Instrumental Analysis for Salts Revision Notes for VCE SSCE Chemistry

Instrumental Analysis for Salts

Introduction to instrumental analysis

Modern chemistry relies on sophisticated instruments that provide faster and more accurate analysis than traditional laboratory techniques. When determining the concentration of salts in samples, instruments called colorimeters and UV-visible spectrophotometers offer several advantages:

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Advantages of instrumental analysis:

They can measure very low concentrations that would be difficult to detect with traditional methods
Results are obtained more quickly
Measurements are more precise and accurate

These instruments work by measuring how light interacts with solutions. The intensity of a solution's color provides information about its concentration, which these instruments can measure with great accuracy.

Spectroscopy

Spectroscopy is the study of how electromagnetic radiation interacts with matter. Understanding this interaction helps us analyze the composition of materials.

Electromagnetic radiation and the electromagnetic spectrum

Light is a form of energy called electromagnetic radiation. Other forms include radio waves, microwaves, infrared radiation, ultraviolet light, and X-rays. The electromagnetic spectrum shows the spread of different types of radiation arranged according to their relative energies and wavelengths.

Visible light occupies only a small portion of the electromagnetic spectrum, corresponding to wavelengths between $400$ nm and $700$ nm. This is the region used in colorimetry and UV-visible spectroscopy for salt analysis.

How color and light absorption work

When light passes through a colored solution, some wavelengths are absorbed while others are transmitted (pass through) or reflected. The color you observe is not the color being absorbed—it's actually the complementary color.

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Understanding color perception:

For example, a solution of potassium permanganate ( $\text{KMnO}_4$ ) appears purple because it absorbs yellow-green light (around $570$ nm). The light that passes through—violet, blue, and red—combines to give the purple color we see.

The more concentrated a solution, the more light it absorbs. A $0.01$ M solution of potassium permanganate appears darker than a $0.001$ M solution because it absorbs more light.

Complementary colors

The relationship between absorbed and observed colors is described by complementary colors. These color pairs are opposite to each other on the color wheel.

Wavelength (nm)	Color absorbed (color of filter)	Color observed
$380-420$	Violet	Yellow
$420-440$	Violet-blue	Yellow-orange
$440-470$	Blue	Orange
$470-500$	Blue-green	Orange-red
$500-520$	Green	Red
$520-550$	Yellow-green	Purple (red + violet)
$550-580$	Yellow	Violet
$580-620$	Orange	Blue
$620-680$	Orange-red	Blue-green
$680-780$	Red	Green

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Memory aid for complementary colors:

Write ROYGBV (Red, Orange, Yellow, Green, Blue, Violet) in order of decreasing wavelengths. Write the complementary colors directly below, starting from Green: GBVROY. This gives you the complementary pairs.

Colorimetry

Colorimetry is a technique that measures the intensity of color in a solution to determine its concentration. Samples may need to be treated with chemical compounds to produce a colored product suitable for analysis.

Components of a colorimeter

A colorimeter consists of three main components:

Light source: Produces light that passes through a filter to select a particular color (wavelength) for the analysis
Sample cell: A glass, plastic, or quartz container that holds the solution being analyzed
Detector and recorder: Electronic device that measures the absorbance of light and displays the result

The filter selects light of the color that the sample absorbs most strongly. For example, if analyzing a solution that appears purple (like $\text{KMnO}_4$ ), you would use a yellow-green filter because that's the complementary color—the color being absorbed.

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The absorbance reading ranges from 0.00 (no absorption) to 1.00 (complete absorption).

Calibration curves

To determine an unknown concentration using colorimetry, you must first create a calibration curve. This involves:

Step 1: Prepare standard solutions

Create a series of standard solutions—solutions with accurately known concentrations. For example, if analyzing nickel(II) sulfate, you might prepare solutions ranging from $0.0$ M to $0.5$ M.

Step 2: Measure absorbances

Measure the absorbance of each standard solution at the selected wavelength.

Example data for nickel(II) sulfate analysis:

Step 3: Plot the calibration curve

Create a graph with:

Concentration on the $x$ -axis (independent variable)
Absorbance on the $y$ -axis (dependent variable)

The calibration curve should show a linear relationship between concentration and absorbance.

