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.

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.

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.

Concentration

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.

• 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.

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.

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.

The image below shows an example of a laboratory setup used for titrations.

Chromatography

Now, let's talk about chromatography.

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.

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!

Acid and Base Testing

To tell whether a solution is an acid or a base, we can use analytical acid and base testing techniques.

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!

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 (SO₄^2-) to precipitate strontium in the form of solid strontium sulfate (SrSO₄).

$$ \text{Fe}^{2+}{(aq)} + \text{OH}^{-}{(aq)} \longrightarrow \text{Fe(OH)}_{2} (s) $$

$$ \text{Sr}^{2+}{(aq)} + \text{SO}^{2-}_{4}{(aq)} \longrightarrow \text{SrSO}_{4} (s) $$

I hope that you feel confident in your understanding of the basics of analytical chemistry and identifying chemicals in solution!