Whenever you work in a chemical laboratory, you're likely to encounter a variety of solutions, including those that contain unknown components. When this happens, you must use Chemical Analysis skills to determine what chemicals are present in solution. So, let's learn the basics of identifying chemicals in solution!
- First, we will learn about what chemical solutions are.
- Then, we will dive into the definition of analytical chemistry and techniques such as conduction testing, titration, Chromatography, acid-base testing, and precipitation.
Chemical Solutions
Before we learn how to identify chemicals in a solution, let's look at the definition of a solution.
A solution is referred to as a homogenous mixture comprised of a solvent plus one or more solutes.
- A homogenous mixture is a mixture that has a uniform composition.
- A solvent is a substance that is present in a solution in the highest amount, and it is used to dissolve the solute.
- A solute is a substance that dissolves into a solvent.
The general chemical equation between a solvent and a solute to form a solution is as follows:
$$ \text{Solute + Solvent }\longrightarrow \text{Solution} $$
Solutes can either be Electrolytes or non-electrolytes. Strong Electrolytes are solutes that dissociate 100% in a given solvent, whereas weak Electrolytes are those that only dissociate slightly. Non-electrolytes do not dissociate at all. Solutes and solvents may be solids, liquids, or gases.
The amount of solute that dissolves in a specific amount of solvent is termed Solubility. When it comes to solids, increasing temperature increases the Solubility of most solids, whereas the opposite happens with gases.
Solubility is defined as the maximum amount of a solute that is able to dissolve in 100 grams of water (H2O).
It is important to note that, although we generally refer to solubility as "in water", it can also refer to other solvents such as ethanol and hexane!
The graph below shows the solubility of some salts in 100 grams of H2O. Sodium chloride (NaCl), for example, has a solubility of around 37 grams per 100 g of H2O at 50 °C.
Figure 1. Solubility curves of different salts in H
2O
Concentration
When we have a solution, we use the units of Concentration to specify the amount of solute dissolved in a solvent to make 1 dm3 (or 1 L) of solution. Usually, the greater the mass or quantity of solute dissolved in a given volume, the greater the Concentration.
Concentration is often measured in terms of Molarity, but there are many other ways to express concentration such as molality, mole fraction, and mass percent.
Molarity (M) is referred to as the moles of solute per liter of solution.
To learn about the different ways to calculate concentration, check out "Molarity and Molality"!
What is analytical chemistry?
We will start with a definition of analytical chemistry.
Analytical Chemistry Definition
Now that we know what chemical solutions are, let's look at the definition of analytical chemistry.
Analytical chemistry is a branch of chemistry that deals with determining the chemical composition of a sample of material.
Analytical chemistry involves two types of analysis: qualitative and quantitative analysis.
Qualitative analysis is performed to determine what is found in a sample.
Quantitative analysis deals with the amount, concentration, or composition of a substance present.
Analytical Chemistry Techniques
To identify chemicals in solution, chemistry can use different analytical chemistry techniques such as Chromatography, titration, precipitation, acid-base testing, and conduction testing. So, let's talk about them!
Conduction Testing
The first test we will explore is called the conduction (or electrical conductivity) test. This test is used by chemists to classify substances as strong electrolytes, weak electrolytes, or non-electrolytes based on their ability to conduct electricity.
Electrolytes are substances that conduct electricity when dissolved in water.
This apparatus consists of a bulb connected in series with two open electrodes, which are placed inside a container containing a sample of the solution being tested. Whenever an electrolyte is present, the light bulb turns on!
Figure 1 shows an example of a conduction test performed on a strong electrolyte, a weak electrolyte, and a non-electrolyte.
Titration
Next, we have the analytical chemistry technique of titration.
Titration is a technique used to find the concentration of an unknown solution, by reacting it with a solution of known concentration.
During titration, the titrant (solution of known concentration) is added in a dropwise manner to the solution with unknown concentration (the analyte) until the reaction reaches its endpoint. To be able to indicate whether an acid-base reaction was reached its endpoint, an indicator is added to the solution.
- The end point is the point where the solution changes color, indicating that the titration should stop.
Indicators are substances that undergo color changes in acidic or basic conditions.
The image below shows an example of a laboratory setup used for Titrations.
To learn more about this, check out "Acid and Base Titration"!
Chromatography
Now, let's talk about chromatography.
Chromatography is an analytical technique that is used to separate a mixture into its components.
The process of chromatography involves two phases. The stationary phase represents a static solid, liquid, or gel, whereas the mobile phase represents the solvent (gas or liquid) used to carry the mixture through the stationary phase.