Step 4: Determine unknown concentration

Measure the absorbance of your unknown sample and use the calibration curve to find the corresponding concentration.

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Critical consideration:

If the unknown sample's absorbance is higher than your highest standard, you must dilute the sample and re-measure. This ensures accurate readings within the calibrated range.

Worked example: Using a calibration curve

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Worked Example: Determining Iron Concentration in Dam Water

Problem: The concentration of iron in dam water is determined by colorimetry. The absorbances of standard solutions and a water sample are shown below. Determine the concentration of iron (as $\text{Fe}^{2+}$ ) in the sample.

Table

Solution:

The calibration curve shows the relationship between iron concentration and absorbance:

Following the procedure:

Plot all standard solution data points
Draw a best-fit line through the points
Find the sample absorbance ( $0.38$ ) on the $y$ -axis
Draw a horizontal line to the calibration curve
Draw a vertical line down to the $x$ -axis
Read the concentration value

Answer: The concentration of iron in the dam water sample is 9 mg mL⁻¹.

UV-visible spectroscopy

A UV-visible spectrophotometer is a more sophisticated instrument than a colorimeter. The key difference is that it uses a monochromator instead of a filter.

Monochromator function

A monochromator splits white light into individual wavelengths, which can be precisely selected using computer control. This provides several advantages:

Exact wavelength selection (not just broad color ranges)
Can measure in both ultraviolet and visible regions
More accurate measurements

Scanning for optimal wavelength

Before analyzing samples, you can scan the solution across multiple wavelengths to determine which wavelength gives the strongest absorbance.

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Scanning advantages:

For example, scanning a chlorophyll solution shows strong absorbance at approximately $420$ nm (violet light) and $660$ nm (red light). You would then conduct your analysis at one of these wavelengths for maximum sensitivity.

The wavelength chosen for analysis should be one where:

The target compound absorbs strongly
Other compounds in the solution do not absorb

Metal complexes

Some metal ions, such as $\text{Fe}^{2+}$ , do not absorb visible light strongly enough for direct analysis. These ions must first be converted into colored metal complexes.

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What is a metal complex?

A metal complex is formed when metal ions bond to molecules or anions called ligands. Transition metals readily form brightly colored complexes suitable for spectroscopic analysis.

Example: Iron(II) phenanthroline complex

When $\text{Fe}^{2+}$ ions react with $1,10$ -phenanthroline (abbreviated as phen), a bright orange complex forms:

The reaction is:

$\text{Fe}^{2+}(\text{aq}) + 3\text{phen}(\text{aq}) \rightarrow \text{Fe(phen)}_3^{2+}(\text{aq})$

This orange complex, $\text{Fe(phen)}_3^{2+}$ , can then be analyzed using colorimetry or UV-visible spectroscopy.

Summary of analysis procedure

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Complete Analysis Procedure:

Form a colored complex (if the metal ion is not strongly colored)
Select the wavelength or filter that corresponds to the light absorbed most strongly
Measure absorbances of standard solutions at the selected wavelength
Plot a calibration curve (absorbance vs. concentration)
Measure the sample absorbance and determine concentration from the calibration curve
Account for any dilutions made during sample preparation

Extracting salts from soil

Soil contains a complex mixture called the soil matrix:

Inorganic particles (minerals: sands, silts, clays)
Organic matter (decaying plant or animal material)
Living organisms (fungi, bacteria, protozoans, invertebrates)

Salts can be located anywhere within this matrix—inside cells, bonded to organic matter, within minerals, or dissolved in water.

Extraction procedure

To analyze salts in soil, they must first be extracted into a liquid:

Accurately weigh a dried soil sample
Add an extraction liquid (often water-based) to dissolve the target salt
Remove solid material from the extract (by filtration)
Dilute the extract to an accurate volume using a volumetric flask
Analyze the extract using colorimetry or UV-visible spectroscopy
Determine concentration using a calibration curve
Calculate the mass or moles of salt per gram of soil

Calculating salt concentration in soil

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Essential formula for soil concentration calculations:

$\text{Concentration in soil} = \frac{\text{concentration of salt in extract} \times \text{volume of extract}}{\text{mass of dried soil}}$

Ensure the volume units match (e.g., both in litres).

