Principles of Quantitative Analysis - SS3 Chemistry Lesson Note

Quantitative analysis is a branch of analytical chemistry that focuses on determining the exact amounts or concentrations of substances present in a sample. This process involves precise measurement and data analysis, relying on well-established principles and techniques. Here, we explore the advanced concepts that underpin quantitative analysis:

Calibration and Standardization:

Calibration is a fundamental principle in quantitative analysis. It involves creating a relationship between the measured signal (e.g., instrument response) and the concentration of the analyte (the substance of interest) in the sample. To achieve this, a series of standards with known concentrations of the analyte is prepared and measured under identical conditions to generate a calibration curve. This curve serves as a reference to quantify the concentration of the analyte in the unknown sample based on its measured signal.

Accuracy and Precision:

Accuracy refers to how close the measured value is to the true or accepted value. Precision, on the other hand, refers to how close repeated measurements of the same sample are to each other. In quantitative analysis, it is essential to achieve both high accuracy and precision to obtain reliable and valid results.

Limit of Detection (LOD) and Limit of Quantification (LOQ):

The limit of detection (LOD) is the lowest concentration of an analyte that can be reliably detected but not necessarily quantified. The limit of quantification (LOQ) is the lowest concentration that can be quantified with acceptable accuracy and precision. These parameters are crucial in determining the sensitivity of an analytical method.

Calibration Curves and Regression Analysis:

Calibration curves are graphical representations of the relationship between analyte concentration and instrument response. They are constructed by plotting the known concentrations of the standards against their corresponding instrument signals. Regression analysis, such as linear regression, is often used to fit the data and obtain an equation that relates the signal to the concentration.

Sample Preparation and Handling:

Accurate quantitative analysis relies on proper sample preparation and handling. Samples must be representative of the whole and free from interference. Precautions are taken to minimise contamination and loss of analytes during preparation.

External and Internal Standards:

In quantitative analysis, external standards are solutions with known concentrations of the analyte added to the sample for calibration. Internal standards, on the other hand, are substances added in a fixed concentration to all samples and standards. Internal standards aid in correcting for potential errors in the analysis, such as variations in sample preparation or instrumental drift.

Gravimetric Analysis and Titration:

The gravimetric analysis involves quantifying analytes based on the mass of a pure precipitate formed through a chemical reaction. Titration is a quantitative method where a known reagent (titrant) is added to the analyte until a specific endpoint is reached. The volume of titrant required to reach the endpoint is used to determine the analyte's concentration.

Spectroscopic Techniques:

Spectroscopic techniques, such as UV-visible spectrophotometry and atomic absorption spectrometry, are widely used in quantitative analysis due to their high sensitivity and selectivity.

In conclusion, the principles of quantitative analysis revolve around accurate calibration, precise measurements, and reliable data analysis. By understanding the relationships between concentration and instrumental response, conducting proper sample preparation, and employing appropriate analytical techniques, chemists can confidently determine the amounts or concentrations of substances in complex samples, contributing to various scientific, industrial, and environmental applications.