Chromatography separates components within a mixture by using the relative affinities of the components.
Affinity is referred to as the degree to which components of a sample are attracted due to their Intermolecular Forces, to the mobile or stationary phase.
- An intermolecular force is a force of attraction between molecules.
If a component possesses a greater affinity (greater attraction) to the mobile phase, it will move up the plate faster, compared to the components that have a stronger affinity to the stationary phase.
- Having a high affinity means that the component interacts strongly with the mobile or stationary phase.
As a general rule, atoms, molecules and ions are more attracted to the mobile or stationary phase that has the same or similar properties.
The simplest type of chromatography is Paper Chromatography. So, let's take a look at how it works to be able to better understand affinity!
Let's say that we wanted to separate dyes from a mixture (ink). In this case, the stationary phase will be the chromatography paper, and the mobile phase will be water.
A pen is first used to make ink dots on the stationary phase. Then, the mobile phase passes through the stationary phase and solubilizes the components. The mobile phase then carries the individual component a certain distance (Rf) through the stationary phase, depending on their attraction to both phases.
The chromatography paper (stationary phase) is derived from cellulose, which contains many polar -OH groups. Water (mobile phase) is also a polar molecule, but more polar than the chromatography paper. So, any component of the sample that has whole or partial charges (ex. ionic or polar substances) will have a higher affinity (or attraction) for the water (mobile phase).
On the other hand, any component of the sample that experiences dispersion forces has a higher affinity for the stationary phase and will not move far from the origin (the line that marks the point where the sample (ink dots) was placed).
Need a more in-depth explanation? Check out "Chromatography"!
Acid and Base Testing
To tell whether a solution is an acid or a base, we can use analytical acid and base testing techniques.
Acids are solutions that increase the concentration of hydrogen ions (H+) in solution.
Bases are solutions that increase the concentration of hydroxide ions (OH-) in solution.
The first technique is using litmus paper. When a blue litmus paper is placed in an acidic solution, it will turn red, while a red litmus paper will turn blue in basic conditions. In neutral solutions, the litmus paper will turn purple!
Another technique is using a universal pH paper. This paper changes color according to the solution's pH.
Now, if you are looking for something more modern, you can use a digital pH meter! A digital pH meter measures the pH of a substance electronically. It works by detecting the amount of hydrogen ions in solution.
- The more hydrogen ions in a solution, the more acidic it will be.
Precipitation
Lastly, we have precipitation. The precipitate test is used by chemists when they suspect that a solution has a certain ion present.
For example, imagine that you have a solution that you believe contains iron (Fe2+) and strontium (Sr2+) in it. You can perform precipitation reactions to confirm the presence of these ions. If a certain precipitate forms, then you were right!
Precipitation reactions are Chemical Reactions where an insoluble product (called a precipitate) is formed.
According to the solubility rules for ionic compounds, we could add a compound containing hydroxide ions (OH-) to a sample of the solution and see if the precipitate iron (II) hydroxide, Fe(OH)2, forms. Similarly, we could add a solution containing sulfate (SO42-) to precipitate strontium in the form of solid strontium sulfate (SrSO4).
$$ \text{Fe}^{2+}{(aq)} + \text{OH}^{-}{(aq)} \longrightarrow \text{Fe(OH)}_{2} (s) $$
$$ \text{Sr}^{2+}{(aq)} + \text{SO}^{2-}_{4}{(aq)} \longrightarrow \text{SrSO}_{4} (s) $$
Looking for solubility rules? Read our explanation on "Solubility"!
I hope that you feel confident in your understanding of the basics of analytical chemistry and identifying chemicals in solution!
Analytical Chemistry - Key takeaways
- A solution is referred to as a homogenous mixture comprised of a solvent plus one or more solutes.
- To identify chemicals in solution, chemistry can use different analytical chemistry techniques such as chromatography, titration, precipitation, acid-base testing, and conduction testing.
- Chromatography is an analytical technique that is used to separate a mixture into its components.
- Precipitation reactions are Chemical Reactions where an insoluble product (called a precipitate) is formed.
References
- Jespersen, N. D., & Kerrigan, P. (2021). AP chemistry premium 2022-2023. Kaplan, Inc., D/B/A Barron’s Educational Series.
- N Saunders, Kat Day, Iain Brand, Claybourne, A., Scott, G., & Smithsonian Books (Publisher. (2020). Supersimple chemistry : the ultimate bite-size study guide. Dk Publishing.
- Swanson, J. (2021). Everything you need to ace chemistry in one big fat notebook. Workman.
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