Analytical Chemistry is the science of obtaining, processing, and communicating information about the composition and structure of matter. It answers the question, "What is it, and how much of it is there?"
Interdisciplinary Nature:
Analytical chemistry is a service science; it's essential to nearly all other scientific fields:
Medicine: Blood tests (e.g., glucose, cholesterol levels), drug analysis, and disease diagnosis.
Environment: Monitoring water and air pollution (e.g., pesticides, heavy metals).
Food Science: Checking for nutritional value, detecting adulterants, and ensuring safety.
Forensics: Analyzing DNA, fibers, and unknown substances at a crime scene.
Industry: Quality control to ensure a product (like plastic, metal, or pharmaceuticals) meets specifications.
Concept of Sampling
Sampling is the process of selecting a small, representative portion of a large bulk material for analysis. It is often the most critical step, as the analysis result is useless if the sample is not representative of the whole.
Example: To analyze a 10-ton shipment of grain for protein, you can't test the whole shipment. You must take small amounts from multiple bags/locations and combine them (a "composite sample") to accurately represent the average protein content.
Homogeneous Sample: A sample that is uniform throughout (e.g., a clear salt solution). Sampling is easy.
Heterogeneous Sample: A sample with non-uniform composition (e.g., a soil sample, a chocolate chip cookie). Sampling is difficult and requires careful procedures (like grinding, mixing, and quartering) to get a representative portion.
Accuracy, Precision, and Errors
Concept
Definition
Example (True Value = 5.0 g)
Accuracy
How close a measured value is to the true or accepted value.
A measurement of 4.9 g is accurate. A measurement of 4.2 g is not.
Precision
How close a set of measurements are to each other (reproducibility).
Measurements of 4.2 g, 4.2 g, and 4.1 g are precise (but not accurate).
Goal: All good analysis must be both accurate and precise.
Sources of Error in Analysis
An error is the difference between the measured value and the true value.
Systematic (or Determinate) Errors:
What: A reproducible error that is always in one direction (e.g., always too high or always too low). It affects accuracy.
Cause: Faulty equipment (e.g., uncalibrated balance), incorrect method, or personal bias (e.g., consistently misreading a burette).
Fix: Can be identified and corrected (e.g., by calibrating equipment).
Random (or Indeterminate) Errors:
What: Unpredictable, small errors that can be positive or negative. They affect precision.
Cause: Natural limitations of measurement (e.g., fluctuation in temperature, electronic noise in an instrument).
Fix: Cannot be eliminated, but can be minimized by taking multiple measurements and calculating an average.
Significant Figures
Significant figures are all the certain digits in a measurement plus one uncertain (estimated) digit. They indicate the precision of a measurement.
Rules for Counting:
Non-zero digits are always significant (e.g., 22.5 has 3).
Captive zeros (between non-zero digits) are significant (e.g., 2.05 has 3).
Leading zeros (before non-zero digits) are NOT significant (e.g., 0.0025 has 2).
Trailing zeros are significant ONLY if there is a decimal point (e.g., 25.0 has 3; 2500 has 2, but 2500. has 4).
Rules for Calculations:
Addition/Subtraction: The answer is limited by the number with the *fewest decimal places*.
(e.g., 12.52 + 5.1 = 17.62 → rounds to 17.6)
Multiplication/Division: The answer is limited by the number with the *fewest significant figures*.
(e.g., 12.52 (4 sig figs) × 5.1 (2 sig figs) = 63.852 → rounds to 64)
Concentration Units
Used to express the amount of solute in a given amount of solvent or solution.
Molarity (M): Moles of solute per liter of solution (mol/L). Used in titrations.
Molality (m): Moles of solute per kilogram of solvent (mol/kg). Temperature-independent.
Normality (N): Gram equivalent weight of solute per liter of solution. (Often Molarity × n-factor).
Parts Per Million (ppm): Milligrams of solute per liter of solution (mg/L). Used for very dilute solutions, like pollutants in water. (1 ppm = 1 mg/kg)
Parts Per Billion (ppb): Micrograms of solute per liter of solution (µg/L). Used for trace-level analysis.
Introduction to Chromatography
Definition: Chromatography is a powerful laboratory technique used to separate the components of a mixture.
Principle:
All chromatographic separations work based on differential partitioning. The mixture is dissolved in a mobile phase (a liquid or gas), which is then passed over a stationary phase (a solid or a liquid coated on a solid).
The components of the mixture separate because they have different affinities (attractions) for the two phases.
A component that is strongly attracted to the stationary phase will move slowly.
A component that is more soluble in the mobile phase will move quickly.
This difference in speed causes the components to separate into distinct bands.
Paper Chromatography
Stationary Phase: A high-quality filter paper (cellulose). The water trapped in the pores of the paper is the true stationary phase.
Mobile Phase: A developing solvent (e.g., a mixture of alcohol and water).
Technique:
1. A spot of the mixture (e.g., ink) is applied near the bottom of the paper.
2. The paper is suspended in a sealed jar with the bottom edge (below the spot) dipped into the mobile phase.
3. The solvent moves up the paper by capillary action, carrying the components of the mixture with it.
4. The components separate into different colored spots.
Thin Layer Chromatography (TLC)
A faster and more efficient version of paper chromatography.
Stationary Phase: A thin layer (e.g., 0.25 mm) of an adsorbent material (like silica gel or alumina) coated onto a glass plate or plastic sheet.
Mobile Phase: A solvent or solvent mixture in a developing tank.
Technique: Similar to paper chromatography. A small spot of the sample is applied, and the plate is placed in a sealed tank with the solvent. The solvent moves up by capillary action, separating the components.
Visualization: If the spots are colorless, they can be visualized by spraying with a reagent (e.g., ninhydrin for amino acids) or by viewing under UV light.
[Image of a TLC plate showing separated spots]
R_f Value (Retardation Factor)
Used in both TLC and paper chromatography to identify components.
R_f = (Distance traveled by the spot) / (Distance traveled by the solvent front)
The R_f value is always between 0 and 1.
It is a characteristic constant for a specific compound, in a specific solvent system, on a specific stationary phase.
By comparing the R_f value of an unknown spot to the R_f value of a known standard, one can identify the unknown.