Worked example: Amount of salt in soil

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Worked Example: Determining Tin Concentration in Soil

Problem: A soil sample suspected of tin contamination was dried and weighed ( $0.298$ g). After acid extraction and dilution to $50$ mL, the tin concentration was measured as $2.74 \text{ mg L}^{-1}$ . Calculate the concentration of tin in the soil.

Solution:

Thinking	Working
Check volume units	Volume: $50$ mL Concentration: $\text{mg L}^{-1}$ (litres)
Convert mL to L	$50 \text{ mL} = 0.050 \text{ L}$
Apply formula	$\frac{2.74 \text{ mg L}^{-1} \times 0.050 \text{ L}}{0.298 \text{ g}}$
Calculate	$= 0.46 \text{ mg g}^{-1}$

Answer: The concentration of tin in the soil is 0.46 mg g⁻¹.

Applications of colorimetry and UV-visible spectroscopy

These techniques are used extensively across many fields:

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Major application areas:

Environmental monitoring:

Measuring nitrate and phosphate levels in water
Determining heavy metal concentrations in soil

Medical testing:

Blood sugar levels
Cholesterol measurements
Hemoglobin concentration in blood

Workplace safety:

Monitoring airborne chromium exposure
Detecting lead in various samples

Specific examples:

Chromium workplace monitoring: Workers carry a pump with a filter that collects air particles. Chromium is extracted from the filter and converted to yellow chromate ions ( $\text{CrO}_4^{2-}$ ) for analysis.

Soil phosphate analysis: Phosphate ions are extracted from soil, then reagents (ammonium molybdate and tin(II) chloride) are added to form a dark-blue complex called phospho-molybdenum blue, which is then analyzed.

Portable colorimeters now allow on-site testing in the field, while laboratory UV-visible spectrophotometers provide more precise measurements for complex analyses.

Case study: Flow injection analysers

Flow Injection Analysis (FIA) is an automated technique designed to handle large numbers of samples efficiently. It solves practical problems encountered when:

Analyzing hundreds of samples
Testing for multiple compounds
Working with very small sample volumes

How FIA works

Components:

Injection port for standards and samples
Sample loop (thin tube collecting a set volume)
Mixing chamber where reagents are added
Detector measuring absorbance at specific wavelengths
Computer software for recording and calculating results
Waste collection system

Procedure:

Standards and samples are pumped into the injection port
A precise volume is collected in the sample loop
The sample is injected into mixing chambers
Reagents are automatically added to create a colored solution
The colored solution flows through the detector
Absorbance values are recorded and processed by computer
Calibration curves are created and sample concentrations calculated automatically
Waste is collected for proper disposal

Field applications

Portable FIA instruments allow researchers to obtain real-time data in the field. These systems are powered by car batteries and enable immediate analysis of water quality parameters.

FIA can analyze many substances including: nitrate, nitrite, phosphate, ammonia, sulfate, sulfide, fluoride, bromide, chloride, cyanide, aluminum, and silica.

Example data from Dandenong Creek monitoring:

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Quality control features:

Blank samples check for contamination
Standard checks verify instrument performance remains consistent during analysis

Exam tip: When interpreting calibration data, always check that blank absorbance is zero and that standard checks fall within acceptable ranges of their expected values.

Summary

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Key Points to Remember:

Colorimeters and UV-visible spectrophotometers measure light absorption to determine solution concentrations
The color you observe is complementary to the color absorbed
Use ROYGBV/GBVROY to remember complementary color pairs
Calibration curves plot absorbance ( $y$ -axis) versus concentration ( $x$ -axis) for standard solutions
UV-visible spectrophotometers use monochromators for precise wavelength selection, making them more accurate than colorimeters
Some metal ions must form colored complexes with ligands before analysis
When calculating salt concentration in soil: $\frac{\text{concentration} \times \text{volume of extract}}{\text{mass of soil}}$
These techniques have wide applications in environmental monitoring, medical testing, and workplace safety

Instrumental Analysis for Salts (VCE SSCE Chemistry): Revision Notes