Chemistry Study Notes: Instrumental Methods & Their Applications

Hey everyone! Welcome to one of the most exciting topics in chemistry – Analytical Chemistry. Think of it as the 'detective work' of science. In this chapter, we'll learn how chemists figure out what is in a sample and how much of it there is. These skills are super important for everything from solving crimes and checking our food for safety to protecting the environment. Let's get our detective hats on and dive in!


Section 1: The Chemist's Toolkit - Identifying Unknowns

Before we use fancy machines, let's look at some classic, hands-on tests that chemists use to identify substances. These are the fundamental skills every chemist needs!

A. The Flame Test: Seeing an Element's True Colours

Have you ever wondered how fireworks get their brilliant colours? It's all about chemistry! When we heat certain metal ions, they give off a characteristic colour of light. We can use this to identify them.

How it works: A clean wire loop is dipped into a sample of the compound and then held in a hot Bunsen flame. The colour of the flame tells us which metal ion is present.

  • Sodium (Na⁺): Intense Golden Yellow
  • Potassium (K⁺): Lilac (a pale purple)
  • Calcium (Ca²⁺): Brick Red
  • Copper (Cu²⁺): Blue-Green
Quick Review: Flame Test Colours

Memory trick: Think of everyday things! Salty crisps (Sodium) are golden. A copper statue turns green.

B. Testing for Common Gases (Molecules)

Here’s how we can identify some common, invisible gases in the lab.

  • Hydrogen (H₂): Use a lit splint. If hydrogen is present, you'll hear a 'squeaky pop'.
  • Oxygen (O₂): Use a glowing splint (one that's been lit and then blown out). If oxygen is present, the splint will relight.
  • Carbon Dioxide (CO₂): Bubble the gas through limewater (calcium hydroxide solution). If CO₂ is present, the limewater will turn milky/cloudy.
  • Chlorine (Cl₂): Use damp blue litmus paper. It will first turn red (because chlorine is acidic in water) and then it will be bleached white.
  • Ammonia (NH₃): Use damp red litmus paper. It will turn blue because ammonia is an alkaline gas.

C. Testing for Ions in Solution

Many substances are dissolved in water, so we need ways to test for the ions they form. We often do this by adding another chemical to see if a solid (a precipitate) forms.

Testing for Cations (Positive Ions)

A common method is to add sodium hydroxide solution, NaOH(aq), drop by drop.

  • Copper(II) (Cu²⁺): Forms a blue precipitate.
  • Iron(II) (Fe²⁺): Forms a dirty green precipitate.
  • Iron(III) (Fe³⁺): Forms a reddish-brown precipitate.
  • Aluminium (Al³⁺), Zinc (Zn²⁺), Calcium (Ca²⁺), Magnesium (Mg²⁺): All form a white precipitate. Don't worry, you'll learn more specific tests to tell these apart if needed!
  • Ammonium (NH₄⁺): Add NaOH(aq) and gently warm the mixture. If NH₄⁺ is present, it will produce ammonia gas, which you can test for with damp red litmus paper (it will turn blue).
Testing for Anions (Negative Ions)
  • Carbonate (CO₃²⁻): Add a dilute acid (like HCl). If carbonate ions are present, you'll see fizzing (effervescence) as CO₂ gas is produced.
  • Chloride (Cl⁻), Bromide (Br⁻), Iodide (I⁻): First, add dilute nitric acid, then add silver nitrate solution.
    • Cl⁻ gives a white precipitate.
    • Br⁻ gives a cream precipitate.
    • I⁻ gives a yellow precipitate.

D. Finding Functional Groups in Organic Compounds

Functional groups are the 'active parts' of organic molecules that determine how they react. We can perform simple tests to find out which ones are present.

  • C=C (in Alkenes): Add bromine water (which is orange-brown). If a C=C bond is present, the bromine water will be decolourised from orange-brown to colourless.
  • -COOH (in Carboxylic Acids): Add a small amount of sodium carbonate. You will see fizzing as CO₂ gas is produced. It's an acid, after all!
  • -CHO (Aldehydes) vs >C=O (Ketones): These can be tricky to tell apart, but we have two great tests!
    1. 2,4-dinitrophenylhydrazine (2,4-DNPH): This test works for BOTH aldehydes and ketones. Adding 2,4-DNPH will produce a bright orange/yellow precipitate if either is present. This is a good test for a 'carbonyl' group in general.
    2. Tollens' Reagent: This test ONLY works for aldehydes. When you warm an aldehyde with Tollens' reagent, a beautiful silver mirror forms on the inside of the test tube. Ketones do not react.
Key Takeaway for Section 1

Simple chemical tests, based on observing colour changes, precipitates, or gas production, are powerful tools for identifying unknown substances. Each test gives us a clue, and by combining clues, we can solve the chemical mystery!


