the molecules of the solute (e.g., sugar in water). In ionic solutions, you'll need to account for the dissociation of the compound into ions.

If the ionic compound fully dissociates in solution, it is considered a strong electrolyte. For example, one mole of NaCl (table salt) will generate 2 moles of dissociated ions in solution, 1 mole of Na+ and one mole of Cl-.

This contrasts with weak electrolytes, which do not fully dissociate, although they may fully dissolve. Acetic acid in water is an example of a fully soluble weak electrolyte, where the equilibrium conditions determine total solute and ion concentrations. When preparing a solution with a specific molarity, use this formula:

Molarity = moles of solute / liters of solution

For weight percent solutions, calculate:

Weight % = (mass of solute / mass of solution) × 100

Remember to adjust your calculations based on the specific type of concentration that is your objective and ensure consistency of the units (molarity, molality, weight percent, etc.).

Adjusting the Solution Volume

Precise volume adjustment may be required while preparing solutions. You'll need to use proper measuring tools and techniques to achieve the desired concentration and total volume accurately.

Using Graduated Cylinders and Solvents

To adjust your solution volume, use a graduated cylinder to measure additional solvents accurately. Add the additional solvent slowly while monitoring the volume markings. For smaller adjustments, pipettes offer greater precision than pouring by hand.

When diluting a sample, add it to the graduated cylinder first. Then carefully add additional solvent up to the desired volume mark. Gently swirl the cylinder to mix thoroughly without creating bubbles.

For dilutions requiring changes in concentration spanning multiple orders of magnitudes, you may need to use multiple additions or dilutions to ensure precision and accuracy. Transfer the initial mixture to a larger container, rinsing the sample cylinder with solvent, and filling the larger container with solvent up to the desired volume. Repeat this process until the final desired concentration is reached.

Ensuring Precision for Total Solution Volume

To achieve high precision, use glassware calibrated for containing or delivering specific volumes. Volumetric flasks are ideal for preparing exact volumes.

When using a volumetric flask:

  1. Add your sample

  2. Fill to just below the calibration mark with solvent

  3. Use a dropper or micropipette to carefully add the final drops

  4. Adjust the liquid level so the bottom of the meniscus aligns with the mark

Remember, temperature affects liquid volume. Prepare and measure solutions at a consistent temperature for accuracy. Always check your final volume at eye level to avoid parallax errors.

Catalysts, Reagents, Heat, and Pressure

Solvent composition directly influences chemical reactions and processes, thus understanding how catalysts, reagents, pressure, and heat interact with solvents can significantly impact reaction outcomes and efficiency.

Catalysis and Reagent Behavior

Catalysts can dramatically alter reaction rates in solvent systems, so it is important to note that catalyst selectivity often depends on solvent composition, affecting both reaction speed and product distribution. For example, polar solvents such as water or methanol can enhance the effectiveness of ionic catalysts by stabilizing charged intermediates or transition states, improving reaction efficiency.

Reagent behavior is similarly influenced by solvent choice. You may observe changes in reagent solubility, reactivity, and stability as you adjust solvent polarity or hydrogen bonding capability. This can be particularly important in multi-step reactions where the solvent can help stabilize intermediate products, allowing the reaction to proceed.

During your selection process, consider these properties, which can significantly impact how catalysts and reagents perform in your reaction system:

  • Solvent polarity

  • Hydrogen bonding ability

  • Dielectric constant

  • Viscosity

The Impact of Pressure and Heat on Solutions

Pressure and heat can profoundly affect solvent behavior and solution properties. You'll notice that increasing temperature often enhances solubility, potentially altering reaction rates and equilibria. This is particularly relevant in crystallization processes, where temperature control is crucial for product purity and yield.

Pressure changes can also influence solvent properties. At higher pressures, you may observe:

  • Increased boiling points

  • Changes in solvent density (at very high pressures)

  • Altered solubility of gases

These effects can be leveraged in supercritical fluid extraction or high-pressure reactions. For instance, supercritical CO2 is used as a green solvent in various industrial processes due to its tunable density, solvent strength, and diffusivity under different supercritical conditions.

By carefully controlling pressure and temperature, you can optimize solvent performance for specific applications, from extraction to chemical synthesis.

Dissolving Solids and Mixing Processes

Dissolving solid solutes and creating evenly distributed mixtures requires proper techniques and thorough integration of components for a uniform solution. To achieve even distribution in solutions, start by adding smaller amounts of the solid solute gradually. This approach prevents clumping and promotes uniform dispersion. Stirring appropriately is important for dissolving solids and certain evaporation techniques.