Section 2: Separation & Purification - Sorting Out the Mess

Real-world samples are almost never pure; they're mixtures. A forensic scientist might find a mixture of soil and glass, or a food chemist might need to separate a vitamin from a fruit juice. Here's how we sort things out.

A. Methods for Solids and Liquids

  • Crystallisation: Used to get a pure solid sample from a solution.

    Example: Getting pure salt from salty water.
    Steps: 1. Gently heat the solution to evaporate some of the solvent and make it more concentrated. 2. Allow the solution to cool slowly. 3. As it cools, the solid will form pure crystals. 4. Filter the crystals out and wash them with a little cold solvent to remove any impurities.

  • Distillation: Used to separate a liquid from a dissolved solid, or to separate liquids with very different boiling points.

    Analogy: When you boil a pot of salty water, the steam that comes off is pure water vapour, leaving the salt behind. Distillation just catches and cools that steam.

  • Fractional Distillation: A more precise version used to separate liquids with closer boiling points (like ethanol and water). It uses a 'fractionating column' with a large surface area (like glass beads) that allows the liquid with the lower boiling point to evaporate and rise to the top first, while the other liquid condenses and falls back down.
  • Liquid-Liquid Extraction: Used to separate a substance from one liquid into another. The two liquids must be immiscible (they don't mix, like oil and water).

    How it works: You shake the mixture in a separating funnel. The substance you want to extract will move into the liquid in which it is more soluble. You can then drain the two layers off separately.

B. Chromatography: Separation by "Stickiness"

This is a super powerful technique for separating complex mixtures, like the different coloured inks in a black pen.

The Main Idea: All chromatography has a stationary phase (a solid or a liquid that stays put, like the paper) and a mobile phase (a liquid or gas that moves, like the solvent). Substances in the mixture are separated because they have different attractions (or 'stickiness') to the stationary phase and different solubilities in the mobile phase.

Analogy: Imagine a race on a muddy field. All runners start at the same line. The runners who are lighter and less likely to get stuck in the mud will travel the furthest. In chromatography, the substances that are more soluble in the mobile phase and less attracted to the stationary phase travel furthest.

Paper & Thin Layer Chromatography (TLC)

After running the chromatogram, we can calculate an Rf value for each spot. This helps to identify the substance.

$$ R_f = \frac{\text{distance travelled by the spot}}{\text{distance travelled by the solvent front}} $$

The Rf value for a specific compound is always the same under the same conditions (same paper, same solvent, same temperature).

C. How Pure Is It? Melting and Boiling Points

This is a simple but very effective way to check the purity of a substance.

  • A pure substance has a sharp, fixed melting point and boiling point. (e.g., pure water boils at exactly 100°C at standard pressure).
  • An impure substance will melt and boil over a range of temperatures. Impurities also tend to lower the melting point and raise the boiling point.
Key Takeaway for Section 2

We can separate mixtures based on differences in physical properties like boiling point, solubility, or attraction to a surface. Checking the melting or boiling point is a quick way to determine the purity of our separated substance.


Section 3: Modern Analytical Instruments

Sometimes, classic tests aren't sensitive enough or the sample is too complex. That's when we bring in the high-tech machines! You don't need to know how they work in detail, just the basic idea and what they tell us.

A. Colorimetry: Measuring Colour Intensity

Basic Principle: This technique measures the concentration of a coloured substance. The more concentrated the solution, the more light it will absorb.

Analogy: Imagine trying to look through a glass of tea. A weak, pale tea is easy to see through (low absorbance). A strong, dark tea is hard to see through (high absorbance).