Use a magnetic stirrer or a mechanical impeller to create a vortex, increasing mass transfer at the solid surface. For viscous liquids, employ a high-speed blender or homogenizer. Temperature can affect solubility, so ensure your desired solution concentration is within the solute’s solubility limits at your operating temperature.

Monitor the process visually. A clear solution indicates complete dissolution. If particles remain, continue stirring, check solubility parameters for your solvent in your solute, or change the temperature if appropriate.

When dissolving solids into liquids, choose the right method based on the solutes properties. For precise concentrations, use volumetric flasks and analytical balances. Always add solute to solvent, not vice versa. This practice prevents violent reactions or splashing, especially when dissolving exothermic substances like concentrated acids or bases. Gradual addition also ensures even mixing and precise concentration control.

Quality Control and Error Prevention

Maintaining precise solvent composition is always recommended for accurate results. Rigorous quality control procedures and error prevention techniques help ensure consistent, reliable outcomes in your experiments.

Procedures to Ensure Balance and Consistency

Calibrate your balance regularly using NIST-referenced test weights to verify accuracy. Always level the balance before use by adjusting its feet until the bubble indicator is centered.

Clean the balance thoroughly between measurements to prevent cross-contamination. Use a soft brush and, if needed, deionized water with lint-free wipes.

Monitor the ambient temperature, as it can affect solvent density and evaporation rates. Maintain a consistent lab environment to minimize fluctuations.

When transferring small amounts, use proper pipetting techniques. Rinse pipette tips with the solvent before dispensing to improve accuracy.

Detecting and Accounting for Errors

Implement a systematic approach to error prevention by focusing on three key pillars: structural measures, procedural safeguards, and human factors.

Develop clear, detailed standard operating procedures (SOPs) for solvent preparation. Include step-by-step instructions and critical checkpoints to catch potential mistakes.

Utilize quality control charts to track balance performance over time. Plot measured values against known standards to detect drift or sudden changes in accuracy.

Perform replicate measurements for critical samples. Calculate the relative standard deviation to assess precision and identify potential outliers.

Consider using internal standards or surrogate compounds to account for errors in sample preparation or instrument performance.

Handling and Safety Considerations

When working with volatile solvents, always wear appropriate personal protective equipment (PPE) including gloves, safety goggles, lab coats, long pants, and closed-toed shoes. Store solvents in clearly labeled, sealed containers in a cool, dry place away from heat sources and direct sunlight. Segregate incompatible chemicals to prevent dangerous reactions.

Keep organic solvents in designated flammable storage cabinets when not in use. Create proper ventilation in storage and work areas to prevent vapor accumulation. Use only spark-proof tools and equipment when handling flammable solvents.

Regularly inspect containers for leaks or damage and use secondary containment where necessary and on waste containers. Have spill kits readily available and train personnel on proper spill response procedures. Maintain accurate inventory records and safety data sheets (SDS) for all solvents in use.

Minimizing Environmental Impact

Given the impact of environmental waste, choose green solvents whenever possible. Consider factors like biodegradability, toxicity, and potential for bioaccumulation when selecting solvents. Implement solvent recycling systems to minimize waste and reduce the need for new solvent production.

Use efficient extraction and purification methods to minimize solvent consumption. Consider alternatives like solvent-free reactions or supercritical CO2 extraction when feasible. Properly dispose of waste solvents through approved channels, never pouring them down drains or venting them to the atmosphere where they can cause environmental pollution.

 

Want to learn more about our solvents? Read more of our articles or visit us at Birch Biotech. 

 

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Disclaimer: The content provided on the Birch Biotech blog is for educational and entertainment purposes only. The information offered here is designed to provide helpful insights and advice related to laboratory practices and supplies.

 Readers are advised to refer to our product-specific quality data sheets and Certificates of Analysis (COAs) available on our website for detailed information on product specifications. It is essential to handle and store all materials according to the safety guidelines and regulatory requirements applicable to your area.

 While we endeavor to ensure the accuracy and relevance of the information published, it should not be used as a substitute for professional advice or official protocols. We encourage all our readers to consult their institution's guidelines, local regulations, and professional standards before implementing any practices discussed here.

 Birch Biotech does not accept liability for any actions undertaken based on the information provided in this blog nor for the misuse of our products. Furthermore, Birch Biotech does not guarantee the completeness, reliability, or timeliness of the information contained on this website.

 This disclaimer is subject to change at any time without notifications.

 

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Lis-Kuberka, J. & Wroclaw. (n.d.). Solutions. https://www.umw.edu.pl/sites/default/files/immchem/files/Optional_courses_Part_3_Solution.pdf

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Seitzman, J. M. (n.d.). General Method for calculating chemical equilibrium Composition. https://seitzman.gatech.edu/classes/ae6765/GenlSolutionChemicalEquilibrium.pdf

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