How it's used to find an unknown concentration:

  1. Make a Calibration Curve: Prepare several solutions of the substance with known concentrations (these are called 'standard solutions').
  2. Measure the absorbance of each standard solution using a colorimeter.
  3. Plot a graph of Absorbance vs. Concentration. This is your calibration curve. It should be a straight line through the origin.
  4. Test the Unknown: Measure the absorbance of your unknown sample.
  5. Find the Concentration: Find your unknown's absorbance on the y-axis of the graph, go across to the line, and then down to the x-axis to read its concentration.

B. Infrared (IR) Spectroscopy: Making Bonds Vibrate

Basic Principle: This machine shoots infrared radiation at a sample. Different chemical bonds (like C=O, O-H, C-H) absorb different, specific frequencies of this radiation, causing them to vibrate. The machine detects which frequencies are absorbed.

What it tells us: It identifies the functional groups present in a molecule. Each functional group has a characteristic 'fingerprint' on the IR spectrum.

How to read an IR spectrum (simplified): You'll be given a data table that tells you which 'wavenumbers' (the units on the x-axis) correspond to which bonds. You just look for major absorptions (the big pointy dips) in those regions.

  • A very broad, large dip around 3200-3600 cm⁻¹ suggests an O-H bond (like in an alcohol or carboxylic acid).
  • A sharp, strong dip around 1700-1750 cm⁻¹ is a dead giveaway for a C=O bond (in an aldehyde, ketone, acid, etc.).

C. Mass Spectrometry: Weighing Molecules and Their Pieces

Basic Principle: This machine basically does three things: 1. It knocks an electron off a molecule to turn it into a positive ion. 2. It accelerates this ion through a magnetic field, which makes it curve. 3. It detects where the ion lands. The key thing is that heavier ions curve less than lighter ions. This allows the machine to separate ions based on their mass-to-charge ratio (m/z), which is essentially their mass.

Analogy: Imagine throwing a bowling ball and a tennis ball with the same force. The lighter tennis ball will be much easier to curve than the heavy bowling ball.

What it tells us:

  • The Molecular Mass: The peak furthest to the right (with the highest m/z value) is usually the 'molecular ion peak' (M⁺). This tells you the relative molecular mass of the entire molecule!
  • The Structure: The machine often breaks the molecule into smaller, charged pieces called fragments. The pattern of these fragments gives us clues about the molecule's structure. For example, if you see a peak at m/z = 43, it might be a CH₃CO⁺ fragment.
Key Takeaway for Section 3

Instrumental methods give us incredibly detailed information about a substance's identity and structure. Colorimetry measures concentration of coloured solutions, IR Spectroscopy finds functional groups, and Mass Spectrometry finds the molecular mass and structural fragments.


Section 4: Analytical Chemistry in the Real World

So, why is all this important? Analytical chemistry is a vital, often invisible, part of our modern society that keeps us safe and healthy.

A. Environmental Protection

Scientists use advanced instruments to monitor our environment. They can measure tiny amounts of harmful pollutants in the air we breathe (like carbon monoxide (CO) from cars or formaldehyde from furniture glue) and in the water we drink, ensuring they stay below safe levels.

B. Forensic Science

This is the "CSI" application! Forensic chemists use analytical techniques to provide legal evidence in criminal investigations. They can:

  • Identify illegal drugs found at a crime scene.
  • Match a tiny paint chip from a hit-and-run to a suspect's car.
  • Analyse fibres left on a victim to link them to a suspect's clothing.
  • Detect traces of explosives after a bombing.

C. Clinical Diagnosis

When you get a blood or urine test at the doctor's office, that's analytical chemistry in action! These tests can:

  • Measure glucose levels to diagnose and manage diabetes.
  • Check for markers that might indicate certain diseases.
  • Determine the levels of different substances (like cholesterol or iron) to assess a person's health.

This allows for the accurate diagnosis, treatment, and prevention of diseases.

Did you know?

The breathalyser used by police is a real-world example of analytical chemistry. It uses a chemical reaction or an electrochemical sensor to measure the amount of ethanol vapour in a person's breath, allowing police to determine if a driver is over the legal alcohol limit.

Final Key Takeaway

From the food we eat to the air we breathe and the justice system we rely on, analytical chemistry plays a crucial role. By combining classic lab tests with modern instrumental methods, chemists can identify substances, determine their quantity, and provide the vital information that helps us make our world safer, healthier, and better understood.